Adsorption mechanism(s) of poly(ethylene oxide) on oxide and silicate surfaces

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Title:
Adsorption mechanism(s) of poly(ethylene oxide) on oxide and silicate surfaces
Alternate title:
Adsorption mechanisms of polyethylene oxide on oxide and silicate surfaces
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xiii, 161 leaves : ill. ; 29 cm.
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English
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Mathur, Sharad, 1966-
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Materials Science and Engineering thesis, Ph. D   ( lcsh )
Dissertations, Academic -- Materials Science and Engineering -- UF   ( lcsh )
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bibliography   ( marcgt )
non-fiction   ( marcgt )

Notes

Thesis:
Thesis (Ph. D.)--University of Florida, 1996.
Bibliography:
Includes bibliographical references (leaves 152-160).
General Note:
Typescript.
General Note:
Vita.
Statement of Responsibility:
by Sharad Mathur.

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University of Florida
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Table of Contents
    Title Page
        Page i
    Acknowledgement
        Page ii
    Table of Contents
        Page iii
        Page iv
        Page v
    List of Figures
        Page vi
        Page vii
        Page viii
        Page ix
    List of Tables
        Page x
        Page xi
    Abstract
        Page xii
        Page xiii
    Chapter 1. Introduction
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
    Chapter 2. Background
        Page 9
        Page 10
        Page 11
        Page 12
        Page 13
    Chapter 3. Experimental
        Page 14
        Page 15
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        Page 31
        Page 32
    Chapter 4. Flocculation and adsorption studies on oxides
        Page 33
        Page 34
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    Chapter 5. Role of surface acidity of oxides in PEO adsorption
        Page 63
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    Chapter 6. Characterization of PEO binding sites
        Page 87
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    Chapter 7. Adsorption and flowcculation behavior of silicates
        Page 109
        Page 110
        Page 111
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        Page 113
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    Chapter 8. Conclusions and future work
        Page 146
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    References
        Page 152
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    Biographical sketch
        Page 161
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        Page 163
        Page 164
Full Text










ADSORPTION MECHANISM(S) OF POLY(ETHYLENE OXIDE) ON
OXIDE AND SILICATE SURFACES












By
SHARAD MATHUR













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 1996













ACKNOWLEDGMENTS

I would like to express my deepest gratitude to Dr. B.M. Moudgil, my major advisor, for his invaluable guidance, assistance and encouragement through this investigation.

My sincere thanks go to Dr. D.O. Shah, Dr. E.D. Whitney, Dr. C.D. Batich, Dr. H. El-Shall and Dr, R.K. Singh for serving on my supervisory committee.

To all my friends- Dr. S.Behl, Dr. R.Damodaran, N.Kulkarni, T.S. Prakash, S.Zhu, R.Kalyanraman, J. Adler and many more involved in the Materials Science and Engineering Department, I would like to express my thanks for their constructive suggestions and cheerful assistance during the course of this work. I also appreciate the experimental help rendered by Adam Bogan, Joseph Puglisi, Matt Guyot, Robert Pekrul and Andrew Gartskiewicz.

I wish to acknowledge the NSF Engineering Research Center for Particle Science and Technology at the University of Florida for providing financial support (through Grant # EEC-94-02989) and a stimulating interdisciplinary research environment.

Last, but not the least, I would like to acknowledge my wife Anuradha for her invaluable assistance during the preparation of this manuscript.














TABLE OF CONTENTS


ACKNOW LEDGMENTS ........................................... ii

A B ST R A C T ................................................... xii

CHAPTERS

1 INTRO DUCTIO N ........................................... 1
Adsorption of Polymers at Solid/Liquid Interface ................... 1
Polym er Adsorption ................................... 1
A pplications .......................................... 3
Polymer Adsorption Mechanisms ......................... 3

2 BACKG RO UND ............................................ 9
Introd uction ............ ..... ........... ... ........ ........ 9
Source of Isolated Hydroxyls on Dolomite ................. 10
Adsorption Mechanism of PEO on Dolomite Samples ........ 10
Deficiencies in the Proposed Adsorption Mechanism of PEO ........ 10 Scope of the Present Study .................................. 13

