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Saving of Time and Energy in the Laundry Process: Importance of Dynamic Surface Tension, Micelle Stability and Surfactan...


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1 SAVING TIME AND ENERGY IN THE LA UNDRY PROCESS: IMPORTANCE OF DYNAMIC SURFACE TENSION, MIC ELLE STABILITY AND SURFACTANT ADSORPTION By DANIEL LARRY CARTER A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2007

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2 2007 Daniel Larry Carter

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3 To my parents, for their continuing support over the years as well as th eir guidance through life. Also, to Dr. Dinesh Shah for being much more than an advisor over the years, for being a wonderful father figure and only as king the best and most from me.

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4 ACKNOWLEDGMENTS This research impart was done with assistan ce from the University of Floridas Summer Science Training Program (UF-SSTP). The author would like to thank th e Procter and Gamble Company for financial support for th is research and the University of Floridas Particle Research and Engineering Center (PERC) fo r use of their labs and equipment. I would also like to thank Mr. Vicente Santamarina, Dr. Shulin Zhang, Mr Rudy Delgado and Mr. Joe Heatherly of the Procter and Gamble Company and Dr. Ranga Narayanan of the Univ ersity of Florida for their stimulating discussions involving th is research. Also, I would like to thank the Center for Surface Science and Engineering (CSSE) group for the w onderful conversations and help that was provided during my stay at the University of Fl orida. I would like to thank my professors at Auburn University and the University of Flor ida for developing my in terest in chemical engineering and their guidance in helping me in my application process to the University of Florida. Finally, I would like to acknowledge Dr. Dinesh Shah, Dr. Ranga Narayanan, Dr. Brij Moudgil, and Dr. Dmitri Kopelivich for their gu idance and assistance fo r being on my doctoral committee and for providing me the privilege of working with them.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4 TABLE OF CONTENTS.............................................................................................................. ...5 LIST OF TABLES................................................................................................................. ..........8 LIST OF FIGURES................................................................................................................ .........9 ABSTRACT....................................................................................................................... ............14 CHAPTER 1 INTRODUCTION..................................................................................................................16 1.1 Introduction............................................................................................................... ........16 1.2 Theory..................................................................................................................... ..........17 1.2.1 Surface Tension Reduction................................................................................17 1.2.2 Role of Adsorbed Micelles and Micellar Stability............................................19 1.2.3 Surfactant Adsorption........................................................................................20 1.3 Textile Chemistry.......................................................................................................... ...22 1.3.1 Chemical Composition of Cotton Fibers...........................................................22 1.3.2 Properties of Cotton...........................................................................................22 1.3.3 Surface Treatments of Cotton Fabrics...............................................................24 1.4 Scientific Approach........................................................................................................ ..25 1.4.1 Surface Tension Reduction using Surfactants...................................................25 1.4.2 Effect of Bulk and Adsorbed Micelles...............................................................27 1.4.3 Surfactant Vesicle Interactions..........................................................................29 1.4.4 Dewatering of Particle Suspensions...................................................................29 1.4.5 Interactions at the Solid-Liquid Inte rface: Role of capillarity in the retention of water in fabrics.........................................................................................31 2 THE RELATIONSHIP OF SURFACE TENSI ON AND THE RESIDUAL MOISTURE CONTENT OF FABRICS......................................................................................................40 2.1 Experimental Background................................................................................................40 2.1.1 Measuring Residual Moisture Content..............................................................42 2.1.2 Surface Tension Measurements.........................................................................43 2.1.3 Materials............................................................................................................43 2.2 Experimental Basis......................................................................................................... ..44 2.2.1 Time Basis.........................................................................................................44 2.2.2 Centrifugation Speed Basis (Effect of Increasing Gravitational Force)............45 2.3 Lowering of Surface Tension by Surfactant Systems.......................................................45 2.3.1 Simple Surfactant Systems................................................................................45

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6 2.3.2 Mixed Surfactant Systems: SDS + C12TAB......................................................46 3 THE EFFECT OF SURFACTANT ADSORPTION ON THE RESIDUAL MOISTURE COTENT OF FABRICS.........................................................................................................51 3.1 Peak in SDS RMC Curve as a Functi on of Increasing SDS Concentration.....................51 3.1.1 Materials............................................................................................................52 3.1.2 Residual Moisture Content (RMC) Measurements............................................52 3.1.3 Surface Tension Measurements.........................................................................53 3.1.4 Adsorption Measurements.................................................................................54 3.2 Dynamics of Residual SDS Solution and the Effect on RMC..........................................54 3.2.1 Surface Tensions of Residual Solutions (Dynamic and Equilibrium) and its Correlation to RMC of Fabrics....................................................................................54 3.2.2 Molecular Mechanism: Explanation of the Peak in the SDS/RMC Curve........57 3.3 Adsorption of SDS onto Cotton Surfaces.........................................................................58 3.4 RMC Peak in Various Surfactant Systems.......................................................................61 3.5 Manipulation of RMC Peak: Fabric Pre-Tr eatment and its Affects on Adsorption of SDS onto Cotton................................................................................................................ .62 4 REDISCOVERING MONOLAYER PENETR ATION: OBTAINING ULTRA-LOW AIR-LIQUID SURFACE TENSIONS...................................................................................77 4.1 Monolayer Penetration......................................................................................................77 4.1.1 Monolayer Penetration Studies..........................................................................79 4.1.2 Reduction of Surface Tension: Monolayer Penetration Results........................80 4.2 Reduction of RMC via Monolayer Penetration................................................................83 4.2.1 Experimental Procedure.....................................................................................83 4.2.2 Small Scale Monolayer Penetration Results......................................................84 4.2.3 Washer Scale Monolayer Penetration Results...................................................84 5 MICELLE STABILITY AND ITS EFFECT ON THE RESIDUAL MOISTURE CONTENT OF FABRICS......................................................................................................94 5.1 Stabilization of Micelles.................................................................................................. .94 5.1.1 Concentration Dependence on Mi celle Stability: 200 mM SDS.......................96 5.1.2 Mixed Surfactant Systems.................................................................................98 5.2 Effect of Dodecyl Sulfate Count erions on the RMC of Fabrics.......................................99 5.2.1 Experimental procedure (Surfactant Synthesis)...............................................100 5.2.2 Molecular mechanisms....................................................................................101 5.3 Chain Length Compatibility...........................................................................................102 5.3.1 Review of Chain Length Compatibility Work.................................................102 5.3.2 SDS + Long Chain Alcohols (CnOH)..............................................................103 5.3.3 SDS + CnTABs................................................................................................104 5.4 Labile Micelles............................................................................................................ ...105 5.4.1 SDS + Polyvinylpyrrolidone (PVP).................................................................106 5.4.2 SDS + Short Chain Alcohols (CnOH)..............................................................106

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7 6 FULL SCALE RESIDUAL MOISTURE CONTENT TESTING UNDER NORMAL CONDITIONS..................................................................................................................... .118 6.1 Various Surfactant Systems in Full Washer Scale.........................................................118 6.1.1 Small Scale Testing..........................................................................................119 6.1.2 Full Scale Washer Testing...............................................................................119 6.2 Vesicle Surfactant Interactions.......................................................................................120 6.2.1 Turbidity..........................................................................................................120 6.2.2 Particle Sizing..................................................................................................121 6.2.3 Molecular Mechanisms between SDS and Vesicles........................................121 6.3 Washer Scale RMC Testing............................................................................................122 7 SUMMARY AND RECOMMENDATIO NS FOR FUTURE WORK................................132 7.1 UF Contributions to the Lowering of Surface Tension: Saving Energy in the Laundry Process................................................................................................................132 7.2 Technological Impact of the Reducti on of RMC in the Laundry Process......................135 7.3 Recommendations for Future Work...............................................................................136 7.3.1 Dynamic Surface Tension................................................................................136 7.3.2 Micelle Stability...............................................................................................136 7.3.3 Monolayer Penetration.....................................................................................138 7.3.4 Surfactant Adsorption......................................................................................138 APPENDIX A ORIENTATION OF ADSORBED SURFACTANT MOLECULES ON COTTON FABRIC......................................................................................................................... .......139 B REMOVAL OF WATER FROM CAPILLA RIES: MATHEMATICAL APPROACH.....141 BIOGRAPHICAL SKETCH.......................................................................................................154

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8 LIST OF TABLES Table page 1-1. Composition of typical cotton fibers......................................................................................39 1-2. Relationship between area per molecule and surface tension................................................39 1-3. Micellar stabilities fo r pure surfactant systems......................................................................39 1-4. Micellar stabilities for SDS mixed surfactant systems...........................................................39 2-1. Surface tensions and co rresponding RMC values for Hanes fabric for various commercially available surfactants....................................................................................50 3-1. The magnitudes of the RMC peak for various fabrics for I) the absolute different in the maximum and minimum of the RMC peak, II) the difference in the maximum and minimum normalized with respect to the RMC maximum and III) the difference in the maximum and minimum normalized wi th respect to the RMC minimum..................76 4-1. RMC of small scale monol ayer penetration with C14S04:DODAB monolayer......................93 5-1. Micellar relaxation times ( 2) for different SDS systems with the addition of cosurfactants.................................................................................................................... ....116 5-2. Physical properties and dime nsionless dynamic surface tension ( ) of different counterions of dodecyl sulfate.........................................................................................117 6-1: RMC for terry cloth fabrics with the a ddition of various addi tives added to 500 ppm solutions of fabric softener...............................................................................................131 6-2: RMC values for large scale washing m achine experiments for various surfactant systems........................................................................................................................ .....131

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9 LIST OF FIGURES Figure page 1-1. Graphic represention of how water is reta ined and removed from laundry in the washing machin......................................................................................................................... .......32 1-2. SEM picture of cotton fabrics............................................................................................. ....32 1-3. Forces involved in capillary rise where FSFT is the force due to surface tension (2cos r ) and Fd is the force due to gravity (mg or 2Hrg )...................................33 1-4. Capillary rise dynamics observed for 1 mM C14E6 surfactant solutions................................33 1-5. The effect of capillary number on the residual oil in porous media.......................................34 1-6. Liquid/gas phenomena exhibiting minima and maxima at 200 mM SDS concentration.......34 1-7. Liquid/liquid and solid/liquid phenomen a exhibiting minima and maxima at 200 mM SDS concentration.............................................................................................................35 1-8. Effect of micellar stabil ity on dynamic surface tension.........................................................35 1-9. Micelles stabilizing a thin film between fabric fibers............................................................35 1-11. Two segments in the cellulose chain....................................................................................36 1-12. Synergism between a cationic Gemini and anionic n-dodecane sulfonate...........................36 1-13. Possible surfactant morphologi es at a solid liquid interface................................................37 1-14. Force required to puncture micel les adsorbed on a mica surface.....................................37 1-15. Moisture content of filter cake as a functio n of surfactant concentration used in slurry pretreatment................................................................................................................... ....38 1-16. Adsorption characteristics of surfactants on kaolin..............................................................38 2-2. Experimental apparatus used to determine the RMC of various fabrics................................47 2-3. RMC as a function of centrifugation time..............................................................................48 2-4. RMC as a function of centrifugation speed............................................................................48 2-5. Relationship between RMC and surface tension....................................................................49 2-6. Relationship between RMC and surface tension for commercial surfactant systems............49

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10 2-7. Residual moisture content and surface tens ion of various weight ratios of SDS to C12TAB............................................................................................................................ ..50 3-1. Experimental apparatus used to dete rmine the residual moisture of fabrics..........................64 3-2. RMC of Hanes 100% cotton fabric as a function of SDS concentration...............................65 3-3. RMC of Hanes 100% cotton fabric as a function of SDS concentration...............................65 3-4. Equilibrium and dynamic surface tension of residual SDS soluti on after exposure to Hanes fabric................................................................................................................... ....66 3-5. Comparison of the RMC of Hanes fabric and the equilibrium surface tension of residual solution after soaking the fabric.........................................................................................66 3-6. RMC of Hanes cotton fabric, terry cloth fabr ic and DOE test fabric as a function of SDS concentration.................................................................................................................. ....67 3-7. RMC and DST of the residual soluti on from the Hanes 100% cotton fabric.........................67 3-8. RMC and DST of the residual soluti on from DOE 50:50 cotton:polyester fabric.................68 3-9. RMC and DST of residual solution from th e Terry Cloth 86:14 cotton:polyester fabric......68 3-10. Indication of the regions associated with the peak in the RMC of Hanes cotton fabric......69 3-11. Molecular mechanism for the adsorption of SDS onto cotton.............................................69 3-12. Calibration curve for MBAS method...................................................................................70 3-13. Adsorption of SDS adsorb ing onto Hanes cotton fabric......................................................70 3-14. RMC of Hanes fabric soaked in C10 fatty acid.....................................................................71 3-15. RMC of Hanes fabric soaked in solutions of a leading fabric softener................................71 3-16. RMC of Hanes fabric soaked in solutions of a leading detergent........................................72 3-17. RMC of Terry cloth fabric soaked in solutions of SDS.......................................................72 3-18. RMC of terry cloth fabric pre-treated with CMC.................................................................73 3-19. RMC of terry cloth fabr ic pre-treated with PAA..................................................................73 3-20. RMC of terry cloth fabr ic pre-treated with PVP..................................................................74 3-21. RMC of Terry cloth fabric pre-treated with C18 Fatty Acid.................................................74 3-22. RMC of Terry cloth fabric pre-treated with C18 Alcohol.....................................................75

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11 3-23. RMC of terry cloth fabr ic pre-treated with DODAB...........................................................75 4-1. Spreading and penetration of an insoluble monolayer...........................................................85 4-2. Equilibrium surface tension of a C16TAB monolayer penetrated with 4 mM C14SO4...........86 4-3. Equilibrium surface tensi on of a stearic acid monolayer penetrated with 4 mM C14TAB.....86 4-4. Equilibrium surface tension of a DDA B monolayer penetrated with 4 mM C14SO4.............87 4-5. Molecular diagram of the transient m onolayer phenomena found in slightly soluble monolayers penetrated with a solubl e surfactant from the subphase.................................87 4-6. Equilibrium surface tension of a DOD AB monolayer penetrated with 4 mM C14SO4..........88 4-7. Equilibrium surface tension of a C20OH monolayer penetrated with 4 mM C14SO4.............88 4-8. Equilibrium surface tension of a Choles terol monolayer penetrated with 4 mM C14SO4......89 4-9. Equilibrium surface tension of a mixed monolayer composed of a 1:5 ratio of C14SO4 to DODAB.......................................................................................................................... ...89 4-10. Equilibrium surface tension of mixe d monolayers of various ratios of C14SO4 to DODAB penetrated with 1000 L of 4 mM solutions of C14SO4.....................................90 4-11. Time study of the equilibrium surf ace tension of a mixed monolayer of C14SO4 and DODAB in a 1:5 Ratio injected with 1000 L of 4 mM C14SO4......................................90 4-12. 2-D hexagonal arrangement of mol ecules at the 1:3 molecular ratio...................................91 4-13. Experimental apparatus used to measure RMC of monolayer penetration..........................91 4-14. Comparison of RMC values for fu ll scale washing machine experiments...........................92 4-15. Comparison of RMC values for fu ll scale washing machine experiments...........................92 5-1. RMC of Hanes 100% cotton fabric as a function of SDS concentration.............................107 5-2. RMC and DST of the residual solu tion from the Hanes 100% cotton fabric.......................107 5-3. Liquid/gas phenomena exhibiting minima and maxima at 200 mM SDS concentration.....108 5-4. Liquid/liquid and solid/liquid phenomen a exhibiting minima and maxima at 200 mM SDS concentration...........................................................................................................108 5-5. RMC of Hanes fabric around the concentrat ion range of most st able micelles of SDS (200 mM)....................................................................................................................... ..109 5-6. Stable micelles trapped in the in terstitial space in be tween fiber strands............................109

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12 5-7. Effect of micellar stabil ity on dynamic surface tension.......................................................110 5-8. RMC of Hanes cotton fabric fo r pure SDS and mixed SDS systems...................................110 5-9. Equilibrium surface tension for dodecyl su lfate surfactants with various counterions........111 5-10. Effect of counterions on the molecular packing of dodecyl sulf ate at the air/liquid interface...................................................................................................................... ......111 5-11. Foam stability for various dodecyl sulf ate counterions at 50 mM total surfactant concentration.................................................................................................................. ..112 5-12. RMC of Hanes cotton fabric for various dodecyl sulfate counter ions at 1 mM total surfactant concentration...................................................................................................112 5-13. RMC of Hanes cotton fabric for various dodecyl sulfate counter ions at 50 mM total surfactant concentration...................................................................................................113 5-14. RMC of cotton fabri cas a function of SDS + CnOHs.........................................................113 5-15. RMC of cotton fabric as a function of SDS + CnOHs........................................................114 5-16. Long relaxation time (2) and RMC of Terry fabric in solutions of PVP..........................115 5-17. Long relaxation time (2) and RMC of Terry fabric in solutions of 100 mM SDS with the addition of short chain alcohols.................................................................................116 6-1: Equilibrium surface tension and RMC of fa bric softener systems with the addition of sodium dodecyl sulfate (SDS).........................................................................................123 6-2: Equilibrium surface tension of fabric softener solutions with the addition of dioctylsulfosuccinate (AOT)............................................................................................124 6-3: Equilibrium surface tension of fabric so ftener solutions with the addition of Dow Corning Q2-5211 Silicone Super Wetter.........................................................................124 6-4: Temperature dependence of the RMC during the final rinse cycle......................................125 6-5: Drying rate curve for terry fabr ics soaked in various solutions...........................................125 6-6: Turbidity measurements of fabric soft ener solutions with the addition of SDS..................126 6-7: Mean particle size of 500 ppm solutions of fabric softener with the addition of SDS.........126 6-8: Particle size and turbidity of 500 ppm fabric softener solutions with the addition of SDS............................................................................................................................ ......127 6-9: Molecular diagram describing the interaction of vesi cles with SDS...................................127

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13 6-10: RMC of Terry cloth fabric with the addition of Cascade...................................................128 6-11: RMC of Terry cloth fabric with the addition of Dow Q2-5211.........................................128 6-12: RMC of Terry cloth fabric with the a ddition of Flexiwet (Q22, RFD-15A, and NF)......129 6-13: RMC of Terry cloth fabric with the addition of Jet Dry.....................................................129 6-14: RMC of Terry cloth fabric with the addition of Sylgard 309.............................................130 6-15: Comparison of RMC values for fu ll scale washing machine experiments........................130

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14 Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy SAVING TIME AND ENERGY IN THE LA UNDRY PROCESS: IMPORTANCE OF DYNAMIC SURFACE TENSION, MIC ELLE STABILITY AND SURFACTANT ADSORPTION By Daniel Larry Carter May 2007 Chair: Dinesh O. Shah Cochair: Ranga Narayanan Major: Chemical Engineering It has been shown that the resi dual moisture content of fabric s at the end of a centrifugation cycle is related to the equilibrium surface tension of the residual solution in the fabric. However, in the case of lowering the surface tension of so lution via increasing concentrations of sodium dodecyl sulfate, a peak is observed in the residual moisture content and the residual moisture of fabrics deviates from the predictions of the LaPlace Equation of capillary rise. This is due to adsorption of surfactant on the fabric and the in crease in dynamic surface tension. Several other molecular mechanisms have been found to affect th e residual moisture cont ent of fabrics such as the following: dynamic surface tensi on, micellar stability, chain le ngth compatibility, surfactantvesicle interactions as well as monolayer penetra tion. In surfactant system s with stable micelles, the dynamic surface tension of solution is high co mpared to labile micellar solutions and the resulting residual moisture content of fabrics is higher than the residual moisture of labile micellar systems. Similarly, when the chain lengt hs of multiple surfactants in a two surfactant system are of the same length, the micellar stabil ity of this system is much higher compared to mismatched chain lengths. The residual moistu re content of fabrics in same chain length surfactant systems has been shown to be higher compared to mismat ched chains. It has also been

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15 established that monolayer penetration can be used to lower the air-liquid surface tension of solution. In this work, we developed a method to lower the air-liquid surf ace tension values to values much lower than published results (about 7 mN/m). This method was then used to reduce the residual moisture content of fabrics in the laundry process.

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16 CHAPTER 1 INTRODUCTION 1.1 Introduction The detergent industry, with a $5.5 billion ma rket in the US, forms a major part of the consumer goods industry. Two large companies, Procter and Gamble and Unilever, essentially cover this market, which implies that a small perc entage gain in the share over its competitor would amount to major gains in revenues, t hus fuelling stiff competition. To win small competitive ground over its rival, companies are re sorting to fierce promotion of brand name by advertisements and sponsorships, matching pr oduct for products, sa turating shelf space by variations of a brand and investing huge am ounts of money on R & D for making bigger and better products. With many of the products alrea dy working at their current optim um in terms of efficiency of cleaning fabrics, companies have started to explore newer variables (other than surfactant formulations) to add appeal to th eir products. In this regard, a re duced drying time of fabric is a key variable to add consumer appeal. The Depa rtment of Energy (DOE) reports that the cloth dryer is typically the second larg est electricity using appliance after the refrigerator, costing about $2.66 billion to op erate annually nationwide.1 Therefore, a conser vative 30% reduction in the drying time of laundry could result in savings of over $1 billion per year in the United States alone. It is of great interest to the DOE to conserve energy; however, the cost savings per consumer will not be a key point driving consumer sales (about $16 per year savings per household with an electric dryer). The key point in finding ways to reduce water content in fabrics is to understand why the water is being held in the fabric. It is believed that water wicks into fabric because of capillary forces operating between fabric fi bers that create capillaries. If capillary forces are indeed the

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17 cause of water retention, then the capillary rise in the fabric s should be governed by the LaPlace equation for capillary rise. These forces can be strong or weak dependi ng on the weave of the fabric as well as the composition of the fabric. For example, cotton fiber holds about 75-80% of initial water after the spin cycle of a washing ma chine. Work has been do ne in this project to decrease the capillary forces to enhance the water removal from fabrics by the reduction of surface tension between the air-liquid interface (Figure 1-1). This methodology can enhance the design of a laundry product that can significantly reduce the amount of residual water in fabric after the spin cycle thus reduc ing the drying time, and saving energy required for drying the fabric. 1.2 Theory 1.2.1 Surface Tension Reduction To reduce the drying time of fabric s, we have identified the role of surface tension as a key parameter of the laundry process. Our rationale is that if we can redu ce the amount of residual water in fabrics during the spin cycle of the washi ng machine, then it will lead to a reduction in drying time during the dryi ng cycle. It is assumed that the wa ter is held by capillaries in the fabric structure created by overlappi ng fabric fibers (Figure 1-2). In Figure 1-3, the height of a liquid column in a capillary is shown to be a balance of the surface tension forces and the force exerted on the cylinder of water in the capillary by gravity. The basis behind our reasoning is found in the LaPlace Equation for capillary rise (Equation 11), where is surface tension, is the contact angle (which is assumed to be zero for most water wettable fabrics), r is the capillary radius and is the solution density. It should be noted that the contact angle and capillary radius are fixed for a given fabric, leading to the idea that the surface tension of solution is the easiest variable to manipulate.

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18 2cos h rg (1-1) As shown in Equation 1-1, the capillary height is a function of the capillary radius. As the capillary radius is decreased, the capillary height is increased as shown in Figure 1-4. As shown in the figure, as the capillary ra dius is decreased from 0.50 mm to 0.10 mm the capillary height is approximately tripled. If the LaPlace Equation for capillary rise can be applied to the removal of water from fabrics then the same trend would be shown in the residual moisture content of fabrics. If the capillary radii are smaller in a fabr ic, then there would be a higher capillary rise in the fabric leading to increased residual moisture content. It has been shown by Preston et al. that water is retained in moist fibers by capillary water held in spaces between fibers and by hydrates of the fiber molecules.2 They have shown that the amount of retained moisture in viscose and cellulo se fibers is proportional to the surface tension of solution. However, their studies only showed the direct relationship between surface tension of solution and the residual moisture content of fiber systems. In our work, we are using a single surfactant in increasing concentration to vary the surface tension and measuring the Residual Moisture Content (RMC) of consumer fabrics instead of indi vidual fiber strands. In comparison, Preston used several different su rfactants at a single concentration to show the relationship between surface tension and capi llary height for fiber bundles. If by adding proper additives, one can subs tantially decrease the surface tension of a solution, then with the same centrifugal forces in the rinse cycle of ex isting machines, we can remove more water. We feel confident about th is approach because of our earlier work in enhanced oil recovery, where we had to eliminat e or reduce similar capilla ry forces responsible for trapping oil in fine pores of an oil reservoi r by reducing the interfacial tensions (Figure 1-5 where capillary number is a dimensionless ratio of viscous forces and surface tension forces).3

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19 However, the analogy should not be drawn too fa r as it is relativel y easy to achieve low interfacial tensions in oil/water systems (as low as 10-3 dynes/cm) as compared to air/water systems. 1.2.2 Role of Adsorbed Mice lles and Micellar Stability After some of the recent Atomic Force Microscopy (AFM) mapping at solid/water interfaces, where investigators have shown ad sorbed surfactant micelles at the interface4, there is a good possibility that surfactant micelles or othe r aggregates are also adsorbing at the fiber surfaces in the capillary, hence modifying capil lary forces. In systems with high micellar stability, very stable films have been observed. With very stable thin liquid films, the RMC should increase due to the higher force required to disrupt these films (Figure 1-9). In this context, micellar stability concepts become importan t. It is planned to investigate the effect of engineered micelles of low and hi gh stability on water retention in fabric to show their effect on the residual water content. Over the years, the Shah research group has shown that the micellar relaxation time is a maximum at a concentration of 200 mM SDS.3, 5-9 This maximum in micellar relaxation time has a dramatic effect on many different properties of SDS solutions (ranging from low foamability, high thin film stability, wetting time, oil solubi lization, etc. (Figure 1-6 and Figure 1-7). The Shah Research group has also shown that micellar ki netics play an important role in detergency. Shah et al. has shown that the efficacy of re moving non-polar compounds from fabrics has been shown to have a strong correlation w ith the relaxation time of micelles.5-10 For example, it was shown by Oh and Shah that using 200 mM SDS (w hich was shown to have the longest micellar relaxation time in the SDS concentration range)6 provided the most efficient removal of an artificial stain created by the depositi on of Orange OT onto fabric samples.9

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20 Since the dynamic surface tension is related to the micellar stability (i.e. higher micellar stability leads to higher dynamic surface tension Figure 1-8) and we have proposed that the surface tension of solution (equilibrium and dynamic) can affect the amount of water retained by fabrics, it would be expected that there will be an increase in the RMC around a SDS concentration of 200 mM (the concentration of highest stability for the SDS system). This maximum is believed to be due to the long rela xation time of the SDS micelles at 200 mM. The long relaxation time of the micelles would lead to a decreased monomer flux from the micelles to the bulk. This decrease in monomer flux would th en be shown as an increase in the dynamic surface tension thus leading to an increase in the RMC (shown in Figure 1-8). Alternatively, another possible explanation to explain the increase in RMC at 200 mM concentrations of SDS could be due to stabilization of thick films on the fabric surface as well as within the interfiber spaces due to relatively stable micelles. It has been shown by Shah et al.11, 12 and Wasan et al.13-18 that layering of micelles or particles can stabil ize thin films (which could possibly explain an increase in the RMC). 1.2.3 Surfactant Adsorption It is proposed that sudden adsorption of su rfactant onto the fabric surface can affect the residual moisture content of fabrics due to th e changes in dynamic surf ace tension of solution. Since it has been shown that cotton has a negativ e zeta potential one might think that an anionic surfactant would have minimal adsorp tion on a negatively charged surface.19-21 However, there have been several papers showing that ability of sodium dodecyl sulfate (SDS) and other anionic surfactants to adsorb onto negatively charged surf aces such as coal fines, cotton and cellulose.19, 22-25 Also, it has been shown by So masundaran et al. that adsorp tion isotherms can show up to four adsorption regions,26 one of them being a sudden increas e of adsorption due to cooperative adsorption of surfactant molecules. If surfactant molecules suddenly adsorb cooperatively on the

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21 solid surface at a critical concen tration, then it must cause a concomitant decrease in monomer concentration in the bulk so lution. Thus, a simple method to determine the monomer concentration below CMC is to measure the surfa ce tension of the residual solution. For a given surfactant below its CMC, the surface tension is a measure of the free monomer concentration of surfactant in solution. However, if the change in surfactant monomer con centration is not very large then the equilibrium surface tension may not change significantly. However, the dynamic surface tension may reflect it more clearly. If there is a sudden increase in adsorption on the fabric surface then there would be less free m onomer available to adsorb on the new air-liquid interface of bubbles create d during the dynamic surface tension measurement. One important aspect in the removal of water fr om fabrics is the ability of surfactants to adsorb onto the textile fibers. Se veral researchers have shown that even though most fibers have a negative charge, the anionic surfactants are sti ll able to adsorb onto th e fibers. As shown in Figure 1-10, sodium dodecyl sulfate is able to ad sorb onto cotton fibers. However, the adsorption continues to increase after the solution CMC (8 mM). Rybicki explains that the increase of adsorption after the CMC is due to a competitive adsorption process between surfactant monomers and micelles. Near the CMC, adsorpti on of monomers and micel les is equally easy, while at higher concentrations micellar adsorption is dominant.19 Another explanation to this phenomenon may be due to fiber swelling. When th e fabric is placed in solutions near the CMC, micellar and monomer adsorption is present. Howeve r, due to the lower number of micelles, the monomers are still able to adsorb into micro-pores that are present when the fibers swell. As the concentration is increased, the micellar phase also increases. Due to the increased numbers of micelles, there is a higher probability that thes e micelles can adsorb and block the micro-pores thus blocking monomers from pene trating into the swollen fibers.

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22 1.3 Textile Chemistry One important factor in unders tanding how the RMC of fabrics can be manipulated is understanding the effect of fiber structure and fiber chemistry on wa ter retention. Since there are many different types of fabrics as well as different fabric weaves, it is important to understand their effect on water retention. 1.3.1 Chemical Composition of Cotton Fibers On average, raw cotton fiber is approximatel y 95% cellulose which is hydrophilic (Table 1-1).27 The remainder of the fibers is noncellu losic materials that are mostly hydrophobic (proteins and waxes). However, these noncellulo sic materials can be selectively removed by using the proper solvents (i.e. chloroform remo ves waxes; ethanol removes waxes, sugars and ashes, etc.).28 Also included in the composition of cotton fibers are various metals which can cause several problems in yarn manufacturing (i.e. silica and metal oxides can cause friction problems in spinning; peroxide bleaching can be affected by ma gnesium salts; copper, calcium and magnesium can interfere with dyeing, etc). 1.3.2 Properties of Cotton One concern in the treatment of cotton fibers is the effect of solvents on the chemical and mechanical properties of the fibers.27 The cellulose produced by the bolls in cotton plants is composed mostly of a polysaccharide cellulose. The polymeric backbone for the cellulose is a linear polymerized form of -D-glucopyranose (Figure 1-11) which is linked at the 1,4-glucosidic oxygen bridge.27 As briefly discussed, cotton is a hydrophilic fibe r as well as porous. It has been shown that when immersed in water, cotton fibers swell and its pores fill with water.27 Due to the small size of the pores, chemical agents used for modifica tion are not always able to penetrate the pores. These modifications are important in the textile business and can include modifications such as

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23 color dyeing, flame resistance, so il release, etc. Therefore, know ledge of these pores is and a chemicals accessibility to these pores is a ne cessity. Several methods have been used in the determination of pore accessibility such as solu te exclusion. Using a series of water-soluble molecular probes (increasing in size) that penetr ate the cotton fiber and do not adsorb onto the fiber surface, the water in poresis exchanged by a solute resulting wi th dilution into the external solution. Then, using chromatogr aphic techniques, the amount of internal volume expansion or contraction can be determined.27 Another aspect of cotton fabr ics is water swelling known as bound water or hydrated water. It has been shown that between 0.1-0.2 g/g of water present in cotton fibers is bound water that can only be removed by thermal methods. One fabric treatment that is widely used in industry is the swelli ng of cotton by sodium hydroxide (called mercerization). 27 This process is used to improve fabric properties such as dye affinity, tensile strength, and smoothness. The great improvement in these properties by mercerization is thought to be due to the increase in pore sizes in the cotton fibers. However, another method of fiber swelling, treatment with liquid ammonia, has been shown to have a low level of large pores in the fibers.27 There are a few other fabric treatments used in industry that can improve fiber properties. Several such methods include et herification (increases wrinkle resistance, water repellency, flame resistance and antimicrobial action) esterification and enzy me modification (for fabric softness and colorfastness of dyes). Fabric dyeing is another importa nt practice in industry that has implication in the removal of water from fabrics. It is general practice to prepare the fabric surface by operations like singeing, desizing, scouring, bleaching and merceriz ation. As stated previously, these treatments are used in the removal of impurities from the co tton fibers (such as waxes, pectins and ashes).

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24 Several dyeing methods exist in i ndustry (such as azoic, direct, re active, sulfur and vat dyeing) but reactive and direct dyeing ar e the most common methods. Dir ect dyeing affixes dyes to the cellulose by hydrogen bonding and van der Waals forces to attach the dye to the fabric surface. The dyes and pigments used in these processes are mostly of water insoluble inorganic or organic composition.27 Since some of the dyes are bonded to the surface or reactively affixed to the fiber structure, changes in th e hydrophilicity of the fiber are possible, hence the implications into the dewatering of fabrics. These dyes can also influence the affinity of ionic surfactants in the adsorption of these molecu les to the fabric surface. 1.3.3 Surface Treatments of Cotton Fabrics As mentioned, there are many types of surface treatments for fabrics (with a main focus on cotton) that are commonly used for consumer co nvenience in order to enhance the properties of the fabric. One important cotton treatment is maki ng the cotton fibers flame resistant. Most of the treatments used in this pr ocess are inorganic based chemicals that are chemisorbed on the fabric (in the case of using multivalent metallic sa lts, the surface charge of the fabric will change therefore changing the adsorption kinetic s of surfactants onto the fabric).28 Another widely used fiber treatment process is treating the fibers for repellency. Several types of treatments are used in the case of different types of repellency (i.e. water, oil, and soil). In the case of water repellency, several diffe rent types of hydrophobic compounds can be used to coat the fabric (ranging from waxes a nd siloxanes, to fluorocarbon treatments).28 As mentioned, the contact angle of the solution on the fabric su rface is one factor in the water removal from fabrics (based on the LaPlace Equation for capillary rise). If the contact angle of the solution is increased, the cos term in the LaPlace equation is decr eased, which would result in a lower

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25 capillary rise or lower water retention. Therefore, any fabric that is treated with any of these repellents will show lower residual moisture due to an increased contact angle. 1.4 Scientific Approach Due to the complex nature of the removal of water from fabrics, several challenges may arise that need to be understood. Technologies ranging from fabric chem istry to the dewatering of coal fines may provide insight to the removal of water from fabrics. There are several things that should be investigated in the process of removing water from fabrics such as surfactant adsorption onto surfaces, the role of vesicle-su rfactant interaction in the lowering of surface tension as well as the role of dynamic processe s such as dynamic surface tension and transient surface tension by monolayer penetration. 1.4.1 Surface Tension Reduction using Surfactants For pure surfactants, either anionic or non-ionic, the surface tension (after critical micelle concentration, cmc) is usually in the range of 30 40 mN/m. For anionics, the surface tension strongly depends on the count er-ion used (Table 1-2).29, 30 In the case of non-ionic surfactant system (CiEj), the surface tens ion changes as a function of the number of ethoxylated groups. A host of mixed surfactant systems show s ynergism in terms of reducing the surface tension, whereby at particular mole ratios of two surfactants, one achieves surface tension values unattainable by an individual surfactant alone. The surface activity is not only enhanced at the gas-liquid interface but also at solid-liquid interfa ce leading to a better wetting of textile fibers and enhanced detergency.31 Working with sodium dodecyl sulfate (SDS) in their chain compa tibility study, Sharma and others32 have shown that the maximum reduction in surface tension of SDS solution occurs by addition of a small amount of dodecanol, i.e. when the chain length of the alcohol and the surfactant are matching. In a related study, Patist et.al.33 showed a maximum reduction in surface

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26 tension when a small amount of dodecyl trimethyl ammonium bromide (DTAB) is added to the SDS solution. Shah et.al.34 have found striking change in propertie s of various systems (eg. lecithincholestrol, stearic acid-stearyl alcohol, decanoi c acid-decanol, potassium oleate-hexanol, SDScetyl pyridinium chloride) at a 1:3 molecular ra tio. Though direct values of surface tension were not reported for these systems, in all cases ther e is indirect evidence (evaporation rate, foam stability, solubilization in microemulsion) that at this ratio there is a maximum crowding of molecules at the interface and the molecules are tightly packed. Other re searchers have reported this synergism for anionic/cationic,35 anionic/zwitterionic,36-38 cationic/zwitterionic,36 nonionic/zwitterionic,36 anionic/cationic-gemini,39 anionic gemini/zwitterionic,40 cationicgemini/nonionic41 and cationic-gemini/sugar surfactants.42 These investigations suggest that properly engineered synergism can he lp reduce surface tension values to 25 mN/m (Figure 112). To further reduce the surface tension, speci al surfactants (i.e. silicone, fluorocarbon surfactant) are needed. Silicone surfactants b ecause of their flexible polymer backbone (Si-O bond) and a preponderance of surf ace-active methyl groups, which can orient in low energy configurations, are capable of re ducing the surface tension to around 19 mN/m. Numerous chemistries such as cationic,43, 44 ring based cationic,43 straight chain45, 46 as well as ring based sulfo and carbo betaines (zwitterioni c), siloxanyl phosphinoxides (amphoteric),43 polyether copolymers (non-ionic),43 bolaform surfactants43, 47 (two hydrophilic groups and only one hydrophobic group) have been developed to expl oit the high surface activity of polydimethyl siloxane (PDMS) backbone some of which we also will be using for th e dewetting experiments.

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27 Similar chemistries have been developed with the fluorocarbon surfactants and the minimum surface tension possible can be ~ 15 mN/m.48 So in between pure, mixed, siloxane based and fluorocarbon surfactant, we have a variety of systems with surface tensi ons in the range (15-40 mN/m) to experiment for our surface tension reduction approach. 1.4.2 Effect of Bulk and Adsorbed Micelles Surfactants are amphiphilic molecules, which te nd to adsorb at any pos sible interface (i.e. air-water, fabric-water). In su rface tension reduction approach, we dealt only with phenomena occurring at the air-water interface. Next we shift our attent ion to surfactant molecules at the fabric-water interface where the adsorbed molecu les may adopt different morphologies such as sparse covering of substrate, monolayer coveri ng, hemimicelles and spherical micelles as shown in Figure 1-13.49 These morphologies depend on the concentr ation, structure of the surface-active molecules and the nature of the substrate (polar/non-polar).49, 50 The effect of the underlying substrate can be observed by comparing atom ic force microscopy (AFM) images of full cylinders meandering across mica surface with changes in direction corresponding to changes in the mica lattice versus spherical micelles on am orphous silica, which lacks these atomic rows4. On the other hand, structure of the surfactant its elf, because of steric packing constraints (and charge as the case might be), may influence the morphologies. Such is the case for cationic gemini surfactant where curved morphologies are not observed and adsorption proceeds in a layer by layer manner.51 Intuitively, any preference of the surfactant-water-fabric system to form bilayer or spherical morphologies (hydrophilic ) should promote capillarity and higher water retention capability. Further the stability of the aggregat es as well as the strength with which these aggregates are adsorbed onto the substrate may al so heavily influence the amount of water that

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28 can be removed from the fabric. Both of these ar eas are currently the focus of scientific pursuit.52 Isolated studies exist on the effect of surfactant concentration on wettability of the fabric but there are no definitive studies for the role of adso rbed aggregates or aggregates in bulk and their properties (stability) on the water retention capability or wettability of fabric.52 A study on scoured (hydrophilic) and unsc oured (hydrophobic) cotton with varying concentration of two surfactants, Tween 20 and Span 20 showed that above their critic al micelle concentration (cmc) values both surfactants enhanced the we tting irrespective of na ture of substrate (hydrophobic/hydrophilic) while the opposite was true for concen trations below their cmc,52 affirming the role of surfactant aggregate mor phology. In a related work, we have shown in a SDS surfactant system (above cmc) that wetting time of cotton is entirely dictated by miceller stability.52 SDS micelles are most stable at a conc entration of 200 mM leading to a smaller monomer flux, thus controlling the time for the aggregate morphologies (a s shown in Figure 1-6) to build up on the fabric surface. The stability of the aggregates can be measured by: 1. using conductivity detection by pressure-jump method for ionic surfactant micelles in bulk53 2. using stopped flow method for non-i onic surfactants micelles in bulk,53 and 3. using atomically smooth surfaces (mica, s ilica, alumina, highly ordered pyrolytic graphite) and AFM for adsorbed surfactant aggregates. Table 1-3 gives micellar stability for sele cted surfactants and Figure 1-14 shows the stability of n-alkyl trimethyl ammonium bromide (alkyl chain length: n = 8, 10, 12, 14, or 16) micelles as measured by AFM.