3 EXPERIM ENTAL .......................................... 14
M ate ria ls . . . . . . .. . . . . . . . . . . . . . . . . . 14
O xide Sam ples ...................................... 14
Silicate Sam ples ..................................... 14
P olym ers ........................................... 19
O ther Chem icals ..................................... 19
M ethod s ...... ........ ........... ..... ...... ... .......... 19
Chem ical Com position ................................ 19
Particle Characterization ............................... 21
PEO Characterization ................................. 23
Surface Chemical Characterization ....................... 26
A FM Studies ........................................ 27
Flocculation Studies .................................. 28
Adsorption Studies ................................... 30

4 FLOCCULATION AND ADSORPTION STUDIES ON OXIDES ....... 33
Introd uction .............................................. 33


iii








Flocculation Studies.................................... 33
Effect of Polymer Molecular Weight ..................... 33
Effect of Dosage.................................. 33
Effect of Floc Detection Technique..................... 36
Electrokinetic Studies .............................. 38
Effect of pH on Flocculation of Oxides with PEO ............38
Adsorption Studies ..................................... 45
Adsorption Kinetics of PEO on Oxides ................... 45
Adsorption Isotherms of PEO on Oxides ................. 45
Adsorption Mechanism of PEO ............................ 52
Effect of Negatively Charged Surface .................. 53
Effect of Hydrated Counter-Ions....................... 53
AFM Studies..................... *,,**............. 54
Role of Specific Surface Binding Sites in PEO
Adsorption on Silica ............................... 62

5 ROLE OF SURFACE ACIDITY OF OXIDES IN PEO ADSORPTION .. 63
Introduction.............. ............................ 63
Accessibility of Surface Sites to PEO Molecules .................63
Concentration of Surface Hydroxyls ..................... 64
Heat of Wetting of Oxides ........................... 66
Nature of Surface Hydroxyls .............................. 66
Point of Zero Charge of Oxides ....................... 66
Correlation between Heat of Wetting and pzc of Oxides.......68
Role of Bronsted Acidity in PEO Adsorption ....................70
Relation Between Type of Oxide and its Point Of Zero Charge 71
Adsorption and Flocculation Behavior of MoO3 and V20.,
with PEO....................................... 73
Role of Lewis Acid Sites ................................. 82
Oxide/PEO/CCI, system ............................ 84
Hematite/Starch/Water System........................ 84

6 CHARACTERIZATION OF PEO BINDING SITES
ON OXIDE SURFACES................................. 86
Introduction .......................................... 86
Surface Hydroxyls on Oxides ............................. 88
Isolated Hydroxyls and PEO Adsorption...................... 92
Effect of Heat Pretreatment............................... 94
Adsorption of PEO on Heat Treated Samples............. 97
Characterization of Surface Acidity of Oxides ..................101
DRIFT Spectra of Adsorbed Pyridine on Oxides ...........101
Acidity of Silanol Groups ........................... 104





iv








Surface Analysis of Silica and Adsorption of PEO ................ 104
Specificity of Hydrogen-Bonding of Isolated Silanols ........ 104 Effect of pH ........................................ 107

7 ADSORPTION AND FLOCCULATION BEHAVIOR OF SILICATES

W ITH PEO ........................................ 109
Introduction ............................................. 109
Adsorption and Flocculation Studies .......................... 109
Flocculation of Silicates ............................... 109
Effect of Flocculant Dosage ........................... 110
Adsorption Studies on Silicates ......................... 113
AFM Studies of Adsorbed Molecules on Tremolite and Augite ...... 122 Surface Characterization of Silicate Minerals ................... 135
Correlation between Isolated Hydroxyls and Adsorption ........... 138
Adsorption Mechanism(s) of PEO on Silicates .................. 140


8 CONCLUSIONS AND FUTURE WORK ....................... 146
S um m ary ............................................... 146
Suggestions for Future W ork ................................ 150

REFERENCES .......................................... 152

BIOGRAPHICAL SKETCH ................................. 161






















v













LIST OF FIGURES

Figure Page

1. 1. Conformation of the adsorbed polymer molecule ................ 2

1.2. Schematic illustrating steric stabilization of particles............... 4

1.3. Schematic of bridging flocculation of particles .................... 5

1.4. Schematic illustrating the selective flocculation process ............. 6

3.1. Crystal structures of silicate samples......................... 18

3.2. Size distribution of PEO samples............................ 25

3.3. Calibration curves for analysis of PEO in solution................ 32

4.1. Flocculation behavior of oxide samples as a function of
molecular weight of PEO (dosage 0.5 mg/g; pH = 9.5) ............. 34

4.2. Flocculation behavior of oxide samples as a function of
dosage of PEO of MW 8,000,000 at pH 9.5.................... 35

4.3. Bed volume of silica sediment as a function of PEO dosage
(MW=5,000,000) at pH-9.5 ................................ 37

4.4. Electrokinetic behavior of oxides as a function of pH
(I = 0.03 kmol/m3) ...................................... 39

4.5. Flocculation behavior of silica samples as a function of pH
(PEO MW = 5,000,000; dosage = 0.5 mg/g).................... 41

4.6. Flocculation behavior of silica A as a function of PEO
molecular weight at different pH (PEO dosage = 0.5 mg/g) .......... 43

4.7. Electrokinetic behavior of silica A with and without PEO
(1=0.03 kmol/m3) ....................................... 44

4.8. Equilibrium adsorption time for PEO on oxides
(PEC MW = 8,000,000; pH = 9.5)........................... 46

vi








4.9. Adsorption isotherms for oxide-PEO system at pH 9.5
(PEO MW = 5,000,000) ...................................... 47
4.10 Adsorption isotherms for oxide-PEO system at pH 3.0
(PEO MW = 5,000,000) ...................................... 48

4.11 Adsorption isotherm of PEO (MW = 8,000,000) on silica A at pH 9.5 51 4.12. AFM image of the bare silica surface ........................... 55

4.13. AFM Tapping Mode topographic image of adsorbed PEO
(MW = 5,000,000 at pH 3.0) ................................. 56

4.14. AFM Tapping Mode topographic image of adsorbed PEO
at pH 9.5 after 1 hour of desorption ........................... 58

4.15. AFM Tapping Mode topographic image of adsorbed PEO
at pH 9.5 after 2 hour of desorption ........................... 59

4.16. Effect of pH on interparticle forces between silica sphere
and a flat plate with and without PEO (MW = 5,000,000) ............ 61

5.1. Schematic of Bronsted acid sites .............................. 69

5.2. Heat of wetting of oxides as a function of their point of zero charge.
(After [Hea65]) ........................................... 70

5.3. Electrokinetic behavior of MoO3 and V205 suspensions as
function of pH ............................................ 75

5.4. Adsorption isotherms of PEO on MoO3 and V205 suspensions
(PEO MW = 5,000,000; pH= 3.0) .............................. 76

5.5. Saturation adsorption density of PEO (MW= 5,000,000) at pH 3.0
as a function of the point of zero charge of oxides ................. 77
5.6. Flocculation behavior of MoO3 and V205 as a function of
molecular weight (dosage = 0.5 mg/g at pH 3.0) ............. 79

5.7. Flocculation of MoO3 and V205 as a function of PEO dosage (pH 3.0). 80 5.8. Schematic showing a Lewis acid site ........................... 84

6.1. Schematic showing surface hydroxylation on various faces of anatase. 88 6.2. DRIFT spectra of oxides in the hydroxyl region ................. 90

vii








6.3. DRIFT spectra of silica B and hematite in the hydroxyl region ....... 91 6.4. DRIFT spectra of heat treated silica samples ..................... 96

6.5. Schematic of i) amorphous silica surface showing the ring structure
and ii) influence of surface curvature on H-bonding ................ 97