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29 By using additives such as tetra-alkyl ammonium chlorides,54 dodecyl alcohol,55 DTAB,33 2-ethyl hexanol and tri-butyl phosphate,56 we have also shown that the stability of SDS micelles can be increased or decreased as shown in Table 1-4. Thus we have enough systems to study any possible correlation between micellar stab ilities and drying time of the fabric. 1.4.3 Surfactant Vesicle Interactions Since the key focus of water removal is duri ng the spinning cycle of a washing machine, an additive that can be introduced into the wash cycle before the final spin occurs is desirable. Currently, consumers use products that enhanc e the fiber softness and are known as fabric conditioners. These products are introduced to the wash cycle dur ing the final rinse before the final spin cycle begins. This implies that the surface tension of this final rinse water is a controlling factor in water removal from fabrics. It has been shown that these fabric conditioners contain long chain cationic surf actants (on average 18 carbon chains ) in both a monoalkyl form and a dialkyl form. Due to the mixture of the monoalkyl and dialkyl su rfactants (known as monoquats and diquats since they are quaternary ammonium salts), they form vesicles. In order to lower the surface tension of th is rinse water, an oppositely char ged (anionic) surfactant should be used since it has been shown that a synergism exists between cationic and anionic surfactants. However, it is possible that these added anionic su rfactants may interact w ith the vesicles already formed in these fabric conditioners. Such inte ractions need to be understood to effectively engineer a system of low surface tension (<20 mN/m). 1.4.4 Dewatering of Particle Suspensions The use of polymers as flocculants for particle suspensions is a widely used practice in the separation and dewatering of solid/liqui d systems containing fine particles.24 Separation by flocculation is appropriate if the desired resu lts are a reduction in sl udge volume with rapid separation. However, if it is desired to obtain a dry solid, the sludge is su bjected to mechanical

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30 forces to aid in the elimination of water. There has been extensive research in the field of flocculation using polymers.24, 57-59 However, not much has been done in the area of correlating adsorption phenomena on the effects of filtration and water removal. Several factors have been investigated in the role of ad sorption phenomena on the effect of residual water in porous media and it has been determined that surface tension plays an important role. The use of surfactants has been investigated to determine the effect of surface tension reduction on the amount of residual water. It has been shown that a substa ntial reduction in the mois ture content of porous systems can be achieved by th e addition of surfactants.24 It has been shown by Singh22 that there is a direct corre lation between the point of zero charge and surface tension reduction in the residual moisture in the dewatering of coal fines. Singh notes that the mechanisms of the reduction of water in the filter cake is complex but that it appears that the reduction of surface tension as well as surface modification in the contact angle by adsorption play an important role. Sim ilar to our study, the Laplace-Young equation for capillary rise is thought to be the controlling mechanism in the retention of water in aqueous slurries. However, Singh points ou t that there is not always a di rect correlation between surface tension and cake moisture indicating that surfact ant adsorption on the liquid-solid interface plays a roll in the dewatering of fine porous media. Using both a catio nic and anionic surfactant, Singh showed that when the surfactant is not as strongly at tracted to the coal partic les (due to a slightly negative surface charge), the cake moisture decreased (Figure 1-15). Since there is a high affinity of the cationic surfactant to the coal particles, most of the surfactant was adsorbed (~90%) and little surfactant was avai lable to adsorb to the air-liquid interface. Due to lower solid surface adsorption for the SDS system, more of the surfact ant could adsorb at the air-liquid interface and thus a lower surface tension could be achieved therefore reducing cake moisture.

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31 Similar studies of surfactant adsorption on kaolin particles ha ve shown results similar to the earlier work done by Singh.57 Besra et al.59 have shown that the equilibrium adsorption isotherm for sodium dodecyl sulfate on kaolin (w hich is a negatively charged surface) shows a Langmuir type curve (Figure 1-16) with a low amount of surfactan t adsorption due to electrostatic repulsion. 1.4.5 Interactions at the Solid-Liquid Interface: Role of capillarity in the retention of water in fabrics Since our rationale is that water is retained in the fabric by capillaries formed between fibers, capillarity plays an important role and sh ould be understood to be able to develop novel methods to remove moisture from fabrics. Se veral studies have been performed on the water transport mechanisms in textiles.60, 61 It has been shown that wate r transport is governed mainly by a modified LaPlace-Young equation that takes in to account viscous te rms in capillary flow.60 However, it was found that this wicking phenomena was not sole ly dependant on fabric type (due to hydrophilicity, cotton shou ld show a higher wicking than a hydrophobic material). It was found by Hollies et al.60 that the fiber roughness as well as weave plays a role in the water transport in fabrics (i.e. the yarn structure is a factor in the transport of liquids in fibers as compared to solely the chemical nature of fibers ). It has been suggested that the use of wicking experiments (measuring the height of water in a fa bric column as a function of time) can be an indication of fiber arrangement60. However, these observations ar e based on the assumptions of no external gravitational force so extending th is information into a dynamic process under centrifugal force may not be approp riate. In the case of centrifugation, the gravitational forces may be much greater than the forces affected by fabric structure (i.e. viscose forces) thus showing more of a relationship between tran sport properties (capil larity, etc.) and the

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32 composition of the fiber used in experimentation. It should also be noted that these experiments were performed using dry fabrics rather th an using fabrics that have been wetted. Figure 1-1. (a) High surface tension, stronger capill ary forces and more amount of trapped water (b) lower surface tension, w eaker capillary forces and less amount of trapped water (c) adsorbed stable micelles may help in trapping water (d) beading of water on surfaces hydrophobized by de-wetting ag ents, causing less residual water Figure 1-2. A) SEM picture of a woven fa bric(used with permission from Schick 1975)62, B) SEM of a scoured cloth(used w ith permission from DeGruy 1973)63.

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33 FSFT Fd H r FSFT Fd H r Figure 1-3. Forces involved in capillary rise where FSFT is the force due to surface tension (2cos r ) and Fd is the force due to gravity (mg or 2Hrg ). Figure 1-4. Capillary rise dynamics observed for 1 mM C14E6 surfactant solutions in hydrophobic capillaries with radii of 0.1 and 0.5 mm.

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34 Capillary Number ( V/ ) 10-610-510-410-310-2 Percent residual oil remaining 0 20 40 60 80 100 Figure 1-5. The effect of capillary number on the residual oil in porous media. Figure 1-6. Liquid/gas phenomena exhibi ting minima and maxima at 200 mM SDS concentration (used with permission from Patist 2002).6

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35 Figure 1-7. Liquid/liquid and so lid/liquid phenomena exhibiting minima and maxima at 200 mM SDS concentration (used with permission from Patist 2002).6 Figure 1-8. Effect of micellar st ability on dynamic surface tension. Figure 1-9. Micelles stabilizing a th in film between fabric fibers.

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36 Figure 1-10. Change in adsorption of sodium dodecyl sulfate on cott on as a function of concentration, Temperature 293K, Time of ad sorption 42 min (used with permission from Rybicki 1984).19 Figure 1-11. Two segments in the cellulose chain. log C -4.5-4.0-3.5-3.0-2.5-2.0-1.5 mN/m 25 30 35 40 45 50 55 60 Surfactant 1 Surfactant 2 Mixed System Figure 1-12. Synergism between a cationic Ge mini (surfactant 1) and anionic n-dodecane sulfonate (surfactant 2) (used w ith permission from Iwasaki 1991).38

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37 Figure 1-13. Possible surfactant mo rphologies at a solid liquid inte rface (a) monolayer formation, (b)bilayer formation (c) micelle to hemimicelle formation (used with permission from Adler 2000).49 Figure 1-14. Force required to puncture micelles adsorbed on a mica surface (used with permission from Adler 2000).49

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38 Figure 1-15. Moisture content of filter cake as a function of surfactant concentration used in slurry pretreatment. Figure 1-16. Adsorption characteristics of surf actants on kaolin (used with permission from Besra 2002).59

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39 Table 1-1. Composition of typical cotton fibers.27 Composition (% of dry weight) Constituent Typical Range Cellulose 95.0 88.0-96.0 Protein (% N x 6.25)a 1.3 1.1-1.9 Pectic Substances 1.2 0.7-1.2 Ash 1.2 0.7-1.6 Wax 0.6 0.4-1.0 Total Sugars 0.3 0.1-1.0 Pigment Trace Others 1.4 aStandard method of estimating percent proteing from nitrogen content (% N) Table 1-2. Relationship between area per molecule and surface tension area/molecule ( 2/molecule) Surface Tension above CMC (mN/m) LiDS 61.3 44.2 NaDS 51.8 40.0 CsDS 44.8 34.4 Table 1-3. Micellar stabilities for pure surfactant systems.64 Surfactant Micellar Stability ( 2) SDS (200 mM) 7 sec Tween 20 6 sec Tween 80 8-10 sec Pure C12(EO)5 10 sec Pure C12(EO)8 4 sec Brij 35 80 sec Triton X-100 3.5 sec Synperionic A50 40 sec Synperionic A7 150 sec Table 1-4. Micellar stabilities fo r SDS mixed surfactant systems.33, 54-56 Surfactant + additive Micellar Stability ( 2) SDS (25 mM) 1 millisec SDS (25 mM) + 1.25 mM Dodecanol 230 millisec SDS (100 mM) 150 millisec SDS (100 mM) + 5 mM DTAB 1350 millisec SDS (100 mM) + 5 mM tetra-ethyl ammonium chloride 2500 millisec SDS (100 mM) + 5 mM tetra-butyl ammonium chloride 50 millisec

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40 CHAPTER 2 THE RELATIONSHIP OF SURFACE TENSI ON AND THE RESIDUAL MOISTURE CONTENT OF FABRICS 2.1 Experimental Background In order to reduce the drying time of fabrics, we have identified the role of surface tension as a key parameter in the reduction of water in the laundry process. Ou r rationale is that if we can reduce the amount of residual water in fabrics duri ng the spin cycle of the washing machine, then a corresponding reduction in drying time during the drying cycle woul d lead to a reduction in th drying costs. It is assumed that the water is held by capillaries in the fabric structure created by overlapping fabric fibers. The basis behind our reasoning is found in the LaPlace Equation for capi llary rise (Figure 2-1 and Equation 2-1), where is surface tension, is the contact angle (w hich is assumed to be zero for hydrophilic fabrics), r is the capillary radius and is the solution density. 2cos h rg (2-1) The amount of fluid that can rise into a capi llary is proportional to the surface tension of the fluid. Equation 2-2 shows that the work required moving a liquid a given distance is proportional to the surface tension. Based on Equation 2-2, if the surface tension () of the fluid is lowered and the work is held constant (the centrifugal force exerted on the fabric during the spin cycle), then the amount of displacement in the capillary, A, must increase to balance the equation. Therefore, based on these two principles, if the surface tension of the rinse water of the final spin cycle is lowered, then more water will be forced out of the fabric. If less water is present in the fabric before placing in it the dr yer, then the time and en ergy required to dry the fabric will be decreased.

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41 WA (2-2) It has been shown by Preston et al.2, 65, 66 that the use of the equa tion for capillary rise is appropriate for use in examining the capillary rise in fiber assemblies. They have shown that the amount of retained moisture in viscose and cellulo se fibers is proportional to the surface tension of solution. However, their studies mainly focu sed on two different surface tension solutions and a fixed centrifugal time. It was shown that the relative mass of water imbibed in capillaries should be linearly proportional to the surface tension of solution. Therefore it is believed that this is an appropriate method to determine the residua l moisture content of different fabric types. However, it should be noted that Preston et al. varied surface tens ion by using different surfactant types. Our work has expanded on this basis to include surface tension variation by varying the concentrations of a single surfactant. Much of Prest ons work was preformed at high gravitational forces (from 1000 5000 g), which is much higher th an the gravitational forces found in a household washing machine (~100 g). On another note, Prestons work never mentioned the implications of his work for use in a laundry product. If by adding proper additives, we can substa ntially decrease the surface tension of a formulation, then with the same centrifugal forces in the spin cycle of existing machines, we can remove more water. This approach was successfully used in of our earlier work in enhanced oil recovery, where we had to eliminate or reduce si milar capillary forces responsible for trapping oil in fine pores of an oil reservoi r by reducing the interfacial tensions.3 Shah et.al.34 have found striking change in propertie s of various systems (eg. lecithincholestrol, stearic acid-stearyl alcohol, decanoi c acid-decanol, potassium oleate-hexanol, SDScetyl pyridinium chloride) at a 1:3 molecular ra tio. Though direct values of surface tension were not reported for these systems, in all cases ther e is indirect evidence (evaporation rate, foam

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42 stability, solubilization in microemu lsion) that at this ratio there is a crowding of molecules at the interface and the molecules are tightly packed. Ot her researchers have reported this synergism for anionic/cationic,67 anionic/zwitterionic,36-38 cationic/zwitterionic,36 non-ionic/zwitterionic,36 anionic/cationic-gemini,39 anionic gemini/zwitterionic,40 cationic-gemini/nonionic41 and cationic-gemini/sugar surfactants.42 These investigations suggest that properly engineered synergism can help reduce surface tension values to ~ 20 mN/m. 2.1.1 Measuring Residual Moisture Content In order to determine how the fabric system r eacts to different variable that could possible affect the residual moisture content (RMC) of fa brics, several experiments were performed to show the effects of centrifugation speed and cen trifugation time as well as testing several different types of fabrics. In order to test th e effect of surface tension on the residual water content of fabrics several assumptions needed to be made. After several force calculations it was determined that the average household washing machine spins with a force about 90 times the force of gravity (or approximately 640 RPM fo r a typical washing machine). For testing purposes each fabric sample was soaked for ten mi nutes and then placed in the centrifuge for ten minutes. The experimental apparatus that was used is shown in Figure 2-2. The setup uses a centrifuge tube with a co pper insert. The copper insert has a cl osed end with the other end flared so that it will not fall inside th e outer tube. The insert also has small holes drilled through it to allow water to drain through the in sert into the collec tion tube (much like how a modern washing machine is designed). After the fabric was soaked and centrifuged, the weight was then taken to determine the Residual Moisture Content (RMC) as shown in Equa tion 2-3. The first sets of experiments were designed to get basic information about how the system acts (such as force and time dependence on the RMC).

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43 100*centrifugeddry dryWeightWeight RMC Weight (2-3) After a basic understanding of how the system acts under different forces, it was desired to determine the relationship between RMC and surface tension. Several sets of experiments were performed using various commercial surfactan ts provided by the manu facturer (DeIonic 100VLF, DeIonic LF60-MOD, and Dow Corning Q25211). All of the commercial surfactants were tested at 1000 ppm (0.1 wt%). Several other surfact ant systems were chosen in this study as well. A leading detergent (at 1500 pp m the normal household dosage in a washing machine) and a leading fabric softener (at 500 ppm household dosage) were also te sted in these experiments. 2.1.2 Surface Tension Measurements The surface tension measurements were made using the Wilhelmy Plate method. The output from a gram-force sensor ho lding the plate is sent to a transducer and then output to a voltage readout. The system was calibrated usin g two known solutions (water at 72.5 mN/m and acetone at 23 mN/m). The platinum plate was heat ed using a torch between each reading to clean off any surfactants or impurities that may have adsorbed onto the pla tinum plate. For the experiments performed in the basis experiments, only equilibrium surface tension was correlated with the residual moisture content of fabrics. 2.1.3 Materials The sodium dodecyl sulfate used in these e xperiments was obtained from the Fisher Scientific Company. Several sets of experi ments were performed using various commercial surfactants provided by the manufacturer (D eIonic 100-VLF, DeIonic LF60-MOD, and Dow Corning Q2-5211). All of the commercial surfac tants were tested at 1000 ppm (0.1 wt%). Several other surfactant systems were chosen in this study as well. A leading detergent (at 1500

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44 ppm, the normal household dosage in a washing m achine) and a leading fabric softener (at 500 ppm, the household dosage) were also tested in these experiments. Several different types of fabric were used in the experiments. For the experimental basis, three samples were used. Two fabrics were 100% cotton (the denim fa bric and plain cotton fabric) and the last fabric was a 65% polyester-cotton blend. For RMC testing, the first type of fabric that was tested is a Department of En ergy (DOE) standard test fabric which is a 50/50 blend of polyester and cotton. A 100% cotton Hanes tee shirt material and an 86/14 cotton polyester terry cloth were also tested. 2.2 Experimental Basis 2.2.1 Time Basis In order to determine the effect of centrifugation time on the residual moisture content (RMC) of fabrics, the centrif ugation speed was held constant while the centrifugation time was varied (from 2 minutes to 45 minutes). As s hown in Figure 2-3, the RMC decreases as the centrifugation time increases. However, a plat eau is observed around 15 minutes. At this RPM (1250 RPM), a point is reached that all of the largest capillaries have released their water (around the 15 minute mark). Water that is trapped in sma ller capillaries cannot be forced from the fabric at this centrifugation speed due to the higher force required to expe l water in smaller capillaries. Also, as the residence time in the centrifuge incr eases, the RMC decreases (Figure 2-3). This is due to the fluid overcoming the viscous forces in the capillaries. Since the force is not being increased, the only factors holding the water in th e fabric are surface tension and viscous forces. As the residence time is increased, the syst em has time to equilibrate and all unbound water can be displaced from the fabric. The remaining wate r left in the fabric is due to the water of hydration or water trapped in side micro-capillaries.

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45 2.2.2 Centrifugation Speed Basis (Effect of Increasing Gravitational Force) In order to determine the effect of centrif ugation speed on the residual moisture content (RMC) of fabrics, the centrif ugation time was held constant wh ile the centrifugation speed was varied (from 1000 RPM to 8000 RPM). It is expected that by the LaPlace Equation for capillary rise, as the gravitational force is increased, the capill ary height should decrease leading to lower residual moisture in the fabric. As shown in Figur e 2-4, the variation of centrifugation speed held constant at a centrifugation time of 10 minutes is shown to decrease in an exponential decay which is expected due to the increase in gravitatio nal force exerted onto the fabric samples. It should be noticed that at about 4500 RPM, there is another plateau th at is observed. It is also shown that the denim and cotton samples (both 100% cotton) show approximately the same RMC as a function of RPM while the polycotton sample is much lower. This is simply due to the face that the polyester samples are much more hydrophobic than their cotton counterparts and thus it is much easier for the hydrophobic fabric to shed water during the centrifugation process (due to an increased contact angle on hydrophobi c surfaces). Since the polyester is hydrophobic the contact angle is increased and the capillary he ight is decreased resulting in a lower RMC. 2.3 Lowering of Surface Tens ion by Surfactant Systems 2.3.1 Simple Surfactant Systems After we had established a basic understandi ng of how the system reacted to different forces, we focused attention on determining the relationship between the RMC and surface tension. To determine whether there was a relatio nship between the surface tension of a solution and the RMC of the fabrics, the RMCs were measur ed for different solution concentrations of the leading detergent. Figure 2-5 shows a smooth trend in th e relationship between the RMC of fabrics and the surface tension. Since the lowe st surface tension achieved using the detergent solutions was ca. 30 mN/m, Dow Corning Q25211 was used as a reference point (at 19.9

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46 mN/m) (see Figure 2-6). If one extrapolates these curves to a surface tension of zero, one might assume that the trapped water was simply th e water of hydration caused by strong hydrogen bonding between the fabric and water. However, ma ny microcapillaries are present in the fabric structure. Under force, these capillaries may clos e due to the crushing of the fabric under load, trapping water inside the fabric structure. Since a clear relationship existed between the surface tension of a solution and the RMC of the fabrics, more experiments were performed us ing a variety of surfactant types to determine whether a general correlation exis ted independent of surfactant t ype. As shown in Figure 2-6, a relationship between the RMC of the fabrics and the solution surface tension existed for various commercial surfactant systems; however, a few discrepancies were present. The range of surfactant types used may account fo r such disturbances in the tre nd. Several different types of surfactants (ionic, nonionic, and s iloxanes as shown in Table 2-1) were used in this experiment, and each type may have had some sort of interact ion with the fabric surface, causing more or less water to be displaced during centrifugation. 2.3.2 Mixed Surfactant Systems: SDS + C12TAB Due to the ready availability of sodium dodecyl sulfate (SDS) and dodecyl trimethyl ammonium bromide (C12TAB) and their opposite charges (a nionic and cationic), the surface tensions and residual moisture conten ts for various ratios of SDS to C12TAB were investigated at a total concentration of 500 part s per million (0.05 wt%, or the typical surfactant concentration during the final rinse cycle of the washing mach ine). At the 3:1 weight ratio of SDS to C12TAB (which is approximately the 3:1 molecular ratio due to similar molecular weights), the lowest RMC of ~50% was achieved at a surface tens ion of 20.5 mN/m as shown in Figure 2-7 (comparable to results fro m Dow Q2-5211 Superwetter with a RMC of 50.6%).

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47 FSFT Fd H r FSFT Fd H r Figure 2-1. Forces involved in capillary rise where FSFT is the force due to surface tension (2cos r ) and Fd is the force due to gravity (mg or 2Hrg ). Figure 2-2. Experimental apparatus used to de termine the RMC of various fabrics. There are holes in the brass insert whic h holds the fabric samples to allow water to drain from the sample tube during centrifugation. Drainage Holes Fabric Sample Water Collection

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48 time, minutes 01020304050 RMC, % 30 35 40 45 50 55 60 Cotton Denim Figure 2-3. RMC as a function of centrifugation time held consta nt at 1250 RPM. A plateau is observed at approximately 15 minutes. RPM 10002000300040005000600070008000 RMC, % 0 10 20 30 40 50 Cotton 65% Polyester Denim Figure 2-4. RMC as a function of centrifugation speed held cons tant at a centrifugation time of 10 minutes. A plateau is obser ved at approximately 4500 RPM.

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49 Equilibrium Surface Tension, mN/m 1020304050607080 RMC, % 20 30 40 50 60 70 80 Hanes Cotton Fabric DOE 50:50 Fabric Figure 2-5. Relationship between RMC and surf ace tension for the detergent system for the Hanes and DOE fabrics at 1000 RPM (~92 times the force of gravity) centrifuged for 10 minutes. Equilibrium Surface Tension, mN/m 020406080 RMC, % 20 30 40 50 60 70 80 Hanes DOE Terries Figure 2-6. Relationship between RMC and surface tension for commercial surfactant systems for Hanes, DOE and Terry Cloth fabrics at 1000 RPM (~92 times the force of gravity) for 10 minutes.

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50 Weight Ratio of SDS to C12TAB SDS8:15:14:13:12:11:21:31:41:51:8C12TAB Surface Tension, mN/m 10 20 30 40 50 60 RMC, % 45 50 55 60 65 70 75 Surface Tension RMC of Cotton Tee-Shirt 3:1 Ratio SDS:C 12 TAB Figure 2-7. Residual moisture content and surfa ce tension of various we ight ratios of SDS to C12TAB centrifuged at 90g for 10 minutes at a total surfactant concentration of 500 ppm. Table 2-1. Surface tensions and corresponding RMC values for Hanes fabric for various commercially available surfactants. Equil. Surface Tension (mN/m) RMC, % Water 72.5 74 Fabric Softener (0.05%) 47.2 65.68 Leading Detergent (0.15%) 30.5 58.53 DeIONIC 100-VLF (0.1%) 27.6 57.43 DeIONIC LF60-MOD (0.1%) 26.1 55.06 SDS:C12TAB (3:1 molecular ratio at 0.1%) 21.2 53.84 Dow Q2-5211 19.9 50.3

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51 CHAPTER 3 THE EFFECT OF SURFACTANT ADSORPTION ON THE RESIDUAL MOISTURE COTENT OF FABRICS 3.1 Peak in SDS RMC Curve as a Functi on of Increasing SDS Concentration It was shown that the RMC of fabrics depends on several different variables such as centrifugation time, centrifugation speed and surface tension of solution.68 However, we have observed that the RMC of fabrics does not completely correlate with the LaPlace equation as expected. Before the critical micelle concentrat ion (CMC) of surfactant solution we investigated, there is a sharp peak in the RMC of fabrics. It is proposed that this increase in RMC is due to the sudden adsorption of surfactant onto the fabric su rface. Since it has been shown that cotton has a negative zeta potential one might think that an anionic surfactant would have minimal adsorption on a negatively charged surface.19-21 However, there have been several studies showing that ability of sodium dodecyl sulfat e (SDS) and other anionic surfact ants to adsorb onto negatively charged surfaces such as coal fines, cotton and cellulose.19, 22-25 Also, it has been shown by Somasundaran et al. that adsorption isotherm s can show up to four adsorption regions,26 one of them being a sudden increase of adsorption du e to cooperative adsorp tion of surfactant molecules, which may explain the peak found in the RMC curves observed in this study. If surfactant molecules suddenly adsorb coopera tively on the solid surf ace at a critical concentration, then it must cause a concomitant decrease in monomer c oncentration in the bulk solution. Thus, a simple method to determine the monomer concentration below CMC is to measure the surface tension of the residual soluti on. For a given surfactant below its CMC, the surface tension is a measure of the free monome r concentration of surfactant in solution. However, if the change is surfactant monome r concentration is not very large then the equilibrium surface tension may not change sign ificantly. However, the dynamic surface tension may reflect this change more clearly. If ther e is a sudden increase in adsorption on the fabric

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52 surface then there would be less free monomer available to adso rb on the new air-liquid interface of bubbles created during the dynamic surface tension measurement. This would thus lead to an increased dynamic surface tension (which s hould be an amplified measurement of the equilibrium surface tension) that should corr espond to the increase in RMC in the same surfactant concentration range. To verify if adsorption is occurring, we have measured the free surfactant monomer concentration by a two phase dye transfer me thod. This method is commonly used in the determination of anionic surfactan ts in wastewater. The method that we used was a separation of methylene blue active substa nces (MBAS) adapted from several different methods.69, 70 3.1.1 Materials The sodium dodecyl sulfate used in these e xperiments was obtained from the Fisher Scientific Company. Experiments were also performed using purified SDS by recrystalization three times in a 50:50 mixtur e of acetone and ethanol. Several different types of fabric were used in the experiments for residual moisture testing. The fabrics used were as follows: a Department of Energy (DOE) standard test fabric (a 50/50 blend of polyester and cotton), a 100% cotton Hanes T-shirt fabric and an 86/14 co tton/polyester terry cloth. 3.1.2 Residual Moisture Content (RMC) Measurements. For measuring the residual moisture, each fabric sample was soaked for ten minutes in surfactant solution and then placed in a DuPont Instruments Sorvall RC-5B centrifuge at 1000 RPM (which corresponds to the force of a house hold washing machine of ~90g) for ten minutes. The experimental apparatus used to hold the fabrics is shown in Figure 3-1. Our setup uses a centrifuge tube with a copper insert that has a clos ed end with the other end flared so that it will not fall inside the outer tube. The insert also ha s small holes drilled through it to allow water to

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53 drain through the insert into the collection t ube (much like how a modern washing machine is designed). After the fabric was soaked and centrifuged, the weight was then measured to determine the residual moisture content (RMC ) as shown in Equation 3-1. %100*centrifugeddry dryWeightWeight RMC Weight (3-1) 3.1.3 Surface Tension Measurements The equilibrium surface tension measurements were made using the Wilhelmy Plate method. The output from a gram-force sensor holding a platinum plate is sent to a transducer and then output to a voltage readout. The system was calibrated using two known solutions (water and acetone at 72.5 and 23 mN/m respectively). Th e platinum plate was heated using a flame between each reading to rem ove surface contamination. Dynamic surface tension was measured usi ng the maximum bubble pressure technique. The pressure required to form a new bubble in solu tion is measured by a pressure transducer and the reading is transmitted to an oscilloscope. For these experiments, fabric was soaked in surfactant solutions for 45 minutes and the dyna mic surface tension of th e residual solution (in the presence of the fabric) was measured. All dynamic surface tension measurements were taken using an 18 gauge needle tip with a gas flow rate of 7.5 cm3/min (which corresponds to 6-15 bubbles per second or approximately 66 to 166 milliseconds per bubble residence time at the needle tip). We chose this flow rate because at higher flow rates, the nitrogen gas forms a continuous jet in the surfactant so lution at the needle tip. At lower flow rates, the results are similar to equilibrium surface tension results.

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54 3.1.4 Adsorption Measurements The actual free concentration of SDS was measured using the MBAS (methylene blue active substance) method. Si nce this method is accura te in the range of 0-25 M concentrations of SDS, each sample of SDS that had the cotton fa bric soaked in it was diluted by 500 times (i.e. 10 L in 5 mL of water) and once the concentration was determined we were able to scale back to the original sample size. Using known concen trations of SDS (between 0-8 mM), a calibration curve was measured by dilution and then the re vised MBAS method as outlined by Chitikela.70 3.2 Dynamics of Residual SDS Solution and the Effect on RMC 3.2.1 Surface Tensions of Residual Solutions (Dynamic and Equilibrium) and its Correlation to RMC of Fabrics In our previous work, the residual moisture content has been shown to be a function of surface tension of solution68. However, as shown in Figure 3-2, the residual moisture does not completely correlate to the equilibrium surface te nsion of pure SDS solutions in the range of 5-8 mM. A small dip in the surface tension at ~6 mM SDS concentration suggests that the sample had a small impurity (presumably dodecyl alcohol) Recent work in our laboratory using purified SDS samples has shown the same RMC peak using purified SDS solutions (Figure 3-3). However, the peak begins to rise at ~5 mM concentrations of SDS with the purified SDS compared to 5.5 mM with the unpurified SDS. There is also no minimum in the purified SDS system compared to the unpurifie d SDS system which we is propos ed to arise from the presence of dodecyl alcohol. It was shown that the RMC of fabrics did not completely correlate with the equilibrium surface tension of SDS as observed in Figure 3-2 and Figure 3-3. Since it is believed that SDS is adsorbing onto the fabric surface, the equilibr ium surface tension of th e residual solution should show an increase in the range where SDS is adsorbing onto the fabric. The equilibrium and

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55 dynamic surface tensions of residual SDS solution were measured after allowing the fabrics to equilibrate in the SDS solutions for 45 minutes. The fabrics were soaked in a 20:1 ratio of the weight of fabric to the volume of SDS soluti on (approximately the same ration of the amount of water to the amount of fabric for a normal lo ad in a household washing machine). For each dynamic surface tension measurement, the nitrogen flow rate was held constant at 7.5 cm3/min (approximately 6-15 bubbles pe r second). It is shown in Figure 3-4 that a small increase was found in the equilibrium surface tension in the co ncentration range of 5-8 mM. Since there is a small decrease in free monomer in solution due to adsorption onto the fabric, the equilibrium surface tension shows a small increase. It wa s shown by the dynamic surface tension of the residual SDS solution (Figure 3-4), that the dynamic surface tension amplifies the small changes seen in the equilibrium surface tension. Since the lowering of surface tension is due to the diffusion of surfactant molecules to the air-li quid interface from the bulk solution (i.e. the lowering of surface tension is a time-dependant process), it is expected that the dynamic surface tension amplifies the changes seen in e quilibrium surface tension. As shown in Figure 3-5, the increase in the equilibrium surface tension fo r the residual SDS solution corresponds with the increase of the RMC of the Hanes fabric presum ably due to the adsorption of SDS onto the fabric surface in the range of 5. 5 to 6.5 mM SDS concentration. It has been shown that a peak exists in th e RMC curve of Hanes fabric soaked in SDS solutions around approximately 7 mM SDS concentration. This peak has also been observed in the RMC of several other type s of test fabrics of varyi ng hydrophobicity as shown in Figure 3-6 (terry cloth and DOE fabrics with 14% polyester and 50% polyeste r respectively). As the fabric becomes more hydrophobic, the absolute RMC magn itude decreases as well as the magnitude of the RMC peaks as shown in Table 3-1. This decr ease in the magnitude of the RMC peak may be

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56 due to the mechanism of adsorption onto the fa bric surface (i.e. mainly hydrophobic interactions with more hydrophobic fabrics compared to hydrogen bonding with hydrophilic fabrics). The lowering of the RMC may be attributed to the increase in the contact angle of liquid with the fiber surface with more hydrophobic fabrics. Du ring the manufacturing process of fabrics, different chemicals and treatments are used. Howe ver, the fabrics that were used in these experiments were thoroughly washed and dried until the surface tension of water after soaking the fabric remained unchanged from pure water. Thus, adsorbed impurities on the fabric surface cannot account for the observed results. We have shown in Figure 3-4 that there is an increas e in the dynamic surface tension of the residual solution afte r the Hanes fabric was soaked. The dynamic surface tension was then measured for the remaining fabrics (DOE and te rry cloth). Each fabric was soaked in SDS solutions allowed to equilibrate for 45 minutes. The dynamic surface tension of the residual solution was then measured (Figure 3-7, Figure 3-8 and Figure 3-9 for the Ha nes, DOE and terry cloth fabric respectively). The flow rate was held constant at 7.5 cm3/min (6-15 bubbles per second or approximately 66 to 166 milliseconds per bubbl e at the needle tip). It is shown in these graphs that a correlation exists between the peaks found in the RMC and the dynamic surface tension of residual solution. Since an increase in surface tension indi cates low adsorption of surfactant at the newly created ai r-liquid interface in the residual solution, the p eaks found in the dynamic surface tension measurements are believed to be indicative of decrease in surfactant concentration due to adsorption onto the fabric surface. An increase in dynamic surface tens ion is due to the reduced ad sorption of surfactant at the air-liquid interface of the new bubble surface created during the measurement. We believe that the decrease in adsorption at the air-liquid interf ace is due to increased adsorption of SDS on the

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57 fabric surface. If there is in creased adsorption of SDS onto the fabric surface due to cooperative adsorption, then it is assumed that there w ould also be a reduction in the free monomer concentration (which has been shown by the incr ease of equilibrium and dynamic surface tension of residual solution as shown in Figure 3-4). 3.2.2 Molecular Mechanism: Explanation of the Peak in the SDS/RMC Curve It is shown in Figure 3-10 and Figure 3-11 the 4 regions associated with the increase in residual moisture content and dynamic surface tens ion. Region A-B is the region of minimal surface adsorption of SDS onto the fabric surface presumably due to a residual negative charge on the fabric surface. The decrease in RMC in this region is due to the increase of free surfactant monomer concentration with low adsorption on the fabric surface. At a concentration of 5.5 mM of SDS, there is a minimum in the RMC and Region B-C begins. This region is due to the sudden increase in adsorption of SDS onto the fabric surface due to a cooperative adsorption phenomenon. Due to electrostatic repulsion betw een the fabric surface and the SDS monomers, there is a barrier to adsorption. However, once several monomers adsorb onto the fabric surface, it provides a cooperative effect promotes SDS ad sorption. This sudden increase in adsorption of the SDS onto the fabric surface reduces the free monomer concentration in the bulk solution thus leading to a reduced amount of free monomer in solution. Hence, less monomer is available to adsorb onto the new air-liquid in terface created during the dynamic surface tension measurement which leads to an increased dynamic surface te nsion. This increase in the dynamic surface tension leads to an increase in the residual mo isture. At approximately a concentration of 6.75 mM of SDS, there is a maximum in the RMC wh ere Region B-C ends and Region C-D begins. It is believed that at this point, complete satu ration of the fabric surf ace by the adsorption of SDS has occurred. Once maximum adsorption has been reached, any additional SDS added into the system will result in an increase in the free m onomer concentration. The increased free monomer

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58 concentration provides the new ai r-liquid interface with higher SD S adsorption thus reducing the dynamic surface tension. At approximately 7.5-8.0 mM concentration of SDS, Region C-D ends and Region D-E begins. This regi on occurs due to the bulk soluti on reaching the critical micelle concentration (CMC). At this point, the free mo nomer concentration remains constant. Since the free monomer concentration is now constant, th e dynamic surface tension and residual moisture should remain constant as well. 3.3 Adsorption of SDS onto Cotton Surfaces It was shown that the RMC of fabrics depends on several different variables such as centrifugation time, centrifugation speed and surface tension of solution.68 However, we have observed that the RMC of fabrics does not comp letely correlate with the reduction of surface tension as predicted by the LaPlace equation for ca pillary rise. We have shown that before the CMC of surfactant solution, there is a sharp p eak in the RMC of fabrics as a function of increasing SDS concentration. It was proposed that this increase in RMC is due to the sudden adsorption of surfactant onto the fabric surface based on secondary data such as the dynamic surface tension of the residual soluti on after the fabric was soaked.71 Since it has b een shown that cotton has a negative zeta potential one might th ink that an anionic surfactant would have minimal adsorption on a ne gatively charged surface.19-21 However, there have been several papers showing that ability of sodium dodecyl sulfate (SDS) and other anionic surfactants to adsorb onto negatively charged surfaces su ch as coal fines, cotton and cellulose.19, 22-25 Also, it has been shown by Somasundaran et al. that adso rption isotherms can show up to four adsorption regions,26 one of them being a sudden increase of adsorption due to cooperative adsorption of surfactant molecules, which may explain the p eak found in the RMC curves observed in this study. We have measured the dynamic surface tens ion of the residual solu tion after soaking the fabric and we have shown that the increase in RM C does correlate very well with an increase in

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59 the dynamic surface tension.71 Our proposed mechanism indicated that there was a cooperative adsorption of SDS onto the fabric surface thus leading to a decrease in the free monomer concentration. This decrease in the free SDS monome r concentration thus leads to an increase in the dynamic surface tension. If there are less surf actant monomers in the system, there will be less monomer available to adsorb onto a ne wly created air-water interface (as in the measurement of dynamic surface tension by the ma ximum bubble pressure technique) which will lead to an increase in the dynamic surface tension. In these experiments, we have measured th e free surfactant monomer concentration by a two phase dye transfer method. This method is commonly used in the de termination of anionic surfactants in wastewater. The method that we used was a separation of methylene blue active substances (MBAS) adapted from several different methods.69, 70 We have shown by equilibrium and dynamic surface tension measurements of the residual solution that the fabric samples have soaked in th at there is a correlation in the peak found in the RMC as a function of increasing c oncentration. However, this has been indirect proof suggesting that there is a decrease in the free monomer c oncentration in the range of the concentrations where the peak exists. Using a two phase dye tr ansfer method (MBAS) we can measure the free monomer concentration of the resi dual solution after the fabric has soaked in the solution. This measurement will provide a detailed measurement of the actual concentration of SDS left in solution after the adsorption of SDS has occurr ed onto the fabric surface. If we can show a correlation with the onset of adsorption with the beginning of the RMC peak as well as show a correlation to the fabric surface becoming saturate d at the same point at which the RMC peak reaches a maximum, we will have direct proof that surfactant adsorption is the cause of the RMC peak of cotton fabric as a functi on on increasing SDS concentration.

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60 As shown in Figure 3-12, we have provided a calibrat ion to determine if this method can be applied to diluted SDS solutions in the concen tration range that we are interested in. For the concentration range we are intere sted in, this method appears to be a good selection to determine the free monomer concentration of SDS after th e residual solution has equilibrated with the fabric (after the SDS has been al lowed to adsorb onto the fabric). Now that we have shown the MBAS method can be applied in determining the free concentration of SDS in the bulk solution, we then measured the free concentration of SDS in the residual solution after the SDS adsorption was allowe d to equilibrate with the fabric surface. As shown in Figure 3-13, the adsorption of SDS onto th e cotton fabric correlated with the peak found in the RMC of cotton as a function on initia l SDS concentration. The adsorption begins slightly before the peak starts to increase whic h we believe is due to impurities in the SDS not adsorbing onto the fabric. These impurities (m ost likely dodecanol) are available to lower the dynamic surface tension in the bulk solution due to limited adsorption of the dodecanol onto the fabric. Once the fabric has become fully saturated with SDS, the RMC of the cotton fabric begins to decrease. This further streng thens our hypothesis that the peak is due to adsorption of SDS onto the fabric. Once the fabric is saturated, any further SDS added into solution stays in the bulk and thus increases the bulk concentration of SDS. If the bulk concentration of SDS is increased after the point of adsorption saturation on the cotton surface, the dynamic surface tension of the residual solution then decreases. This maximum in the adsorption isotherm correlates with the peak found in the RMC curve. Once this maximum is reached, the RMC starts to decrease due to the increased amount of SDS in the bulk solution. We have shown that not only the equilibrium surface tension of solution has an effect on the RMC of fabrics. Therefore, finding a solu tion with the lowest eq uilibrium surface tension

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61 does not necessarily mean that we have identified a system that will result in the lowest RMC. We have shown that the reduction of RMC has many different aspects. Not only does equilibrium surface tension affects the RMC of fabrics but the dynamic surface tension also plays a very important role on the removal of water from fabrics. 3.4 RMC Peak in Various Surfactant Systems Due to the fact that we investigated a m odel system to determine how increasing the concentration of a common surfactant (SDS in this case) had an effect on the RMC of fabrics, the need to determine if this phenomenon existed in other surfactant systems (such as other anionic systems, cationic systems or actual detergent syst ems). The first systems investigated were fatty acid surfactants with varying chai n lengths (n = 10-14). Shown in Figure 3-14, the same peak observed in the SDS experiments was observed for the C10 fatty acid system (similar to other fatty acids). Of more interest to commercial aspects, the same experiments were repeated for a leading detergent and a leading fabric softener to determ ine if the same peaks ex isted in the RMC curves as a function of increasing concentration. As shown in Figure 3-15 for the leading fabric softener, there exists a sharp peak at approxi mately 1100 ppm of total product concentration. Since this is at about twice th e concentration of a normal dosage in the washing machine, it normally would not be of concern. However, many consumers overdose their machines with fabric softener and this could possibly lead to longer drying times due to the higher RMC values. Likewise, for the leading detergent (Figure 3-16), there is a pe ak at approximately 400 ppm (about 1/3 of the normal dosage for a washing mach ine). However, it is unknow n if there exists a synergism of adsorption if there is surfactant carryover from the detergent into the final rinse cycle when the liquid fabric softener is added. In the case with a mixed surfactant system, the possibility exists to shift the peak found in th e fabric softener RMC curve at a lower value.

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62 3.5 Manipulation of RMC Peak: Fabric Pre-Trea tment and its Affects on Adsorption of SDS onto Cotton Since our goal is to lower the RMC of the fabr ic at the end of the laundry spin cycle, the manipulation of the peak found in the RMC values in the SDS system should be investigated. Specifically, one would like to either shift the p eak to a range that woul d be out of the normal values for a detergent system, re duce the magnitude of the peak found in the RMC, or find a way to eliminate the peak completely. Since the pe ak found in the RMC curve is due to a sudden adsorption phenomena of the SDS onto the fabric surface, if the fabric su rface could be modified in a way that could alter the ad sorption of SDS (such as increasi ng the anionic charge density or reducing the amount of available sites for the SD S to adsorb onto), the RMC peak found in the SDS system could be shifted, reduced or even eliminated. In order to manipulate the adsorption of SD S onto the fabric, various surfactants and insoluble long chain compounds were coated on the fabric. The fabric was soaked in various solutions of surfactants and polym ers which were solubilized in et hanol. The fabric was allowed to equilibrate with the solution for 30 minutes. The fabric was then removed from the solution and allowed to dry. If we can manipulate how mu ch SDS is adsorbed onto the fabric, then the dynamic surface tension of the residual SDS solu tion can be changed and thus the RMC can be changed. Since we changed from a Hanes tee shir t fabric to a 100% cotton terry cloth fabric, the baseline for the SDS system wa s measured and is shown in Figure 3-17. As shown earlier, the peak in the RMC curve is still pr esent indicating that the SDS is adsorbing on the terry fabric as well (which is as expected due to the terry fabric being 100% cotton). The first experiments performed measured the RMC of fabric coated with polymers. The fabric was soaked in carboxy met hyl cellulose (CMC as shown in Figure 3-18), poly acrylamide (PAA as shown in Figure 3-19) and polyvinylpyrr olidone (PVP as shown in Figure 3-20). It

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63 should be pointed out that in ea ch case, the magnitude of the RMC peak is much less compared to the untreated fabric (with the exception of PVP). The adsorption of CMC and PAA both reduced the magnitude of the RMC peak indica ting that the amount of SDS adsorbed onto the fabric has been reduced and thus the dynamic su rface tension decreases as the concentration of SDS increases. This may be due to several r easons such as the elimination of available adsorption sites due to the adsorption of the pol ymer onto the fabric surface, or, since both CMC and PAA are both negatively charged polymers, the adsorption of these polymers on the fabric possibly increased the total anioni c charge density on the fabric and thus resulted in a higher electrostatic repulsion between the fabric and the SDS. The next set of experiments involved the us e of long chain alcohols, fatty acids and insoluble surfactants (C18 fatty acid, C18 alcohol and dioc tyldecyldimethylamm onium bromide or DODAB). With the use of the octadecanoic acid, the RMC of the terry cloth fabric is a smooth curve and the peak has been almost completely removed (Figure 3-21) which is thought to be due to the tight packing of th e octadecanoic acid adsorbing on the fabric which increases the charge density (to a higher net negative charge) and thus repuls es the anionic SDS. When the experiments were repeated with the octadecanol the RMC peak has been reduced but is still present (Figure 3-22) which is thought to be due to th e reduction of availabl e adsorption sites and not electrostatic repulsion due to the lack of a charge on th e octadecanol. The last set of experiments involved the fabric being soaked in DODAB. However, the RMC was not reduced near as much with the use of DODAB as compared to the octadecanoic acid (Figure 3-23) possibly due to a bilayer formation of DODAB on the fabric providing adsorption sites for the SDS to adsorb on).

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64 It is believed that the adsorption of these various compounds interfer es with the adsorption of SDS onto the fabric either due to the reduction of available ad sorption sites or by an increased charge density increasing the electrostatic repulsion. In the case of octadecanoic acid, electrostatic repulsion exists which would repel the SDS from adsorbing onto the fabric surface and thus the RMC would then follow the same tr end as the equilibrium surface tension. That being said, if one can manipulate the amount of su rfactant that is being adsorbed onto the fabric, the RMC can thus be greatly reduced compared to the fabric systems where SDS was allowed to adsorb onto the fabric. Drainage Holes Fabric Sample Water Collection Drainage Holes Fabric Sample Water Collection Figure 3-1. Experimental apparatus used to determine the residual moisture of fabrics.

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65 Initial SDS Conc. mM 0123456789101112131415 RMC, % 50 55 60 65 70 75 80 85 90 95 Equi. Surface Tension, mN/m 25 30 35 40 45 50 55 60 65 70 RMC of Hanes Fabric Equi. Surface Tension of SDS Figure 3-2. RMC of Hanes 100% co tton fabric as a function of SD S concentration plotted with equilibrium surface tension of pure SDS solutions. Initial SDS Conc. mM 012345678910 RMC, % 65 70 75 80 85 90 Equil. Surface Tension, mN/m 35 40 45 50 55 60 65 RMC Hanes Fabric EtOH Purified SDS Surface Tension Figure 3-3. RMC of Hanes 100% co tton fabric as a function of SD S concentration plotted with equilibrium surface tension of ethanol:acetone purified SDS solutions.

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66 Initial SDS Conc, mM 0123456789101112 Equi. Surface Tension, mN/m 30 32 34 36 38 40 42 Dynamic Surface Tension, mN/m 56 58 60 62 64 66 68 70 72 74 Equilibrium Surface Tension Dynamic Surface Tension Figure 3-4. Equilibrium and dynamic surface tensi on of residual SDS solution after exposure to Hanes fabric. Initial SDS Conc. mM 012345678910 RMC, % 55 60 65 70 75 80 85 90 E q ui. Surface Tension, mN/m 30 32 34 36 38 40 42 Hanes RMC Equi. Surface Tension of Residual Solution Figure 3-5. Comparison of the RMC of Hanes fa bric and the equilibrium surface tension of residual solution after soaking the fabric.

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67 Initial SDS Conc. mM 024681012 RMC, % 30 40 50 60 70 80 90 Hanes 100% Cotton Terries 86% Cotton DOE 50% Cotton CMCRMC max RMC min Figure 3-6. RMC of Hanes cotton fa bric, terry cloth fabric and DOE test fabric as a function of SDS concentration showing the maxi mum and minimum in the RMC peak. Initial SDS Conc. mM 024681012 RMC, % 50 55 60 65 70 75 80 85 90 D y namic Surface Tension, mN/m 50 55 60 65 70 75 RMC Hanes Fabric Dynamic Surface Tension CMC Figure 3-7. RMC and DST of the residual solution from the Hanes 100% cotton fabric soaked in SDS solutions.

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68 Initial SDS Conc. mM 024681012 D y namic Surface Tension mN/m 35 40 45 50 55 60 65 70 RMC, % 28 30 32 34 36 38 40 42 44 Dynamic Surface Tension RMC DOE Fabric CMC Figure 3-8. RMC and DST of the residual solu tion from DOE 50:50 cotton:polyester fabric soaked in SDS solutions. Initial SDS Conc. mM 024681012 Dynamic Surface Tension, mN/m 40 45 50 55 60 65 70 75 RMC, % 45 50 55 60 65 70 75 80 Dynamic Surface Tension RMC Terry Cloth Fabric CMC Figure 3-9. RMC and DST of residual solution from the Terry Cloth 86:14 cotton:polyester fabric soaked in SDS solutions.

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69 SDS Conc., mM 024681012 RMC, % 55 60 65 70 75 80 85 90 Hanes RMC A B C D E Figure 3-10. Indication of the regions associated with the peak in the RMC of Hanes cotton fabric. A-B) B-C) D-E) C-D) Fabric Fabric Fabric Fabric Air Air Air Air A-B) B-C) D-E) C-D) Fabric Fabric Fabric Fabric Air Air Air Air Air Air Air Figure 3-11. A-B) High adsorption of surfactan t monomer at the airliquid interface and low adsorption on the fabric-li quid interface resultin g in a low dynamic surface tension BC) Sudden adsorption due to cooperative adso rption on the fabric surface resulting in a decreased monomer concentration in the bulk solution and decreased adsorption at the air-liquid interf ace C-D) Maximum adsorption is reached at the fabric-liquid interface and increased adsorption is occurri ng at the air-liquid interface D-E) The CMC is reached and the monomer concentra tion is stable resulting in a constant dynamic surface tension and RMC.

PAGE 70

70 Molar Concentration of SDS 01020304050 Absorbance 0 1 2 3 4 5 Calibration Curve Passed functional range of this method due to saturation of the methylene blue complexes i.e. conc of SDS was higher than methylene blue 20.08520.0924 0.9947 yx R Figure 3-12. Calibration curve for MBAS method in the range of SDS concentrations tested in these experiments. Initial SDS Concentration, mM 012345678910 mg SDS / gram Fabric 0 1 2 3 4 5 6 7 8 RMC, % 50 55 60 65 70 75 80 85 90 Adsorption RMC Figure 3-13. Adsorption of SDS adsorbing onto Hanes cotton fabric. Adsorption was measured from the residual SDS solution after the cotton fabric was soaked for 30 minutes.

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71 Total Conc. mM 86889092949698100 RMC, % 45 50 55 60 65 70 75 CMC Figure 3-14. RMC of Hanes fabric soaked in C10 fatty acid (centrifuged for 10 minutes at 1000 RPM). Conc. ppm 0200400600800100012001400 RMC, % 50 55 60 65 70 75 80 85 90 Hanes RMC in Fabric Softener CMC Figure 3-15. RMC of Hanes fabric soaked in solutions of a leadi ng fabric softener (centrifuged for 10 minutes at 1000 RPM).

PAGE 72

72 Conc. ppm 020040060080010001200 RMC, % 60 65 70 75 80 85 90 RMC of Detergent CMC Figure 3-16. RMC of Hanes fabric soaked in solutions of a lead ing detergent (centrifuged for 10 minutes at 1000 RPM). Initial SDS Conc., mM 012345678910 RMC, % 50 55 60 65 70 75 80 85 90 95 100 Equil. Surface Tension mN/m 25 30 35 40 45 50 55 60 65 70 75 Terry Cloth RMC Equil. Surface Tension Figure 3-17. RMC of Terry cloth fabr ic soaked in solutions of SDS.

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73 Initial SDS Conc., mM 012345678910 RMC, % 60 65 70 75 80 85 90 95 100 105 110 Equil. Surface Tension mN/m 30 35 40 45 50 55 60 65 70 75 Terry Cloth Soaked in CMC Equil. Surface Tension Figure 3-18. RMC of terry cloth fabric pre-treated with CMC. Initial SDS Conc., mM 012345678910 RMC, % 50 55 60 65 70 75 80 85 90 95 100 Equil. Surface Tension mN/m 25 30 35 40 45 50 55 60 65 70 75 Terry Cloth RMC PAA Modified Equil. Surface Tension Figure 3-19. RMC of terry cloth fabric pre-treated with PAA.

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74 Initial SDS Concentration (mM) 01234567891011 RMC, % 55 60 65 70 75 80 85 90 95 100 Equilibrium Surface Tension mN/m 20 30 40 50 60 70 80 Terry Cloth with PVP Treatment Equilbirum Surface Tension Figure 3-20. RMC of terry cloth fabric pre-treated with PVP. Initial SDS Conc., mM 012345678910 RMC, % 50 55 60 65 70 75 80 85 90 95 100 Equil. Surface Tension mN/m 25 30 35 40 45 50 55 60 65 70 75 Terry Cloth Soaked in C18Acid RMC Equil. Surface Tension Figure 3-21. RMC of Terry clot h fabric pre-treated with C18 Fatty Acid.

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75 Initial SDS Conc., mM 012345678910 RMC, % 55 60 65 70 75 80 85 90 95 100 Equil. Surface Tension mN/m 25 30 35 40 45 50 55 60 65 70 75 Terry Cloth RMC C18OH Modified Equil. Surface Tension Figure 3-22. RMC of Terry clot h fabric pre-treated with C18 Alcohol. Initial SDS Conc., mM 012345678910 RMC, % 60 65 70 75 80 85 90 95 Equil. Surface Tension mN/m 25 30 35 40 45 50 55 60 65 70 75 Terry Cloth RMC DODAB Modified Equil. Surface Tension Figure 3-23. RMC of terry cloth fabric pre-treated with DODAB.

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76 Table 3-1. The magnitudes of the RMC peak for va rious fabrics for I) the absolute different in the maximum and minimum of the RMC peak II) the difference in the maximum and minimum normalized with respect to the RMC maximum and III) the difference in the maximum and minimum normalized with respect to the RMC minimum. I II III maxmin R MCRMC maxmin max R MCRMC RMC maxmin min R MCRMC RMC Hanes 17.45% 22.72% 29.40% Terries 10.23% 16.74% 20.01% DOE 4.18% 12.13% 13.89%

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77 CHAPTER 4 REDISCOVERING MONOLAYER PENETRAT ION: OBTAINING ULTRA-LOW AIRLIQUID SURFACE TENSIONS 4.1 Monolayer Penetration When an insoluble monolayer has a surfact ant laden subphase below the monolayer, the surfactant from the subphase can ad sorb into the monolayer thus penetrating the monolayer. This adsorption of surfactant penetrating the monolay er thus changes in the surface tension and surface pressure by changing the effective area per mo lecule in the monolayer (tighter packing in the monolayer results in a decrease in the surface tension). As discussed by Datwani et al., early models for equilibrium monolayer penetrati on related the surface pressure in the mixed monolayer to the adsorbed amount of the sol uble surfactant from the subphase which is a function of the bulk concentration of the surfactant in the subphase.72 Work has been done by Schulmans research group which revolutionized the study of monolayers (static monolayers and monolayer penetration)73-75. Matalon et al. developed a method in 1949 to measure surface pressures resu lting from the interaction of an insoluble monolayer with surfactants ad sorbing into the monolayer fr om the underlying bulk solution.73 Schulmans group also researched many other aspects of monolayers and penetration of monolayers such as follows : penetration of monola yers with surfactants,74-76 interactions of monolayers with metal ions,77-79 complex formation and ster ic effects in monolayers,80, 81 and the ionic structure and the effect s of unsaturation on monolayers.82-84 There has been very many papers publishe d on the various aspects of monolayer penetration such as the e ffect of charged surfactan ts penetrating monolayers,72 mathematical evaluations of monolayer penetration,85 the thermodynamics and kinetics monolayer penetration,86-91 and many other various papers on monolayer penetration.72, 82, 92-98 Due to the wide range of papers discussing monolayer pene tration, the mathematics governing penetration

PAGE 78

78 will not be discussed. However, based on the similarity of dioctadecyldimethylammonium bromide (DODAB) to the active surfactant in ma ny commercially available fabric softeners,99-101 we chose to use DODAB as the insoluble surfact ant to be used in the insoluble monolayer. Several different surfactants, sodium dodecyl sulfate (SDS) and sodium tetradecyl sulfate (C14SO4), were chosen to use as the penetrating surf actant based on their similarities to the active surfactants in detergents. We have shown that with the use of monolay er penetration that we can now lower the equilibrium surface tension to values lower than pr eviously achieved with silicone super wetters (lower than 19 mN/m). We have also shown that the ability to lower the surface to low values and maintain that low value depends on the type of insoluble monolayer we spread. When an ionic monolayer is used and penetrated with an oppositely charged surfac tant from the subphase, we have shown that this type of system is more effective in lowering surface tension compared to the use of penetrating a nonionic monolayer wi th an ionic surfactant from the subphase. We believe that this is due to electrostatic at traction between the charged monolayer and the oppositely charged surfactant from the subphase. A nother aspect we investigated in monolayer penetration is the use of a mi xed monolayer (a mixture of th e insoluble surfactant with the penetrating surfactan t in the subphase). We have found that the mixed monolayer penetrated with the soluble component of the mixed monolayer provi ded excellent results in the ability to lower surface tension (to values less than 10 mN/m). From our previous work, we have shown that there is a correlation between equilibrium surface tension and the residual mois ture content (RMC) of fabrics.68, 71 We have shown that the lower the surface tension, the more water can be shed from fabrics during the centrifugation process. Based on this work, we are now trying to reach air-liquid surface tension values of

PAGE 79

79 lower than 10 mN/m using monolay er penetration. Using monolayer penetration, we have been able to measure air-liquid surface tension values as low as ~8 mN/m and we have determined a method to use monolayer penetration in the reduction of RMC of fabrics. 4.1.1 Monolayer Penetration Studies As discussed by Welzel et al., there are two methods to stud y monolayer penetration: 1) the penetrant is injected beneat h the already spread monolayer and 2) the monolayer film is spread onto the penetrating solution.93 In these studies, we have used both methods to measure the effects of monolayer penetration. The first method was used in the sm all scale surface tension measurements and the second method was used in the full scale washing machine experiments. Penetration studies were done by solubilizing the insoluble monol ayer in a mixture of 1:1:3 volume mixture of methanol, chloroform and hexa ne in a total concentr ation of 0.5 wt%. The solubilized solution was then 5L of this monolayer solution was placed onto the surface of a Petri dish filled with 5 mL of water using a microsyringe (see Figure 4-). The penetrating surfactant solutions in various concentrations were then injected beneath the monolayer in different volumetric amounts. Meanwhile, the surf ace tension was monitored using the Wilhelmy plate method. However, unlike previous surface tension methods, the output from the voltage sensor from the Wilhelmy plate was input to a computer using a WinDAQ data acquisition card. Using this method, the surface tension can be measured as a function of time while the monolayer is being penetrated with the surfactant from the subphase. For the washer scale tests, ethanol was used to solubilize the monol ayer instead of the methanol, hexane and chloroform solvent due to ethanol being more environmentally friendly and the ethanol is not as corro sive to the washing machine co mponents compared to the other solvents. Also, the penetrating surfactant was alre ady present in the fabric before the monolayer

PAGE 80

80 was spread on the fabric due to the difficulty in injecting the subphase surfactant beneath the monolayer in full scale te sting (see Figure 4-13). The equilibrium surface tension measurements were made using the Wilhelmy Plate method. The output from a gram-force sensor holding a platinum plate is sent to a transducer and then output to a voltage readout. This voltage readout was captured using a computer with a WinDAQ data acquisition card. Th e system was calibrated using two known solutions (water and acetone at 72.5 and 23 mN/m respectively). Th e platinum plate was heated using a flame between each reading to rem ove surface contamination. 4.1.2 Reduction of Surface Tension: Monolayer Penetration Results We first experimented with spreading a monolayer of C16TAB dissolved in the universal solvent (1:1:3 volume mixture of methanol, chloroform and hexane) on the Petri dish filled with water. Once the solvent was allowed to evaporate, we began to monitor the surface tension. We then injected solutions of 4 mM C14SO4 beneath the monolayer and continued to measure the surface tension. As shown in Figure 4-2, the surface tension reaches a minimum immediately after the penetrating surfactan t solution was injected. Dependi ng on the amount of penetrant injected into the Petri dish, the surface tension stays at a low value and then jumps up to higher values (which are a lower va lue than the surface tension of the pure monolayer) due to solubilization of the monolayer. We also repeated the same experiment using stearic acid as the insoluble monolayer penetrated with a subphase su rfactant of an opposite ch arge compared to the stearic acid (C14TAB was used at a cationic penetrant). As shown in Figure 4-3, we show the same trend in surface tension as shown in th e other monolayer penetration experiments. Immediately after the penetrating surfactant is in jected into the system, the equilibrium surface tension drops to a minimum value and then slowly increases to another equilibrium value. We then repeated the same experiment with DDAB (didodecyldimethylammonium bromide) as the

PAGE 81

81 monolayer with C14SO4 as the penetrating surfactant. As shown in Figure 4-4, the equilibrium surface tension follows the same trend as the C16TAB monolayer system. The surface tension goes to a minimum value and then increases. We have called this a transient monolayer and we believe that this phenomenon is due to the solubi lization of the monolayer after the penetrating surfactant has penetrated the monolayer. Both C16TAB and DDAB have slight solubility in water and with the addition of C14SO4, we believe that the penetrating surfactant helps solubilize small amounts of the monolayer into the solution and t hus increasing the equilibrium surface tension. Then, as shown in Figure 4-5, the monolayer which has been penetrated with the C14SO4 starts to solubilize and dissolves into the solution thus leaving the interface le ss tightly packed and increasing the equilibrium surface tension. The lo w surface tension values are due to the supersaturation of the air-liquid in terface due to the Coulombic interaction between the cationic monolayer and the anionic penetrating su rfactant. Using the monolayer penetration method in th ese preliminary experiments, we obtained equilibrium surface tension values at the air-liqui d interface of ~ 17 mN/m. Previously, values as low as ~18-19 mN/m at the air-liquid inte rface have been obtained using conventional surfactants. After we ran the preliminary monolayer penetr ation studies, we then looked into using a monolayer composed of longer ch ain surfactants and al cohols that are insoluble in water and would theoretically not exhibi t the transient mono layer phenomenon that we observed with slightly soluble monolayers. We first looked at DODAB (the C18 version of the DDAB used in previous experiments) penetrated with C14SO4. As shown in Figure 4-6, the equilibrium surface tension of a DODAB monolay er penetrated with C14SO4 reaches a minimum value of

PAGE 82

82 approximately 15 mN/m. This value is lower than values obtained at th e air-liquid interface for other methods (18-19 mN/m for fluo ro-surfactants and siloxanes). We then chose to use cholesterol and C20OH as the insoluble monolayer and we continued to use C14SO4 to penetrate the monolayer. As shown in Figure 4-7, the surface tension of a C20OH monolayer penetrated with C14SO4 is shown to decrease to a value of about 25 mN/m. However, we were still attempti ng to achieve surface tensions of much lower values (less than 10 mN/m). We then used cholesterol as th e insoluble monolayer penetrated with C14SO4. However, the surface tension was only reduced to about 20 mN/m in this system (Figure 4-8). Using a pure monolayer of DODAB penetrated with 1 mL of 4 mM C14SO4, a surface tension of 13.5 mN/m was achieved for an indefi nite amount of time. It was thought that if a mixed monolayer was formed that tighter packing would be pres ent in the monolayer due to electrostatic interactions betw een the headgroups. The system th at was investigated was the tetradecyl sodium sulfate (C14SO4) with dioctyldecyl dimet hylammonium bromide (DODAB). Ratios of 1:10, 1:5, 1:3, and 1:2 of the C14SO4:DODAB were investigated. The first systems tested were the monolayer compositions that we re higher in DODAB concentration (the 1:10, 1: 5, 1:3 and 1:2 ratios of C14SO4:DODAB). As shown in Figure 4-10, the 1:10 ratio of C14SO4:DODAB monolayer with C14SO4 injected beneath the monolayer resulted in a surface tension as low as 19 mN/m. Increasing the C14SO4 amount to a ratio of 1:5, the surface tension dropped to approximately 8.5 mN/m with 1000 L of C14SO4 injected beneath the monolayer (Figure 49). Since it has been well do cumented that tightest packing occurs at a 1:3 ratio, the surface tension of a monolayer at this ratio was measured. However, the minimal surface tension for the 1:3 system was no t as low as the 1:5 ra tio monolayer. It is believed that this is due to molecular packing tr ying to achieve the 1:3 ratio. At a lower ratio of

PAGE 83

83 C14SO4:DODAB (the 1:5 system), there is somewh at tight packing. This packing can be optimized by the addition of more C14SO4 beneath the monolayer. The addition of the 4 mM C14SO4 results in what we believe is a 1:3 ratio of C14SO4:DODAB in the monolayer after this addition of C14SO4 beneath the monolayer. However, when the monolayer is already at its tightest packing at a 1:3 ratio of C14SO4:DODAB, there is not sufficient room for more C14SO4 to penetrate the monolayer resulting in a higher surface tension than the previous system. When higher ratios (1:2) of C14SO4:DODAB were tested, there wa s no monolayer present after spreading with universal solvent. Since the C14SO4 is soluble in water, once the monolayer is spread it is being solubilized into so lution due to the in creased amount of C14SO4 resulting in a higher surface tension (approximately the surface te nsion of water) that re duces with the addition of C14SO4. 4.2 Reduction of RMC via Monolayer Penetration 4.2.1 Experimental Procedure Large scale tests were done in a Whirlpool washing machine. Using a strobe-scope, we measured the RPM of the washing machine spin cycle to be 640 RPM or about 90 gs which is comparable to the force we tested in the sma ll scale centrifuge. However, the centrifugation time in the washing machine is 6 minutes compared to 10 minutes in the small scale testing. As shown in Figure 4-13, the method we used to test the RM C in the washing machine is as follows: 1) the fabric is soaked in the penetra ting surfactant subphase solution, 2) the fabric is placed in the washing machine and the spin cycle is starte d, 3) once the spin cycle reaches speed, the penetrating solution is poured ont o the fabric and the washing machine is allowed to complete the spin cycle.

PAGE 84

84 4.2.2 Small Scale Monolayer Penetration Results Several experiments were performed in the lab scale to determine the effectives of monolayer penetration in the redu ction of RMC. The first experiments were run to determine the baseline of the system. In Table 4-1, the RMC from pure water to 4 mM C14SO4 solutions reduced from 82% to 60%. The next experiment s were to determine how effective monolayer penetration would be to lower the RMC. The first monolayer penetration experiment was performed by soaking the fabric in 4 mM C14SO4 and then spraying the fabric once (with ~2 mL) of the 1:5 C14SO4:DODAB monolayer solution solubilized in ethanol. The resulting RMC was reduced to 60.7%. However, due to the large amount of bulk solution in the fabric, the RMC wasnt reduced as much as expected based on the surface tension of that monolayer study. The next experiment was to soak the fabric in the 4 mM C14SO4 and then centrifuge the sample for 5 minutes to remove most of the bulk water. The fabric was then taken ou t of the centrifuge and then sprayed with 2 mL of the 1:5 monolayer so lution solubilized in ethanol. The resulting RMC was 55% which is comparable to the results from the silicone super wetter (50%) or about a 27% reduction in the RMC from that of pure water. 4.2.3 Washer Scale Monolayer Penetration Results After showing that monolayer penetration was promising in the reduction of RMC in fabric, we then expanded the experiments into full washing machine scale. The fabric was first soaked in SDS solutions (due to the expense of C14SO4) and the 1:5 SDS and DODAB monolayer was solubilized in ethanol. The fabr ic was placed in the washing machine and the spin cycle was started. Once the washing machine reached full speed in the spin cycle, 100 mL of the monolayer penetration solu tion (at 0.1% total concentrati on) was poured onto the fabric during the centrifugation process. Once the spin cycle finished, the RMC was determined after weighing the fabric.

PAGE 85

85 In Figure 4-14, we compared the effectiveness of changing the penetrant to SDS as well as changing the monolayer to SDS and DODAB from C14SO4. As shown in Figure 4-14, the RMC was reduced to ~68% for both monolayer penetr ation systems. We then performed the same experiments to determine if the RMC followed the surface tension trend which was observed in the surface tension studies of the mixed monolayer s. As shown in Figure 4-15, the RMC of the terry fabrics closely followed the surface tension tr end from the 1:10 ratio to the 1:3 ratio of SDS to DODAB with the 1:5 ratio showing the minimu m in RMC (and surface tension). This method may prove very valuable in the reduction of RMC and en ergy in laundry systems. Figure 4-1. Spreading and penetrat ion of an insoluble monolayer.

PAGE 86

86 1000 uL Time, sec 051015202530354045Equil. Surface Tension, mN/m 15 20 25 30 35 40 45 500 uL, 750 uL time, sec 01020304050607080 1000 uL C14SO 4 750 uL C14SO 4 500 uL C14SO 4 Figure 4-2. Equilibrium surface tension of a C16TAB monolayer penetrated with 4 mM C14SO4. time, sec. 01020304050607080 Surface Tension, mN/m 20 25 30 35 40 45 50 55 60 65 250 mL C14TAB 500 mL C14TAB 750 mL C14TAB 750 L 250 L 500 L Figure 4-3. Equilibrium surface tension of a st earic acid monolayer penetrated with 4 mM C14TAB.

PAGE 87

87 time, sec 0102030405060708090100 Surface Tension, mN/m 14 16 18 20 22 24 26 28 30 32 34 36 250 L C14SO 4 500 L C14SO 4 750 L C14SO 4 1000 L C14SO 4 250 L 500 L 750 L 1000 L Figure 4-4. Equilibrium surface tension of a DDAB monolayer penetrated with 4 mM C14SO4. Figure 4-5. Molecular diagram of the transient monolayer phenom ena found in slightly soluble monolayers penetrated with a solu ble surfactant from the subphase.

PAGE 88

88 Time, sec 020406080100120140160180200 Equil. Surface Tension, mN/m 10 15 20 25 30 35 40 45 50 55 250 L C14SO 4 500 L C14SO 4 1000 L C14SO 4 250 L 500 L 1000 L Figure 4-6. Equilibrium surface tension of a DODAB monolayer penetrated with 4 mM C14SO4. time, sec 01020304050607080 Equil. Surface Tension, mN/m 20 25 30 35 40 45 50 55 60 65 70 250 uL C14SO4 500 uL C14SO4 750 uL C14SO4 1000 uL C14SO4 1000 L 750 l 500 L 250 L Figure 4-7. Equilibrium surface tension of a C20OH monolayer penetrated with 4 mM C14SO4.

PAGE 89

89 Time, sec 010203040506070 Equil. Surface Tension, mN/m 18 20 22 24 26 28 30 32 34 36 38 250 uL C14SO4 500 uL C14SO4 750 uL C14SO4 1000 uL C14SO4 250 L 500 L 750 L 1000 L Figure 4-8. Equilibrium surface tension of a C holesterol monolayer penetrated with 4 mM C14SO4. time, sec 0102030405060708090100 Surface Tension, mN/m 5 10 15 20 25 30 35 250 L 500 L 750 L 1000 L 1000 L 750 L 500 L 250 L Figure 4-9. Equilibrium surface te nsion of a mixed monolayer co mposed of a 1:5 ratio of C14SO4 to DODAB penetrated with 4 mM solutions of C14SO4.

PAGE 90

90 Time, sec 020406080100120140160180200 Equil. Surface Tension, mN/m 0 10 20 30 40 50 60 70 80 1:10 1:5 1:3 1:2 1000 L Injections of 4 mM C 14 SO 4 Ratio of C14SO4 to DODAB 1:2 1:3 1:10 1:5 Figure 4-10. Equilibrium surface tension of mixed monolayers of various ratios of C14SO4 to DODAB penetrated with 1000 L of 4 mM solutions of C14SO4. time, sec 0100200300400500600700800 Equil. Surface Tension, mN/m 5 10 15 20 25 30 1:5 Molecular Ratio of DODAB to C14SO4injected with 1000 L of C14SO4 Figure 4-11. Time study of the equilibrium surface tension of a mixed monolayer of C14SO4 and DODAB in a 1:5 Ratio injected with 1000 L of 4 mM C14SO4.

PAGE 91

91 Figure 4-12. 2-D hexagonal arrangement of molecu les at the 1:3 molecular ratios in the mixed surfactant system of C14SO4 and DODAB. Figure 4-13. A) Fabric soaked in penetrating sub phase solution is placed in the washing machine, B) The spin cycle is started and allowed to come to full speed, C) Monolayer solution is poured over the fabric a nd the spin cycle is allowed to complete and the RMC is measured.

PAGE 92

92 RMC, % 60 65 70 75 80 85 90 95 100 Pure Water 150 ppm Detergent 4 mM SDS + 100 mL EtOH 4 mM SDS 1:5 Ratio + SDS 1:5 Ratio + C14SO4 1:5 Ratio + 150 ppm Detergent SDS:DODAB Monolayer SDS:C14SO4Monolayer SDS:DODAB Monolayer Figure 4-14. Comparison of RMC values for full scale washing machine experiments (150 ppm of detergent is the standard of compar ison) showing the reduction in RMC using monolayer penetration (red bars). RMC, % 60 70 80 90 100 Pure Water 150 ppm Detergent 4 mM SDS + 100 mL EtOH 1:3 Ratio + SDS 1:5 Ratio + SDS 1:7 Ratio + SDS 1:10 Ratio + SDS Pre-Soaked in 4 mM SDS Monolayer Ratios of SDS:DODAB Figure 4-15. Comparison of RMC values for full scale washing machine experiments (150 ppm of detergent is the standard of compar ison) showing the reduction in RMC using monolayer penetration (red bars).

PAGE 93

93 Table 4-1. RMC of small scale monolayer penetration with C14S04:DODAB monolayer. RMC, % Absolute Change,% Relative Change, % Pure Water 82.10 0.00 0.00 C14SO4 60.77 -21.33 -25.98 C14SO4 sprayed once with 1:5 C14SO4:DODAB monolayer solution 60.68 -21.42 -26.09 C14SO4 centrifuged for 5 minutes, sprayed with 1:5 C14SO4:DODAB and centrifuged for 5 more minutes 55.10 -27.00 -32.89 1 wt% Dow Q2-5211 50.30 -31.80 -38.73

PAGE 94

94 CHAPTER 5 MICELLE STABILITY AND ITS EFFECT ON THE RESIDUAL MOISTURE CONTENT OF FABRICS 5.1 Stabilization of Micelles It has been shown earlier that micellar st ability depends on surf actant concentration.8 The Shah Research group has shown that the micellar st ability depends on surfactant concentration. It has been shown that a maximum mi cellar stability for SDS solutions exists at 200 mM due to the small intermicellar distance, resulting in a strong repulsion between the micelles.3, 5-9 Therefore, the micelles become more rigid as the surfactan t concentration increases. This maximum in micellar relaxation time has a dramatic effect on many different propert ies of SDS solutions (ranging from low foamability, high thin film stab ility, wetting time, oil solubilization, etc.).5 The Shah Research group has also shown that micellar kinetics play an important role in detergency.5 Shah et al. has shown that the effi cacy of removing non-polar compounds from fabrics has been shown to have a strong corre lation with the relaxation time of micelles.5-10 For example, it was shown by Oh and Shah that us ing 200 mM SDS (which was shown to have the longest micellar relaxation time in the SDS concentration range)6 provided the most efficient removal of an artificial stain created by th e deposition of Orange OT onto fabric samples.9 However, the micellar stability can also be influenced by the addition of an alcohol.102 It has been shown that the maximu m micellar stability fo r SDS/alcohol mixtures exists for the system SDS/C12OH system, where the chain lengths of the surfactant and the alcohol are the same and thus where the van der Waals inter action between the hydrophobic tails is maximum (hydrophobic-hydrophobic inte ractions between the tail groups).103-106 In this study, we have found that at a concen tration of 200 mM of SD S, there is peak found in the residual moisture content (RMC) of fabrics. As shown by Patist et al.,5 we believe that this correlates to the wetting time of fabrics and is due to the increase in dynamic surface tension

PAGE 95

95 resulting from high micelle stability. We ha ve shown that not only the equilibrium surface tension plays a role in the removal of water from fabrics that the availa bility of monomer to adsorb onto the air-liquid in terface plays a role as well.107 For systems with high micellar stability, the monomer flux to the air-liquid interf ace will be less than a system with low micelle stability due to higher availabil ity of surfactant monomers to ad sorb on a newly created air-liquid interface which would lead to a higher dynamic su rface tension. At the 2 00 mM concentration of SDS, there is a maximum in micelle stability which would account for less monomer flux and less monomer to adsorb on the new air-liquid interface of bubbles creat ed during the dynamic surface tension measurement. This would thus lead to an increased dynamic surface tension that should correspond to the increase in RMC in the same surfactant concentration range. There has been a lot of work from the Sh ah Research Group showing that the micellar stability of various surfactant systems can be significantly influenced by the addition of cosurfactants.5-7, 54, 64, 106, 108-111 The increase in micellar stability of mixed surfactant systems is due to synergism shown between oppositely ch arged headgroups or hydrophobic-hydrophobic interactions between the surfact ant tail groups (as shown with a non-ionic co-surfactant). Due to coulombic or hydrophobic interactio ns, the stability of mixed surf actant systems can be tailored to varying degrees of stability. It is typically generalized that micelles are of ten drawn as static structures of spherical nature composed surfactant molecules with polar head groups exposed to the aqueous solution protecting the hydrophobic ta ils of the surfact ant in the micelle core. However, micelles are in dynamic equilibrium with individual surfactant mo lecule monomers that are constantly being exchanged between the bulk surfactant solution and the micelles. Additionally, the micelles themselves are continuously disi ntegrating and re-forming. The kine tics of this process has been

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96 evaluated by Aniansson,53, 112, 113 and the relevance of micella r relaxation time to various technological processes for single surfactant syst ems such as sodium dodecyl sulfate (SDS) in water has been extensively studied by Shah and co-workers.114 The kinetics of micellization has been studied by various techni ques such as stopped flow,115 temperature jump,116 pressure jump117 and ultrasonic absorption.118, 119 Based on previous RMC work pertaining to th e adsorption of SDS onto Hanes fabric, we have shown that an increase in the dynamic su rface tension (or a decrease in the free surfactant monomer content in the bulk solution) leads to an increase in the RMC of fabric.71 This work continued on to show that phenomena in the bulk surfactant solution that can alter the available free surfactant monomer concentration (i.e. anythi ng that can change the dynamic surface tension of the bulk solution) can influence the RMC of fabric at the end of a laundry spin cycle. Surfactant systems that have a long micellar rela xation time (i.e. micellar systems that are very stable) have been shown to have a high dyna mic surface tension and thus a higher RMC. 5.1.1 Concentration Dependence on Micelle Stability: 200 mM SDS In our previous work, the residual moisture content has been shown to be a function of surface tension of solution.68 However, as shown in Figure 5-1, the residual moisture does not completely correlate to the equilibrium surface te nsion of pure SDS solutions in the range of 5-8 mM. A small dip in the surface tension at ~6 mM SDS concentration suggests that the sample had a small impurity (presumably dodecyl alcoho l). Recent experiments in our laboratory using purified SDS samples have shown the same RM C peak. Due to adsorption of SDS onto the fabric surface, the dynamic surface tension of the residual solution increased thus leading to an increase in the RMC (Figure 5-2). We have shown over the past years that st able micelles greatly affect many difference aspects of surfactant systems.5, 7 For the SDS system at 200 mM (most stable micelles for SDS),

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97 we have shown that there is a large increase in bubble volume, si ngle film stability, detergency effectiveness, emulsion droplet size, benzene so lubilization etc (Figure 5-3 and Figure 5-4). It has also been shown with there SDS solutions that at 200 mM that there is a decrease in foamability and time to solubilize benzene in solution (Figure 5-3 and Figure 5-4). Since the relaxation time of surfactants play su ch a large role in ma ny different properties of surfactant systems, the RMC of Hanes fabric around the concentration range of highest SDS micellar stability was measured (from 125-250 mM concentrations of SDS). Since the dynamic surface tension is related to the micellar stabilit y (i.e. higher micellar st ability leads to higher dynamic surface tension as shown in Figure 5-7), it would be expected that there will be an increase in the RMC around a SDS concentration of 200 mM. In Figure 5-5, we have shown that at 200 mM concentration of SDS that there is peak shown in the RMC of Hanes fabric. This peak is believed to be due to the long relaxati on time of the SDS micelle s at 200 mM. The long relaxation time of the micelles would lead to a decreased monomer flux from the micelles to the bulk. Since the micellar stability is high for 200 mM SDS, there is less free monomer flux from the micelle to the bulk solution t hus causing an increase in the DS T. This decrease in monomer flux would then be shown as an increase in the dynamic surface tension this leading to an increase in the RMC. Alternatively, another po ssible explanation to explain the increase in RMC at 200 mM concentrations of SDS could be due to stabilization of thin films on the fabric surface as well as films in the interfiber spaces due to relatively stable micelles as demonstrated in Figure 5-6. It has been shown by Shah et al.11, 12 and Wasan et al.13-18 that layering of micelles or particles can stabilize thin films (which c ould possibly explain an increase in the RMC). However, it is a possibility that the increase in RMC at 200 mM SD S is due to a combined effect

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98 of thin film stability due to layering of micelles as well as the increase in dynamic surface tension from the reduction in mono mer flux from the stable micelles. 5.1.2 Mixed Surfactant Systems It has been well documented by the Shah Resear ch group that the stab ility of SDS micelles can be greatly influence by th e addition of co-surfactants.106, 109, 110, 120 For this study, we chose to add C12TAB and C12OH to the SDS solutions in order to increase the stability of the mixed micellar system compared to the stability of th e pure SDS system. As shown in Table 5-1, the micellar relaxation times of the systems we chose to test show an increase in micellar relaxation time from ~1 ms to as high as 2,000 ms. Ba sed on our results for the 200 mM SDS system (highest stability over the entire SDS concentration range), we w ould expect that the RMC of the cotton fabric will show the same trend as the micellar relaxation time increases shown in Table 5-1. Shown in Figure 5-8, we have shown that th ere is an increase in RMC from the pure SDS system to the mixed system of SDS and C12TAB. The RMC increased from the pure system to the addition of C12OH and then further increased with the SDS and C12TAB system (RMC of ~62, 67 and 80% respectively). The increase in micellar relaxation time for the SDS and C12OH system arises from the strong ion-dipole in teraction between the SDS and the dodecanol. The large increase in relaxation time with the addition of C12TAB to the SDS results from the strong electrostatic (anionic-cat ionic) interactions betw een the head groups of th e surfactants resulting in a very tightly packed interf ace (at the air-liquid and micellar interfaces). These increases in relaxation times also result in an increase in the dynamic surface tension of each solution. Very stable micelles have little monomer flux to th e bulk solution and thus for any new air-liquid interface created, there is a small amount of free monomer available to adsorb at the new

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99 interface to lower the dynamic surface tension. As we have previously discussed, an increase in the dynamic surface tension will then cause an increase in the RMC of fabrics. We have shown that many differe nt factors affect the RMC of fabric/surfactant systems. It was shown that adsorption phenomena play an impor tant factor in laundry processes. It should also be noted that the dynamic surface tension has been shown to play a large role in the manipulation of the RMC of fabrics. Based on th e results that have been presented, if the magnitude of adsorption of surfactant onto the fabr ic or if the micellar kinetics of the surfactant system used can be significantly changed, the magnitude of the RMC can thus be significantly altered (i.e. increased relaxation times for in creased RMC or decreased relaxation times for lower RMC). 5.2 Effect of Dodecyl Sulfate Counterions on the RMC of Fabrics Based on previous RMC work pertaining to th e adsorption of SDS onto Hanes fabric, we have shown that an increase in the dynamic su rface tension (or a decrease in the free surfactant monomer content in the bulk soluti on) leads to an increase in the RMC of fabric. This work has continued to show that phenomena in the bulk surf actant solution that can a lter the available free surfactant monomer concentration (i.e. anything that can change the dynamic surface tension of the bulk solution) can influence th e RMC of fabric at the end of a laundry spin cycle. Surfactant systems that have a long micellar relaxation time (i.e. mi cellar systems that are very stable) have been shown to have a high dynamic surface tension and thus a higher RMC. The increase in RMC with stable micellar systems is because micelles must be broken down into monomers to be available to adsorb onto the newly created air-liquid interface and thus reduce the dynamic surface tension (and RMC).121 If the micelles are stable, the monomer flux from the micelle is very low and the dynamic surface tension is high resulting in an increase in RMC.