6.6. DRIFT spectra showing effect of heat treatment on the surface
hydroxylation of alum ina A .................................. 99

6.7. Adsorption isotherms of PEO for heat treated oxides at pH 9.5
(PEO MW =5,000,000) ..................................... 100

6.8. DRIFT spectra of pyridine treated MoO3, V20, and SiO2 samples ... 104

6.9. Plot of the change in frequency of isolated silanols against the
specific heat of adsorption for several vapors adsorbed on silica
surface (data after [Kis65] and And [65a] ....................... 107

7.1. Flocculation of silicates as a function of molecular weight of PEO
(pH=9.5; dosage=lmg/g) .................................. 110

7.2. Flocculation behavior of augite as a function of PEO dosage
at pH 9 .5 ................ .... ............. ....... ...... 113
7.3. Flocculation behavior of tremolite as function of PEO dosage
at p H = 9 .5 ............................................. 115

7.4. Kinetics of PEO adsorption on tremolite and augite ............... 116

7.5. Adsorption isotherms of PEO on chain and orthosilicates at pH 9.5
(PEO MW = 5,000,000) ................................... 117

7.6. Adsorption isotherms of PEO on clays at pH 9.5
(PEO MW = 5,000,000) ................................... 118

7.7. AFM image of bare tremolite surface .......................... 124

7.8. AFM image of adsorbed PEO on tremolite ...................... 125

7.9. AFM Friction image of adsorbed PEO on tremolite .............. 126

7.10. Histogram of parking area of PEO molecules on tremolite ......... 127

7.11 AFM image of bare augite surface ............................ 131


viii








7.12. AFM image of adsorbed PEO on augite ........................ 132

7.13. AFM Friction image of adsorbed PEO on augite ................. 133

7.14. Histogram of parking area of PEO molecules on augite ........... 134

7.15. DRIFT spectra of chain and layered silicates in the hydroxyl region... 137 7.16. DRIFT spectra of orthosilicate minerals in the hydroxyl region ....... 138

7.17. Adsorption of PEO on clays by interaction of the ether oxygen of PEO
with the hydration shell of the exchangeable ion (Bronsted acid site). 143


































ix














LIST OF TABLES

Table Page

2.1. Correlation between the intensity of the isolated hydroxyl groups on
dolomite samples with flocculation and the saturation adsorption
density of PEO of 5,000,000 MW [Beh93a] ..................... 11

3.1. Oxide samples and their sources............................ 15

3.2. Structural Units Observed in Crystalline Silicates (After [Kin76]) ...... 16 3.3. Silicate samples selected and their idealized chemical composition. . 17 3.4. Characteristics of the Polymers used in this study................ 20

3.5. Physical Characteristics of the oxide samples ................... 22

3.6. Physical characteristics of the silicate samples.................. 24

3.7. Dimensions of the flocculation cell........................... 29

4.1. Isoelectric Point of Oxides determined by Electrokinetic Studies ...... 40

4.2. Saturation adsorption density of PEO (MW = 5,000,000) on oxide
samples at different pH .................................. 49

5.1. Concentration of surface hydroxyl groups [And82] ................ 65

5.2. Heat of Wetting values for oxides in water [Che59; Hea65] .........67

5.3. Probable ranges of pzc of different types of oxides [Par65J .......... 73

5.4. Dissolution behavior of MoO3 and V205 powders and the
adsorption of dissolved ions on other oxides .................... 82

6.1. Surface hydroxyls on different oxides and the saturation
adsorption density of PEO ................................ 94



x








6.2. Infrared bands of pyridine in the 1400-1 700 cm-'
region of the spectrum .................................. 103

7.1. Critical PEO molecular weight for flocculation of the silicate
minerals at pH 9.5 ..................................... 112

7.2. Saturation adsorption density of PEO (MW = 5,000,000)
on chain and orthosilicates............................... 120

7.3. Estimated surface areas from Hg-porosimetry and the calculated
saturation adsorption densities for tremolite and augite ...........122