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100 For the purpose of this work, we have i nvestigated the effects of using different counterions for the dodecyl sulfat e surfactant. The substitution of one kind of counterion with another counterion has the potential to alter the interactions betw een both the counterions and the surface-active molecules. By changing the degree of binding of counterions to the surface-active portions of the surfactant molecule can greatly influence the surface active chemical properties of the surfactant.122 One key aspect of the solution that is significantly influenced by a change in the counterion is the equilibrium surface tension30 In terms of affecting the CMC of the dodecyl sulfate surfactants, the CMCs of LiDS, NaDS, CsDS, and Mg(DS)2 are reported by Mukerjee to be 8.92, 8.32, 6.09, 0.88 mM, respectively, at 25C.123 5.2.1 Experimental procedure (Surfactant Synthesis) Lithium dodecyl sulfate (99% purity) is purchased from Ac ros (Orlando, FL), sodium dodecyl sulfate (99% purity) from MP Biomedi cals, Inc. Magnesium dodecyl sulfate (98% purity) from Pfaltz and Bauer (Waterbury, CT). Cesium dodecyl sulfate is prepared in our laboratory with the same procedure as shown by Kim et al.122. Chlorosulfonic acid (Aldrich, Milwaukee, WI, 553.5 mM) is added to dodecan ol drop by drop with vigorous mixing at 25C under a nitrogen atmosphere. The su lfation reaction is performed ve ry slowly (40 min and cooled with ice) since the sulfation pr ocess is highly exothermic. After the sulfation pro cess, nitrogen gas is used to purge the reaction mixture to remove HCl produced during the reaction. Aqueous CsOH solution (Aldrich, 50.0 wt%) is added to the reaction mixture in a 1:1 molar ratio to neutralize the acid. The CsDS is recrystallized three times with a 50:50 mixture (by volume) of ethanol and acetone, keeping the solution below 5C.

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101 5.2.2 Molecular mechanisms Figure 5-9 shows the equilibrium surface tensio n values for the dodecy l sulfate surfactants (with Li, Na, Cs and Mg counterions) at both 1 mM and 50 mM molecular concentrations of surfactants. It is well known that the lowering of surface tension is due to efficient molecular packing at the air-liquid interface (tighter packing results in lower surface tension). As shown in Table 5-2, we have indicated the Ionic Radius (), Hydrate Radius (), Area per Molecule (2/molecule) and the dimensionless dynamic surface tension (, as indicated in Equation 5-1). With tighter molecular packing (as shown in Figure 5-10), lower surface tensions can be obtained. deq weq (5-1) The dimensionless dynamic surface tension (Equa tion 5-1) is used to show the importance of micellar break up in the measurement of dynamic surface tension where d is the dynamic surface tension, eq is the equilibrium surface tension (as measured by the Wilhelmy plate method) and w is the surface tension of pure water.30 The value of = 0 (or d = eq) indicates that the surfactant adsorption under dynamic cond ition is the same as that under equilibrium conditions and the micelles are labile as well as the monomers are diffusing fast, whereas = 1 ( d = w) indicates no surfactant is present at the interface under the dynamic conditions existing during the bubbling process implying either th e presence of relatively stable micelles or monomers with high charac teristic diffusion time. Previous work on foam studies by Pandey et al. has shown that at 50 mM concentrations of the dodecyl sulfate su rfactants using Li+, Na+, Cs+ and Mg++ the foam is most stable using Mg(DS)2 as shown in Figure 5-11.30 However, we have shown mixed RMC results with the use

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102 of various counterions and cotton fabric. The RMC was measured at 1 mM total surfactant concentration and as shown in Figure 5-12, the RMC decreases in order of decreasing equilibrium surface tension. Since we are below th e CMC of the surfactant s (or right at the CMC in the case of Mg(DS)2), micellar kinetics do not play a role in the removal of water from fabrics. However, as shown in Figure 5-13, we have meas ured the RMC of Hanes cotton fabric at 50 mM total surfactant concentr ation (which is well above the CMC for each surfactant system) and we have shown that the RMC increases in or der of increasing micellar relaxation time, 2. We have already shown that the RMC of fabr ics is a function of equilibrium and dynamic surface tension.68, 71, 124 From the data shown here along with the work done on adsorption of SDS onto the fabric surface (causing an increase in the dynamic surface tension)71, we have identified micellar kinetics to be a very impor tant parameter in the reduction of RMC from fabrics. Micellar kinetics has a huge potential to save millions if not billions of dollars per year (in the US alone) in consumer dr ying costs. There is a huge pote ntial to save even more money with this information extrapolat ed into industrial applications. 5.3 Chain Length Compatibility 5.3.1 Review of Chain Le ngth Compatibility Work The Shah Group has shown that chain length comp atibility is an important factor in many mixed surfactant systems.54, 106, 109, 110, 125 As surfactant molecule s (or other hydrocarbon molecules in mixed systems) adsorb at interf aces, the molecular packi ng at the interface is influenced by the matching or mismatching of the alkyl chains of the surfactants and cosurfactants adsorbing at the interface. In general, the chain length of surfactants used in a given mixture must be the same to maximize lateral molecu lar interactions. It has been reported that as the difference in chain lengths of mixed surfactants increases, the spacing between the adjacent surfactant molecule increases.126 Although these changes are very small, they have a very large

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103 effect upon the interfacial and bulk properties of th e solutions, e.g., foamability, foam stability, surface tension, surface viscosity, contact angle, bubble size, fluid displacement in porous media, and microemulsion stability.32, 126-129 The effect of chain length compatibility is particularly important to interfacial properties and technologie s, such as surface tension, surface viscosity, foamability, lubrication, contact angle, bubble size, environmental remediation, enhanced oil recovery, water solubiliza tion in microemulsions, and microemulsion stability.3, 106, 126 5.3.2 SDS + Long Chain Alcohols (CnOH) The residual moisture content (RMC) of SDS/CnOH (n = 8, 10, 12, 14, and 16) mixtures was measured by our established method 68. Based on the previous work of Patist showing the micellar stability and the chain length compatibility of SDS/CnOH systems (where SDS and C12OH showed the greatest micellar stability, highest foam stability, highest surface viscosity, etc.110) we have shown a correlation of chain length compatibility between SDS and CnOH with the RMC of cotton fabrics. Patist et al.110 have shown the following conclusions from their work on SDS + long chain alcohols: 1. Long chain alcohols (CnOH for n = 8, 10, 12, 14, and 16) stabilize SDS micelles, up to approximately 150 mM SDS (dependi ng on the carbon chain length of the alcohol) due to the strong ion-dipole in teraction between th e negatively charged SDS head group and the hydroxyl group of the alcohol. Beyond this critical concentration the chain length compatibili ty starts playing an important role. Therefore, only C12OH will cause a furthe r increase in micellar stability, whereas the mismatch in chain length between the other alcohols and the SDS results in a disruption of the molecular packing in the micelle, thereby decreasing the stability. 2. The effect of adding C12OH is most pronounced when the stability of pure SDS micelles is very low, i.e., at low SDS concentrations (25 mM). At higher SDS concentrations, the micellar stability increases, which makes the effect of C12OH less pronounced. 3. The effect of micellar stability plays an important role in processes involving a rapid increase in surface area. If enough tim e is allowed for the interface to form, the dynamic surface tension approaches the equilibrium surface tension and thus

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104 more foam is generated (more in case of SDS/C12OH mixtures). However, in very high speed processes, the micellar stability and thus the time it takes for micelles to break up, determines the rate of adso rption of surfactant molecules and therefore higher surface tensions will be attained for SDS/C12OH solutions. In that case less foam is generated, even though the e quilibrium surface tension of the SDS/C12OH system is lower. In conclusion, different methods of foaming can produce opposite results as illustrated by the foam-abi lity measurements in this study. Based on this work, we measured the RMC of mixed surfactant syst ems of SDS with the addition of long chain alcohols (CnOH for n= 8, 10, 12, 14, and 16). As shown in Figure 5-14, there is a maximum in the RMC with the SDS/C12OH system. When the hydrophobic tail groups were of the same chain length, there was a maximum in RMC which corresponds to the maximum found in micellar relaxati on time as shown by Patist et al.110 We have shown that an increase in the micellar stability results in an increase in the dynamic surface tension of solution due to a reduction in the monomer flux from the stable micelles to the newly created air/liquid interface. 5.3.3 SDS + CnTABs The residual moisture content (RMC) of SDS/CnTAB (n = 8, 10, 12, 14, and 16) mixtures was measured by our established method 68. Based on the previous work of Patist et al.54 showing the micellar stability and the chain length compatibility of SDS/CnTAB systems (where SDS and C12TAB showed the greatest micellar st ability, highest foam stability, highest surface viscosity, etc.) we have show n a correlation of chain length compatibility between SDS and CnTAB with the RMC of cotton fabrics. Patist et al.54 have shown the following conclusions from their work on SDS + alkyltrimethylammonium bromides (CnTAB for n = 8, 10, 12, 14, and 16): 1. For mixed solutions of anionic and cationic surfactants (SDS + CnTAB), the surface properties depend upon the chain length of the individual surfactant molecules.

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105 2. For mixed surfactant systems, minimu m surface tension, maximum surface viscosity, highest micellar stability, maximum foam stability, and minimum foamability were observed when both surfactants in the system had the same chain length (SDS + C12TAB). 3. The chain length compatibility and th e Coulombic interaction in SDS/C12TAB mixtures result in a closest packing of mo lecules in both micelles as well as at the air/water interface. Based on this work, we measured the RMC of mixed surfactant syst ems of SDS with the addition of alkyltrimethylammonium bromides (CnTAB for n= 8, 10, 12, 14, and 16). As shown in Figure 5-15, there is a maximum in the RMC with the SDS/C12TAB system. When the hydrophobic tail groups were of th e same chain length, there was a maximum in RMC which corresponds to the maximum f ound in micellar relaxation time as shown by Patist et al.54 These results are relevant to laundry process as consumers often use the fabric softeners which are cationic molecules after washing the laundry with detergents containing negatively charged surfactant molecules. The residual deterg ent which carries over into the rinse cycle may combine with the fabric softener molecules and produce stable micelles or a tightly packed film at the air water interf ace in the fabric. If a stable mixed micelle between the anionic detergent and the cationic fabric softener is formed, the result could be an increase in the RMC of the laundry. This would then lead to an increase in drying time and thus drying energy costs. 5.4 Labile Micelles It has been shown by Leung et al.102 that by the addition of eith er short chain alkanols or a polymer such as polyvinylpyrrolidone that the mi cellar relaxation time of SDS at 100 mM could be could be reduced or the micelle could beco me labilized. For the addition of a short chain alkanol, this labilizing effect was due to either the stabilization of the micelle nuclei from a decrease in the micelle nucleus size or it wa s due to the reduction of the activation energy required to form a micelle nucleus.102 Likewise, by the addition of a polymer (such as PVP), the

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106 micelle relaxation time decreased due to the polymer creating a nucleating site for micelle nuclei. These results indicated that the micelle relaxation time was rate limited by the rate at which micelle nuclei are created. 5.4.1 SDS + Polyvinylpyrrolidone (PVP) In Figure 5-16, the micelle relaxation time and the RMC of the same SDS and polymer (PVP) system is shown. As predicted by previous results in that the RMC of the fabrics was directly related to the stability of micelles, thes e results showed the same trend as the relaxation time indicated in Figure 5-16. As the micelles become labile, the free monomer content is thus increased and is available to lower the dynami c surface tension. This decrease in dynamic surface tension results in a lower RMC. 5.4.2 SDS + Short Chain Alcohols (CnOH) Similar to the results of the SDS/PVP system, the addition of short chain alcohols has been shown to reduce the micelle relaxation time. As shown in Figure 5-17, the relaxation times and the corresponding RMC values for cotton fabric as a function of 1-alkanol concentration indicates that as the micelle relaxation time decreases (i.e. micelles become more labile), the RMC decreases as well. This is also due to an increase in the free monomer content in the bulk solution to lower the dynamic surface tension. Howeve r, the lowering of the thin film stability by stabile micelles could possibly explain the same results.

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107 Initial SDS Conc. mM 0123456789101112131415 RMC, % 50 55 60 65 70 75 80 85 90 95 Equi. Surface Tension, mN/m 25 30 35 40 45 50 55 60 65 70 RMC of Hanes Fabric Equi. Surface Tension of SDS Figure 5-1. RMC of Hanes 100% co tton fabric as a function of SD S concentration plotted with equilibrium surface tension of pure SDS solutions. Initial SDS Conc. mM 024681012 RMC, % 50 55 60 65 70 75 80 85 90 D y namic Surface Tension, mN/m 50 55 60 65 70 75 RMC Hanes Fabric Dynamic Surface Tension CMC Figure 5-2. RMC and DST of the residual solution from the Hanes 100% cotton fabric soaked in SDS solutions.

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108 Figure 5-3. Liquid/gas phenomena exhibi ting minima and maxima at 200 mM SDS concentration. Figure 5-4. Liquid/liquid and so lid/liquid phenomena exhibiting minima and maxima at 200 mM SDS concentration.

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109 Initial SDS Conc., mM 100120140160180200220240260 RMC, % 64 66 68 70 72 74 76 78 Figure 5-5. RMC of Hanes fabric around the concen tration range of most stable micelles of SDS (200 mM) (i.e. stable micelles can increas e the RMC from ~68% to ~76% in SDS solutions). Figure 5-6. Stable micelles trapped in the inte rstitial space in between fiber strands. Stable micelles could help stabilize the thin liquid film in this interstitial space leading to an increase in RMC.

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110 Figure 5-7. Effect of micellar st ability on dynamic surface tension. More stable micelles result in less monomer flux thus causing an incr ease in the dynamic surface tension. RMC % 60 65 70 75 80 85 110 mM SDS 100 mM SDS + 10 mM C 12 OH 100 mM SDS + 10 mM C 12 TAB Figure 5-8. RMC of Hanes cotton fabric for pure SDS and mixed SDS systems (with the addition of either C12OH or C12TAB).

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111 Equilibrium Surface Tension, mN/m 0 10 20 30 40 50 60 70 1 mM Concentration 50 mM Concentration LiDSNaDS CsDS Mg(DS)2 Figure 5-9. Equilibrium surface tension for dodecyl sulfate surfactants with various counterions for 1 mM and 50 mM concentrations. Figure 5-10. Effect of counterions on the molecular packing of dodecyl sulfate at the air/liquid interface. Area per molecule (Am) is in the following order for the chosen counterions: Am Li+>Am Na+>Am Cs+>Am Mg++.30

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112 Foam Stability (hrs) 0 10 20 30 40 50 60 50 mM Surfactant Concentrations LiDSNaDS CsDS Mg(DS)2 Figure 5-11. Foam stability for various dodecyl sulfate counterions at 50 mM total surfactant concentration.30 RMC, % 60 62 64 66 68 70 72 74 Hanes Fabric RMC LiDSNaDS CsDS Mg(DS)21 mM Surfactant Concentration Figure 5-12. RMC of Hanes cotton fabric for vari ous dodecyl sulfate counterions at 1 mM total surfactant concentration.

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113 RMC, % 70 72 74 76 78 80 82 Hanes Fabric RMC LiDSNaDS CsDS Mg(DS)250 mM Surfactant Concentration Figure 5-13. RMC of Hanes cotton fabric for vari ous dodecyl sulfate counte rions at 50 mM total surfactant concentration. RMC, % 54 56 58 60 62 64 66 68 70 72 Pure 100 mM SDS SDS + C8OH SDS + C10OH SDS + C12OH SDS + C14OH SDS + C16OH 95 mM SDS + 5 mM CnOH Figure 5-14. RMC of cotton fabr ic (centrifuged for 10 minutes at 1000 RPM) as a function of SDS + CnOHs (for n= 8, 10, 12, 14, and 16) w ith a reference to the RMC of cotton fabric at 100 mM pure SDS.

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114 RMC, % 54 56 58 60 62 64 66 68 70 72 Pure 100 mM SDS SDS + C8TAB SDS + C10TAB SDS + C12TAB SDS + C14TAB SDS + C16TAB100 mM SDS + 5 mM C n TAB Figure 5-15. RMC of cotton fabr ic (centrifuged for 10 minutes at 1000 RPM) as a function of SDS + CnOHs (for n = 8, 10, 12, 14, and 16) with a reference to the RMC of cotton fabric at 100 mM pure SDS.

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115 Concentration of PVP (wt%) 0.010.1110 RMC, % 40 50 60 70 80 90 100 RMC of Terry Cloth 2 (sec) 0.0001 0.001 0.01 0.1 1 Relaxation time of SDS + PVP Figure 5-16. Long relaxation time (2) and RMC of Terry fabric in solutions of PVP.

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116 CONCENTRATION OF 1-ALKANOLS 0200400600800100012001400 RMC, % 40 50 60 70 80 Methanol Ethanol Propanol Butanol Pentanol Hexanol 2 (sec) 0.0001 0.001 0.01 0.1 1 10 Methanol Ethanol Propanol Butanol Pentanol Hexanol 100 mM SDS 100 mM SDS Figure 5-17. Long relaxation time (2) and RMC of Terry fabric in solutions of 100 mM SDS with the addition of short chain alcohols. Table 5-1. Micellar relaxation times (2) for different SDS systems with the addition of cosurfactants.54, 109-111 Micelle Stability 2 (ms) 100 mM SDS 100 100 mM SDS + 10 mM C12OH 900 100 mM SDS + 10 mM C12TAB 2000

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117 Table 5-2. Physical properties and dimensionless dynamic surface tension ( ) of different counterions of dodecyl sulfate.30 Ion Ionic Radius () Hydrated Radius () A rea per molecule (2/molecule) Parameter Li+ 0.60 3.82 61.0 0.138 Na+ 0.94 3.58 51.5 0.131 Cs+ 1.69 3.29 44.5 0.202 Mg++ 0.65 4.28 38.7 0.353

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118 CHAPTER 6 FULL SCALE RESIDUAL MOISTURE CONTENT TESTING UNDER NORMAL CONDITIONS 6.1 Various Surfactant Systems in Full Washer Scale We have shown that the reduction of equi librium and dynamic surface tension and the micellar stability play very important roles in reducing the residual moisture of fabrics68, 71. Since the goal is to reduce the RMC of the fabric durin g the final spin cycle of the washing machine, the use of a fabric softener along with the a ddition of a co-surfactan t to reduce the surface tension of solution may show promise in the redu ction of RMC. Our rational was due to the fact that before the final spin cycle of the washing machine, many people add fabric softeners to the final rinse to enhance the properties of the fabric once it has been dried. Shah et al.34 has found striking change in properties of various systems (eg. lecithincholestrol, stearic acid-stearyl alcohol, decanoi c acid-decanol, potassium oleate-hexanol, SDScetyl pyridinium chloride) at a 1:3 molecular ra tio. Though direct values of surface tension were not reported for these systems, in all cases ther e is indirect evidence (evaporation rate, foam stability, solubilization in microemu lsion) that at this ratio there is a crowding of molecules at the interface and the molecules are tightly packed. Ot her researchers have reported this synergism for anionic/cationic,35 anionic/zwitterionic,36-38 cationic/zwitterionic,36 non-ionic/zwitterionic,36 anionic/cationic-gemini,39 anionic gemini/zwitterionic,40 cationic-gemini/nonionic41 and cationicgemini/sugar surfactants.42 These investigations suggest that properly en gineered synergism can help reduce surface tension values to 20 mN/m. Since it has been well documented that synergism exists in mixed surfactant systems, ani onic co-surfactants were chosen to use in the reduction of surface tension in fabric softener systems.

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119 6.1.1 Small Scale Testing The surface tension of a leading fabric soften er solutios (500 ppm solutions which is the common household dosage in the washing machine) with the addition of various co-surfactants was measured to determine if one of the syst ems would produce a low surface tension value that could significantly decrease the am ount of water retained by the fabr ic at the end of the final spin cycle. With the addition of SDS to fabric soften er solutions, the surface te nsion initial decreases and then increases once again and then levels o ff (Figure 6-1). The same trend was observed in the fabric softener and diocty l sulfosuccinate (AOT) system (Figure 6-2). This phenomenon is further discussed in this chapter. The surface tension of the fabric softener solu tions with the addition of a silicone super wetting surfactant was also measured. However, the effectiveness of silicone surfactants has been shown to decrease depending on the pH of so lution. At higher pH va lues, the functionality of silicone surfactants decreases and the surface tension lowering power of silioxanes decreases. In this study, we measured the initial surface te nsion of solution once the Dow Q2-5211 silicone super wetter was added to the fabric softener and then once again measured the surface tension after allowing the solution to sit for 4 days. As shown in Figure 6-3, the initial equilibrium surface tension is lowered to ~20 mN/m. After the solution has aged for 4 days, the surface tension has increased from 20 mN /m to as high as 40 mN/m indi cating the effectiveness of the silicone surfactant has been altered. 6.1.2 Full Scale Washer Testing Based on the research that was performed in small scale, several systems were chosen to measure the RMC in large scale. As shown in Ta ble 6-1, the RMC for the 3:1 molecular ratio of SDS to C12TAC (C12TAB was used in the lab) was on the order of the RMC obtained for the detergent control. Also, the RMC of the fabric softener systems with the addition of SDS and

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120 AOT was reduced to values lower than the leadin g detergent control. The washing machine used in these studies was a Whirl pool Model LSQ9010LWO with a spin cycle speed of 640 RPM and a drum diameter of 21 inches. The final spin cycle was on average 360 seconds (6 minutes). The dependence of RMC on the temperature of the rinse water was also observed. As shown in Figure 6-4, the RMC decreases from about 94% to about 79% from a temperature change from 60F to 100F. This is due to the reduction of surface tension as the temperature increases as well as the swelling of fibers allowi ng more water to be displaced due to a lessened capillary force from larger capillaries. Also, th e drying curve was measured for the same systems as discussed previously (SDS:C12TAC, SDS in fabric softener and AOT in fabric softener). In Figure 6-5, the drying rates are shown to be almost superimposed upon each other with no distinguishable differences between the three systems. This is e xpected since the RMC values for each system are approximately the same. 6.2 Vesicle Surfactant Interactions As previously discussed, the surface tension va lues of 500 ppm solutions of fabric softener with the addition of SDS showed an interesting trend. We believe that this is due to the interaction of the anionic co-surfactant with the vesicles in the fabric softener system. As shown in Figure 6-1, the surface tension a nd RMC of Hanes fabr ic was measured for 500 ppm solutions of fabric softener with th e addition of SDS. Several experiments were performed to determine the molecular mechanisms responsible for the changes in surface tension that we observed. Since vesicles compose most normal fabric softener systems, the turbidity and particle sizing of the interac tion between fabric softener system and SDS was measured. 6.2.1 Turbidity The turbidity of fabric softener solutions with the addition of SDS was measured and shown in Figure 6-6. The initial turbidity was measured immediately after the addition of the

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121 SDS to the 500 ppm fabric softener solutions. Th e turbidity was then m onitored over a period of 7 days. It was noticed that after 24 hours, th e turbidity measurements stabilized and were approximately the same from 1 to 7 days. This is an indication that the system has equilibrated. Due to the large increase in turbidity at the 1:1 ratio of SDS to the active surfactant in the fabric softener, it is believed that some sort of agglom eration is occurring due to charge shielding of the vesicles. 6.2.2 Particle Sizing From the turbidity measurements, we believ e that the increase in surface tension and turbidity is due to an agglomeration of the fabric softener vesicles due to charge shielding at the 1:1 ratio of SDS to the active surf actant in the fabric softener. In Figure 6-7, it is shown that the mean particle size of 500 ppm solutions of the fabric softener increases from 200 nm with no SDS present (which corresponds to particle size data verifying th at the vesicles are 200 nm in size) to over 600 nm at the 1:1 ratio of SDS to the actives in the fabric softener. This validates our assumption that the increase in surface tension and turbidity was due to the agglomeration of vesicles due to charge shielding. 6.2.3 Molecular Mechanisms between SDS and Vesicles It has been shown that the surface tension, turbidity and mean particle size of 500 ppm fabric softener solutions with the addition of SD S show a maximum at the 1:1 ratio of SDS to the active surfactant in the fabric so ftener (Figure 6-8). It is believ ed that this is due to charge neutralization of the vesicles allowing the vesi cles to agglomerate and cause an increase in turbidity and particle size. A diagram showing the interactions of SDS with the vesicles at the 1:1 ratio is shown in Figure 6-9.

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122 6.3 Washer Scale RMC Testing In order to reduce the residual mo isture content of fabrics at th e end of the final spin cycle, several various common surfactant systems were investigated. The use of adding the following common surfactant solutions was investigated: Cascade (Figur e 6-10), Dow Q2-5211 (Figure 611), Flexiwets (Figure 6-12), Jet Dry (Figure 6-13) and Sylgard 309 (Figure 6-14). As shown in those figures, the RMC was reduced to as low as 60-65% RMC (60% with Jet Dry and 65% with Cascade). The use of the active surfactant in t hose solutions may prove to be useful in the reduction of RMC at the end of the laundry spin cycle. However, the mechanism behind the lowering of RMC is unknown at this time. As sh own in Figure 6-10, the surface tension of the Cascade solutions is in the high 20 mN/m range (about 27-29 mN/m) but the resulting RMC is as low as 65%. This indicates that the surface tension is not playing as an important role in this system. As shown in the LaPlace equation, the co ntact angle of water with the fabric can be changed and affect the capillar y height (and thus the RMC). If the fabric becomes hydrophobic, water is more easily shed from the fabric dur ing the centrifugation process even if the surface tension of solution is higher. This is the mechan ism which we believe is responsible for the low RMC values with these commercially available surfactant systems. As shown in Figure 6-15, the lowest surface tension achieved with monolayer penetration in the washing machine is about 68%. However, using Jet Dry additive in the washing machine, the RMC was reduced to 61%. As shown in Table 6-2, a RMC value of 56% would need to be reached to obtain the ultimate goal of a 30% reduction in the RMC (compared to the RMC reduction of 80% RMC for the 150 ppm carryover of the detergent into the rinse water). Table 62 lists the various methods used in reducing th e RMC and the various RMC values obtained for each method. There were several methods used su ch as monolayer penetration and soaking the fabric in solution. Also, it s hould be pointed out that the RM C is only reduced to about 70%

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123 using the silicone super wetter in the washi ng machine. Based on these results, monolayer penetration is a promising method to redu ce the RMC of laundry us ing surfactants and monolayer materials that are pres ently used in the wash cycle. ppm SDS 050100150200250300 RMC, % 56 58 60 62 64 66 68 Molecular Ratio SDS:Fabric Softener Actives Pure Downy1:11.9:12.8:13.75:14.7:15.6:1 Equil. Surface Tension, mN/m 30 35 40 45 50 55 60 RMC Surface Tension 1:1 Ratio SDS::Softener 3:1 Ratio SDS:Softener Figure 6-1: Equilibrium surface tension and RMC of fabric softener systems with the addition of sodium dodecyl sulfate (SDS).

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124 ppm AOT 0255075100125150175200 Equil. Surface Tension, mN/m 25 30 35 40 45 50 55 Molecular Ratio AOT:Softener Pure Downy1:3.31:1.651:1.11.2:11.5:11.8:12.1:12.4:1 1:4.11:1.18 1:2.1 Figure 6-2: Equilibrium surface tension of fabr ic softener solutions with the addition of dioctylsulfosuccinate (AOT). ppm Dow Q2-5211 0100200300400500600 Equil. Surface Tension, mN/m 10 20 30 40 50 60 0 Days 4 Days Figure 6-3: Equilibrium surface tension of fabric softener solutions wi th the addition of Dow Corning Q2-5211 Silicone Super Wetter.

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125 Temp, oF 406080100120140160 RMC, % 76 78 80 82 84 86 88 90 92 94 96 Terry Fabric RMC Figure 6-4: Temperature de pendence of the RMC during the final rinse cycle. Time, minutes 0102030405060 RMC, % 0 20 40 60 80 3:1 SDS:C12TAC 50 ppm SDS in softener 60 ppm AOT in softener Figure 6-5: Drying rate curv e for terry fabrics soaked in various solutions.

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126 ppm SDS 050100150200250300Turbidity, NTU 20 40 60 80 100 120 140 160 180 200 Ratio SDS:Softener Pure Softener1:11.9:12.8:13.75:14.7:15.6:1 0 Day 1 Day 4 Day 7 Day 1:1 SDS:Softener 3:1 SDS:Softener Figure 6-6: Turbidity measuremen ts of fabric softener soluti ons with the addition of SDS. Solutions were aged up to 7 days. Fa bric softener solutions were 500 ppm. Mean particle size 500 ppm fabric softener solutions with addition of SDSppm SDS 050100150200250300mean partcile size, nm 100 200 300 400 500 600 700 Ratio SDS:Softener Pure Softener1:11.9:12.8:13.75:14.7:15.6:1 4 Days 7 Days Figure 6-7: Mean particle size of 500 ppm solutions of fabric soft ener with the addition of SDS.

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127 ppm SDS 050100150200250300 Turbidity, NTU 20 40 60 80 100 120 140 160 180 200 Particle Size, nm 100 200 300 400 500 600 700 Figure 6-8: Particle size and turb idity of 500 ppm fabric softener solutions with the addition of SDS. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + -1:1 SDS-Downy Actives Ratio1:1 Packing resulting in complete charge neutralization Agglomeration of Vesicles + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + -1:1 SDS-Downy Actives Ratio1:1 Packing resulting in complete charge neutralization Agglomeration of Vesicles Figure 6-9: Molecular diagram describing the interaction of vesicles with SDS at 500 ppm concentrations of fabric softener at a 1:1 ratio of SDS to the ac tive surfactant in the fabric softener. At the 1:1 ratio, ch arge neutralization occurs and allows agglomeration of the vesicles.

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128 Cascade Conc. wt% 0.00.51.01.52.02.5 RMC, % 60 65 70 75 80 85 90 95 100 Equi. Surface Tension, mN/m 27.0 27.5 28.0 28.5 29.0 29.5 30.0 30.5 31.0 SFT RMC Figure 6-10: RMC of Terry cloth fabr ic with the addition of Cascade. Dow Q2-5211 Conc. wt% 0.00.20.40.60.81.01.2 RMC, % 65 70 75 80 85 90 Figure 6-11: RMC of Terry cloth fabric with the addition of Dow Q2-5211.

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129 Conc. ppm 020040060080010001200 RMC, % 90 100 110 120 130 140 Flexiwet Q-22 Flexiwet RFD-15A Flexiwet NF Figure 6-12: RMC of Terry cloth fabric with the addition of Flexiwet (Q -22, RFD-15A, and NF). Jet Dry Conc. wt% 0123456 RMC, % 60 65 70 75 80 85 90 95 100 Figure 6-13: RMC of Terry cloth fabr ic with the addition of Jet Dry.

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130 Sylgard 309 Conc. wt% 0.00.51.01.52.02.5 RMC, % 65 70 75 80 85 90 95 100 Figure 6-14: RMC of Terry cloth fabric with the addition of Sylgard 309. Full Scale Washing Machine Tests RMC, % 60 70 80 90 100 Pure Water 150 ppm Detergent 4 mM SDS + 100 mL EtOH 1:3 Ratio + SDS 1:5 Ratio + SDS 1:7 Ratio + SDS 1:10 Ratio + SDS Pre-Soaked in 4 mM SDS Monolayer Ratios of SDS:DODAB Figure 6-15: Comparison of RMC values for full scale washing machine experiments (150 ppm of detergent is the standard of compar ison) showing the reduction in RMC using monolayer penetration (red bars).

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131 Table 6-1: RMC for terry cloth fabrics with th e addition of various a dditives added to 500 ppm solutions of fabric softener. RMC, % Absolute Change, % Relative Change, % Water 91.87 0.00 0.00 Detergent Control (0.15%) 76.67 -15.20 -16.55 3:1 SDS:C12TAC 77.96 -13.91 -15.14 50 ppm SDS 76.53 -15.34 -16.70 50 ppm AOT 72.91 -18.96 -20.64 Table 6-2: RMC values for large scale washing machine experiments for various surfactant systems. RMC% SFT, mN/m Relative RMC Change wrt Detergent % Application Contact AngleUltimae Goal of 30% RMC Reduction 56.00%--30.17%-Jet Dry (2%) 62.96%29-21.49%Solution Cascade Clean and Clear (0.75%) 65.55%28.5-18.26%Solution 1:5 C14SO4:DODAB Monolayer on 4 mM C14SO4 67.16%--16.25%Monola y er 1:5 SDS:DODAB Monolayer on 4 mM SDS 68.53%--14.54%Monola y er Mr. Clean Car Wash (1%) 70.78%27-11.73%Solution Dow Q2-5211 SuperWetter (1%) 70.81%19.9-11.70%Solution 1:7 SDS:DODAB Monolayer on 4 mM SDS 73.66%--8.14%Monolayer 4 mM SDS with 100 mL Ethanol Splash 74.73%--6.81%Monolayer 1:5 SDS:DODAB Monolayer on 150 ppm Detergent 76.36%--4.78%Monolayer 1:10 SDS:DODAB Monolayer on 4 mM SDS 76.68%--4.38%Monolayer 500 ppm Fabric Softener + 150 ppm Detergent 77.57%--3.27%Solution 1500 ppm Detergent 80.19%300.00%Solution 1:3 SDS:DODAB Monolayer on 4 mM SDS 80.45%-0.32%Monolayer 500 ppm Fabric Softener 85.77%446.96%Solution Pure 4 mM SDS 88.63%6410.53%Solution Water 93.55%72.516.66%Solution

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132 CHAPTER 7 SUMMARY AND RECOMMENDATIONS FOR FUTURE WORK 7.1 UF Contributions to the Lowering of Sur face Tension: Saving Energy in the Laundry Process As discussed, by lowering the residual moistu re of household laundry during the final rinse process can have an enormous impact on the amount of time and energy that is utilized in the washing and drying cycles. As the amount of water is reduced in the fabr ic, the amount of time and energy required to dry the fabr ic is significantly reduced. We have shown that it is not simply the ai r/liquid surface tension that can affect the RMC of laundry fabrics. There are ind eed many different parameters that must be investigated to determine how the RMC can be reduced. By look ing at individual aspects of how to remove water from fabrics, such as monolayer penetr ation, micelle stability, vesicle/surfactant interactions and static and dynami c surface tension, we can identif y efficient ways to reduce the RMC. The main rationale behind reducing the moisture content of fabrics was that it could be assumed that the fabric behaves similar to a singl e capillary (or a distribution of capillaries). The LaPlace Equation for capillary rise was used and it was shown that as one reduces the equilibrium surface tension of solution, the height that fluid can rise in that same capillary reduces as well. Taking several different surfactant systems at various air/ liquid surface tensions, the RMC was measured and it was found that as the air/liquid surface tens ion decreases, so does the RMC of the fabric. To further strengthen the point that the RMC is a function of equilibrium surface tension, the sodium dodecyl sulfate system was investigated from low concentrations to concentrations above the critical micelle concen tration (where the equilibrium surface tension no longer changes). As the concentration on SDS increases, the equilibr ium surface tension of solution is reduced (up until the critical micelle concentration is reached). One would expect

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133 based on the LaPlace Equation that th e residual moisture of fabric s would follow the same trend. However, below the critical micelle concentr ation (8 mM), a large peak in the RMC was observed contrary to what was predicted. It was then found that by measuring the dynamic surface tension of the residual solution as a functi on of initial SDS concentration (after the fabric had been allowed to soak) that the same peak was found in the RMC curve was found at the exact same concentration in the dynamic su rface tension as a func tion of initial SDS concentration for the residual solution. Further st udies showed that this increase was due to a cooperative adsorption phenomenon of the SDS ad sorbing onto the cotton fabric. Due to the adsorption of SDS onto the fabric, the free mono mer concentration of the SDS in solution was reduced and thus resulted in an increase in the dynamic surface tension (since dynamic surface tension is a measure of the availa bility of surfactant monomer in solution). This increase in the dynamic surface tension indicated that it was not simply the equilibrium surface tension that affects the RMC of solution that since the centrifugation process in the washing machine was on a fast time scale (~90g or 640 RPM in a wash ing machine), the dynami c surface tension of solution provides a better indication for the results of RMC. It was shown that by simply changing the equi librium surface tension of the residual water does not always result in lowe ring the RMC and that the dynamic surface tension of solution provides a much better understandi ng of the amount of water retain ed by the fabric. We have shown that by changing the micellar stability of the residual solution we can significantly alter the amount of water retained by fabrics due either to thin film stabiliza tion or the reduction of free monomer in solution due to more stable micelles having a lower monomer flux compared to less stable micelles. As micelles become more and more stable (and monomer flux becomes less and less), more water is thus retained in the fabric due to an increase in the dynamic surface

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134 tension of solution. It was s hown that by only changing the co unter-ion on the dodecyl sulfate anion that the micellar stability increased with the molecular packing at the various interfaces. As micelle stability increased by changing the counter-ion from lithium to sodium to magnesium, the RMC of the same solutions increased as well indicating that the RMC is in fact a function of micellar stability. Based on the same idea, it has been shown that by exposing the fabric to certain concentrations of surfactant so lution can also change the RMC. For the sodium dodecyl sulfate system, it is known that at a concentration of 200 mM that the micella r stability reaches a maximum and then decreases once again at high er concentrations. Due to the higher micellar stability at 200 mM solutions of SDS, the RMC also showed an increase at the same concentration. To find an effective way to lower the RM C of fabrics, monolayer penetration was investigated. By spreading an insoluble monolay er on the air/liquid surface and injecting a cosurfactant beneath that surface, the equilibrium surface tension can be significantly reduced. By using a mixed monolayer of DODAB and C14SO4, the surface tension could be lowered to values as low as 7 mN/m (compared to about 20 mN/m for conventional surfactant systems). This technique was then used to try and lower the RMC of fabric du ring the final spin cycle. By soaking the fabric in the pene trating surfactan t solution and then pouri ng the monolayer solution onto the fabric during the final spin cycle, the RMC was reduced almost to 30% for that of the model system (150 ppm of detergent carryover into the final rinse and spin cycle). Due to the synergism shown in reducing th e equilibrium surface tension of solution between anionic and cationic surfac tants, the use of an anionic su rfactant in the final rinse cycle of the washing machine along with the use of a liquid fabric softener (composed of cationic

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135 quaternary ammonium salts that fo rm vesicles in solution) was investigated to determine if the equilibrium surface tension of solution could be re duced and thus reduce the RMC of the fabrics. However, it was shown that with the addition of SDS (depending on concentration) that the combined system of the fabric softener and th e SDS resulted in much higher equilibrium surface tension values than expected. Also, those same systems tended to be much more turbid and eventually those systems phase separated. This sy stem was further investigated by using particle size analysis and by measuring zeta potential. It was found that at a 1:1 ratio between the SDS and the actives in the softener (or DODAB for the model system) that the particle size vastly increased and the zeta potential for the part icles was reduced to a value of 0 mV. These measurements indicate that at a 1:1 concentration, there is complete charge neutralization on the vesicles and the particles aggl omerate and the surface tension increases. This increase in surface tension results resulted in the increase in the RMC. 7.2 Technological Impact of the Reduc tion of RMC in the Laundry Process By decrease the amount of water used in the washing process, the amount of energy can be significantly reduced. In the US alone, a cons ervative 30% reduction in the RMC can possibly save over $1 billion per year. This savings in energy can also help re duce our dependence on oil products by the reduction of energy consumption. However, these results are only for household and consumer usage of energy in the US. If this technology was expanded for industrial use, there exists a huge potential for further cost savings. This technology can also be expanded into use for any type of industr y that requires the dewatering of porous media. For example, in th e pulp and paper industry, the amount of energy used to dry the pulp into paper is enormous. The use of such a technology (such as monolayer penetration) on the drying of paper pulp into pa per products could result in large energy savings.

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136 There also exists opportunities to save energy in fields such as dewatering of wastewater sludge to the processing of sugar from sugar cane. 7.3 Recommendations for Future Work 7.3.1 Dynamic Surface Tension In order to better understand the effects of dynamic surface tension on the RMC of fabrics, the use of a higher resolution (<10 ms per bubble in terms of bubble frequency) bubble tensiometer could be used to measure the DS T at higher bubble fre quencies and to also determine if a correlation between dynamic surf ace tension and micelle st ability can be found. Due to the lack of resolution of the DST when measuring high concentrations of surfactant (due to the amount of micelles surr ounding the newly crea ted air bubble), higher bubble rates could help determine if the increase in micelle stabil ity causes an increase in the DST and thus an increase in the RMC or if it is simply a layering of micelles in the thin film which causes the increase in RMC. 7.3.2 Micelle Stability Since it was found that the stab ility of micelles co uld greatly affect the amount of water that fabrics retained, further study should be done to determine how various micellar systems change the RMC. For example, the differen ce between stabilizing micelles by changing the concentration and changing the stability by ch anging counter-ions or adding a co-surfactant could be investigated. In such an investigation, the use of a pressure jump or a stopped flow apparatus could be used to determine the st ability of unknown systems (such as a detergent formulation or a fabric softener formulation) or for other various systems. Also, the stability of a micelle can be engineered and thus verified using such an apparatus and then the resulting affect on RMC could be determined. Essentially, determ ining the difference in stabilizing micelles by

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137 changing concentration, by cha nging the surfactant counter-i on or by adding a co-surfactant would be the ideal goal for such experiments. It was discussed that the stability of a mice lle could be changed by varying the counter-ion. By changing the ion from sodium to magnesium the stability increased greatly and the RMC followed the same trend. However, it is unknown if this phenomena is due to stabilized micelles or adsorption of the divalent magnesium on the fa bric surface and thus f acilitating adsorption of the dodecyl sulfate anion on the fabric due to the charge reversal from anionic to cationic on the fabric surface from the magnesium ion. One wa y to determine which factor is affecting the RMC, once the bulk concentration of Mg(DS)2 after the fabric has equilibrated with the surfactant solution could be measured using the MBAS method. The measurement of the bulk concentration would help determine if the adsorp tion of the counter-ion on the fabric is causing the increase or if it is truly a micelle stabilization phenomenon. It was also shown that the by th e addition of short chain alcohols or a polymer such as PVP to the SDS system that the micelles became less stable or labile. This decrease in stability corresponded with a decrease in the RMC of the fabric. However, the last two points on this curve did not completely corres pond to the relaxation times. Th is was explained due to the increase in viscosity of the solution due to the polymer. With higher viscosity, longer centrifugation times would be needed to remove all of the residual water. Since the centrifugation time is kept constant at 10 minutes the solutions of higher viscosity are no longer under steady state conditions at th e end of the centrifugation proce ss (after 10 minutes). It would be of interest to determine how the viscosity of the solution affects the RMC. Simple experiments could be performed by varying the viscosity of solu tion with a viscosity enhancing aid and then measuring the resul ting RMC. Another explanation th at could result in the higher

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138 RMC values for those last two data points could be due to salting out e ffects. Due to the amount of water molecules needed to hydrate the PVP, th ere are now fewer molecules of water to help solubilize the SDS. This causes an effective incr ease in the surfactant concentration and thus could cause the system to approach a pseudo 200 mM concentration of SDS in the polymer system. This would then cause stabilization of th e thin film and thus cause an increase in the RMC. 7.3.3 Monolayer Penetration There exists huge possibilities in finding monol ayer penetration syst ems that could result in much lower pseudo-equilibrium air/liquid surf ace tensions compared to the current values found in monolayer penetration (~ 7-8 mN/m). One such experiment that could be performed is the use of an anionic surfactan t as the insoluble monolayer co mpared to the current cationic system and then penetrating with a cationic surfacta nt. This leads to the use of a fabric softening type system for the monolayer and the use of possibly the carryover surf actant from the wash cycle penetrating to lower the surface tension. A nother useful experiment would be to determine the final molecular ratio in the penetrated system. This would help determine if the 1:3 ratio occurred when a 1:5 ratio monolayer is penetrated by SDS from the subphase. 7.3.4 Surfactant Adsorption Since it was shown that the adsorption of SD S onto the fabric surface could be modified by first pre-treating the fabric with a hydrophobic a dditive, determining the adsorption isotherms of the SDS onto the treated fabric would be of intere st. Likewise, the contact angle of the resulting treated and washed fabric can be measured to dete rmine the effect of such a pre-treatment. Since it would be ideal to have no adsorption of SD S onto the fabric and still have a hydrophilic surface (since hydrophobic towels would ha ve difficulty in removing water).

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139 APPENDIX A ORIENTATION OF ADSORBED SURFAC TANT MOLECULES ON COTTON FABRIC It was shown in this work that SDS does indeed adsorb onto the surface of cotton fabric. However, it should be noted that the actual m echanism for how the SDS is adsorbing onto the fabric is unknown. There exists the possibility that the surfactant is being adsorbed in monolayers, bilayers, hemimicelles or a combin ation of the previous. Since the original molecular mechanism for the RMC peak indicated the adsorption of the SDS occurred in a monolayer with the hydrophobic ta il of the surfactant being oriented towards the water phase, there would be concern of the fabric becomi ng hydrophobic and thus he lping reduce the RMC. However, this phenomenon is not seen in the RMC values since the RMC actually increases rather than decreases. Based on this idea, a nd knowing the amount of SD S adsorbed onto the fabric and the average surface area for the fabric (~50 m2/g)130, the amount of coverage can be calculated as shown in the tabl e below. There was approximately 1.1 mM SDS (from a 30 mL solution onto 1.5 grams of fabric) adsorbed onto the fabric. Based on that information and using 50 2/molecule for the area of a SDS molecule, the area of the SDS on the fabric can be calculated and thus translated in to percentage coverage of the SDS onto the fabric which is roughly 13% coverage. Due to this low amount of coverage on the fabric surface, one can assume that there is not enough of the SDS adsorb ing on the fabric to significantly change the fabric wettability. Likewise, the adsorption of the SDS onto the cotton fabric may not be due to the SDS adsorbing onto the hydrophilic si tes. Due to the low amount of SDS adsorption, the SDS could possible be adsorbing on the hydrophobic and ca tionic sites that exist on the cotton fabric surface.

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140 mMolar SDS Adsorbed onto Fabric 1.1 mM mMoles of SDS 0.033 mMoles Volume of solution 30 mL Area of SDS molecule 50 2/molecule Area of SDS on fabric 9.933E+20 2 Surface Area of Fabric 50 m2/gram grams of fabric 1.5 g Total Surface Area of Fabric 75 m2 Coverage of SDS on Fabric 9.933 m2 Ratio of Coverage 0.13244 m2 SDS per m2 fabric Percentage Coverage 13.24 %

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141 APPENDIX B REMOVAL OF WATER FROM CAPILLA RIES: MATHEMATICAL APPROACH The rationale behind this research was that the RMC of fabrics is related to the height that a liquid can rise into a capillary. However, the LaPlace Equati on does not tell us everything that we may need. If you have ever taken straws of various diameters and submerged them into water, you may have noticed with smaller straws th at more water is retained inside that straw. However, if the radius of the straw is continua lly increased, a critical radius is reached and no more water can be held inside th e straw no matter what else is done This critical value is given as follows (where once the water level or capillar y height is below 0.001 cm, all of the water is considered to be expelled): 2 0.001**cR cmg It is noticed that this criti cal radius is a function of the surface tension of solution. By lowering the surface tension, more and more water will come out of a given radius capillary (or the lower the surface tension, the smaller the radius required to hold the same height of water). By assuming several values for radius and deviation (average radius size of 125 m with a deviation of 40 m), a normal distribution can be ge nerated and plotted along with the cumulative value of the distribution of water he ld in a capillary. We can then calculate the critical radius for a given surface tension and we can determine approximately how many of the capillaries will be empty and how many will still have fluid in them. This rough figure can give an estimation of the amount of water held insi de the fabric. Shown below is the relative volume distribution plotted along with th e cumulative distribution. It is shown that for a surface tension value of 72.5 mN/m that approximately 70% of the capillaries in the distribution are still retaining their water (or all capillaries smaller than the critical radius of 150 m). However, once

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142 the surface tension is lowered to 20 mN/m, we now find that only a very small percentage of capillaries are holding their wate r (roughly 5%) due to a change in the critical radius from roughly 150 m to 40 m. Capillary Radius ( m) 050100150200250300 Relative Volume 0 1000 2000 3000 4000 5000 6000 7000 SFT = 20 Cumulative 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 SFT =72.5 It was shown that for cotton soaked in water (surface tension is equal to 72.5 mN/m), that the RMC was roughly 80-90%. This value is near what was predicted by the normal distribution. However, for cotton soaked in solutions of low surface tension values, the RMC was only reduced to approximately 60%. This is far away from the 5% that was predicted by the normal distribution. However, many different parameters come into play once the fabric is being centrifuged. The model which was presented is considering the static LaPlace Equation which assumes that the capillary is in contact with the liquid source. During centrifugation, the capillaries in the fabric are no longer in cont act with a base liquid. Another parameter which would affect the amount of water retained is how the capillaries and th e fabric respond to

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143 centrifugation. When the fabric is being centrif uged, capillaries are being closed off and the water is trapped inside the fibe rs. Another explanation to the di fferences in the model and the actual system is that based on the average capill ary size and deviation, it was assumed that the distribution was a normal distribu tion. However, in a real fiber system, there are many microcapillaries inside the actual fibe r strand which could be responsibl e for the high amount of water retention. In addition, the wate r of hydration for the cotton fibe rs is not considered in these calculations.

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144 LIST OF REFERENCES 1. DOE, End-Use Consumption of Electrici ty by End-Use and Appliance. In 2001. 2. Preston, J. M.; Nimkar, M. V.; Gundavda, S. P., Capillary and imbibed water in assemblies of moist fibres. Journal of the Textile Institute 1951, 42, T79-T90. 3. Shah, D. O., Micelles, Microemulsions, and Monolayers. Marcel Dekker Publication: New York, 1998. 4. Manne, S.; Gaub, H. E., Molecular Organiza tion of Surfactants at Solid-Liquid Interfaces. Science 1995, 270, (5241), 1480-1482. 5. Patist, A.; Jha, B. K.; Oh, S. G.; Shah, D. O., Importance of Micellar Relaxation Time on Detergent Properties. Journal of Surfactants and Detergents 1999, 2, (3), 317-324. 6. Patist, A.; Kanicky, J. R.; Shukla, P. K.; Shah, D. O., Importance of Micellar Kinetics in Relation to Technological Processes. Journal of Colloid and Interface Science 2002, 245, 1-15. 7. Patist, A.; Oh, S. G.; Leung, R.; Shah, D. O., Kinetics of micellizat ion: its significance to technological processes. Colloids and Surfaces 2001, 176, 3-16. 8. Oh, S. G.; Shah, D. O., Effect of Micel lar Lifetime on the Wetting Time of Cotton in Sodium Dodecyl Sulfate Solutions. Langmuir 1992, 8, 1232-1233. 9. Oh, S. G.; Shah, D. O., The Effect of Mi cellar Lifetime on the Rate of Solubilization and Detergency in Sodium Dodecyl Sulfate Solutions. Journal of the American Oil Chemists' Society 1993, 70, (7), 673-678. 10. Lpez-Montilla, J. C.; James, M. A.; Oscar, D. C.; Shah, D. O., Surfactants and Protocols to Induce Spontaneous Emulsifi cation and Enhance Detergency. Journal of Surfactants and Detergents 2005, 8, 45-53. 11. Oh, S. G.; Klein, S. P.; Shah, D. O., E ffect of Micellar Lifetime on the Bubble Dynamics in Sodium Dodecyl-Sulfate Solutions. Aiche Journal 1992, 38, (1), 149-152. 12. Patel, S. S.; Kumar, K.; Shah, D. O.; Delfi no, J. J., Effect of su rfactant concentration and film area on the stability of films of surfactant solutions. Journal of Colloid and Interface Science 1996, 183, (2), 603-606. 13. Sethumadhavan, G.; Nikolov, A.; Wasan, D., Stability of films with nanoparticles. Journal of Colloid and Interface Science 2004, 272, (1), 167-171. 14. Kumar, K.; Nikolov, A. D.; Wasan, D. T., Ef fect of Film Curvature on Drainage of Thin Liquid Films. Journal of Colloid and Interface Science 2002, 256, 194-200.

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145 15. Sethumadhavan, G.; Bindal, S.; Nikolov, A.; Wasan, D., Stability of thin liquid films containing polydisperse particles. Colloids and Surfaces a-Physicochemical and Engineering Aspects 2002, 204, (1-3), 51-62. 16. Wasan, D. T.; Nikolov, A.; Trokhymchuk, A.; Henderson, D., Colloidal suspensions confined to a film: Local structure and film stability. Condensed Matter Physics 2001, 26, 361-374. 17. Sethumadhavan, G. N.; Nikolov, A. D.; Wasa n, D. T., Stability of liquid films containing monodisperse colloidal particles. Journal of Colloid and Interface Science 2001, 240, (1), 105-112. 18. Wasan, D. T.; Nikolov, A. D.; Lobo, L. A.; Koczo, K.; Edwards, D. A., Foams, Thin-Films and Surface Rheological Properties. Progress in Surface Science 1992, 39, (2), 119-154. 19. Rybicki, E.; Paryjczak, T., Adsorp tion of Anionic Surfactants on Cotton. Tenside Detergents 1984, 21, (4), 184-189. 20. Ribitsch, V.; Stana-Kleinscheck, K., Charact erizing Textile Fiber Surfaces with Streaming Potential Measurements. Textile Research Journal 1998, 68, (10), 701-707. 21. Stana-Kleinscheck, K.; Ribitsch, V., Electroki netic properties of pro cessed cellulose fibers. Colloids and Surfaces A: Pysiochemical Engineering Aspects 1998, 140, 127-138. 22. Singh, B. O., The role of surfactant adsorp tion in the improved dewa tering of fine coal. Fuel 1999, 78, 501-506. 23. Singh, B. P., The Influence of Surface Phe nomena on the Dewatering of Fine Clean Coal. Filtration and Separation 1997, 34, (2), 159-163. 24. Singh, B. P.; Besra, L.; Reddy, P. S. R.; Sengupta, D. K., Use of surfactants to aid in the dewatering of fine clean coal. Fuel 1998, 77, (12), 1349-1356. 25. Paria, S.; Manohar, C.; Khilar, K. C., E ffect of cationic surfactant on the adsorption characteristics of anionic surfactant on cellulose surface. Colloids and Surfaces aPhysicochemical and Engineering Aspects 2004, 232, (2-3), 139-142. 26. Somasundaran, P.; Fuerstenau, D. W., M echanisms of Alkyl Sulfonate Adsorption at Alumina-Water Interface. Journal of Physical Chemistry 1966, 70, (1), 90. 27. Lewin, M.; Pearce, E. M., Handbook of fiber chemistry. 2nd ed.; Marcel Dekker: New York, 1998; p xxiv, 1083. 28. Lewin, M.; Sello, S. B., Chemical Processing of Fibers and Fabrics: Functional Finishes. Marcel Dekker, Inc.: New York, 1984; Vol. II.

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146 29. Oh, S. G.; Shah, D. O., Effect of Counter -Ions on the Interfacial Tension and Emulsion Droplet Size in the Oil-Wa ter Dodecyl Sulfate System. Journal of Physical Chemistry 1993, 97, (2), 284-286. 30. Pandey, S.; Bagwe, R. P.; Shah, D. O., Effect of counterions on surface and foaming properties of dodecyl sulfate. Journal of Colloid and Interface Science 2003, 267, 160-166. 31. Friedli, F. E., Detergency of specialty surfactants. M. Dekker: New York, 2001; p x, 284. 32. Sharma, M. K.; Shah, D. O.; Brigham, W. E., Correlation of Chain Length Compatibility and Surface Properties of Mixed Foaming Agents with Fluid Displacement Efficiency and Effective Air Mobility in Porous Media. Industrial & Engineering Chemistry Fundamentals 1984, 23, (2), 213-220. 33. Patist, A.; Chhabra, V.; Pagidipati, R.; Sh ah, R.; Shah, D. O., Effect of chain length compatibility on micellar stability in sodi um dodecyl sulfate / al kyltrimethylammonium bromide solutions. Langmuir 1997, 13, (3), 432-434. 34. Shah, D. O., Significance of 1:3 Mol ecular Ratio in Mixed Surfactant Systems. Journal of Colloid and Interface Science 1971, 37, (4), 744. 35. Shambil, F.; Schuwuger, M. J., Interfacial and colloidal properties. In Surfactants in Consumer Products: Theory, Technology and Application, Springer-Verlag: Heidelberg, 1987. 36. Hines, J. D.; Thomas, R. K.; Garret, P. R.; Rennie, G. K., Investigation of mixing in binary surfactant solutions by surface tension and neutron reflection: Strongly interacting anionic/zwitterionic mixtures. Journal of Physical Chemistry B 1998, 102, (44), 88348846. 37. Rosen, M. J.; Zhu, B. Y., Synergism in binary mixtures of surfactants. 3. Betaine containing systems. Journal of Colloid and Interface Science 1984, 99, (2), 427-434. 38. Iwasaki, T.; Ogawa, M.; Esumi, K.; Megur o, K., Interactions between betaine type zwitterionic and anionic surfactants in mixed micelles. Langmuir 1991, 7, (1), 30-35. 39. Liu, L.; Rosen, M. J., The interaction of so me novel diquaternary ge mini surfactants with ionic surfactants. Journal of Colloid and Interface Science 1996, 179, (2), 454-459. 40. Rosen, M. J.; Gao, T.; Nakatsuji, Y.; Ma suyama, A., Synergism in binary systems of surfactants. 12. Mixtures cont aining surfactants with 2 hydrop hillic and 2 or 3 hydrophobic groups. Colloids and Surfaces A: Pysioc hemical Engineering Aspects 1994, 88, (1), 1-11. 41. Esumi, K.; Miyazaki, M.; Arai, T.; Koide, Y., Mixed micellar properties of cationic gemini surfactants and a nonionic surfactant. Colloids and Surfaces A: Pysiochemical Engineering Aspects 1998, 135, (1-3), 117-122.

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147 42. Li, F.; Rosen, M. J.; Sulthana, S. B., Surface properties of cationic gemini surfactants and their interaction with alkylglucoside or maltoside surfactants. Langmuir 2001, 17, (4), 1037-1042. 43. Schmaucks, G., Novel siloxa ne surfactant structures. In Silicone Surfactants, Hill, R., Ed. Marcel Dekker: New York, 1999; Vol. 86. 44. Snow, S. A., Synthesis, characterization, stability, aqueous surface activity, and aqueous solution aggregation of the novel, cationi c siloxane surfactan ts (Me3SiO)2Si(Me)(CH2)3+NMe2(CH2)2OR X(R = hydrogen, a cetyl, N-phenylcarbamyl; X = chloride, bromide, iodide, nitrate, methyl sulfate). Langmuir 1993, 9, (2), 424-430. 45. Snow, S. A.; Fenton, W. N.; Owen, M. J ., Zwitterionic organofunctional siloxanes as aqueous surfactants: synthesis and characteri zation of betaine func tional siloxanes and their comparison to sulfobetaine functional siloxanes. Langmuir 1991, 7, (5), 868-871. 46. Snow, S. A.; Fenton, W. N.; Owen, M. J., Synthesis and characterization of zwitterionic silicone sulfobetaine surfactants. Langmuir 1990, 6, (2), 385-391. 47. Bhattacharya, S.; Haldar, J., Molecular desi gn of surfactants to ta ilor its aggregation properties. Colloids and Surfaces A: Pysiochemical Engineering Aspects 2002, 205, (1-2), 119-126. 48. DuPont, Zonyl Fluorosurfactants In DuPont: Wilmington, DE, 1987. 49. Adler, J. J.; Singh, P. K.; Patist, A.; Rabinovich, Y. I.; Shah, D. O.; Moudgil, B. M., Correlation of Particulate Disp ersion Stability with the Strength of Self-Assembled Surfactant Films. Langmuir 2000, 16, (18), 7255-7262. 50. Fielden, M. L.; Claesson, P. M.; Verrall, R. E., Investigating the Ad sorption of the Gemini Surfactant "12-2-12" onto Mica Using At omic Force Microscopy and Surface Force Apparatus Measurements. Langmuir 1999, 15, (11), 3924-3934. 51. Myers, D., Surface Activity and Surfactant Structures. In Surfaces, Interfaces, and Colloids, John Wiley & Sons: New York, 1999; pp 21-39. 52. Kim, C.; Hsieh, Y.-L., Wetting and absorb ency of nonionic surfactant solutions on cotton fabrics. Colloids and Surfaces A: Pysioc hemical Engineering Aspects 2001, 187, 385-397. 53. Aniansson, E. A. G.; Wall, S. N.; Almgren, M.; Hoffmann, H.; Kielmann, I.; Ulbricht, W.; Zana, R.; Lang, J.; Tondre, C., Theory of Ki netics of Micellar Equ ilibria and Quantitative Interpretation of Chemical Relaxation Studies of Micellar Solutions of Ionic Surfactants. Journal of Physical Chemistry 1976, 80, (9), 905-922. 54. Patist, A.; Huibers, P. D.; Deneka, B.; Shah, D. O., Effect of tetraalkylammonium chlorides on foaming properties of sodium dodecyl sulfate solutions. Langmuir 1998, 14, (16), 4471-4474.

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148 55. Patist, A.; Axelberd, T.; Shah, D. O., Eff ect of long chain alcohols on micellar relaxation time and foaming properties of s odium dodecyl sulfate solutions. Journal of Colloid and Interface Science 1998, 208, 259-265. 56. Jha, B. K.; Patist, A.; Shah, D. O., Effect of antifoaming agents on the micellar stability and foamability of sodium dodecyl sulfate solutions. Langmuir 1999, 15, (9), 3042-3044. 57. Besra, L.; Sengupta, D. K.; Roy, S. K.; Ay, P ., Influence of surfactants on flocculation and dewatering of kaolin suspensions by catio nic polyacrylamide (PAM-C) flocculant. Separation and Purifi cation Technology 2003, 30, 251-264. 58. Besra, L.; Sengupta, D. K.; Roy, S. K.; Ay P., Studies on flocculation and dewatering of kaolin suspensions by anioni c polyacrylamide flocculant in the presence of some surfactants. International Journal Mineral Processing 2002, 66, 1-28. 59. Besra, L.; Sengupta, D. K.; Roy, S. K.; Ay, P., Polymer adsorption: its correlation with flocculation and dewatering of kaolin su spension in the presence and absence of surfactants. International Journal of Mineral Processing 2002, 66, (1-4), 183-202. 60. Hollies, N. R. S.; Kaessinger, M. M.; Bogaty H., Water Transport Mechanisms in Textile Materials. Part I: The Role of Yarn Roughness in Capillary-Type Penetration. Textile Research Journal 1956, 26, (11), 829-835. 61. Hollies, N. R. S.; Kaessinger, M. M.; Wa tson, B. S.; Bogaty, H., Water Transport Mechanisms in Textile Materials. Part II: Cap illary-Type Penetration in Yarns and Fabrics. Textile Research Journal 1957, 27, (1), 8-13. 62. Schick, M. J., Surface characteristics of fibers and textiles. M. Dekker: New York, 1975; p 2 v. (xii, 669 ). 63. DeGruy, I. V.; Carra, J. H.; Goynes, W. R., The fine structure of co tton; an atlas of cotton microscopy. M. Dekker: New York ,, 1973; p xi, 224. 64. Patist, A. Tailoring micellar stability to c ontrol interfacial properties and behaviour of dispersed systems. Ph.D, Universi ty of Florida, Gainesville, 1998. 65. Clark, J. F.; Preston, J. M., Some effects of temperatures on wet viscose rayon. I. Water imbition and swelling. Journal of the Textile Institute 1956, 47, (8), T413-16. 66. Preston, J. M.; Nimkar, M. V., Measur ing the swelling of fibers in water. Journal of the Textile Institute 1949, 40, P674-86. 67. Schambil, F.; Schwuger, M. J., Inte rfacial and colloidal properties. In Surfactants in Consumer Products: Theory, Technology and Application, Springer-Verlag: Heidelberg, 1987; pp 133-196. 68. Carter, D. L.; Shah, D. O., The Role of Surface Tension on the Residual Water Content of Fabrics. Journal of Surfactants and Detergents 2005, 8, 91-94.

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149 69. 5540 C. Anionic Surfactants as MBAS. In Standard Methods for the Examination of Water and Wastewater 20th Edition, Clesceri, L. S.; Greenberg, A. E.; Eaton, A. D., Eds. American Public Health Association: Ba ltimore, Maryland, 1998; pp 5-47 5-49. 70. Chitikela, S.; Dentel, S. K.; Allen, H. E ., Modified Method for the Analysis of Anionic Surfactants as Methylene-Blue Active Substances. Analyst 1995, 120, (7), 2001-2004. 71. Carter, D. L.; Draper, M. C.; Peterson, R. N.; Shah, D. O., Importance of dynamic surface tension to the residual wa ter content of fabrics. Langmuir 2005, 21, (22), 10106-10111. 72. Datwani, S. S.; Stebe, K. J., Monolayer penetration by a charged amphiphile: equilibrium and dynamics. Colloids and Surfaces a-Physicoche mical and Engineering Aspects 2001, 192, (1-3), 109-129. 73. Matalon, R.; Schulman, J. H., A new me thod of studying mechanical properties of penetrated monolayers. Journal of Colloid Science 1949, 4, 89-90. 74. Doty, P.; Schulman, J. H., Formation of lipoprotein monolayers. I. Preliminary investigation on the adsorption of pr oteins onto lipide monolayers. Discussions of the Faraday Society 1949, 6, 21-27. 75. Matalon, R.; Schulman, J. H., Penetration of Sodium Cetyl Sulphate into Cetyl Alcohol Effect of Salt on the Time-Penetration Curves. Transactions of the Faraday Society 1947, 43, (8-9), 479-485. 76. Matalon, R.; Schulman, J. H., Formation of Lipo-Protein Monolayers .2. Mechanism of Adsorption, Solution and Penetration. Discussions of the Faraday Society 1949, (6), 27-44. 77. Wolstenholme, G. A.; Schulman, J. H., Metalmonolayer interactions in aqueous systems. I. Interaction of monolayer s of long-chain polar compounds with metal ions in the underlying solution. Transactions of the Faraday Society 1950, 46, (475-87). 78. Wolstenholme, G. A.; Schulman, J. H., Metalmonolayer interactions in aqueous systems. II. Adsorption of long-chain compounds from aqueous solution onto evaporated metal films. Transactions of the Faraday Society 1950, 46, 488-97. 79. Wolstenholme, G. A.; Schulman, J. H., Metalmonolayer interactions in aqueous systems. III. Steric effects with branch ed-chain fatty acid monolayers. Transactions of the Faraday Society 1951, 47, 788-94. 80. Goddard, E. D.; Schulman, J. H., Molecu lar interaction in monolayers. I. Complex formation. Journal of Colloid Science 1953, 8, 309-28. 81. Goddard, E. D.; Schulman, J. H., Molecular in teraction in monolayers. II. Steric effects in the nonpolar portion of the molecules. Journal of Colloid Science 1953, 8, 329-40. 82. Shah, D. O.; Schulman, J. H., Ioni c Structure of Lecithin Monolayers. Journal of Lipid Research 1967, 8, (3), 227.

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150 83. Shah, D. O.; Schulman, J. H., Influence of calcium, cholesterol, and unsaturation on lecithin monolayers. Journal of Lipid Research 1967, 8, (3), 215-26. 84. Shah, D. O.; Schulman, J. H., Ioni c structure of sphingomyelin monolayers. Biochimica et Biophysica Acta, Biomembranes 1967, 135, (1), 184-7. 85. Sundaram, S.; Stebe, K. J., Equations for the equilibrium surface pressure increase on the penetration of an insoluble m onolayer by a soluble surfactant. Langmuir 1996, 12, (8), 2028-2034. 86. Hall, D. G., Thermodynamics of monolayer pe netration: A criterion for ascertaining when the adsorption of the penetrating species is negligible. Langmuir 2000, 16, (12), 54945495. 87. Hall, D. G., Thermodynamics of Monolayer Penetration. Langmuir 1986, 2, (6), 809-812. 88. Motomura, K.; Hayami, Y.; Aratono, M.; Matuura, R., Thermodynamics of multicomponent monolayers : IV. Monolayer penetration. Journal of Colloid and Interface Science 1982, 87, (2), 333-338. 89. Barnes, G. T., On the kinetics of monolayer penetration. Journal of Colloid and Interface Science 1978, 63, (1), 162-163. 90. Sanchez, C. C.; Fernandez, M. C.; RodriguezNino, M. R.; RodriguezPatino, J. M., Thermodynamic and Dynamic Characteristics of Monoglyceride Monolayers Penetrated by b-Casein. Langmuir 2006, 22, (9), 4215-4224. 91. Santos Magalhaes, N. S.; de Oliveira, H. M.; Baszkin, A., Motomura's modified equation for surfactant penetration into spread monolayers. Colloids and Surfaces A: Physicochemical and Engineering Aspects 1996, 118, (1-2), 63-73. 92. Dynarowicz-Latka, P.; Seoane, R.; Mi nones, J.; Velo, M.; Minones, J., Study of penetration of amphotericin B into cholesterol or ergos terol containing dipalmitoyl phosphatidylcholine Langmuir monolayers. Colloids and Surfaces B-Biointerfaces 2003, 27, (2-3), 249-263. 93. Welzel, P. B.; Cammenga, H. K., Equilib rium penetration of DMPC monolayers by sodium cholate. Journal of Colloid and Interface Science 1998, 207, (1), 70-77. 94. Fang, H.; Shah, D. O., The effect of surf actant monolayers on the heat transfer through air/water and oil/water interfaces using IR imaging technique. Journal of Colloid and Interface Science 1998, 205, (2), 531-534. 95. Charron, J. R.; Tilton, R. D., Penetration of insoluble lipid monol ayers at the air-water interface by water-soluble block c opolymers and homopolymers. Langmuir 1997, 13, (21), 5524-5527.

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151 96. Tajima, K.; Koshinuma, M.; Nakamura, A., Equilibrium Penetration of N-Dodecyl-BetaAlanine into the Leci thin and Dilaurin Monolayers .1. Interaction between Polar Head Groups. Langmuir 1991, 7, (11), 2764-2773. 97. Panaiotov, I. I.; Terminassiansaraga, L. ; Albrecht, G., Penetration into Insoluble Monolayers .2. Surface-Tension and Surface Pre ssure Studies with Soluble Vinblastine Sulfate and Spread Egg Lecithin. Langmuir 1985, 1, (4), 395-399. 98. Mcgregor, M. A.; Barnes, G. T., Equilibri um Penetration of Monolayers .5. Sodium Docosyl Sulfate-Sodium Dodecyl Sulfonate System. Journal of Colloid and Interface Science 1978, 65, (2), 291-295. 99. Tsai, P.-C.; Ding, W.-H., Determination of alkyltrimethylammonium surfactants in hair conditioners and fabric softeners by gas ch romatography-mass spectrometry with electronimpact and chemical ionization. Journal of Chromatography A 2004, 1027, (1-2), 103-108. 100. Puchta, R.; Krings, P.; Sandkuehler, P., A new generation of softeners. Tenside, Surfactants, Detergents 1993, 30, (3), 186-191. 101. Goncalves da Silva, A. M.; Romao, R. I. S., Mixed monolayers involving DPPC, DODAB and oleic acid and their interaction with nicotinic acid at the air-water interface. Chemistry and Physics of Lipids 2005, 137, (1-2), 62-76. 102. Leung, R.; Shah, D. O., Dynamic Properties of Micellar Solutions .1. Effects of ShortChain Alcohols and Polymers on Micellar Stability. Journal of Colloid and Interface Science 1986, 113, (2), 484-499. 103. Shah, D. O.; Shiao, S. Y., Effect of Chai n-Length Compatibility on Molecular Area, InterMolecular Spacing, Dispersion Energies and Evaporation Resistance of Mixed Monolayers. Berichte Der Bunsen-Gesellschaft-Physical Chemistry Chemical Physics 1978, 82, (9), 882-882. 104. Sharma, M. K.; Shiao, S. Y.; Bansal, V. K.; Shah, D. O., Effect of Chain-Length Compatibility on Monolayers, Foams, a nd Macroemulsions and Microemulsions. Acs Symposium Series 1985, 272, 87-103. 105. Sharma, M. K.; Shiao, S. Y.; Bansal, V. K.; Shah, D. O., Chain-Length Compatibility Effects in Monolayers, Macroemulsions and Microemulsions. Abstracts of Papers of the American Chemical Society 1983, 186, (Aug), 51-Inde. 106. Shiao, S. Y.; Chhabra, V.; Patist, A.; Free, M. L.; Huibers, P. D. T.; Gregory, A.; Patel, S.; Shah, D. O., Chain length compatibility effects in mixed surfactant systems for technological applications. Advances in Colloid and Interface Science 1998, 74, 1-29. 107. Carter, D. L.; Draper, M. C.; Peterson, R. N.; Shah, D. O., Importance of Dynamic Surface Tension to the Residual Wa ter Content of Fabrics. Langmuir 2005.

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152 108. Patist, A.; Devi, S.; Shah, D. O., Importan ce of 1 : 3 molecular ratio on the interfacial properties of mixed surfactant systems. Langmuir 1999, 15, (21), 7403-7405. 109. Patist, A.; Axelberd, T.; Shah, D. O., Eff ect of chain length co mpatibility on micellar stability and foaming properties of sodium dodecyl sulfate long chain alcohol mixtures. Abstracts of Papers of the American Chemical Society 1998, 215, U410-U410. 110. Patist, A.; Axelberd, T.; Shah, D. O., Eff ect of long chain alcohols on micellar relaxation time and foaming properties of s odium dodecyl sulfate solutions. Journal of Colloid and Interface Science 1998, 208, (1), 259-265. 111. Patist, A.; Chhabra, V.; Pagidipati, R.; Sh ah, R.; Shah, D. O., Effect of chain length compatibility on micellar stability in sodi um dodecyl sulfate/al kyltrimethylammonium bromide solutions. Langmuir 1997, 13, (3), 432-434. 112. Aniansson, E. A. G.; Wall, S. N., Ki netics of Step-Wise Micelle Association. Journal of Physical Chemistry 1974, 78, (10), 1024-1030. 113. Aniansson, E. A. G.; Wall, S. N., Kinetics of step-wise micelle association. Correction and improvement. Journal of Physical Chemistry 1975, 79, (8), 857-858. 114. Oh, S. G.; Shah, D. O., Micellar Lifetim e Its Relevance to Various Technological Processes. Journal of Dispersion Science and Technology 1994, 15, (3), 297-316. 115. James, A. D.; Robinson, B. H.; White, N. C., Dynamics of Sm all Molecule Micelle Interactions Charge and Ph Effects on Kine tics of Interaction of Dyes with Micelles. Journal of Colloid and Interface Science 1977, 59, (2), 328-336. 116. Tondre, C.; Lang, J.; Zana, R., Use of Dy es for Kinetic Study of Micellar Equilibria. Journal of Colloid and Interface Science 1975, 52, (2), 372-379. 117. Hoffmann, H.; Nagel, R.; Platz, G.; Ulbr icht, W., Kinetics of Micelle Formation of Alkylpyridinium Halides. Colloid and Polymer Science 1976, 254, (9), 812-834. 118. Frindi, M.; Michels, B.; Zana, R., Ultras onic-Absorption Studies of Surfactant Exchange between Micelles and the Bulk Phase in Aqueous Micellar Solutions of Amphoteric Surfactants. Journal of Physical Chemistry 1994, 98, (26), 6607-6611. 119. Kato, S.; Harada, S.; Sahara, H., Ultrasoni c Relaxation and Volume tric Studies of the Micelle Monomer Exchange Process in Aque ous-Solutions of the Nonionic Surfactants C(7)E(4), C(8)E(4) and C(8)E(5). Journal of Physical Chemistry 1995, 99, (33), 1257012575. 120. Patist, A.; Huibers, P. D. T.; Deneka, B. ; Shah, D. O., Effect of tetraalkylammonium chlorides on foaming properties of sodium dodecyl sulfate solutions. Langmuir 1998, 14, (16), 4471-4474.

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153 121. Oh, S. G.; Shah, D. O., Relationship between Micellar Lifetime and Foamability of Sodium Dodecyl-Sulfate and Sodium Dodecyl-Sulfate 1-Hexanol Mixtures. Langmuir 1991, 7, (7), 1316-1318. 122. Kim, D. H.; Oh, S. G.; Cho, C. G., Effects of Cs and Na ions on the interfacial properties of dodecyl sulfate solutions. Colloid and Polymer Science 2001, 279, 39-45. 123. Mukerjee, P.; Mysels, K. J., Critical Micelle Concentrations of Aqueous Surfactant Systems. National Bureau of Standards: Washington, DC, 1971. 124. Carter, D. L.; Shah, D. O. In Saving Energy and Time in Dr ying of Laundry: Molecular Mechanisms to Decrease the Residual Water Cont ent of Fabric at the End of the Washing Cycle, International Detergency Conference, Dsseldorf, May 30th, 2005; Dsseldorf, 2005. 125. Rabinovich, Y. I.; Pandey, S.; Shah, D. O.; Moudgil, B. M., Effect of the chain-length compatibility of surfactants and mechanical properties of mixed micelles on surfaces. Langmuir 2006, 22, (16), 6858-6862. 126. Shiao, S. Y. Thesis: The effect of the ch ain length compatibility on the properties of the mixed surfactant systems. Universi ty of Florida, Gainesville, 1976. 127. Oh, S. G.; Shah, D. O., The Effect of Mi cellar Lifetime on the Rate of Solubilization and Detergency in Sodium Dodecyl-Sulfate Solutions. Journal of the American Oil Chemists Society 1993, 70, (7), 673-678. 128. Chattopadhyay, A. K.; Shah, D. O.; Ghaich a, L., Double-Tailed Surfactants and Their Chain-Length Compatibility in Water-in-Oil Emulsions. Langmuir 1992, 8, (1), 27-30. 129. Shah, D. O.; Shiao, S. Y., Chain-Length Compatibility and Molecular Area in Mixed Alcohol Monolayers. Advances in Chemistry Series 1975, (144), 153-164. 130. Sommers, R. A. A Surface Area Study of Cotton Dried From Li quid Carbon Dioxide at Zero Surface Tension. Georgia Tech, 1963.

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154 BIOGRAPHICAL SKETCH Daniel Carter was born in Ha milton, Alabama and went to high school at Bob Jones High School in Madison, Alabama. Daniel was accepted to Auburn in his junior year of high school as an early admissions student passing over his senior year to attend college. He then completed his Bachelor of Science degree in chemical engineer ing and was then accepted to the University of Florida. In the fall of 2002, Dani el was chosen by Professor Dinesh O. Shah to work on the Fast Dry project funded by Procter and Gamble. In May 2007, Daniel was awarded the Doctor of Philosophy.


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Permanent Link: http://ufdc.ufl.edu/UFE0017567/00001

Material Information

Title: Saving of Time and Energy in the Laundry Process: Importance of Dynamic Surface Tension, Micelle Stability and Surfactant Absorption
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0017567:00001

Permanent Link: http://ufdc.ufl.edu/UFE0017567/00001

Material Information

Title: Saving of Time and Energy in the Laundry Process: Importance of Dynamic Surface Tension, Micelle Stability and Surfactant Absorption
Physical Description: Mixed Material
Copyright Date: 2008

Record Information

Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
System ID: UFE0017567:00001


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Full Text





SAVING TIME AND ENERGY INT THE LAUNDRY PROCESS: IMPORTANCE OF
DYNAMIC SURFACE TENSION, MICELLE STABILITY AND SURFACTANT
ADSORPTION




















By

DANIEL LARRY CARTER


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2007

































O 2007 Daniel Larry Carter


































To my parents, for their continuing support over the years as well as their guidance through life.
Also, to Dr. Dinesh Shah for being much more than an advisor over the years, for being a
wonderful father figure and only asking the best and most from me.









ACKNOWLEDGMENTS

This research impart was done with assistance from the University of Florida' s Summer

Science Training Program (UF-SSTP). The author would like to thank the Procter and Gamble

Company for financial support for this research and the University of Florida' s Particle Research

and Engineering Center (PERC) for use of their labs and equipment. I would also like to thank

Mr. Vicente Santamarina, Dr. Shulin Zhang, Mr. Rudy Delgado and Mr. Joe Heatherly of the

Procter and Gamble Company and Dr. Ranga Narayanan of the University of Florida for their

stimulating discussions involving this research. Also, I would like to thank the Center for Surface

Science and Engineering (CSSE) group for the wonderful conversations and help that was

provided during my stay at the University of Florida. I would like to thank my professors at

Auburn University and the University of Florida for developing my interest in chemical

engineering and their guidance in helping me in my application process to the University of

Florida. Finally, I would like to acknowledge Dr. Dinesh Shah, Dr. Ranga Narayanan, Dr. Brij

Moudgil, and Dr. Dmitri Kopelivich for their guidance and assistance for being on my doctoral

committee and for providing me the privilege of working with them.












TABLE OF CONTENTS



page

ACKNOWLEDGMENT S .............. ...............4.....


T ABLE OF CONTENT S............... ...............5


LIST OF TABLES ................ ...............8............ ....


LIST OF FIGURES .............. ...............9.....


AB S TRAC T ............._. .......... ..............._ 14...


CHAPTER


1 INTRODUCTION ................. ...............16.......... ......


1 .1 Introducti on ................. ...............16........... ...
1.2 Theory ................. .......... ............ ...... .........1
1.2.1 Surface Tension Reduction .................. ................ ...............17. ....
1.2.2 Role of Adsorbed Micelles and Micellar Stability ................. .....................19
1.2.3 Surfactant Adsorption ................ ...............20........... ....
1.3 Textile Chemistry .............. .. ............. .............2
1.3.1 Chemical Composition of Cotton Fibers .............. ...............22....
1.3.2 Properties of Cotton ................ ............. ...............22 ....
1.3.3 Surface Treatments of Cotton Fabrics .............. ...............24....
1.4 Scientific Approach ..................... ................ .......2
1.4.1 Surface Tension Reduction using Surfactants .............. ....................2
1.4.2 Effect of Bulk and Adsorbed Micelles ................. .............. ......... .....27
1.4.3 Surfactant Vesicle Interactions .............. ...............29....
1.4.4 Dewatering of Particle Suspensions............... .. ..... .................2
1.4.5 Interactions at the Solid-Liquid Interface: Role of capillarity in the
retention of water in fabrics ................ ...............31........... ...


2 THE RELATIONSHIP OF SURFACE TENSION AND THE RESIDUAL MOISTURE
CONTENT OF FABRICS............... ...............40


2.1 Experimental Background ................. .. ......... ...........4
2.1.1 Measuring Residual Moisture Content .............. ...............42....
2. 1.2 Surface Tension Measurements .............. ...............43....
2.1.3 M materials .............. ...............43....
2.2 Experimental Basis ................ ...............44........... ....
2.2.1 Tim e Basis .............. .............. ... .. ... ................4
2.2.2 Centrifugation Speed Basis (Effect of Increasing Gravitational Force)........... .45
2.3 Lowering of Surface Tension by Surfactant Systems............... ...............45
2.3.1 Simple Surfactant Systems .............. ...............45....