7.4. Type of hydroxyl groups on silicates along with PEO
saturation adsorption density.............................. 140

































xi













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


ADSORPTION MECHANISM(S) OF POLY(ETHYLENE OXIDE) ON OXIDE AND SILICATE SURFACES


By

SHARAD MATHUR

December, 1996


Chairman: Dr. Brij M. Moudgil
Major Department: Materials Science and Engineering



The surface modification of solids by adsorption of polymers is critical to a number of industrial processes and products. In a solvent medium, polymer adsorption on solid substrate is exploited to disperse or aggregate the particulate slurries. Solid/solid separations using selective flocculation technique rely on the specificity of the polymer for the aggregating particles. Hydrogen bonding has been suggested to be the primary adsorption mechanism for non-ionic polymers and associated with nonselectivity. However, literature survey of PEO-oxide system suggested poly(ethylene oxide) (PEO), a non-ionic polymer, to be substrate specific.

In this study the adsorption of PEO on various oxides and silicates and their flocculation behavior was systematically investigated to understand the adsorption xii








mechanism(s). Surface characterization of the solid substrates was performed through DRIFT, AFMV, adsorption and electrokinetic measurements to identify the adsorption mechanisms. It was shown that the adsorption of PEO is substrate specific indicating that hydrogen bonding is strongly dependent on the surface chemical nature of the substrate. It was determined that strong Bronsted acid sites on the surface interact with the ether oxygen, a Lewis base, of PEO to induce adsorption and subsequently flocculation of the particles.

In the oxidelPEO system, highly acidic oxides such as SiO2, MoO3, and V205 strongly adsorb PEO and exhibit flocculation. On the other hand, relatively basic oxides with a point of zero charge (pzc) greater than that of silica such as TiO2, Fe2O3, A63 and MgO, did not exhibit significant adsorption of PEO. Further, dissolved ions and charge characteristics were shown not to affect the adsorption and flocculation behavior of oxides. It was revealed for the silicate/PEO system that the connectivity of the silicate tetrahedra is essential in generation of strong Bronsted acid sites capable of interacting with PEO.

The concept of strong Bronsted sites being essential for PEO adsorption provided the commonality for the binding sites identified earlier, viz., the isolated silanols and exchangeable ions. It also showed that rather than the isolated nature of the surface hydroxyls their acid strength is of prime importance in interacting with the ether oxygen of PEO. Further, the effect of pH on adsorption of PEO and similar non-ionic polymers on silica could be explained within the framework of the established adsorption mechanism. Additionally, the results will be beneficial to identification/synthesis of selectively adsorbing polymers with applications in processing of mineral fines, controlled drug delivery systems and ultrapurification of fines.

xiii














CHAPTER 1

INTRODUCTION

Adsorption of Polymers at Solid/Liquid Interface Polymer Adsorption

A flexible polymer molecule such as poly(ethylene oxide) (PEO) in solution has a dynamically changing conformation which can be described as a random coil. The size of the coil is dependent upon the solvent quality, the polymer concentration, and characteristics of the polymer chain [Spe92]. Upon adsorption of the polymer molecule at the solid solution interface, the conformation of the polymer may change from that in the solution state. The equilibrium conformation is a compromise between the enthalpic factors (which tend to maximize the segment/surface contacts) and entropic ones (trying to maintain a thick layer with many degrees of freedom). The adsorbed polymer conformation is described in terms of trains, loops and tails (see Figure 1. 1). The segments bonded to the surface comprise the trains, the segments between the trains constitute the loops and the tails are the free ends of the polymer molecule. In principle, the trains, loops and tails can be distinguished by the difference in their mobility by techniques such as Small Angle Neutron Scattering, Nuclear Magnetic Resonance and Electron Spin Resonance.





























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3

Applications

Adsorbed polymer molecules at the solid/solution interface play a very important role in many industrial and biological products and processes. For instance, polymers are used as dispersants or flocculants to stabilize or aggregate particulate slurries, respectively.