2.3.2 Mixed Surfactant Systems: SDS + C12TAB .............. ..... ............... 4

3 THE EFFECT OF SURFACTANT ADSORPTION ON THE RESIDUAL MOISTURE
CO TENT OF FABRIC S............... ...............5 1


3.1 Peak in SDS RMC Curve as a Function of Increasing SDS Concentration ................... ..51
3.1.1 M materials .............. ............ ............. ...........5
3.1.2 Residual Moisture Content (RMC) Measurements............... ..............5
3.1.3 Surface Tension Measurements .............. ...............53....
3.1.4 Ad sorption M easurements ..................... ............ ........5
3.2 Dynamics of Residual SDS Solution and the Effect on RMC. .................. ................ ...54
3.2.1 Surface Tensions of Residual Solutions (Dynamic and Equilibrium) and its
Correlation to RM C of Fabrics .......... .. .... .................... ..................5
3.2.2 Molecular Mechanism: Explanation of the Peak in the SDS/RMC Curve........57
3.3 Adsorption of SDS onto Cotton Surfaces ....__ ......_____ .......___ ..........5
3.4 RMC Peak in Various Surfactant Systems .............. ... .... ........... ......_. ..........6
3.5 Manipulation of RMC Peak: Fabric Pre-Treatment and its Affects on Adsorption of
SDS onto Cotton .............. ...............62....

4 REDISCOVERING MONOLAYER PENETRATION: OBTAINING ULTRA-LOW
AIR-LIQUID SURFACE TEN SION S ................. ......... ...............77......

4. 1 M onolayer Penetration ................. .............. ...............77. ....
4.1.1 M onolayer Penetration Studies ................. .. ............ ... ......... ............7
4. 1.2 Reduction of Surface Tension: Monolayer Penetration Results ................... .....80
4.2 Reduction of RMC via Monolayer Penetration ................. ...............83........... ..
4.2.1 Experimental Procedure ................. ... ......... ...............83......
4.2.2 Small Scale Monolayer Penetration Results ................. .......... ...............84
4.2.3 Washer Scale Monolayer Penetration Results ................ ........................84

5 MICELLE STABILITY AND ITS EFFECT ON THE RESIDUAL MOISTURE
CONTENT OF FABRICS............... ...............94

5.1 Stabilization of M icelles ................. ....... ..._... ........._.. .. .... ....... ........9
5.1.1 Concentration Dependence on Micelle Stability: 200 mM SDS .......................96
5.1.2 M ixed Surfactant Systems ............... ...... ........ ...............9
5.2 Effect of Dodecyl Sulfate Counterions on the RMC of Fabrics ................. ................. .99
5.2.1 Experimental procedure (Surfactant Synthesis)............... ..............10
5.2.2 M olecular mechanisms .............. ...............101....
5.3 Chain Length Compatibility .............. ............... ................10
5.3.1 Review of Chain Length Compatibility Work............... ...............102.
5.3.2 SDS + Long Chain Alcohols (CnOH) ................ ...............103........... ..
5.3.3 SDS + CnTABs .............. ...............104....
5.4 Labile M icelles .............. .......... .................10
5.4.1 SDS + Polyvinylpyrrolidone (PVP) ....__ ......_____ ...........___........10
5.4.2 SDS + Short Chain Alcohols (CnOH) ....._____ ..... ... .__ .........._.......0











6 FULL SCALE RESIDUAL MOISTURE CONTENT TESTING UNDER NORMAL
CONDITIONS ................. ...............118................


6. 1 Various Surfactant Systems in Full Washer Scale ................. ............................118
6.1.1 Small Scale Testing ................. ...............119...............
6. 1.2 Full Scale W asher Testing .............. ...............119............ ...
6.2 Vesicle Surfactant Interactions ........._.. ..... ._ __ ...............120...
6.2.1 Turbidity .............. ...............120....
6.2.2 Particle Sizing .............. ..... ... .. ......... .. ........ .. ...........12
6.2.3 Molecular Mechanisms between SDS and Vesicles ................. ................ ..121
6.3 Washer Scale RMC Testing............... ...............122

7 SUMMARY AND RECOMMENDATIONS FOR FUTURE WORK ............... .... .........._.132


7.1 UF Contributions to the Lowering of Surface Tension: Saving Energy in the
Laundry Process ................... .............. .. ............ .... ... ... .. ..........3
7.2 Technological Impact of the Reduction of RMC in the Laundry Process. ................... ..13 5
7.3 Recommendations for Future Work .............. ...............136....
7.3.1 Dynamic Surface Tension............... ...............136
7.3.2 M icelle Stability ................. ...............136................
7.3.3 Monolayer Penetration ................. ...............138................
7.3.4 Surfactant Adsorption ................ ...............138....._.__.....

APPENDIX

A ORIENTATION OF ADSORBED SURFACTANT MOLECULES ON COTTON
FABRIC ................. ...............139................

B REMOVAL OF WATER FROM CAPILLARIES: MATHEMATICAL APPROACH .....141

BIOGRAPHICAL SKETCH ................. ...............154......... ......










LIST OF TABLES


Table page

1-1. Composition of typical cotton fibers. ............. ...............39.....

1-2. Relationship between area per molecule and surface tension .............. ....................3

1-3. Micellar stabilities for pure surfactant systems. ............. ...............39.....

1-4. Micellar stabilities for SDS mixed surfactant systems. .............. ...............39....

2-1. Surface tensions and corresponding RMC values for Hanes fabric for various
commercially available surfactants ................. ...............50........... ....

3-1. The magnitudes of the RMC peak for various fabrics for I) the absolute different in the
maximum and minimum of the RMC peak, II) the difference in the maximum and
minimum normalized with respect to the RMC maximum and III) the difference in
the maximum and minimum normalized with respect to the RMC minimum. .................76

4-1. RMC of small scale monolayer penetration with C14SO4:DODAB monolayer. ................... ..93

5-1. Micellar relaxation times (z2) for different SDS systems with the addition of co-
surfactants. .............. .. ...............116......... ......

5-2. Physical properties and dimensionless dynamic surface tension (0) of different
counterions of dodecyl sulfate. ..........._ ..... .__ ...............117...

6-1: RMC for terry cloth fabrics with the addition of various additives added to 500 ppm
solutions of fabric softener. ........... ..... .. ...............131.

6-2: RMC values for large scale washing machine experiments for various surfactant
systems ................. ...............131.....__ .....










LIST OF FIGURES


Figure page

1-1. Graphic represention of how water is retained and removed from laundry in the washing
machine ...._ ................. ...............32.......

1-2. SEM picture of cotton fabrics............... ...............32

1-3. Forces involved in capillary rise where FSFT is the force due to surface tension
(2xir y cos B ) and Fd is the force due to gravity (mg or Hair2 pg ). .........._..._. ...............33

1-4. Capillary rise dynamics observed for 1 mM C14E6 Surfactant solutions ............... ...............33

1-5. The effect of capillary number on the residual oil in porous media............... .................3

1-6. Li qui d/gas phenomena exhibiting minima and maxima at 200 mM SD S concentrate on.......3 4

1-7. Liquid/liquid and solid/liquid phenomena exhibiting minima and maxima at 200 mM
SDS concentration .............. ...............35....

1-8. Effect of micellar stability on dynamic surface tension. ................ ....._.__............_....35

1-9. Micelles stabilizing a thin film between fabric fibers. ............. ...............35.....

1-11. Two segments in the cellulose chain. ................ ...............36.._._._

1-12. Synergism between a cationic Gemini and anionic n-dodecane sulfonate ................... ........36

1-13. Possible surfactant morphologies at a solid liquid interface .............. ....................3

1-14. Force required to "puncture" micelles adsorbed on a mica surface .............. ...................37

1-15. Moisture content of filter cake as a function of surfactant concentration used in slurry
pretreatm ent. ............. ...............38.....

1-16. Adsorption characteristics of surfactants on kaolin. ....._.__._ ............ ......._._. .....3

2-2. Experimental apparatus used to determine the RMC of various fabrics. ............. ..... ........._.47

2-3. RMC as a function of centrifugation time ........... ....._ ... ...............4

2-4. RMC as a function of centrifugation speed. .............. ...............48....

2-5. Relationship between RMC and surface tension............... ...............49

2-6. Relationship between RMC and surface tension for commercial surfactant systems............ 49











2-7. Residual moisture content and surface tension of various weight ratios of SDS to
C 12TAB .............. ...............50....

3-1. Experimental apparatus used to determine the residual moisture of fabrics. ................... ......64

3-2. RMC of Hanes 100% cotton fabric as a function of SDS concentration. ............. ...... ........._65

3-3. RMC of Hanes 100% cotton fabric as a function of SDS concentration. ............. ...... ........._65

3-4. Equilibrium and dynamic surface tension of residual SDS solution after exposure to
Hanes fabric. ............. ...............66.....

3-5. Comparison of the RMC of Hanes fabric and the equilibrium surface tension of residual
solution after soaking the fabric............... ...............66.

3-6. RMC of Hanes cotton fabric, terry cloth fabric and DOE test fabric as a function of SDS
concentration ................. ...............67.................

3-7. RMC and DST of the residual solution from the Hanes 100% cotton fabric. ................... .....67

3-8. RMC and DST of the residual solution from DOE 50:50 cotton:polyester fabric .................68

3-9. RMC and DST of residual solution from the Terry Cloth 86: 14 cotton:polyester fabric. .....68

3-10. Indication of the regions associated with the peak in the RMC of Hanes cotton fabric. .....69

3-11. Molecular mechanism for the adsorption of SDS onto cotton .............. .....................6

3-12. Calibration curve for MBAS method .............. ...............70....

3-13. Adsorption of SDS adsorbing onto Hanes cotton fabric .............. ...............70....

3-14. RMC of Hanes fabric soaked in Clo fatty acid ................. ...............71...........

3 -15. RMC of Hanes fabric soaked in solutions of a leading fabric softener .............. ..............71

3-16. RMC of Hanes fabric soaked in solutions of a leading detergent ..........._..................72

3-17. RMC of Terry cloth fabric soaked in solutions of SDS. ............. ...............72.....

3 -18. RMC of terry cloth fabric pre-treated with CMC. ......___ ..... ... __ ......_.....7

3 -19. RMC of terry cloth fabric pre-treated with PAA ................. ...............73...........

3-20. RMC of terry cloth fabric pre-treated with PVP. ..........._ ..... .__ ......__ ........7

3-21. RMC of Terry cloth fabric pre-treated with Cls Fatty Acid .................... ...............7

3-22. RMC of Terry cloth fabric pre-treated with Cls Alcohol. ................... ............... 7










3-23. RMC of terry cloth fabric pre-treated with DODAB. ................ ................ ......... .75

4-1. Spreading and penetration of an insoluble monolayer. ............. ...............85.....

4-2. Equilibrium surface tension of a C16TAB monolayer penetrated with 4 mM C14SO4...........8s6

4-3. Equilibrium surface tension of a stearic acid monolayer penetrated with 4 mM C14TAB.....86

4-4. Equilibrium surface tension of a DDAB monolayer penetrated with 4 mM C14SO4.............8s7

4-5. Molecular diagram of the transient monolayer phenomena found in slightly soluble
monolayers penetrated with a soluble surfactant from the subphase. .............. .... .........._.87

4-6. Equilibrium surface tension of a DODAB monolayer penetrated with 4 mM C14SO4..........88s

4-7. Equilibrium surface tension of a C200H monolayer penetrated with 4 mM C14SO4. ............88

4-8. Equilibrium surface tension of a Cholesterol monolayer penetrated with 4 mM C14SO4......89g

4-9. Equilibrium surface tension of a mixed monolayer composed of a 1:5 ratio of C14SO4 to
DODAB .............. ...............89....

4-10. Equilibrium surface tension of mixed monolayers of various ratios of C14SO4 to
DODAB penetrated with 1000 CIL of 4 mM solutions of C14SO4. ....... ............... 90

4-11i. Time study of the equilibrium surface tension of a mixed monolayer of C14SO4 and
DODAB in a 1:5 Ratio injected with 1000 CIL of 4 mM C14SO4. ...... ...............90

4-12. 2-D hexagonal arrangement of molecules at the 1:3 molecular ratio. .............. ... ........._...91

4-13. Experimental apparatus used to measure RMC of monolayer penetration ................... .......91

4-14. Comparison of RMC values for full scale washing machine experiments. ................... .......92

4-15. Comparison of RMC values for full scale washing machine experiments. ................... .......92

5-1. RMC of Hanes 100% cotton fabric as a function of SDS concentration ................... ..........107

5-2. RMC and DST of the residual solution from the Hanes 100% cotton fabric ....................107

5-3. Liquid/gas phenomena exhibiting minima and maxima at 200 mM SDS concentration.....1 08

5-4. Liquid/liquid and solid/liquid phenomena exhibiting minima and maxima at 200 mM
SDS concentration. ............. ...............108....

5-5. RMC of Hanes fabric around the concentration range of most stable micelles of SDS
(200 m M ) .............. ...............109....

5-6. Stable micelles trapped in the interstitial space in between fiber strands ............................109











5-7. Effect of micellar stability on dynamic surface tension. ........___ ......__ ................110

5-8. RMC of Hanes cotton fabric for pure SDS and mixed SDS systems............... ................11

5-9. Equilibrium surface tension for dodecyl sulfate surfactants with various counterions ........111

5-10. Effect of counterions on the molecular packing of dodecyl sulfate at the air/liquid
interface............... ...............11

5-11. Foam stability for various dodecyl sulfate counterions at 50 mM total surfactant
concentration. ........... ........... ...............112.....

5-12. RMC of Hanes cotton fabric for various dodecyl sulfate counterions at 1 mM total
surfactant concentration. ........... ......__ ...............112......

5-13. RMC of Hanes cotton fabric for various dodecyl sulfate counterions at 50 mM total
surfactant concentration. ........... ......__ ...............113......

5-14. RMC of cotton fabrics a function of SDS + CnOHs ......___ .... ... .___ ........_._.....1

5-15. RMC of cotton fabric as a function of SDS + CnOHs ................. ....___ ...............11

5-16. Long relaxation time (z2) and RMC of Terry fabric in solutions of PVP. ................... ...... 115

5-17. Long relaxation time (z2) and RMC of Terry fabric in solutions of 100 mM SDS with
the addition of short chain alcohols. ........... _.....__ ...............116

6-1: Equilibrium surface tension and RMC of fabric softener systems with the addition of
sodium dodecyl sulfate (SDS). ............. ...............123....

6-2: Equilibrium surface tension of fabric softener solutions with the addition of
dioctyl sulfosuccinate (AOT)............... ...............124.

6-3: Equilibrium surface tension of fabric softener solutions with the addition of Dow
Corning Q2-5211 Silicone Super Wetter ................. ...............124........... ..

6-4: Temperature dependence of the RMC during the final rinse cycle ................. ................. 125

6-5: Drying rate curve for terry fabrics soaked in various solutions. ............. .....................12

6-6: Turbidity measurements of fabric softener solutions with the addition of SDS. ........._......126

6-7: Mean particle size of 500 ppm solutions of fabric softener with the addition of SDS.........126

6-8: Particle size and turbidity of 500 ppm fabric softener solutions with the addition of
SD S. ............. ...............127....

6-9: Molecular diagram describing the interaction of vesicles with SDS ................. ............... 127










6-10: RMC of Terry cloth fabric with the addition of Cascade. ......___ .... ... ._ ..............128

6-11:. RMC of Terry cloth fabric with the addition of Dow Q2-5211. ................ ................. ..128

6-12: RMC of Terry cloth fabric with the addition of Flexiwet (Q-22, RFD-15A, and NF). .....129

6-13: RMC of Terry cloth fabric with the addition of Jet Dry ................. .........................129

6-14: RMC of Terry cloth fabric with the addition of Sylgard 309. .............. .....................130

6-15: Comparison of RMC values for full scale washing machine experiments ................... .....130









Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

SAVING TIME AND ENERGY IN THE LAUNDRY PROCESS: IMPORTANCE OF
DYNAMIC SURFACE TENSION, MICELLE STABILITY AND SURFACTANT
ADSORPTION

By

Daniel Larry Carter

May 2007

Chair: Dinesh O. Shah
Cochair: Ranga Narayanan
Major: Chemical Engineering

It has been shown that the residual moisture content of fabrics at the end of a centrifugation

cycle is related to the equilibrium surface tension of the residual solution in the fabric. However,

in the case of lowering the surface tension of solution via increasing concentrations of sodium

dodecyl sulfate, a peak is observed in the residual moisture content and the residual moisture of

fabrics deviates from the predictions of the LaPlace Equation of capillary rise. This is due to

adsorption of surfactant on the fabric and the increase in dynamic surface tension. Several other

molecular mechanisms have been found to affect the residual moisture content of fabrics such as

the following: dynamic surface tension, micellar stability, chain length compatibility, surfactant-

vesicle interactions as well as monolayer penetration. In surfactant systems with stable micelles,

the dynamic surface tension of solution is high compared to labile micellar solutions and the

resulting residual moisture content of fabrics is higher than the residual moisture of labile

micellar systems. Similarly, when the chain lengths of multiple surfactants in a two surfactant

system are of the same length, the micellar stability of this system is much higher compared to

mismatched chain lengths. The residual moisture content of fabrics in same chain length

surfactant systems has been shown to be higher compared to mismatched chains. It has also been









established that monolayer penetration can be used to lower the air-liquid surface tension of

solution. In this work, we developed a method to lower the air-liquid surface tension values to

values much lower than published results (about 7 mN/m). This method was then used to reduce

the residual moisture content of fabrics in the laundry process.









CHAPTER 1
INTRODUCTION

1.1 Introduction

The detergent industry, with a $5.5 billion market in the US, forms a major part of the

consumer goods industry. Two large companies, Procter and Gamble and Unilever, essentially

cover this market, which implies that a small percentage gain in the share over its competitor

would amount to maj or gains in revenues, thus fuelling stiff competition. To win small

competitive ground over its rival, companies are resorting to fierce promotion of brand name by

advertisements and sponsorships, matching product for products, saturating shelf space by

variations of a brand and investing huge amounts of money on R & D for making "bigger and

better" products.

With many of the products already working at their current optimum in terms of efficiency

of cleaning fabrics, companies have started to explore newer variables (other than surfactant

formulations) to add appeal to their products. In this regard, a reduced drying time of fabric is a

key variable to add consumer appeal. The Department of Energy (DOE) reports that the cloth

dryer is typically the second largest electricity using appliance after the refrigerator, costing

about $2.66 billion to operate annually nationwide.l Therefore, a conservative 30% reduction in

the drying time of laundry could result in savings of over $1 billion per year in the United States

alone. It is of great interest to the DOE to conserve energy; however, the cost savings per

consumer will not be a key point driving consumer sales (about $16 per year savings per

household with an electric dryer).

The key point in finding ways to reduce water content in fabrics is to understand why the

water is being held in the fabric. It is believed that water wicks into fabric because of capillary

forces operating between fabric fibers that create capillaries. If capillary forces are indeed the









cause of water retention, then the capillary rise in the fabrics should be governed by the LaPlace

equation for capillary rise. These forces can be strong or weak depending on the weave of the

fabric as well as the composition of the fabric. For example, cotton fiber holds about 75-80% of

initial water after the spin cycle of a washing machine. Work has been done in this proj ect to

decrease the capillary forces to enhance the water removal from fabrics by the reduction of

surface tension between the air-liquid interface (Figure 1-1). This methodology can enhance the

design of a laundry product that can significantly reduce the amount of residual water in fabric

after the spin cycle thus reducing the drying time, and saving energy required for drying the

fabric.

1.2 Theory

1.2.1 Surface Tension Reduction

To reduce the drying time of fabrics, we have identified the role of surface tension as a key

parameter of the laundry process. Our rationale is that if we can reduce the amount of residual

water in fabrics during the spin cycle of the washing machine, then it will lead to a reduction in

drying time during the drying cycle. It is assumed that the water is held by capillaries in the

fabric structure created by overlapping fabric fibers (Figure 1-2).

In Figure 1-3, the height of a liquid column in a capillary is shown to be a balance of the

surface tension forces and the force exerted on the cylinder of water in the capillary by gravity.

The basis behind our reasoning is found in the LaPlace Equation for capillary rise (Equation 1-

1), where y is surface tension, 6 is the contact angle (which is assumed to be zero for most water

wettable fabrics), r is the capillary radius and p is the solution density. It should be noted that the

contact angle and capillary radius are fixed for a given fabric, leading to the idea that the surface

tension of solution is the easiest variable to manipulate.










h= 27o9(1-1)
rpg

As shown in Equation 1-1, the capillary height is a function of the capillary radius. As the

capillary radius is decreased, the capillary height is increased as shown in Figure 1-4. As shown

in the Eigure, as the capillary radius is decreased from 0.50 mm to 0.10 mm the capillary height is

approximately tripled. If the LaPlace Equation for capillary rise can be applied to the removal of

water from fabrics then the same trend would be shown in the residual moisture content of

fabrics. If the capillary radii are smaller in a fabric, then there would be a higher capillary rise in

the fabric leading to increased residual moisture content.

It has been shown by Preston et al. that water is retained in moist fibers by capillary water

held in spaces between fibers and by hydrates of the fiber molecules.2 They have shown that the

amount of retained moisture in viscose and cellulose fibers is proportional to the surface tension

of solution. However, their studies only showed the direct relationship between surface tension

of solution and the residual moisture content of fiber systems. In our work, we are using a single

surfactant in increasing concentration to vary the surface tension and measuring the Residual

Moisture Content (RMC) of consumer fabrics instead of individual fiber strands. In comparison,

Preston used several different surfactants at a single concentration to show the relationship

between surface tension and capillary height for fiber bundles.

If by adding proper additives, one can substantially decrease the surface tension of a

solution, then with the same centrifugal forces in the rinse cycle of existing machines, we can

remove more water. We feel confident about this approach because of our earlier work in

enhanced oil recovery, where we had to eliminate or reduce similar capillary forces responsible

for trapping oil in fine pores of an oil reservoir by reducing the interfacial tensions (Figure 1-5

where capillary number is a dimensionless ratio of viscous forces and surface tension forces).3









However, the analogy should not be drawn too far as it is relatively easy to achieve low

interfacial tensions in oil/water systems (as low as 10-3 dynes/cm) as compared to air/water

sy stem s.

1.2.2 Role of Adsorbed Micelles and Micellar Stability

After some of the recent Atomic Force Microscopy (AFM) mapping at solid/water

interfaces, where investigators have shown adsorbed surfactant micelles at the interface4, there is

a good possibility that surfactant micelles or other aggregates are also adsorbing at the fiber

surfaces in the capillary, hence modifying capillary forces. In systems with high micellar

stability, very stable films have been observed. With very stable thin liquid films, the RMC

should increase due to the higher force required to disrupt these films (Figure 1-9). In this

context, micellar stability concepts become important. It is planned to investigate the effect of

engineered micelles of low and high stability on water retention in fabric to show their effect on

the residual water content.

Over the years, the Shah research group has shown that the micellar relaxation time is a

maximum at a concentration of 200 mM SDS.3, 5-9 This maximum in micellar relaxation time has

a dramatic effect on many different properties of SDS solutions (ranging from low foamability,

high thin film stability, wetting time, oil solubilization, etc. (Figure 1-6 and Figure 1-7). The

Shah Research group has also shown that micellar kinetics play an important role in detergency.

Shah et al. has shown that the efficacy of removing non-polar compounds from fabrics has been

shown to have a strong correlation with the relaxation time of micelles.5-1 For example, it was

shown by Oh and Shah that using 200 mM SDS (which was shown to have the longest micellar

relaxation time in the SDS concentration range)6 prOVided the most efficient removal of an

artificial stain created by the deposition of Orange OT onto fabric samples.9









Since the dynamic surface tension is related to the micellar stability (i.e. higher micellar

stability leads to higher dynamic surface tension Figure 1-8) and we have proposed that the

surface tension of solution (equilibrium and dynamic) can affect the amount of water retained by

fabrics, it would be expected that there will be an increase in the RMC around a SDS

concentration of 200 mM (the concentration of highest stability for the SDS system). This

maximum is believed to be due to the long relaxation time of the SDS micelles at 200 mM. The

long relaxation time of the micelles would lead to a decreased monomer flux from the micelles to

the bulk. This decrease in monomer flux would then be shown as an increase in the dynamic

surface tension thus leading to an increase in the RMC (shown in Figure 1-8). Alternatively,

another possible explanation to explain the increase in RMC at 200 mM concentrations of SDS

could be due to stabilization of thick films on the fabric surface as well as within the interfiber

spaces due to relatively stable micelles. It has been shown by Shah et al.11, 12 and Wasan et al.13-18

that layering of micelles or particles can stabilize thin films (which could possibly explain an

increase in the RMC).

1.2.3 Surfactant Adsorption

It is proposed that sudden adsorption of surfactant onto the fabric surface can affect the

residual moisture content of fabrics due to the changes in dynamic surface tension of solution.

Since it has been shown that cotton has a negative zeta potential one might think that an anionic

surfactant would have minimal adsorption on a negatively charged surface.19-2 However, there

have been several papers showing that ability of sodium dodecyl sulfate (SDS) and other anionic

surfactants to adsorb onto negatively charged surfaces such as coal fines, cotton and cellulose.19,

22-25 Also, it has been shown by Somasundaran et al. that adsorption isotherms can show up to

four adsorption regions,26 One of them being a sudden increase of adsorption due to cooperative

adsorption of surfactant molecules. If surfactant molecules suddenly adsorb cooperatively on the









solid surface at a critical concentration, then it must cause a concomitant decrease in monomer

concentration in the bulk solution. Thus, a simple method to determine the monomer

concentration below CMC is to measure the surface tension of the residual solution. For a given

surfactant below its CMC, the surface tension is a measure of the free monomer concentration of

surfactant in solution. However, if the change in surfactant monomer concentration is not very

large then the equilibrium surface tension may not change significantly. However, the dynamic

surface tension may reflect it more clearly. If there is a sudden increase in adsorption on the

fabric surface then there would be less free monomer available to adsorb on the new air-liquid

interface of bubbles created during the dynamic surface tension measurement.

One important aspect in the removal of water from fabrics is the ability of surfactants to

adsorb onto the textile fibers. Several researchers have shown that even though most fibers have

a negative charge, the anionic surfactants are still able to adsorb onto the fibers. As shown in

Figure 1-10, sodium dodecyl sulfate is able to adsorb onto cotton fibers. However, the adsorption

continues to increase after the solution CMC (8 mM). Rybicki explains that the increase of

adsorption after the CMC is due to a competitive adsorption process between surfactant

monomers and micelles. Near the CMC, adsorption of monomers and micelles is equally easy,

while at higher concentrations micellar adsorption is dominant.19 Another explanation to this

phenomenon may be due to fiber swelling. When the fabric is placed in solutions near the CMC,

micellar and monomer adsorption is present. However, due to the lower number of micelles, the

monomers are still able to adsorb into micro-pores that are present when the fibers swell. As the

concentration is increased, the micellar phase also increases. Due to the increased numbers of

micelles, there is a higher probability that these micelles can adsorb and block the micro-pores

thus blocking monomers from penetrating into the swollen fibers.









1.3 Textile Chemistry

One important factor in understanding how the RMC of fabrics can be manipulated is

understanding the effect of fiber structure and fiber chemistry on water retention. Since there are

many different types of fabrics as well as different fabric weaves, it is important to understand

their effect on water retention.

1.3.1 Chemical Composition of Cotton Fibers

On average, raw cotton fiber is approximately 95% cellulose which is hydrophilic (Table

1-1).27 The remainder of the fibers is noncellulosic materials that are mostly hydrophobic

(proteins and waxes). However, these noncellulosic materials can be selectively removed by

using the proper solvents (i.e. chloroform removes waxes; ethanol removes waxes, sugars and

ashes, etc.).28 Also included in the composition of cotton fibers are various metals which can

cause several problems in yarn manufacturing (i.e. silica and metal oxides can cause friction

problems in spinning; peroxide bleaching can be affected by magnesium salts; copper, calcium

and magnesium can interfere with dyeing, etc).

1.3.2 Properties of Cotton

One concern in the treatment of cotton fibers is the effect of solvents on the chemical and

mechanical properties of the fibers.27 The cellulose produced by the bolls in cotton plants is

composed mostly of a polysaccharide cellulose. The polymeric backbone for the cellulose is a

linear polymerized form of P-D-glucopyranose (Figure 1-11) which is linked at the 1,4-P-

glucosidic oxygen bridge."

As briefly discussed, cotton is a hydrophilic fiber as well as porous. It has been shown that

when immersed in water, cotton fibers swell and its pores fill with water.27 Due to the small size

of the pores, chemical agents used for modification are not always able to penetrate the pores.

These modifications are important in the textile business and can include modifications such as









color dyeing, flame resistance, soil release, etc. Therefore, knowledge of these pores is and a

chemical's accessibility to these pores is a necessity. Several methods have been used in the

determination of pore accessibility such as solute exclusion. Using a series of water-soluble

molecular probes (increasing in size) that penetrate the cotton fiber and do not adsorb onto the

fiber surface, the water in poresis exchanged by a solute resulting with dilution into the external

solution. Then, using chromatographic techniques, the amount of internal volume expansion or

contraction can be determined.27 Another aspect of cotton fabrics is water swelling known as

bound water or hydrated water. It has been shown that between 0. 1-0.2 g/g of water present in

cotton fibers is bound water that can only be removed by thermal methods.

One fabric treatment that is widely used in industry is the swelling of cotton by sodium

hydroxide (called mercerization). 27 This process is used to improve fabric properties such as dye

affinity, tensile strength, and smoothness. The great improvement in these properties by

mercerization is thought to be due to the increase in pore sizes in the cotton fibers. However,

another method of fiber swelling, treatment with liquid ammonia, has been shown to have a low

level of large pores in the fibers.27

There are a few other fabric treatments used in industry that can improve fiber properties.

Several such methods include etherifieation (increases wrinkle resistance, water repellency,

flame resistance and antimicrobial action) esterifieation and enzyme modification (for fabric

softness and colorfastness of dyes).

Fabric dyeing is another important practice in industry that has implication in the removal

of water from fabrics. It is general practice to prepare the fabric surface by operations like

singeing, desizing, scouring, bleaching and mercerization. As stated previously, these treatments

are used in the removal of impurities from the cotton fibers (such as waxes, pectins and ashes).









Several dyeing methods exist in industry (such as azoic, direct, reactive, sulfur and vat dyeing)

but reactive and direct dyeing are the most common methods. Direct dyeing affixes dyes to the

cellulose by hydrogen bonding and van der Waals forces to attach the dye to the fabric surface.

The dyes and pigments used in these processes are mostly of water insoluble inorganic or

organic composition.27 Since some of the dyes are bonded to the surface or reactively affixed to

the fiber structure, changes in the hydrophilicity of the fiber are possible, hence the implications

into the dewatering of fabrics. These dyes can also influence the affinity of ionic surfactants in

the adsorption of these molecules to the fabric surface.

1.3.3 Surface Treatments of Cotton Fabrics

As mentioned, there are many types of surface treatments for fabrics (with a main focus on

cotton) that are commonly used for consumer convenience in order to enhance the properties of

the fabric. One important cotton treatment is making the cotton fibers flame resistant. Most of

the treatments used in this process are inorganic based chemicals that are chemisorbed on the

fabric (in the case of using multivalent metallic salts, the surface charge of the fabric will change

therefore changing the adsorption kinetics of surfactants onto the fabric).28

Another widely used fiber treatment process is treating the fibers for repellency. Several

types of treatments are used in the case of different types of repellency (i.e. water, oil, and soil).

In the case of water repellency, several different types of hydrophobic compounds can be used to

coat the fabric (ranging from waxes and siloxanes, to fluorocarbon treatments).28 As mentioned,

the contact angle of the solution on the fabric surface is one factor in the water removal from

fabrics (based on the LaPlace Equation for capillary rise). If the contact angle of the solution is

increased, the cos Bterm in the LaPlace equation is decreased, which would result in a lower









capillary rise or lower water retention. Therefore, any fabric that is treated with any of these

repellents will show lower residual moisture due to an increased contact angle.

1.4 Scientific Approach

Due to the complex nature of the removal of water from fabrics, several challenges may

arise that need to be understood. Technologies ranging from fabric chemistry to the dewatering

of coal fines may provide insight to the removal of water from fabrics. There are several things

that should be investigated in the process of removing water from fabrics such as surfactant

adsorption onto surfaces, the role of vesicle-surfactant interaction in the lowering of surface

tension as well as the role of dynamic processes such as dynamic surface tension and transient

surface tension by monolayer penetration.

1.4.1 Surface Tension Reduction using Surfactants

For pure surfactants, either anionic or non-ionic, the surface tension (after critical micelle

concentration, cmc) is usually in the range of 30 40 mN/m. For anionics, the surface tension

strongly depends on the counter-ion used (Table 1-2).29, 30 In the case of non-ionic surfactant

system (CiEj), the surface tension changes as a function of the number of ethoxylated groups.

A host of mixed surfactant systems show synergism in terms of reducing the surface

tension, whereby at particular mole ratios of two surfactants, one achieves surface tension values

unattainable by an individual surfactant alone. The surface activity is not only enhanced at the

gas-liquid interface but also at solid-liquid interface leading to a better wetting of textile fibers

and enhanced detergency.31

Working with sodium dodecyl sulfate (SDS) in their chain compatibility study, Sharma

and others32 have shown that the maximum reduction in surface tension of SDS solution occurs

by addition of a small amount of dodecanol, i.e. when the chain length of the alcohol and the

surfactant are matching. In a related study, Patist et.al.33 Showed a maximum reduction in surface









tension when a small amount of dodecyl trimethyl ammonium bromide (DTAB) is added to the

SDS solution.

Shah et.al.34 have found striking change in properties of various systems (eg. lecithin-

cholestrol, stearic acid-stearyl alcohol, decanoic acid-decanol, potassium oleate-hexanol, SDS-

cetyl pyridinium chloride) at a 1:3 molecular ratio. Though direct values of surface tension were

not reported for these systems, in all cases there is indirect evidence (evaporation rate, foam

stability, solubilization in microemulsion) that at this ratio there is a maximum crowding of

molecules at the interface and the molecules are tightly packed. Other researchers have reported

this synergism for amiomc/catiomic,3 amiomc/zwitteriomic,363 cationic/zwitteriomc,,3 non-

iomic/zwitteriomic,3 amiomc/caltiongmc-gemim amiome gemimi/zwitteriomic,4 caltiomic-

gemini/nonionic41 and cationic-gemini/sugar surfactants.42 These investigations suggest that

properly engineered synergism can help reduce surface tension values to ~ 25 mN/m (Figure 1-

12).

To further reduce the surface tension, special surfactants (i.e. silicone, fluorocarbon

surfactant) are needed. Silicone surfactants because of their flexible polymer backbone (Si-O

bond) and a preponderance of surface-active methyl groups, which can orient in low energy

configurations, are capable of reducing the surface tension to around ~ 19 mN/m. Numerous

chemistries such as cationic,43, 44 ring based cationic,43 Straight chain45, 46 as well as ring based

sulfo and carbo betaines (zwitterionic), siloxanyl phosphinoxides (amphoteric),43 pOlyether

copolymers (non-ionic),43 bolaform surfactants43, 47 (two hydrophilic groups and only one

hydrophobic group) have been developed to exploit the high surface activity of polydimethyl

siloxane (PDMS) backbone some of which we also will be using for the dewetting experiments.









Similar chemistries have been developed with the fluorocarbon surfactants and the minimum

surface tension possible can be ~ 15 mN/m.48

So in between pure, mixed, siloxane based and fluorocarbon surfactant, we have a variety

of systems with surface tensions in the range (15-40 mN/m) to experiment for our surface

tension reduction approach.

1.4.2 Effect of Bulk and Adsorbed Micelles

Surfactants are amphiphilic molecules, which tend to adsorb at any possible interface (i.e.

air-water, fabric-water). In surface tension reduction approach, we dealt only with phenomena

occurring at the air-water interface. Next we shift our attention to surfactant molecules at the

fabric-water interface where the adsorbed molecules may adopt different morphologies such as

sparse covering of substrate, monolayer covering, hemimicelles and spherical micelles as shown

in Figure 1-13.49 These morphologies depend on the concentration, structure of the surface-active

molecules and the nature of the substrate (polar/non-polar).49, 50 The effect of the underlying

substrate can be observed by comparing atomic force microscopy (AFM) images of full

cylinders meandering across mica surface with changes in direction corresponding to changes in

the mica lattice versus spherical micelles on amorphous silica, which lacks these atomic rows4.

On the other hand, structure of the surfactant itself, because of steric packing constraints (and

charge as the case might be), may influence the morphologies. Such is the case for cationic

gemini surfactant where curved morphologies are not observed and adsorption proceeds in a

layer by layer manner."

Intuitively, any preference of the surfactant-water-fabric system to form bilayer or

spherical morphologies hydrophilicc) should promote capillarity and higher water retention

capability. Further the stability of the aggregates as well as the strength with which these

aggregates are adsorbed onto the substrate may also heavily influence the amount of water that









can be removed from the fabric. Both of these areas are currently the focus of scientific pursuit.52

Isolated studies exist on the effect of surfactant concentration on wettability of the fabric but

there are no definitive studies for the role of adsorbed aggregates or aggregates in bulk and their

properties (stability) on the water retention capability or wettability of fabric.52 A study on

scoured hydrophilicc) and unscoured (hydrophobic) cotton with varying concentration of two

surfactants, Tween 20 and Span 20 showed that above their critical micelle concentration (cmc)

values both surfactants enhanced the wetting irrespective of nature of substrate

(hydrophobic/hydrophilic) while the opposite was true for concentrations below their cmc,52

affirming the role of surfactant aggregate morphology. In a related work, we have shown in a

SDS surfactant system (above cmc) that wetting time of cotton is entirely dictated by miceller

stability.52 SDS micelles are most stable at a concentration of 200 mM leading to a smaller

monomer flux, thus controlling the time for the aggregate morphologies (as shown in Figure 1-6)

to build up on the fabric surface.

The stability of the aggregates can be measured by:

1. using conductivity detection by pressure-jump method for ionic surfactant micelles

in bulk53

2. using stopped flow method for non-ionic surfactants micelles in bulk,53 and

3. using atomically smooth surfaces (mica, silica, alumina, highly ordered pyrolytic

graphite) and AFM for adsorbed surfactant aggregates.

Table 1-3 gives micellar stability for selected surfactants and Figure 1-14 shows the

stability of n-alkyl trimethyl ammonium bromide alkyll chain length: n = 8, 10, 12, 14, or 16)

micelles as measured by AFM.









By using additives such as tetra-alkyl ammonium chlorides,54 dodecyl alcohol,55 DTAB,33

2-ethyl hexanol and tri-butyl phosphate,56 we have also shown that the stability of SDS micelles

can be increased or decreased as shown in Table 1-4. Thus we have enough systems to study any

possible correlation between micellar stabilities and drying time of the fabric.

1.4.3 Surfactant Vesicle Interactions

Since the key focus of water removal is during the spinning cycle of a washing machine,

an additive that can be introduced into the wash cycle before the final spin occurs is desirable.

Currently, consumers use products that enhance the fiber softness and are known as fabric

conditioners. These products are introduced to the wash cycle during the final rinse before the

final spin cycle begins. This implies that the surface tension of this final rinse water is a

controlling factor in water removal from fabrics. It has been shown that these fabric conditioners

contain long chain cationic surfactants (on average 18 carbon chains) in both a monoalkyl form

and a dialkyl form. Due to the mixture of the monoalkyl and dialkyl surfactants (known as

monoquats and diquats since they are quaternary ammonium salts), they form vesicles. In order

to lower the surface tension of this rinse water, an oppositely charged (anionic) surfactant should

be used since it has been shown that a synergism exists between cationic and anionic surfactants.

However, it is possible that these added anionic surfactants may interact with the vesicles already

formed in these fabric conditioners. Such interactions need to be understood to effectively

engineer a system of low surface tension (<20 mN/m).

1.4.4 Dewatering of Particle Suspensions

The use of polymers as flocculants for particle suspensions is a widely used practice in the

separation and dewatering of solid/liquid systems containing fine particles.24 Separation by

flocculation is appropriate if the desired results are a reduction in sludge volume with rapid

separation. However, if it is desired to obtain a dry solid, the sludge is subjected to mechanical









forces to aid in the elimination of water. There has been extensive research in the Hield of

flocculation using polymers.24, 57-59 However, not much has been done in the area of correlating

adsorption phenomena on the effects of filtration and water removal. Several factors have been

investigated in the role of adsorption phenomena on the effect of residual water in porous media

and it has been determined that surface tension plays an important role. The use of surfactants

has been investigated to determine the effect of surface tension reduction on the amount of

residual water. It has been shown that a substantial reduction in the moisture content of porous

systems can be achieved by the addition of surfactants.24

It has been shown by Singh22 that there is a direct correlation between the point of zero

charge and surface tension reduction in the residual moisture in the dewatering of coal Eines.

Singh notes that the mechanisms of the reduction of water in the fi1ter cake is complex but that it

appears that the reduction of surface tension as well as surface modification in the contact angle

by adsorption play an important role. Similar to our study, the Laplace-Young equation for

capillary rise is thought to be the controlling mechanism in the retention of water in aqueous

slurries. However, Singh points out that there is not always a direct correlation between surface

tension and cake moisture indicating that surfactant adsorption on the liquid-solid interface plays

a roll in the dewatering of Eine porous media. Using both a cationic and anionic surfactant, Singh

showed that when the surfactant is not as strongly attracted to the coal particles (due to a slightly

negative surface charge), the cake moisture decreased (Figure 1-15). Since there is a high affinity

of the cationic surfactant to the coal particles, most of the surfactant was adsorbed (~90%) and

little surfactant was available to adsorb to the air-liquid interface. Due to lower solid surface

adsorption for the SDS system, more of the surfactant could adsorb at the air-liquid interface and

thus a lower surface tension could be achieved therefore reducing cake moisture.









Similar studies of surfactant adsorption on kaolin particles have shown results similar to

the earlier work done by Singh." Besra et al.59 have shown that the equilibrium adsorption

isotherm for sodium dodecyl sulfate on kaolin (which is a negatively charged surface) shows a

Langmuir type curve (Figure 1-16) with a low amount of surfactant adsorption due to

electrostatic repulsion.

1.4.5 Interactions at the Solid-Liquid Interface: Role of capillarity in the retention of
water in fabrics

Since our rationale is that water is retained in the fabric by capillaries formed between

fibers, capillarity plays an important role and should be understood to be able to develop novel

methods to remove moisture from fabrics. Several studies have been performed on the water

transport mechanisms in textiles.60, 61 It has been shown that water transport is governed mainly

by a modified LaPlace-Young equation that takes into account viscous terms in capillary flow.60

However, it was found that this wicking phenomena was not solely dependant on fabric type

(due to hydrophilicity, cotton should show a higher wicking than a hydrophobic material). It was

found by Hollies et al.60 that the fiber roughness as well as weave plays a role in the water

transport in fabrics (i.e. the yarn structure is a factor in the transport of liquids in fibers as

compared to solely the chemical nature of fibers). It has been suggested that the use of wicking

experiments (measuring the height of water in a fabric column as a function of time) can be an

indication of fiber arrangements. However, these observations are based on the assumptions of

no external gravitational force so extending this information into a dynamic process under

centrifugal force may not be appropriate. In the case of centrifugation, the gravitational forces

may be much greater than the forces affected by fabric structure (i.e. viscose forces) thus

showing more of a relationship between transport properties capillarityy, etc.) and the











composition of the fiber used in experimentation. It should also be noted that these experiments

were performed using dry fabrics rather than using fabrics that have been wetted.


C erntriugal


Perfo rated inner tub of
a washing muachine


II' I


lnl


Figure 1-1. (a) High surface tension, stronger capillary forces and more amount of trapped water
(b) lower surface tension, weaker capillary forces and less amount of trapped water
(c) adsorbed stable micelles may help in trapping water (d) beading of water on
surfaces hydrophobized by de-wetting agents, causing less residual water


1-2.i A)S Mpcur fawve arcusdulhpemsinfrmSuklas ,B


1 2 SEM i t r of a scoure clothc(used ith\ pe rm i~ rlssi o n from D ehruy 1973 ~)6


























Figure 1-3. Forces involved in capillary rise where FSFT is the force due to surface tension
(2xiry cos B) and Fd is the force due to gravity (mg or Hzir pg ).


O 5 10 IS 20 25

Figure 1-4. Capillary rise dynamics observed for 1 mM C14E6 Surfactant solutions in hydrophobic
capillaries with radii of 0. 1 and 0.5 mm.


















80 -





S40-


S20-



106 105 104 1(P 102
Capillary Number (pV/o)

Figure 1-5. The effect of capillary number on the residual oil in porous media.


Frelu~encynof Bubble



Volume of Single Bubble


Single Film Stability

Foamability



Slow Micellar Relaxalion


S200 mM

sDs CONCENTRATION


Figure 1-6. Liquid/gas phenomena exhibiting minima and maxima at 200 mM SDS
concentration (used with permission from Patist 2002).6


























































I I Fsb~ic P1~EB I I
~
?C=+~~='lu. =r~3. =+ ~
~i=' ;il' +il' ''
.I~. +C .f+,
C1~ b ,
;i +;i +
''~3. =r~3. *
I;~ 1+;1~ 1;111 1;1~ II;
,fr~l =:r~l ..f.,. 13f',. f'+l
,I .I g. =r ;: :
1' +~ r 3~
~c~t~' ~' ~
~; ~
Y :+ ~
~.;r'=;;


Timeto Reach Saltuatllo
ofSDS Solution by Beanzen


Rmval Orange OT dye


Soldb lzeation Rate of


Drople Size in Emulsions

Wetting Time


SDS CONCENTIMION



Figure 1-7. Liquid/liquid and solid/liquid phenomena exhibiting minima and maxima at 200 mM
SDS concentration (used with permission from Patist 2002).6


Lery untal miced





~(7 "

V'en stable~r micalles


I_


High D.S.T.


Figure 1-8. Effect of micellar stability on dynamic surface tension.


Figure 1-9. Micelles stabilizing a thin film between fabric fibers.















90 h [ 1 5 1











Fiue11.Cag nasrto fsdu ddclslaeo otna ucino












HHOO




Figure 1-11. Two sgme ntas rpin n the celuoseu chain. lae ncttna afnti






RybSrfatan 194)'

E c~r 45 -0
E31 P











Figure 1-12. Syrtneris bewe aincGmn sratn )adaincnddcn

sulfonate (surfac rfatant 2)(sdwt emsin rmIaai19)3
























Figure 1-13. Possible surfactant morphologies at a solid liquid interface (a) monolayer formation,
(b)bilayer formation (c) micelle to hemimicelle formation (used with permission from
Adler 2000).49














10 11 2 '18 14 15 10
Chain length of cationic surfactant
Figure 1-14. Force required to "puncture" micelles adsorbed on a mica surface (used with
permission from Adler 2000).49













1Vateurr a St kPcs


50l *








10


0 2 3 A,.s 9.2 12 a IS ~ & 1 2.0
C:9riFncentratior male drm- MID-


Figure 1-15. Moisture content of filter cake as a function of surfactant concentration used in
slurry pretreatment.

Eqluilibrum DOnCentration, mcngli
012345678








54
$9 ACTAlB





O W( 100 15 200 25( 300 35( 400 450 530

Eqlulllbdlurn concentraion, mg~llt

Figure 1-16. Adsorption characteristics of surfactants on kaolin (used with permission from
Besra 2002).59










Table 1-1. Composition of typical cotton fibers.27
Consttuent Composition (% of dry weight)
Typical Range
Cellulose 95.0 88.0-96.0
Protein (% N x6.25)a 1.3 1.1-1.9
Pectic Substances 1.2 0.7-1.2
Ash 1.2 0.7-1.6
Wax 0.6 0.4-1.0
Total Sugars 0.3 0.1-1.0
Pigment Trace
Others 1.4
aStandard method of estimating percent protein from nitrogen content (% N)

Table 1-2. Relationship between area per molecule and surface tension
arealmolecule Surface Tension
(A2/mOlecule) above CMC (mN/m)
LiDS 61.3 44.2
NaDS 51.8 40.0
CsDS 44.8 34.4


Table 1-3. Micellar stabilities for pure surfactant systems.64
Micellar
Surfactant Stability( (2)
SDS (200 mM) 7 sec
Tween 20 6 sec
Tween 80 8-10 sec
Pure C12(EO)5 10 sec
Pure C12(EO)8 4 sec
Brij 35 80 sec
Triton X-100 3.5 sec
Synperionic A50 40 sec
Synperionic A7 150 sec


mixed surfactant systems.33, 54-56
Micellar Stability


Table 1-4. Micellar stabilities for SDS


Surfactant + additive


SDS (25 mM)
SDS (25 mM) + 1.25 mM Dodecanol
SDS (100 mM)
SDS (100 mM) + 5 mM DTAB
SDS (100 mM) + 5 mM tetra-ethyl ammonium chloride
SDS (100 mM) + 5 mM tetra-butyl ammonium chloride


1 millisec
230 millisec
150 millisec
1350 millisec
2500 millisec
50 millisec









CHAPTER 2
THE RELATIONSHIP OF SURFACE TENSION AND THE RESIDUAL MOISTURE
CONTENT OF FABRICS

2.1 Experimental Background

In order to reduce the drying time of fabrics, we have identified the role of surface tension

as a key parameter in the reduction of water in the laundry process. Our rationale is that if we can

reduce the amount of residual water in fabrics during the spin cycle of the washing machine, then

a corresponding reduction in drying time during the drying cycle would lead to a reduction in th

drying costs. It is assumed that the water is held by capillaries in the fabric structure created by

overlapping fabric fibers.

The basis behind our reasoning is found in the LaPlace Equation for capillary rise (Figure

2-1 and Equation 2-1), where y is surface tension, 6 is the contact angle (which is assumed to be

zero for hydrophilic fabrics), r is the capillary radius and p is the solution density.


h = 27cs9(2-1)
rpg

The amount of fluid that can rise into a capillary is proportional to the surface tension of

the fluid. Equation 2-2 shows that the work required moving a liquid a given distance is

proportional to the surface tension. Based on Equation 2-2, if the surface tension (y) of the fluid

is lowered and the work is held constant (the centrifugal force exerted on the fabric during the

spin cycle), then the amount of displacement in the capillary, AA, must increase to balance the

equation. Therefore, based on these two principles, if the surface tension of the rinse water of the

final spin cycle is lowered, then more water will be forced out of the fabric. If less water is

present in the fabric before placing in it the dryer, then the time and energy required to dry the

fabric will be decreased.









W = 7 *A (2-2)

It has been shown by Preston et al.2, 65, 66 that the use of the equation for capillary rise is

appropriate for use in examining the capillary rise in fiber assemblies. They have shown that the

amount of retained moisture in viscose and cellulose fibers is proportional to the surface tension

of solution. However, their studies mainly focused on two different surface tension solutions and

a fixed centrifugal time. It was shown that the relative mass of water imbibed in capillaries

should be linearly proportional to the surface tension of solution. Therefore it is believed that this

is an appropriate method to determine the residual moisture content of different fabric types.

However, it should be noted that Preston et al. varied surface tension by using different

surfactant types. Our work has expanded on this basis to include surface tension variation by

varying the concentrations of a single surfactant. Much of Preston' s work was preformed at high

gravitational forces (from 1000 5000 g), which is much higher than the gravitational forces

found in a household washing machine (~100 g). On another note, Preston's work never

mentioned the implications of his work for use in a laundry product.

If by adding proper additives, we can substantially decrease the surface tension of a

formulation, then with the same centrifugal forces in the spin cycle of existing machines, we can

remove more water. This approach was successfully used in of our earlier work in enhanced oil

recovery, where we had to eliminate or reduce similar capillary forces responsible for trapping

oil in fine pores of an oil reservoir by reducing the interfacial tensions.3

Shah et.al.34 have found striking change in properties of various systems (eg. lecithin-

cholestrol, stearic acid-stearyl alcohol, decanoic acid-decanol, potassium oleate-hexanol, SDS-

cetyl pyridinium chloride) at a 1:3 molecular ratio. Though direct values of surface tension were

not reported for these systems, in all cases there is indirect evidence (evaporation rate, foam









stability, solubilization in microemulsion) that at this ratio there is a crowding of molecules at the

interface and the molecules are tightly packed. Other researchers have reported this synergism

for amiomc/catiomic,6 amiomc/zwitteriomic,363 cationic/zwitteriomc,,3 non-10mic/zwitt eriomc,,3

amiomc/catiomic-gemimi,3 amiome gemimi/zwitteriomic,4 caltionic-gemini/nonio mcc4 and

cationic-gemini/sugar surfactants.42 These investigations suggest that properly engineered

synergism can help reduce surface tension values to ~ 20 mN/m.

2.1.1 Measuring Residual Moisture Content

In order to determine how the fabric system reacts to different variable that could possible

affect the residual moisture content (RMC) of fabrics, several experiments were performed to

show the effects of centrifugation speed and centrifugation time as well as testing several

different types of fabrics. In order to test the effect of surface tension on the residual water

content of fabrics several assumptions needed to be made. After several force calculations it was

determined that the average household washing machine spins with a force about 90 times the

force of gravity (or approximately 640 RPM for a typical washing machine). For testing

purposes each fabric sample was soaked for ten minutes and then placed in the centrifuge for ten

minutes. The experimental apparatus that was used is shown in Figure 2-2. The setup uses a

centrifuge tube with a copper insert. The copper insert has a closed end with the other end flared

so that it will not fall inside the outer tube. The insert also has small holes drilled through it to

allow water to drain through the insert into the collection tube (much like how a modern washing

machine is designed).

After the fabric was soaked and centrifuged, the weight was then taken to determine the

Residual Moisture Content (RMC) as shown in Equation 2-3. The first sets of experiments were

designed to get basic information about how the system acts (such as force and time dependence

on the RMC).










Weighteentrtfuge Weightdry
R2MC =100* (2-3)
Weightdr

After a basic understanding of how the system acts under different forces, it was desired to

determine the relationship between RMC and surface tension. Several sets of experiments were

performed using various commercial surfactants provided by the manufacturer (Delonic 100-

VLF, Delonic LF60-MOD, and Dow Corning Q2-5211i). All of the commercial surfactants were

tested at 1000 ppm (0.1 wt%/). Several other surfactant systems were chosen in this study as well.

A leading detergent (at 1500 ppm the normal household dosage in a washing machine) and a

leading fabric softener (at 500 ppm household dosage) were also tested in these experiments.

2.1.2 Surface Tension Measurements

The surface tension measurements were made using the Wilhelmy Plate method. The

output from a gram-force sensor holding the plate is sent to a transducer and then output to a

voltage readout. The system was calibrated using two known solutions (water at 72.5 mN/m and

acetone at 23 mN/m). The platinum plate was heated using a torch between each reading to clean

off any surfactants or impurities that may have adsorbed onto the platinum plate. For the

experiments performed in the basis experiments, only equilibrium surface tension was correlated

with the residual moisture content of fabrics.

2.1.3 Materials

The sodium dodecyl sulfate used in these experiments was obtained from the Fisher

Scientific Company. Several sets of experiments were performed using various commercial

surfactants provided by the manufacturer (Delonic 100-VLF, Delonic LF60-MOD, and Dow

Corning Q2-5211i). All of the commercial surfactants were tested at 1000 ppm (0. 1 wt%).

Several other surfactant systems were chosen in this study as well. A leading detergent (at 1500










ppm, the normal household dosage in a washing machine) and a leading fabric softener (at 500

ppm, the household dosage) were also tested in these experiments.

Several different types of fabric were used in the experiments. For the experimental basis,

three samples were used. Two fabrics were 100% cotton (the denim fabric and plain cotton

fabric) and the last fabric was a 65% polyester-cotton blend. For RMC testing, the first type of

fabric that was tested is a Department of Energy (DOE) standard test fabric which is a 50/50

blend of polyester and cotton. A 100% cotton Hanes tee shirt material and an 86/14 cotton

polyester terry cloth were also tested.

2.2 Experimental Basis

2.2.1 Time Basis

In order to determine the effect of centrifugation time on the residual moisture content

(RMC) of fabrics, the centrifugation speed was held constant while the centrifugation time was

varied (from 2 minutes to 45 minutes). As shown in Figure 2-3, the RMC decreases as the

centrifugation time increases. However, a plateau is observed around 15 minutes. At this RPM

(1250 RPM), a point is reached that all of the largest capillaries have released their water (around

the 15 minute mark). Water that is trapped in smaller capillaries cannot be forced from the fabric

at this centrifugation speed due to the higher force required to expel water in smaller capillaries.

Also, as the residence time in the centrifuge increases, the RMC decreases (Figure 2-3). This is

due to the fluid overcoming the viscous forces in the capillaries. Since the force is not being

increased, the only factors holding the water in the fabric are surface tension and viscous forces.

As the residence time is increased, the system has time to equilibrate and all unbound water can

be displaced from the fabric. The remaining water left in the fabric is due to the water of

hydration or water trapped inside micro-capillaries.









2.2.2 Centrifugation Speed Basis (Effect of Increasing Gravitational Force)

In order to determine the effect of centrifugation speed on the residual moisture content

(RMC) of fabrics, the centrifugation time was held constant while the centrifugation speed was

varied (from 1000 RPM to 8000 RPM). It is expected that by the LaPlace Equation for capillary

rise, as the gravitational force is increased, the capillary height should decrease leading to lower

residual moisture in the fabric. As shown in Figure 2-4, the variation of centrifugation speed held

constant at a centrifugation time of 10 minutes is shown to decrease in an exponential decay

which is expected due to the increase in gravitational force exerted onto the fabric samples. It

should be noticed that at about 4500 RPM, there is another plateau that is observed. It is also

shown that the denim and cotton samples (both 100% cotton) show approximately the same

RMC as a function of RPM while the polycotton sample is much lower. This is simply due to the

face that the polyester samples are much more hydrophobic than their cotton counterparts and

thus it is much easier for the hydrophobic fabric to shed water during the centrifugation process

(due to an increased contact angle on hydrophobic surfaces). Since the polyester is hydrophobic

the contact angle is increased and the capillary height is decreased resulting in a lower RMC.

2.3 Lowering of Surface Tension by Surfactant Systems

2.3.1 Simple Surfactant Systems

After we had established a basic understanding of how the system reacted to different

forces, we focused attention on determining the relationship between the RMC and surface

tension. To determine whether there was a relationship between the surface tension of a solution

and the RMC of the fabrics, the RMCs were measured for different solution concentrations of the

leading detergent. Figure 2-5 shows a smooth trend in the relationship between the RMC of

fabrics and the surface tension. Since the lowest surface tension achieved using the detergent

solutions was ca. 30 mN/m, Dow Corning Q2-5211 was used as a reference point (at 19.9










mN/m) (see Figure 2-6). If one extrapolates these curves to a surface tension of zero, one might

assume that the trapped water was simply the water of hydration caused by strong hydrogen

bonding between the fabric and water. However, many microcapillaries are present in the fabric

structure. Under force, these capillaries may close due to the crushing of the fabric under load,

trapping water inside the fabric structure.

Since a clear relationship existed between the surface tension of a solution and the RMC of

the fabrics, more experiments were performed using a variety of surfactant types to determine

whether a general correlation existed independent of surfactant type. As shown in Figure 2-6, a

relationship between the RMC of the fabrics and the solution surface tension existed for various

commercial surfactant systems; however, a few discrepancies were present. The range of

surfactant types used may account for such disturbances in the trend. Several different types of

surfactants (ionic, nonionic, and siloxanes as shown in Table 2-1) were used in this experiment,

and each type may have had some sort of interaction with the fabric surface, causing more or less

water to be displaced during centrifugation.

2.3.2 Mixed Surfactant Systems: SDS + C12TAB

Due to the ready availability of sodium dodecyl sulfate (SDS) and dodecyl trimethyl

ammonium bromide (C12TAB) and their opposite charges (anionic and cationic), the surface

tensions and residual moisture contents for various ratios of SDS to C12TAB were investigated at

a total concentration of 500 parts per million (0.05 wt%, or the typical surfactant concentration

during the final rinse cycle of the washing machine). At the 3:1 weight ratio of SDS to C12TAB

(which is approximately the 3:1 molecular ratio due to similar molecular weights), the lowest

RMC of ~50% was achieved at a surface tension of 20.5 mN/m as shown in Figure 2-7

(comparable to results from Dow Q2-5211 Superwetter with a RMC of 50.6%).





Figure 2-1. Forces involved in capillary rise where FSFT is the force due to surface tension


(2Kiry cos B) and Fd is the force due to gravity (mg or Hair2pg)


'






I L
: i


--Drainage Holes








Fabric Sample





-Water Collection


Figure 2-2. Experimental apparatus used to determine the RMC of various fabrics. There are
holes in the brass insert which holds the fabric samples to allow water to drain from
the sample tube during centrifugation.


FSFT





Fd


































10 20 30 40

time, minutes


--~-0--- Cotton
SDenim


Figure 2-3. RMC as a function of centrifugation time held constant at 1250 RPM. A plateau is
observed at approximately 15 minutes.


--O-- Cotton
65% Polyester
-*- -* Denlm


1000 2000 3000 4000 5000 6000 7000 8000
RPM


Figure 2-4. RMC as a function of centrifugation speed held constant at a centrifugation time of
10 minutes. A plateau is observed at approximately 4500 RPM.


































UOI I I I I I I I
10 20 30 40 50 60 70 80

Equilibrium Surface Tension, mN/m


Figure 2-5. Relationship between RMC and surface tension for the detergent system for the
Hanes and DOE fabrics at 1000 RPM (~92 times the force of gravity) centrifuged for
10 minutes.


ZO ,


60


O 50


40


30


-0 Hanes Cotton Fabric
+ DOE 50:50 Fabric


SHanes
- O- DOE
STerries


70


60


O 50
-


40
3-


II P
8$----Q--


Equilibrium Surface Tension, mN/m


Figure 2-6. Relationship between RMC and surface tension for commercial surfactant systems
for Hanes, DOE and Terry Cloth fabrics at 1000 RPM (~92 times the force of gravity)
for 10 minutes.














































Water 72.5 74
Fabric Softener (0.05%) 47.2 65.68
Leading Detergent (0.15%) 30.5 58.53
DelONIC 100O-VLF (0.1%) 27.6 57.43
DelONIC LF60-MOD (0.1%) 26.1 55.06
SDS:C12TAB (3:1 molecular
21.2 53.84
ratio at 0.1%)
Dow Q2-5211 19.9 50.3


S70


/ C6 5


60 O


55


50


45
1:3 1:4 1:5 1:8 C12TAB


50

z \.
i40 .









10
SDS 8:1


13 1Ratio
SSDS C12TAB
5:1 4:1 3:1 2:1 1:2


Weight Ratio of SDS to C12TAB


Figure 2-7. Residual moisture content and surface tension of various weight ratios of SDS to
C12TAB centrifuged at 90g for 10 minutes at a total surfactant concentration of 500
ppm.

Table 2-1. Surface tensions and corresponding RMC values for Hanes fabric for various
commercially available surfactants.


Equil. Surface
Tension (mN/m)


RMC, %









CHAPTER 3
THE EFFECT OF SURFACTANT ADSORPTION ON THE RESIDUAL MOISTURE
COTENT OF FABRICS

3.1 Peak in SDS RMC Curve as a Function of Increasing SDS Concentration

It was shown that the RMC of fabrics depends on several different variables such as

centrifugation time, centrifugation speed, and surface tension of solution.68 However, we have

observed that the RMC of fabrics does not completely correlate with the LaPlace equation as

expected. Before the critical micelle concentration (CMC) of surfactant solution we investigated,

there is a sharp peak in the RMC of fabrics. It is proposed that this increase in RMC is due to the

sudden adsorption of surfactant onto the fabric surface. Since it has been shown that cotton has a

negative zeta potential one might think that an anionic surfactant would have minimal adsorption

on a negatively charged surface.19-2 However, there have been several studies showing that

ability of sodium dodecyl sulfate (SDS) and other anionic surfactants to adsorb onto negatively

charged surfaces such as coal fines, cotton and cellulose.19, 22-25 Also, it has been shown by

Somasundaran et al. that adsorption isotherms can show up to four adsorption regions,26 One Of

them being a sudden increase of adsorption due to cooperative adsorption of surfactant

molecules, which may explain the peak found in the RMC curves observed in this study. If

surfactant molecules suddenly adsorb cooperatively on the solid surface at a critical

concentration, then it must cause a concomitant decrease in monomer concentration in the bulk

solution. Thus, a simple method to determine the monomer concentration below CMC is to

measure the surface tension of the residual solution. For a given surfactant below its CMC, the

surface tension is a measure of the free monomer concentration of surfactant in solution.

However, if the change is surfactant monomer concentration is not very large then the

equilibrium surface tension may not change significantly. However, the dynamic surface tension

may reflect this change more clearly. If there is a sudden increase in adsorption on the fabric









surface then there would be less free monomer available to adsorb on the new air-liquid interface

of bubbles created during the dynamic surface tension measurement. This would thus lead to an

increased dynamic surface tension (which should be an amplified measurement of the

equilibrium surface tension) that should correspond to the increase in RMC in the same

surfactant concentration range.

To verify if adsorption is occurring, we have measured the free surfactant monomer

concentration by a two phase dye transfer method. This method is commonly used in the

determination of anionic surfactants in wastewater. The method that we used was a separation of

methylene blue active substances (MBAS) adapted from several different methods.69, 70

3.1.1 Materials

The sodium dodecyl sulfate used in these experiments was obtained from the Fisher

Scientific Company. Experiments were also performed using purified SDS by recrystalization

three times in a 50:50 mixture of acetone and ethanol.

Several different types of fabric were used in the experiments for residual moisture testing.

The fabrics used were as follows: a Department of Energy (DOE) standard test fabric (a 50/50

blend of polyester and cotton), a 100% cotton Hanes T-shirt fabric and an 86/14 cotton/polyester

terry cloth.

3.1.2 Residual Moisture Content (RMC) Measurements.

For measuring the residual moisture, each fabric sample was soaked for ten minutes in

surfactant solution and then placed in a DuPont Instruments Sorvall RC-5B centrifuge at 1000

RPM (which corresponds to the force of a household washing machine of ~90g) for ten minutes.

The experimental apparatus used to hold the fabrics is shown in Figure 3-1. Our setup uses a

centrifuge tube with a copper insert that has a closed end with the other end flared so that it will

not fall inside the outer tube. The insert also has small holes drilled through it to allow water to









drain through the insert into the collection tube (much like how a modern washing machine is

designed) .

After the fabric was soaked and centrifuged, the weight was then measured to determine

the residual moisture content (RMC) as shown in Equation 3-1.

RiW%=10'Weight~~~ -Weight ~ 31
Weightdr

3.1.3 Surface Tension Measurements

The equilibrium surface tension measurements were made using the Wilhelmy Plate

method. The output from a gram-force sensor holding a platinum plate is sent to a transducer and

then output to a voltage readout. The system was calibrated using two known solutions (water

and acetone at 72.5 and 23 mN/m respectively). The platinum plate was heated using a flame

between each reading to remove surface contamination.

Dynamic surface tension was measured using the maximum bubble pressure technique.

The pressure required to form a new bubble in solution is measured by a pressure transducer and

the reading is transmitted to an oscilloscope. For these experiments, fabric was soaked in

surfactant solutions for 45 minutes and the dynamic surface tension of the residual solution (in

the presence of the fabric) was measured. All dynamic surface tension measurements were taken

using an 18 gauge needle tip with a gas flow rate of 7.5 cm3/min (Which corresponds to 6-15

bubbles per second or approximately 66 to 166 milliseconds per bubble residence time at the

needle tip). We chose this flow rate because at higher flow rates, the nitrogen gas forms a

continuous jet in the surfactant solution at the needle tip. At lower flow rates, the results are

similar to equilibrium surface tension results.









3.1.4 Adsorption Measurements

The actual free concentration of SDS was measured using the MBAS (methylene blue

active substance) method. Since this method is accurate in the range of 0-25 CIM concentrations

of SDS, each sample of SDS that had the cotton fabric soaked in it was diluted by 500 times (i.e.

10 CIL in 5 mL of water) and once the concentration was determined we were able to scale back

to the original sample size. Using known concentrations of SDS (between 0-8 mM), a calibration

curve was measured by dilution and then the revised MBAS method as outlined by Chitikela.70

3.2 Dynamics of Residual SDS Solution and the Effect on RMC

3.2.1 Surface Tensions of Residual Solutions (Dynamic and Equilibrium) and its
Correlation to RMC of Fabrics

In our previous work, the residual moisture content has been shown to be a function of

surface tension of solution68. However, as shown in Figure 3-2, the residual moisture does not

completely correlate to the equilibrium surface tension of pure SDS solutions in the range of 5-8

mM. A small dip in the surface tension at~-6 mM SDS concentration suggests that the sample

had a small impurity (presumably dodecyl alcohol). Recent work in our laboratory using purified

SDS samples has shown the same RMC peak using purified SDS solutions (Figure 3-3).

However, the peak begins to rise at ~5 mM concentrations of SDS with the purified SDS

compared to 5.5 mM with the unpurified SDS. There is also no minimum in the purified SDS

system compared to the unpurified SDS system which we is proposed to arise from the presence

of dodecyl alcohol.

It was shown that the RMC of fabrics did not completely correlate with the equilibrium

surface tension of SDS as observed in Figure 3-2 and Figure 3-3. Since it is believed that SDS is

adsorbing onto the fabric surface, the equilibrium surface tension of the residual solution should

show an increase in the range where SDS is adsorbing onto the fabric. The equilibrium and









dynamic surface tensions of residual SDS solution were measured after allowing the fabrics to

equilibrate in the SDS solutions for 45 minutes. The fabrics were soaked in a 20: 1 ratio of the

weight of fabric to the volume of SDS solution (approximately the same ration of the amount of

water to the amount of fabric for a normal load in a household washing machine). For each

dynamic surface tension measurement, the nitrogen flow rate was held constant at 7.5 cm3/min

(approximately 6-15 bubbles per second). It is shown in Figure 3-4 that a small increase was

found in the equilibrium surface tension in the concentration range of 5-8 mM. Since there is a

small decrease in free monomer in solution due to adsorption onto the fabric, the equilibrium

surface tension shows a small increase. It was shown by the dynamic surface tension of the

residual SDS solution (Figure 3-4), that the dynamic surface tension amplifies the small changes

seen in the equilibrium surface tension. Since the lowering of surface tension is due to the

diffusion of surfactant molecules to the air-liquid interface from the bulk solution (i.e. the

lowering of surface tension is a time-dependant process), it is expected that the dynamic surface

tension amplifies the changes seen in equilibrium surface tension. As shown in Figure 3-5, the

increase in the equilibrium surface tension for the residual SDS solution corresponds with the

increase of the RMC of the Hanes fabric presumably due to the adsorption of SDS onto the

fabric surface in the range of 5.5 to 6.5 mM SDS concentration.

It has been shown that a peak exists in the RMC curve of Hanes fabric soaked in SDS

solutions around approximately 7 mM SDS concentration. This peak has also been observed in

the RMC of several other types of test fabrics of varying hydrophobicity as shown in Figure 3-6

(terry cloth and DOE fabrics with 14% polyester and 50% polyester respectively). As the fabric

becomes more hydrophobic, the absolute RMC magnitude decreases as well as the magnitude of

the RMC peaks as shown in Table 3-1. This decrease in the magnitude of the RMC peak may be









due to the mechanism of adsorption onto the fabric surface (i.e. mainly hydrophobic interactions

with more hydrophobic fabrics compared to hydrogen bonding with hydrophilic fabrics). The

lowering of the RMC may be attributed to the increase in the contact angle of liquid with the

fiber surface with more hydrophobic fabrics. During the manufacturing process of fabrics,

different chemicals and treatments are used. However, the fabrics that were used in these

experiments were thoroughly washed and dried until the surface tension of water after soaking

the fabric remained unchanged from pure water. Thus, adsorbed impurities on the fabric surface

cannot account for the observed results.

We have shown in Figure 3-4 that there is an increase in the dynamic surface tension of

the residual solution after the Hanes fabric was soaked. The dynamic surface tension was then

measured for the remaining fabrics (DOE and terry cloth). Each fabric was soaked in SDS

solutions allowed to equilibrate for 45 minutes. The dynamic surface tension of the residual

solution was then measured (Figure 3-7, Figure 3-8 and Figure 3-9 for the Hanes, DOE and terry

cloth fabric respectively). The flow rate was held constant at 7.5 cm3/min (6-15 bubbles per

second or approximately 66 to 166 milliseconds per bubble at the needle tip). It is shown in these

graphs that a correlation exists between the peaks found in the RMC and the dynamic surface

tension of residual solution. Since an increase in surface tension indicates low adsorption of

surfactant at the newly created air-liquid interface in the residual solution, the peaks found in the

dynamic surface tension measurements are believed to be indicative of decrease in surfactant

concentration due to adsorption onto the fabric surface.

An increase in dynamic surface tension is due to the reduced adsorption of surfactant at the

air-liquid interface of the new bubble surface created during the measurement. We believe that

the decrease in adsorption at the air-liquid interface is due to increased adsorption of SDS on the










fabric surface. If there is increased adsorption of SDS onto the fabric surface due to cooperative

adsorption, then it is assumed that there would also be a reduction in the free monomer

concentration (which has been shown by the increase of equilibrium and dynamic surface tension

of residual solution as shown in Figure 3-4).

3.2.2 Molecular Mechanism: Explanation of the Peak in the SDS/RMC Curve

It is shown in Figure 3-10 and Figure 3-11 the 4 regions associated with the increase in

residual moisture content and dynamic surface tension. Region A-B is the region of minimal

surface adsorption of SDS onto the fabric surface presumably due to a residual negative charge

on the fabric surface. The decrease in RMC in this region is due to the increase of free surfactant

monomer concentration with low adsorption on the fabric surface. At a concentration of 5.5 mM

of SDS, there is a minimum in the RMC and Region B-C begins. This region is due to the

sudden increase in adsorption of SDS onto the fabric surface due to a cooperative adsorption

phenomenon. Due to electrostatic repulsion between the fabric surface and the SDS monomers,

there is a barrier to adsorption. However, once several monomers adsorb onto the fabric surface,

it provides a cooperative effect promotes SDS adsorption. This sudden increase in adsorption of

the SDS onto the fabric surface reduces the free monomer concentration in the bulk solution thus

leading to a reduced amount of free monomer in solution. Hence, less monomer is available to

adsorb onto the new air-liquid interface created during the dynamic surface tension measurement

which leads to an increased dynamic surface tension. This increase in the dynamic surface

tension leads to an increase in the residual moisture. At approximately a concentration of 6.75

mM of SDS, there is a maximum in the RMC where Region B-C ends and Region C-D begins. It

is believed that at this point, complete saturation of the fabric surface by the adsorption of SDS

has occurred. Once maximum adsorption has been reached, any additional SDS added into the

system will result in an increase in the free monomer concentration. The increased free monomer









concentration provides the new air-liquid interface with higher SDS adsorption thus reducing the

dynamic surface tension. At approximately 7.5-8.0 mM concentration of SDS, Region C-D ends

and Region D-E begins. This region occurs due to the bulk solution reaching the critical micelle

concentration (CMC). At this point, the free monomer concentration remains constant. Since the

free monomer concentration is now constant, the dynamic surface tension and residual moisture

should remain constant as well.

3.3 Adsorption of SDS onto Cotton Surfaces

It was shown that the RMC of fabrics depends on several different variables such as

centrifugation time, centrifugation speed, and surface tension of solution.68 However, we have

observed that the RMC of fabrics does not completely correlate with the reduction of surface

tension as predicted by the LaPlace equation for capillary rise. We have shown that before the

CMC of surfactant solution, there is a sharp peak in the RMC of fabrics as a function of

increasing SDS concentration. It was proposed that this increase in RMC is due to the sudden

adsorption of surfactant onto the fabric surface based on secondary data such as the dynamic

surface tension of the residual solution after the fabric was soaked.n Since it has been shown that

cotton has a negative zeta potential one might think that an anionic surfactant would have

minimal adsorption on a negatively charged surface.19-2 However, there have been several

papers showing that ability of sodium dodecyl sulfate (SDS) and other anionic surfactants to

adsorb onto negatively charged surfaces such as coal fines, cotton and cellulose.19, 22-25 Also, it

has been shown by Somasundaran et al. that adsorption isotherms can show up to four adsorption

regions,26 One of them being a sudden increase of adsorption due to cooperative adsorption of

surfactant molecules, which may explain the peak found in the RMC curves observed in this

study. We have measured the dynamic surface tension of the residual solution after soaking the

fabric and we have shown that the increase in RMC does correlate very well with an increase in









the dynamic surface tension.n Our proposed mechanism indicated that there was a cooperative

adsorption of SDS onto the fabric surface thus leading to a decrease in the free monomer

concentration. This decrease in the free SDS monomer concentration thus leads to an increase in

the dynamic surface tension. If there are less surfactant monomers in the system, there will be

less monomer available to adsorb onto a newly created air-water interface (as in the

measurement of dynamic surface tension by the maximum bubble pressure technique) which will

lead to an increase in the dynamic surface tension.

In these experiments, we have measured the free surfactant monomer concentration by a

two phase dye transfer method. This method is commonly used in the determination of anionic

surfactants in wastewater. The method that we used was a separation of methylene blue active

substances (MBAS) adapted from several different methods.69, 70

We have shown by equilibrium and dynamic surface tension measurements of the residual

solution that the fabric samples have soaked in that there is a correlation in the peak found in the

RMC as a function of increasing concentration. However, this has been indirect proof suggesting

that there is a decrease in the free monomer concentration in the range of the concentrations

where the peak exists. Using a two phase dye transfer method (MBAS) we can measure the free

monomer concentration of the residual solution after the fabric has soaked in the solution. This

measurement will provide a detailed measurement of the actual concentration of SDS left in

solution after the adsorption of SDS has occurred onto the fabric surface. If we can show a

correlation with the onset of adsorption with the beginning of the RMC peak as well as show a

correlation to the fabric surface becoming saturated at the same point at which the RMC peak

reaches a maximum, we will have direct proof that surfactant adsorption is the cause of the RMC

peak of cotton fabric as a function on increasing SDS concentration.









As shown in Figure 3-12, we have provided a calibration to determine if this method can

be applied to diluted SDS solutions in the concentration range that we are interested in. For the

concentration range we are interested in, this method appears to be a good selection to determine

the free monomer concentration of SDS after the residual solution has equilibrated with the

fabric (after the SDS has been allowed to adsorb onto the fabric).

Now that we have shown the IVBAS method can be applied in determining the free

concentration of SDS in the bulk solution, we then measured the free concentration of SDS in the

residual solution after the SDS adsorption was allowed to equilibrate with the fabric surface. As

shown in Figure 3-13, the adsorption of SDS onto the cotton fabric correlated with the peak

found in the RMC of cotton as a function on initial SDS concentration. The adsorption begins

slightly before the peak starts to increase which we believe is due to impurities in the SDS not

adsorbing onto the fabric. These impurities (most likely dodecanol) are available to lower the

dynamic surface tension in the bulk solution due to limited adsorption of the dodecanol onto the

fabric. Once the fabric has become fully saturated with SDS, the RIVC of the cotton fabric begins

to decrease. This further strengthens our hypothesis that the peak is due to adsorption of SDS

onto the fabric. Once the fabric is saturated, any further SDS added into solution stays in the bulk

and thus increases the bulk concentration of SDS. If the bulk concentration of SDS is increased

after the point of adsorption saturation on the cotton surface, the dynamic surface tension of the

residual solution then decreases. This maximum in the adsorption isotherm correlates with the

peak found in the RIVC curve. Once this maximum is reached, the RIVC starts to decrease due to

the increased amount of SDS in the bulk solution.

We have shown that not only the equilibrium surface tension of solution has an effect on

the RIVC of fabrics. Therefore, finding a solution with the lowest equilibrium surface tension









does not necessarily mean that we have identified a system that will result in the lowest RMC.

We have shown that the reduction of RMC has many different aspects. Not only does

equilibrium surface tension affects the RMC of fabrics but the dynamic surface tension also

plays a very important role on the removal of water from fabrics.

3.4 RMC Peak in Various Surfactant Systems

Due to the fact that we investigated a model system to determine how increasing the

concentration of a common surfactant (SDS in this case) had an effect on the RMC of fabrics, the

need to determine if this phenomenon existed in other surfactant systems (such as other anionic

systems, cationic systems or actual detergent systems). The first systems investigated were fatty

acid surfactants with varying chain lengths (n = 10-14). Shown in Figure 3-14, the same peak

observed in the SDS experiments was observed for the Clo fatty acid system (similar to other

fatty acids).

Of more interest to commercial aspects, the same experiments were repeated for a leading

detergent and a leading fabric softener to determine if the same peaks existed in the RMC curves

as a function of increasing concentration. As shown in Figure 3-15 for the leading fabric

softener, there exists a sharp peak at approximately 1 100 ppm of total product concentration.

Since this is at about twice the concentration of a normal dosage in the washing machine, it

normally would not be of concern. However, many consumers overdose their machines with

fabric softener and this could possibly lead to longer drying times due to the higher RMC values.

Likewise, for the leading detergent (Figure 3-16), there is a peak at approximately 400 ppm

(about 1/3 of the normal dosage for a washing machine). However, it is unknown if there exists a

synergism of adsorption if there is surfactant carryover from the detergent into the final rinse

cycle when the liquid fabric softener is added. In the case with a mixed surfactant system, the

possibility exists to shift the peak found in the fabric softener RMC curve at a lower value.









3.5 Manipulation of RMC Peak: Fabric Pre-Treatment and its Affects on Adsorption of
SDS onto Cotton

Since our goal is to lower the RMC of the fabric at the end of the laundry spin cycle, the

manipulation of the peak found in the RMC values in the SDS system should be investigated.

Specifically, one would like to either shift the peak to a range that would be out of the normal

values for a detergent system, reduce the magnitude of the peak found in the RMC, or Eind a way

to eliminate the peak completely. Since the peak found in the RMC curve is due to a sudden

adsorption phenomena of the SDS onto the fabric surface, if the fabric surface could be modified

in a way that could alter the adsorption of SDS (such as increasing the anionic charge density or

reducing the amount of available sites for the SDS to adsorb onto), the RMC peak found in the

SDS system could be shifted, reduced or even eliminated.

In order to manipulate the adsorption of SDS onto the fabric, various surfactants and

insoluble long chain compounds were coated on the fabric. The fabric was soaked in various

solutions of surfactants and polymers which were solubilized in ethanol. The fabric was allowed

to equilibrate with the solution for 30 minutes. The fabric was then removed from the solution

and allowed to dry. If we can manipulate how much SDS is adsorbed onto the fabric, then the

dynamic surface tension of the residual SDS solution can be changed and thus the RMC can be

changed. Since we changed from a Hanes tee shirt fabric to a 100% cotton terry cloth fabric, the

baseline for the SDS system was measured and is shown in Figure 3-17. As shown earlier, the

peak in the RMC curve is still present indicating that the SDS is adsorbing on the terry fabric as

well (which is as expected due to the terry fabric being 100% cotton).