In order to disperse particles, a complete surface coverage of the polymer molecules is required in a good solvent medium so that interpenetration of the adsorbed polymer layers during interparticle collision leads to a net repulsive force. The stabilization of a suspension by this mechanism is termed as steric stabilization and is schematically illustrated in Figure 1.2. Dispersion of particles is important in ceramic and mineral processing, formulation of inks and paints, cosmetics, pharmaceutical and food industry.

A partial coating of the polymer on a particle may lead to its adsorption on a bare surface of the other colliding particle. This is the origin of bridging flocculation illustrated schematically in Figure 1.3. The flocculation of particles forms the basis for both solid/liquid and solid/solid separations. The flocculation phenomenon, when it is confined to only specific type of particles in a multi-component particulate systems is termed selective flocculation (see Figure 1.4). Polymer Adsorption Mechanisms

In order to optimize the effectiveness and efficiency of the polymer as a dispersant/flocculant, it is important to understand the underlying adsorption mechanism(s). This can be illustrated by way of example of the selective flocculation process. The process comprises of (1) dispersion of the fine particles








4






























Figure 1.2. Schematic illustrating steric stabilization of particles.








5































D

Figure 1.3. Schematic of bridging flocculation of particles.







6




oO oO O o 0 OO O
OO O O O Ooo Oo
0 0 0 00 ,-0' 0 O0 "0
0 J.00 -0 00000
0~00 00 0o,0 0 0 0 g.o;0o
0 0 0 O 0 + Polymer 0

ooo hoo o 0 o
ogo 0" CDog 0
0 0
000 000 00
000 000 0 00 oOO

Dispersion Selective Flocculation

000 0 0
o 0 0 O00

+ 0 oo 00
OO o

0000
So0 0o oo0000
__ 00 00 0 0

Separation of Flocs by Sedimentation or Flotation


Figure 1.4. Schematic illustrating the selective flocculation process.








7

(2) selective adsorption of the polymer on the flocculating component and formation of the flocs, (3) floc growth which is generally achieved by conditioning at low shear, and (4) floc separation either through sedimentation/elutriation/sieving or flotation followed by cleaning of flocs by repeated redispersion and flocculation, if necessary. Among these steps, the second step, i.e., selective adsorption of the polymer is the most difficult to control and by far is the limiting step to the success of the selective flocculation technique [Aft87;Mou9l].

Adsorption of the polymer on a particular surface is the result of the interactions between the functional groups and the binding sites on the surface. Flocculants are high molecular weight polymers which can adsorb through a number of mechanisms such as van der Waals, electrostatic interactions, hydrogen bonding, hydrophobic interactions and chemical bonding. Clearly, the surface property chosen (hydrophobicity, hydrogen bonding, surface charge or chemical bonding) must represent the one with the greatest difference between the flocculating and non-flocculating particles.

Hydrogen bonding has been accepted as a ubiquitous mechanism for adsorption of polymers. This generalization was borne out of the fact that the oxide surfaces are rich in hydroxyl groups and the polymers contain a functional group such as the ether oxygen, or alcohol, or amide groups capable of hydrogen bonding. Recent investigations for the starch-hematite system, however, showed that the adsorption mechanism is not hydrogen bonding as was believed but a specific interaction between the Fe sites on the surface and the functional groups of the polymer [Pra9l ,We95].








8

The aim of the present study was to understand the adsorption mechanism(s) of PEO on oxide and silicate surfaces since the molecule was reported to adsorb onto silica [Rub76;Che85;Kil84;Kok9O] and certain silicates [Sch85:Stan9O;Hog85] and not on other oxides such as hematite and alumina [Kok90;Sha86]. The elucidation of the adsorption mechanism(s) of PEO was expected to provide a unifying explanation for the experimental observations documented in the literature on PEO adsorption at oxide(silicate)-solution interface. This is also expected to lead to guidelines for identification/synthesis of selectively adsorbing polymers.