The first experiments performed measured the RMC of fabric coated with polymers. The

fabric was soaked in carboxy methyl cellulose (CMC as shown in Figure 3-18), poly acrylamide

(PAA as shown in Figure 3-19) and polyvinylpyrrolidone (PVP as shown in Figure 3-20). It









should be pointed out that in each case, the magnitude of the RMC peak is much less compared

to the untreated fabric (with the exception of PVP). The adsorption of CMC and PAA both

reduced the magnitude of the RMC peak indicating that the amount of SDS adsorbed onto the

fabric has been reduced and thus the dynamic surface tension decreases as the concentration of

SDS increases. This may be due to several reasons such as the elimination of available

adsorption sites due to the adsorption of the polymer onto the fabric surface, or, since both CMC

and PAA are both negatively charged polymers, the adsorption of these polymers on the fabric

possibly increased the total anionic charge density on the fabric and thus resulted in a higher

electrostatic repulsion between the fabric and the SDS.

The next set of experiments involved the use of long chain alcohols, fatty acids and

insoluble surfactants (Cls fatty acid, Cls alcohol and dioctyldecyldimethylammonium bromide or

DODAB). With the use of the octadecanoic acid, the RMC of the terry cloth fabric is a smooth

curve and the peak has been almost completely removed (Figure 3-21) which is thought to be

due to the tight packing of the octadecanoic acid adsorbing on the fabric which increases the

charge density (to a higher net negative charge) and thus repulses the anionic SDS. When the

experiments were repeated with the octadecanol, the RMC peak has been reduced but is still

present (Figure 3-22) which is thought to be due to the reduction of available adsorption sites and

not electrostatic repulsion due to the lack of a charge on the octadecanol. The last set of

experiments involved the fabric being soaked in DODAB. However, the RMC was not reduced

near as much with the use of DODAB as compared to the octadecanoic acid (Figure 3-23)

possibly due to a bilayer formation of DODAB on the fabric providing adsorption sites for the

SDS to adsorb on).










It is believed that the adsorption of these various compounds interferes with the adsorption

of SDS onto the fabric either due to the reduction of available adsorption sites or by an increased

charge density increasing the electrostatic repulsion. In the case of octadecanoic acid,

electrostatic repulsion exists which would repel the SDS from adsorbing onto the fabric surface

and thus the RMC would then follow the same trend as the equilibrium surface tension. That

being said, if one can manipulate the amount of surfactant that is being adsorbed onto the fabric,

the RMC can thus be greatly reduced compared to the fabric systems where SDS was allowed to

adsorb onto the fabric.



we wDrainage Holes

***





Fat~rnc Sample





-water collection



Figure 3-1. Experimental apparatus used to determine the residual moisture of fabrics.














95

90

85

80

~P75

70

65

60

55

50


70

65

60 -E

55

50

45

40 1

35 0

30

25


0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Initial SDS Conc. mM


Figure 3-2. RMC of Hanes 100% cotton fabric as a function of SDS concentration plotted with
equilibrium surface tension of pure SDS solutions.


35



234567891


Initial SDS Conc. mM

Figure 3-3. RMC of Hanes 100% cotton fabric as a function of SDS concentration plotted with
equilibrium surface tension of ethanol:acetone purified SDS solutions.















42


40-


38 -








~30

0 1 2 4


+ Equilibrium Surface Tension
+ Dynarric Surface Tension


- 72

E

- 68

- 66~




-62 o

-60


9 10 11 12


Initial SDS Conc, mM


Figure 3-4. Equilibrium and dynamic surface tension of residual SDS solution after exposure to
Hanes fabric.


Of 1-eSIOUaI 601Ulton E

E
-38














30
5678910


O
r
~71)


01234


Initial SDS Cone. mM

Figure 3-5. Comparison of the RMC of Hanes fabric and the equilibrium surface tension of
residual solution after soaking the fabric.






























































... I


I ~I
I ICM
0 2 4 6 8 10


Initial SDS Conc. mMV

Figure 3-6. RMC of Hanes cotton fabric, terry cloth fabric and DOE test fabric as a function of
SDS concentration showing the maximum and minimum in the RMC peak.


- 75

E
Z
-70 E

o

-65'



- 60 't

cg


-o


--0-- RMCHanesFabric
-6 Dynaric Surface Tension


O 70

65

60



55


0 2 4 6 8 10 12

Initial SDS Conc. mM


Figure 3-7. RMC and DST of the residual solution from the Hanes 100% cotton fabric soaked in
SDS solutions.















E
652
E
r
60 o
v,
r
56 a,


50 co
't
3
V)
45
o
E
co


42
















28


0 2 4 6 8 10 12


Initid SDS Con. mrM

Figure 3-8. RMC and DST of the residual
soaked in SDS solutions.




U


75 -\ --0-- RMC Terry

70-1 1







so |
~6I

5I CMC


solution from DOE 50:50 cotton:polyester fabric





75

surface Tension
Cloth Fabric 70 E
Z
E









45


0 2 4 6 8 10 12

Initial SDS Conc. mM

Figure 3-9. RMC and DST of residual solution from the Terry Cloth 86: 14 cotton:polyester
fabric soaked in SDS solutions.


















p 75-
r B D E
t' 70 -

65

60-

55
0 2 4 6 8 10 12
SDS Conc., mMN

Figure 3-10. Indication of the regions associated with the peak in the RMC of Hanes cotton
fabric.


Fabric IFabric
Figure 3-11. A-B) High adsorption of surfactant monomer at the air-liquid interface and low
adsorption on the fabric-liquid interface resulting in a low dynamic surface tension B-
C) Sudden adsorption due to cooperative adsorption on the fabric surface resulting in
a decreased monomer concentration in the bulk solution and decreased adsorption at
the air-liquid interface C-D) Maximum adsorption is reached at the fabric-liquid
interface and increased adsorption is occurring at the air-liquid interface D-E) The
CMC is reached and the monomer concentration is stable resulting in a constant
dynamic surface tension and RMC.















1 Callbration Curve


.p.~~ .. 7 Adsorption


y = 0.0852x -0.0 924 i j

R2 = 0.9947 Passed functional
range of this method
due to saturation of the
methylene blue complexes
i.e. conc of SDS was higher
than methylene blue


10 20 30

gLMolar Concentration of SDS


40 50


Figure 3-12. Calibration curve for MBAS method in the range of SDS concentrations tested in
these experiments.


L.
-1
as 4




-
0


70 O




60

55

50


0 1 23 45 67

Initial SDS Concentration, mM


8 9 10


Figure 3-13. Adsorption of SDS adsorbing onto Hanes cotton fabric. Adsorption was measured
from the residual SDS solution after the cotton fabric was soaked for 30 minutes.

















70 i


65 -


O 60 1


55 i


50 -


45 -
8


CMC

6 88 90 92 94 96 98


Total Conc. mM


Figure 3-14. RMC of Hanes fabric soaked in Clo fatty acid (centrifuged for 10 minutes at 1000
RPM).


O 70

65

60

55

50


0 200 400 600 800 1000 1200 1400

Conc. ppm


Figure 3-15. RMC of Hanes fabric soaked in solutions of a leading fabric softener (centrifuged
for 10 minutes at 1000 RPM).


















85 -( I


80-


O 75-



70-


65-


so CMC
0 200 400 600 800 1000 1200

Conc. ppm



Figure 3-16. RMC of Hanes fabric soaked in solutions of a leading detergent (centrifuged for 10
minutes at 1000 RPM).


75

70

65

60 cp

55 "

50 m

45 co)

40


Terry Cloth RMC
--0-- Equil Surface Tension


0 1 23 45 67


8 9 10


Initial SDS Conc., mM



Figure 3-17. RMC of Terry cloth fabric soaked in solutions of SDS.










































III III III II


75
Terry Cloth Soaked In CMC
---Equil Surface Tension ~70

65

60 -P



55 m

50


0 1 23 45 67

Initial SDS Conc., mM


8 9 10


Figure 3-18. RMC of terry cloth fabric pre-treated with CMC.


STerry Cloth RMC PAA Modified
- -0 Equil Surface Tension


-60

-55

-50

-45

-40

- -O -35
- 30


0 1 23 45 67

Initial SDS Conc., mM


8 9 10


Figure 3-19. RMC of terry cloth fabric pre-treated with PAA.
















-80
Terry Cloth with PVP Treatment
- 0- Equilbirum Surface Tension
-70


-60 a,


mE
-50 2z


01234567891011

Initial SDS Concentration (mM)



Figure 3-20. RMC of terry cloth fabric pre-treated with PVP.


STerry Cloth Soaked In C18Acid RMC
- 0 Equil Surface Tension


0 1 23 45 6 7 890

Initial SDS Conc., mM



Figure 3-21. RMC of Terry cloth fabric pre-treated with Cls Fatty Acid.





--~- Eqll~rfac~enlon75
STerry Cloth RMC C180H Modified 7

S65


60 Cp

55

50 mZ

45 1

40

35

30

25


0 1 23 45 67

Initial SDS Conc., mM


8 9 10


Figure 3-22. RMC of Terry cloth fabric pre-treated with Cls Alcohol.


q~ Terry Cloth RMC -DODAB Modified
$- -0 -- Equil Surface Tension


95

90

85
S8-

S7-



70


0 1 23 45 67

Initial SDS Conc., mM


8 9 10


Figure 3 -23. RMC of terry cloth fabric pre-treated with DODAB.










Table 3-1. The magnitudes of the RMC peak for various fabrics for I) the absolute different in
the maximum and minimum of the RMC peak, II) the difference in the maximum and
minimum normalized with respect to the RMC maximum and III) the difference in
the maximum and minimum normalized with respect to the RMC minimum.
I II III

RM~Cm RM\~Cm 1WC RM~C
ElRMC lRMC max mi max mmn
max mm RM~C RM~C

Hanes 17.45% 22.72% 29.40%
Terries 10.23% 16.74% 20.01%
DOE 4.18% 12.13% 13.89%









CHAPTER 4
REDISCOVERING MONOLAYER PENETRATION: OBTAINING ULTRA-LOW AIR-
LIQUID SURFACE TENSIONS

4.1 Monolayer Penetration

When an insoluble monolayer has a surfactant laden subphase below the monolayer, the

surfactant from the subphase can adsorb into the monolayer thus penetrating the monolayer. This

adsorption of surfactant penetrating the monolayer thus changes in the surface tension and

surface pressure by changing the effective area per molecule in the monolayer (tighter packing in

the monolayer results in a decrease in the surface tension). As discussed by Datwani et al., early

models for equilibrium monolayer penetration related the surface pressure in the mixed

monolayer to the adsorbed amount of the soluble surfactant from the subphase which is a

function of the bulk concentration of the surfactant in the subphase.72

Work has been done by Schulman's research group which revolutionized the study of

monolayers (static monolayers and monolayer penetration)73-75. Matalon et al. developed a

method in 1949 to measure surface pressures resulting from the interaction of an insoluble

monolayer with surfactants adsorbing into the monolayer from the underlying bulk solution.73

Schulman's group also researched many other aspects of monolayers and penetration of

monolayers such as follows: penetration of monolayers with surfactants,74-76 interactions of

monolayers with metal ions,77-79 COmplex formation and steric effects in monolayers,so, sl and the

ionic structure and the effects of unsaturation on monolayers.82-84

There has been very many papers published on the various aspects of monolayer

penetration such as the effect of charged surfactants penetrating monolayers,72 mathematical

evaluations of monolayer penetration,"' the thermodynamics and kinetics monolayer

penetration,86-9 and many other various papers on monolayer penetration.72, 82, 92-98 Due to the

wide range of papers discussing monolayer penetration, the mathematics governing penetration









will not be discussed. However, based on the similarity of dioctadecyldimethylammonium

bromide (DODAB) to the active surfactant in many commercially available fabric softeners,99-101

we chose to use DODAB as the insoluble surfactant to be used in the insoluble monolayer.

Several different surfactants, sodium dodecyl sulfate (SDS) and sodium tetradecyl sulfate

(C14SO4), were chosen to use as the penetrating surfactant based on their similarities to the active

surfactants in detergents.

We have shown that with the use of monolayer penetration that we can now lower the

equilibrium surface tension to values lower than previously achieved with silicone super wetters

(lower than 19 mN/m). We have also shown that the ability to lower the surface to low values

and maintain that low value depends on the type of insoluble monolayer we spread. When an

ionic monolayer is used and penetrated with an oppositely charged surfactant from the subphase,

we have shown that this type of system is more effective in lowering surface tension compared to

the use of penetrating a nonionic monolayer with an ionic surfactant from the subphase. We

believe that this is due to electrostatic attraction between the charged monolayer and the

oppositely charged surfactant from the subphase. Another aspect we investigated in monolayer

penetration is the use of a mixed monolayer (a mixture of the insoluble surfactant with the

penetrating surfactant in the subphase). We have found that the mixed monolayer penetrated with

the soluble component of the mixed monolayer provided excellent results in the ability to lower

surface tension (to values less than 10 mN/m).

From our previous work, we have shown that there is a correlation between equilibrium

surface tension and the residual moisture content (RMC) of fabrics.68, 71 We have shown that the

lower the surface tension, the more water can be shed from fabrics during the centrifugation

process. Based on this work, we are now trying to reach air-liquid surface tension values of









lower than 10 mN/m using monolayer penetration. Using monolayer penetration, we have been

able to measure air-liquid surface tension values as low as ~8 mN/m and we have determined a

method to use monolayer penetration in the reduction of RMC of fabrics.

4.1.1 Monolayer Penetration Studies

As discussed by Welzel et al., there are two methods to study monolayer penetration: 1)

the penetrant is inj ected beneath the already spread monolayer and 2) the monolayer film is

spread onto the penetrating solution.93 In these studies, we have used both methods to measure

the effects of monolayer penetration. The first method was used in the small scale surface tension

measurements and the second method was used in the full scale washing machine experiments.

Penetration studies were done by solubilizing the insoluble monolayer in a mixture of 1:1:3

volume mixture of methanol, chloroform and hexane in a total concentration of 0.5 wt%. The

solubilized solution was then 5 CIL of this monolayer solution was placed onto the surface of a

Petri dish filled with 5 mL of water using a microsyringe (see Figure 4-). The penetrating

surfactant solutions in various concentrations were then inj ected beneath the monolayer in

different volumetric amounts. Meanwhile, the surface tension was monitored using the Wilhelmy

plate method. However, unlike previous surface tension methods, the output from the voltage

sensor from the Wilhelmy plate was input to a computer using a WinDAQ data acquisition card.

Using this method, the surface tension can be measured as a function of time while the

monolayer is being penetrated with the surfactant from the subphase.

For the washer scale tests, ethanol was used to solubilize the monolayer instead of the

methanol, hexane and chloroform solvent due to ethanol being more environmentally friendly

and the ethanol is not as corrosive to the washing machine components compared to the other

solvents. Also, the penetrating surfactant was already present in the fabric before the monolayer









was spread on the fabric due to the difficulty in inj ecting the subphase surfactant beneath the

monolayer in full scale testing (see Figure 4-13).

The equilibrium surface tension measurements were made using the Wilhelmy Plate

method. The output from a gram-force sensor holding a platinum plate is sent to a transducer and

then output to a voltage readout. This voltage readout was captured using a computer with a

WinDAQ data acquisition card. The system was calibrated using two known solutions (water and

acetone at 72.5 and 23 mN/m respectively). The platinum plate was heated using a flame

between each reading to remove surface contamination.

4.1.2 Reduction of Surface Tension: Monolayer Penetration Results

We first experimented with spreading a monolayer of C16TAB dissolved in the universal

solvent (1:1:3 volume mixture of methanol, chloroform and hexane) on the Petri dish filled with

water. Once the solvent was allowed to evaporate, we began to monitor the surface tension. We

then inj ected solutions of 4 mM C14SO4 beneath the monolayer and continued to measure the

surface tension. As shown in Figure 4-2, the surface tension reaches a minimum immediately

after the penetrating surfactant solution was inj ected. Depending on the amount of penetrant

inj ected into the Petri dish, the surface tension stays at a low value and then jumps up to higher

values (which are a lower value than the surface tension of the pure monolayer) due to

solubilization of the monolayer. We also repeated the same experiment using stearic acid as the

insoluble monolayer penetrated with a subphase surfactant of an opposite charge compared to the

stearic acid (C14TAB was used at a cationic penetrant). As shown in Figure 4-3, we show the

same trend in surface tension as shown in the other monolayer penetration experiments.

Immediately after the penetrating surfactant is inj ected into the system, the equilibrium surface

tension drops to a minimum value and then slowly increases to another equilibrium value. We

then repeated the same experiment with DDAB (didodecyldimethylammonium bromide) as the









monolayer with C14SO4 aS the penetrating surfactant. As shown in Figure 4-4, the equilibrium

surface tension follows the same trend as the C16TAB monolayer system. The surface tension

goes to a minimum value and then increases. We have called this a transient monolayer and we

believe that this phenomenon is due to the solubilization of the monolayer after the penetrating

surfactant has penetrated the monolayer. Both C16TAB and DDAB have slight solubility in water

and with the addition of C14SO4, we believe that the penetrating surfactant helps solubilize small

amounts of the monolayer into the solution and thus increasing the equilibrium surface tension.

Then, as shown in Figure 4-5, the monolayer which has been penetrated with the C14SO4 Starts to

solubilize and dissolves into the solution thus leaving the interface less tightly packed and

increasing the equilibrium surface tension. The low surface tension values are due to the super-

saturation of the air-liquid interface due to the Coulombic interaction between the cationic

monolayer and the anionic penetrating surfactant.

Using the monolayer penetration method in these preliminary experiments, we obtained

equilibrium surface tension values at the air-liquid interface of ~ 17 mN/m. Previously, values as

low as ~18-19 mN/m at the air-liquid interface have been obtained using conventional

surfactants .

After we ran the preliminary monolayer penetration studies, we then looked into using a

monolayer composed of longer chain surfactants and alcohols that are insoluble in water and

would theoretically not exhibit the transient monolayer phenomenon that we observed with

slightly soluble monolayers. We first looked at DODAB (the Cls version of the DDAB used in

previous experiments) penetrated with C14SO4. As shown in Figure 4-6, the equilibrium surface

tension of a DODAB monolayer penetrated with C14SO4 reaches a minimum value of










approximately 15 mN/m. This value is lower than values obtained at the air-liquid interface for

other methods (18-19 mN/m for fluoro-surfactants and siloxanes).

We then chose to use cholesterol and C200H as the insoluble monolayer and we continued

to use C14SO4 to penetrate the monolayer. As shown in Figure 4-7, the surface tension of a

C200H monolayer penetrated with C14SO4 is shown to decrease to a value of about 25 mN/m.

However, we were still attempting to achieve surface tensions of much lower values (less than 10

mN/m). We then used cholesterol as the insoluble monolayer penetrated with C14SO4. However,

the surface tension was only reduced to about 20 mN/m in this system (Figure 4-8).

Using a pure monolayer of DODAB penetrated with 1 mL of 4 mM C14SO4, a Surface

tension of 13.5 mN/m was achieved for an indefinite amount of time. It was thought that if a

mixed monolayer was formed that tighter packing would be present in the monolayer due to

electrostatic interactions between the headgroups. The system that was investigated was the

tetradecyl sodium sulfate (C14SO4) with dioctyldecyl dimethylammonium bromide (DODAB).

Ratios of 1:10, 1:5, 1:3, and 1:2 of the C14SO4:DODAB were investigated.

The first systems tested were the monolayer compositions that were higher in DODAB

concentration (the 1:10, 1:5, 1:3 and 1:2 ratios of C14SO4:DODAB). As shown in Figure 4-10,

the 1:10 ratio of C14SO4:DODAB monolayer with C14SO4 inj ected beneath the monolayer

resulted in a surface tension as low as 19 mN/m. Increasing the C14SO4 amOunt to a ratio of 1:5,

the surface tension dropped to approximately 8.5 mN/m with 1000 CIL of C14SO4 injected

beneath the monolayer (Figure 4-9). Since it has been well documented that tightest packing

occurs at a 1:3 ratio, the surface tension of a monolayer at this ratio was measured. However, the

minimal surface tension for the 1:3 system was not as low as the 1:5 ratio monolayer. It is

believed that this is due to molecular packing trying to achieve the 1:3 ratio. At a lower ratio of










C14SO4:DODAB (the 1:5 system), there is somewhat tight packing. This packing can be

optimized by the addition of more C14SO4 beneath the monolayer. The addition of the 4 mM

C14SO4 TOSults in what we believe is a 1:3 ratio of C14SO4:DODAB in the monolayer after this

addition of C14SO4 beneath the monolayer. However, when the monolayer is already at its

tightest packing at a 1:3 ratio of C14SO4:DODAB, there is not sufficient room for more C14SO4

to penetrate the monolayer resulting in a higher surface tension than the previous system. When

higher ratios (1:2) of C14SO4:DODAB were tested, there was no monolayer present after

spreading with universal solvent. Since the C14SO4 is soluble in water, once the monolayer is

spread it is being solubilized into solution due to the increased amount of C14SO4 TOSulting in a

higher surface tension (approximately the surface tension of water) that reduces with the addition

of C14SO4

4.2 Reduction of RMC via Monolayer Penetration

4.2.1 Experimental Procedure

Large scale tests were done in a Whirlpool washing machine. Using a strobe-scope, we

measured the RPM of the washing machine spin cycle to be 640 RPM or about 90 g' s which is

comparable to the force we tested in the small scale centrifuge. However, the centrifugation time

in the washing machine is 6 minutes compared to 10 minutes in the small scale testing. As shown

in Figure 4-13, the method we used to test the RMC in the washing machine is as follows: 1) the

fabric is soaked in the penetrating surfactant subphase solution, 2) the fabric is placed in the

washing machine and the spin cycle is started, 3) once the spin cycle reaches speed, the

penetrating solution is poured onto the fabric and the washing machine is allowed to complete

the spin cycle.









4.2.2 Small Scale Monolayer Penetration Results

Several experiments were performed in the lab scale to determine the effective of

monolayer penetration in the reduction of RMC. The first experiments were run to determine the

baseline of the system. In Table 4-1, the RMC from pure water to 4 mM C14SO4 Solutions

reduced from 82% to 60%. The next experiments were to determine how effective monolayer

penetration would be to lower the RMC. The first monolayer penetration experiment was

performed by soaking the fabric in 4 mM C14SO4 and then spraying the fabric once (with ~2 mL)

of the 1:5 C14SO4:DODAB monolayer solution solubilized in ethanol. The resulting RMC was

reduced to 60.7%. However, due to the large amount of bulk solution in the fabric, the RMC

wasn't reduced as much as expected based on the surface tension of that monolayer study. The

next experiment was to soak the fabric in the 4 mM C14SO4 and then centrifuge the sample for 5

minutes to remove most of the bulk water. The fabric was then taken out of the centrifuge and

then sprayed with 2 mL of the 1:5 monolayer solution solubilized in ethanol. The resulting RMC

was 55% which is comparable to the results from the silicone super wetter (50%) or about a 27%

reduction in the RMC from that of pure water.

4.2.3 Washer Scale Monolayer Penetration Results

After showing that monolayer penetration was promising in the reduction of RMC in

fabric, we then expanded the experiments into full washing machine scale. The fabric was first

soaked in SDS solutions (due to the expense of C14SO4) and the 1:5 SDS and DODAB

monolayer was solubilized in ethanol. The fabric was placed in the washing machine and the

spin cycle was started. Once the washing machine reached full speed in the spin cycle, 100 mL

of the monolayer penetration solution (at 0. 1% total concentration) was poured onto the fabric

during the centrifugation process. Once the spin cycle finished, the RMC was determined after

weighing the fabric.









In Figure 4-14, we compared the effectiveness of changing the penetrant to SDS as well as

changing the monolayer to SDS and DODAB from C14SO4. As shown in Figure 4-14, the RMC

was reduced to ~68% for both monolayer penetration systems. We then performed the same

experiments to determine if the RMC followed the surface tension trend which was observed in

the surface tension studies of the mixed monolayers. As shown in Figure 4-15, the RMC of the

terry fabrics closely followed the surface tension trend from the 1:10 ratio to the 1:3 ratio of SDS

to DODAB with the 1:5 ratio showing the minimum in RMC (and surface tension). This method

may prove very valuable in the reduction of RMC and energy in laundry systems.







SCo-Su rfacta nt



Surfactant Monolayer


Figure 4-1. Spreading and penetration of an insoluble monolayer.











1000 uL Time, sec
0 5 10 15 20 25


30 35 40 45


0 10 20 30 40 50 60 70 80


500 uL, 750 uL time, sec


Figure 4-2. Equilibrium surface tension of a C16TAB monolayer penetrated with 4 mM C14SO4



65
o 250 mLC14TAB
60-
S500 mLC14TAB
55 0 750 mLC14TAB

z 50-



4 40 -1 750ig5 I1L

35 250 pL

30-

25 500 pL

20
0 10 20 30 40 50 60 70 80

time, sec.


Figure 4-3. Equilibrium surface tension of a stearic acid monolayer penetrated with 4 mM
C14TAB .











36
34-
32-
E 30 a 250 pL
E 28-
S26 -c
r ~500 pL
S24 -<
$ ~750 pL
22-

a 20 + 250 sL C14SO4
18 ~~eeF 1000 pL o 500 stL C14SO4
16 -1 os 750 stL C14SO4
14 +0 1000 sL C14SO4
0 10 20 30 40 50 60 70 80 90 100

time, sec
Figure 4-4. Equilibrium surface tension of a DDAB monolayer penetrated with 4 mM C14SO4

Initial Monolayer






-Transient M~onolayer







Equilibrated Monolayer





Figure 4-5. Molecular diagram of the transient monolayer phenomena found in slightly soluble
monolayers penetrated with a soluble surfactant from the subphase.
































0 20 40 60 80 100 120 140 160 180 200

Time, sec
Figure 4-6. Equilibrium surface tension of a DODAB monolayer penetrated with 4 mM C14SO4


o 250 uL C14SO4
a 500 uL C14SO4
o 750 uL C14SO4
0 1000 uL C14SO4




250 pL

-- 500 pL

75075 pl



1000 pL


0 10 20 30 40 50 60 70 80

time, sec
Figure 4-7. Equilibrium surface tension of a C200H monolayer penetrated with 4 mM C14SO4















88













250 uL C14SO4
500 uL C14SO4
750 uL C14SO4
1000 uL C14SO4



bi 250 pL

500 pL


7g 50 pL

1000 pL


36 -

E 34

E 32

,30

S28

S26

24

W 22

20

18


0 10 20 30 40 50 60 70


Time, sec
surface tension of a Cholesterol monolayer penetrated with 4 mM


Figure 4-8.







30 -


Equilibrium
C14SO4


0 10 20 30 40 50 60 70 80 90 100

time, sec
Figure 4-9. Equilibrium surface tension of a mixed monolayer composed of a 1:5 ratio of C14SO4
to DODAB penetrated with 4 mM solutions of C14SO4





























__S~gp~T L_


Time, sec
Figure 4-10. Equilibrium surface tension of mixed monolayers of various ratios of C14SO4 to
DODAB penetrated with 1000 CIL of 4 mM solutions of C14SO4




30 -1 1:5 Molecular Ratio of DODAB to C14SO4
injected with 1000 pL of C14SO4





5 20-


S15-


W 10-



0 100 200 300 400 500 600 700 800

time, sec
Figure 4-11. Time study of the equilibrium surface tension of a mixed monolayer of C14SO4 and
DODAB in a 1:5 Ratio injected with 1000 CIL of 4 mM C14SO4


1000 pL Injections of 4 mM C14SO4 Ratio of
C14SO4 to DODAB
o 1:10
o 1:5
S1:3
+ 1:2

8dLC~b1:2
CC~e 1:3



~aarD 1:10 1:5


n


0 20 40 60 80 100 120 140 160


180 200



















Fiue41.2Dhxgna ragmn fmlcue tte13mlcla aisi h ie



















Figure 4-13. A)D Feabrc onake irn penerting soeubph as e solulcuarrtion spae in thewahn macine,










B) The spin cycle is started and allowed to come to full speed, C) Monolayer solution
is poured over the fabric and the spin cycle is allowed to complete and the RMC is
measured.





















































Pre-Soaked in 4 mM SDS


SMonolayer Ratios of SDS:DODAB


100


CZ.C'CZ. CII:I:ICI~E

~b:ni~j.rr


C2.C~C2.C C~I:I:I
II
hR:n:ij.~r


60 I


I I


Pure 150 ppm 4 mM SDS 4 mM 1:5 Ratio 1:5 Ratio 1:5 Ratio
Water Detergent + SDS + + + 150 ppm
100 mL EtOH SDS C14SO4 Detergent

Figure 4-14. Comparison of RMC values for full scale washing machine experiments (150 ppm
of detergent is the standard of comparison) showing the reduction in RMC using
monolayer penetration (red bars).



100


90 -




o 80 -




70 -




60 -


Pure
Water


1:5 Ratio 1:7 Ratio 1:10 Ratio

SDS SDS SDS


150 ppm 4 mM SDS
Detergent +
100 mL EtOH


1:3 Ratio

SDS


Figure 4-15. Comparison of RMC values for full scale washing machine experiments (150 ppm
of detergent is the standard of comparison) showing the reduction in RMC using
monolayer penetration (red bars).











Table 4-1. RMC of small scale monolayer penetration with C14SO4:DODAB monolayer.
Absolute Relative
RMC, %
'Change,% Change, %
Pure Water 82.10 0.00 0.00
C14SO4 60.77 -21.33 -25.98
C14SO4 sprayed once with 1:5 C14SO4:DODAB
60.68 -21.42 -26.09
monolayer solution
C14SO4 centrifuged for 5 minutes, sprayed with
1:5 C14SO4:DODAB and centrifuged for 5 more 55.10 -27.00 -32.89
minutes
1 wt% Dow Q2-5211 50.30 -31.80 -38.73









CHAPTER 5
MICELLE STABILITY AND ITS EFFECT ON THE RESIDUAL MOISTURE CONTENT OF
FABRICS

5.1 Stabilization of Micelles

It has been shown earlier that micellar stability depends on surfactant concentration.8 The

Shah Research group has shown that the micellar stability depends on surfactant concentration. It

has been shown that a maximum micellar stability for SDS solutions exists at 200 mM due to the

small intermicellar distance, resulting in a strong repulsion between the micelles.3, 5-9 Therefore,

the micelles become more rigid as the surfactant concentration increases. This maximum in

micellar relaxation time has a dramatic effect on many different properties of SDS solutions

(ranging from low foamability, high thin film stability, wetting time, oil solubilization, etc.).'

The Shah Research group has also shown that micellar kinetics play an important role in

detergency.' Shah et al. has shown that the efficacy of removing non-polar compounds from

fabrics has been shown to have a strong correlation with the relaxation time of micelles."-0For

example, it was shown by Oh and Shah that using 200 mM SDS (which was shown to have the

longest micellar relaxation time in the SDS concentration range)6 prOVided the most efficient

removal of an artificial stain created by the deposition of Orange OT onto fabric samples.9

However, the micellar stability can also be influenced by the addition of an alcohol.102I

has been shown that the maximum micellar stability for SDS/alcohol mixtures exists for the

system SDS/C120H system, where the chain lengths of the surfactant and the alcohol are the

same and thus where the van der Waals interaction between the hydrophobic tails is maximum

(hydrophobic-hydrophobic interactions between the tail groups).103-106

In this study, we have found that at a concentration of 200 mM of SDS, there is peak found

in the residual moisture content (RMC) of fabrics. As shown by Patist et al.,5 we believe that this

correlates to the wetting time of fabrics and is due to the increase in dynamic surface tension









resulting from high micelle stability. We have shown that not only the equilibrium surface

tension plays a role in the removal of water from fabrics that the availability of monomer to

adsorb onto the air-liquid interface plays a role as well.107 For systems with high micellar

stability, the monomer flux to the air-liquid interface will be less than a system with low micelle

stability due to higher availability of surfactant monomers to adsorb on a newly created air-liquid

interface which would lead to a higher dynamic surface tension. At the 200 mM concentration of

SDS, there is a maximum in micelle stability which would account for less monomer flux and

less monomer to adsorb on the new air-liquid interface of bubbles created during the dynamic

surface tension measurement. This would thus lead to an increased dynamic surface tension that

should correspond to the increase in RMC in the same surfactant concentration range.

There has been a lot of work from the Shah Research Group showing that the micellar

stability of various surfactant systems can be significantly influenced by the addition of co-

surfactants.5-7, 54, 64, 106, 108-111 The increase in micellar stability of mixed surfactant systems is due

to synergism shown between oppositely charged headgroups or hydrophobic-hydrophobic

interactions between the surfactant tail groups (as shown with a non-ionic co-surfactant). Due to

coulombic or hydrophobic interactions, the stability of mixed surfactant systems can be tailored

to varying degrees of stability.

It is typically generalized that micelles are often drawn as static structures of spherical

nature composed surfactant molecules with polar head groups exposed to the aqueous solution

protecting the hydrophobic tails of the surfactant in the micelle core. However, micelles are in

dynamic equilibrium with individual surfactant molecule monomers that are constantly being

exchanged between the bulk surfactant solution and the micelles. Additionally, the micelles

themselves are continuously disintegrating and re-forming. The kinetics of this process has been









evaluated by Aniansson,53 112, 113 and the relevance of micellar relaxation time to various

technological processes for single surfactant systems such as sodium dodecyl sulfate (SDS) in

water has been extensively studied by Shah and co-workers.114 The kinetics of micellization has

been studied by various techniques such as stopped flow, "' temperature jump,116 preSSUTO

jump" and ultrasomec absorption.ls 1

Based on previous RMC work pertaining to the adsorption of SDS onto Hanes fabric, we

have shown that an increase in the dynamic surface tension (or a decrease in the free surfactant

monomer content in the bulk solution) leads to an increase in the RMC of fabric." This work

continued on to show that phenomena in the bulk surfactant solution that can alter the available

free surfactant monomer concentration (i.e. anything that can change the dynamic surface tension

of the bulk solution) can influence the RMC of fabric at the end of a laundry spin cycle.

Surfactant systems that have a long micellar relaxation time (i.e. micellar systems that are very

stable) have been shown to have a high dynamic surface tension and thus a higher RMC.

5.1.1 Concentration Dependence on Micelle Stability: 200 mM SDS

In our previous work, the residual moisture content has been shown to be a function of

surface tension of solution.68 However, as shown in Figure 5-1, the residual moisture does not

completely correlate to the equilibrium surface tension of pure SDS solutions in the range of 5-8

mM. A small dip in the surface tension at ~6 mM SDS concentration suggests that the sample

had a small impurity (presumably dodecyl alcohol). Recent experiments in our laboratory using

purified SDS samples have shown the same RMC peak. Due to adsorption of SDS onto the

fabric surface, the dynamic surface tension of the residual solution increased thus leading to an

increase in the RMC (Figure 5-2).

We have shown over the past years that stable micelles greatly affect many difference

aspects of surfactant systems.5 For the SDS system at 200 mM (most stable micelles for SDS),









we have shown that there is a large increase in bubble volume, single fi1m stability, detergency

effectiveness, emulsion droplet size, benzene solubilization etc (Figure 5-3 and Figure 5-4). It

has also been shown with there SDS solutions that at 200 mM that there is a decrease in

foamability and time to solubilize benzene in solution (Figure 5-3 and Figure 5-4).

Since the relaxation time of surfactants play such a large role in many different properties

of surfactant systems, the RMC of Hanes fabric around the concentration range of highest SDS

micellar stability was measured (from 125-250 mM concentrations of SDS). Since the dynamic

surface tension is related to the micellar stability (i.e. higher micellar stability leads to higher

dynamic surface tension as shown in Figure 5-7), it would be expected that there will be an

increase in the RMC around a SDS concentration of 200 mM. In Figure 5-5, we have shown that

at 200 mM concentration of SDS that there is peak shown in the RMC of Hanes fabric. This peak

is believed to be due to the long relaxation time of the SDS micelles at 200 mM. The long

relaxation time of the micelles would lead to a decreased monomer flux from the micelles to the

bulk. Since the micellar stability is high for 200 mM SDS, there is less free monomer flux from

the micelle to the bulk solution thus causing an increase in the DST. This decrease in monomer

flux would then be shown as an increase in the dynamic surface tension this leading to an

increase in the RMC. Alternatively, another possible explanation to explain the increase in RMC

at 200 mM concentrations of SDS could be due to stabilization of thin films on the fabric surface

as well as films in the interfiber spaces due to relatively stable micelles as demonstrated in

Figure 5-6. It has been shown by Shah et al.11, 12 and Wasan et al.13-18 that layering of micelles or

particles can stabilize thin films (which could possibly explain an increase in the RMC).

However, it is a possibility that the increase in RMC at 200 mM SDS is due to a combined effect










of thin film stability due to layering of micelles as well as the increase in dynamic surface

tension from the reduction in monomer flux from the stable micelles.

5.1.2 Mixed Surfactant Systems

It has been well documented by the Shah Research group that the stability of SDS micelles

can be greatly influence by the addition of co-surfactantS.106, 109, 110, 120 For this study, we chose to

add C12TAB and C120H to the SDS solutions in order to increase the stability of the mixed

micellar system compared to the stability of the pure SDS system. As shown in Table 5-1, the

micellar relaxation times of the systems we chose to test show an increase in micellar relaxation

time from ~1 ms to as high as 2,000 ms. Based on our results for the 200 mM SDS system

(highest stability over the entire SDS concentration range), we would expect that the RMC of the

cotton fabric will show the same trend as the micellar relaxation time increases shown in Table

5-1.

Shown in Figure 5-8, we have shown that there is an increase in RMC from the pure SDS

system to the mixed system of SDS and C12TAB. The RMC increased from the pure system to

the addition of C120H and then further increased with the SDS and C12TAB system (RMC of

~62, 67 and 80% respectively). The increase in micellar relaxation time for the SDS and C120H

system arises from the strong ion-dipole interaction between the SDS and the dodecanol. The

large increase in relaxation time with the addition of C12TAB to the SDS results from the strong

electrostatic (anionic-cationic) interactions between the head groups of the surfactants resulting

in a very tightly packed interface (at the air-liquid and micellar interfaces). These increases in

relaxation times also result in an increase in the dynamic surface tension of each solution. Very

stable micelles have little monomer flux to the bulk solution and thus for any new air-liquid

interface created, there is a small amount of free monomer available to adsorb at the new









interface to lower the dynamic surface tension. As we have previously discussed, an increase in

the dynamic surface tension will then cause an increase in the RMC of fabrics.

We have shown that many different factors affect the RMC of fabric/surfactant systems. It

was shown that adsorption phenomena play an important factor in laundry processes. It should

also be noted that the dynamic surface tension has been shown to play a large role in the

manipulation of the RMC of fabrics. Based on the results that have been presented, if the

magnitude of adsorption of surfactant onto the fabric or if the micellar kinetics of the surfactant

system used can be significantly changed, the magnitude of the RMC can thus be significantly

altered (i.e. increased relaxation times for increased RMC or decreased relaxation times for

lower RMC).

5.2 Effect of Dodecyl Sulfate Counterions on the RMC of Fabrics

Based on previous RMC work pertaining to the adsorption of SDS onto Hanes fabric, we

have shown that an increase in the dynamic surface tension (or a decrease in the free surfactant

monomer content in the bulk solution) leads to an increase in the RMC of fabric. This work has

continued to show that phenomena in the bulk surfactant solution that can alter the available free

surfactant monomer concentration (i.e. anything that can change the dynamic surface tension of

the bulk solution) can influence the RMC of fabric at the end of a laundry spin cycle. Surfactant

systems that have a long micellar relaxation time (i.e. micellar systems that are very stable) have

been shown to have a high dynamic surface tension and thus a higher RMC. The increase in

RMC with stable micellar systems is because micelles must be broken down into monomers to

be available to adsorb onto the newly created air-liquid interface and thus reduce the dynamic

surface tension (and RMC).121 If the micelles are stable, the monomer flux from the micelle is

very low and the dynamic surface tension is high resulting in an increase in RMC.









For the purpose of this work, we have investigated the effects of using different

counterions for the dodecyl sulfate surfactant. The substitution of one kind of counterion with

another counterion has the potential to alter the interactions between both the counterions and the

surface-active molecules. By changing the degree of binding of counterions to the surface-active

portions of the surfactant molecule can greatly influence the surface active chemical properties

of the surfactant.122 One key aspect of the solution that is significantly influenced by a change in

the counterion is the equilibrium surface tension30 In terms of affecting the CMC of the dodecyl

sulfate surfactants, the CMCs of LiDS, NaDS, CsDS, and Mg(DS)2 are TepOrted by Mukerjee to

be 8.92, 8.32, 6.09, 0.88 mM, respectively, at 250C.123

5.2.1 Experimental procedure (Surfactant Synthesis)

Lithium dodecyl sulfate (99% purity) is purchased from Acros (Orlando, FL), sodium

dodecyl sulfate (99% purity) from MP Biomedicals, Inc. Magnesium dodecyl sulfate (98%

purity) from Pfaltz and Bauer (Waterbury, CT). Cesium dodecyl sulfate is prepared in our

laboratory with the same procedure as shown by Kim et al.122. Chlorosulfonic acid (Aldrich,

Milwaukee, WI, 553.5 mM) is added to dodecanol drop by drop with vigorous mixing at 250C

under a nitrogen atmosphere. The sulfation reaction is performed very slowly (40 min and cooled

with ice) since the sulfation process is highly exothermic. After the sulfation process, nitrogen

gas is used to purge the reaction mixture to remove HCI produced during the reaction. Aqueous

CsOH solution (Aldrich, 50.0 wt%/) is added to the reaction mixture in a 1:1 molar ratio to

neutralize the acid. The CsDS is recrystallized three times with a 50:50 mixture (by volume) of

ethanol and acetone, keeping the solution below 50C.