CHAPTER 2

BACKGROUND

Introduction

Previous studies in the silica/PEO and silica/poly(vinyl alcohol) (PVA) systems suggested the isolated silanols to be the principal adsorption sites for PEO on silica [Rub76;Tad78;Che85;Kha88]. The extension of this concept to isolated hydroxyls was examined for the dolomite-apatite system by Behl and Moudgil [Beh93d]. The DRIFT spectra of apatite and dolomite revealed that isolated hydroxyl groups are exclusive to the dolomite A surface, whereas hydrogen-bonded hydroxyl groups are present on both apatite and dolomite surface [Beh93a]. These investigators hydpothesized that any polymer capable of hydrogen bonding such as polyacrylic acid (PAA), polyacrylamide (PAM) and polyethyleneoxide (PEO) therefore should be capable of flocculating the two materials. Among these, PEO being a weak flocculant [Sch87], only the material with stronger interactions with the polymer molecules may be expected to flocculate. Experiments performed with 5,000,000 MW PEO revealed that irrespective of the amount of polymer added, flocculation of apatite was not observed. On the other hand, instantaneous flocculation of dolomite occurred justifying the assumption of a specific interaction of the ether oxygen of PEO with the isolated hydroxyls on dolomite [Beh93d].





9








10

The importance of the isolated hydroxyl groups was further shown through a correlation between their intensity on dolomite samples collected from different sources and the adsorption and flocculation with PEO (see Table 1) [Beh93d]. The dolomite samples showed similar bulk chemical composition, and it was even possible to separate one dolomite from another dolomite provided one of the dolomite could be flocculated with PEO [Mou95a]. Source of Isolated Hydroxyls on Dolomite

The fact that some dolomite showed the isolated hydroxyls while the others did not was unexplained [Beh93d;Mou95a]. A detailed characterization of the dolomite samples, described below, was attempted by Moudgil et al. [Mou95b] to determine the cause of the isolated OH on some dolomite samples and its absence on others. DRIFT and X-ray Diifraction (XRD) studies revealed the presence of a coating of palygorskite clay on the flocculating dolomite samples. Adsorption Mechanism of PEO on Dolomite Samples

Hoghooghi [Hog85] has shown that PEO is an excellent flocculant for palygorskite. The mechanism of adsorption of PEO on dolomite A is expected to be similar to the hydrogen bonding mechanism proposed for silica. The isolated OH on dolomite A may act as proton donor in hydrogen bonding to ether oxygen of PEO.

Deficiencies in the Proposed Adsorption Mechanism of PEO

The correlation between saturation adsorption density of PEO and flocculation of dolomite samples with the presence of isolated hydroxyls suggested that the assumption of isolated hydroxyls being the principal adsorption sites for the








11
















Table 2.1. Correlation between the intensity of the isolated hydroxyl groups on dolomite samples with flocculation and the saturation adsorption density of PEO of 5,000,000 MW [Beh93a].

Dolomite Flocculation Saturation Intensity of
(dosage 1 mg/g) adsorption Isolated OH at density, mg/in2 3619 cm-1 A 98.5 2.18 High
B 92.5 1.93 High

C 69.3 1.16 Medium
D 72.5 1.19 Medium
E 0 0.88 None

F 0 0.39 None








12

ether oxygen of PEO is correct. However, the presence of isolated OH on dolomite samples was determined to be due to the coating of palygorskite clay. The flocculation behavior of clays such as montmorillonite and palygorskite has been extensively studied by Scheiner and co-workers [Sch86;Sch87;Bro89]. These investigators attributed the adsorption mechanism of PEO to a hydrogen bonding mechanism involving the water shell around the exchangeable cations on the clays [Sch86;Bro89]. Although the isolated hydroxyls constitute the surface of other oxides such as alumina and hematite yet flocculation of these oxides was not observed [Kok90]. The presence of isolated hydroxyls on the surface of these oxides has been shown through vibrational spectroscopy [Hai67;Tsy72;Mor76]. Additionally, other possible mechanisms for adsorption of PEO suggested in the literature e.g. electrostatic interactions with a positively charged surface, and complex binding with adsorbed ions such as K', Cd*, Mg'etc. [Bai76; Kje8l; Ana87; Beh93d;Pra95] have not been examined in detail to explain the flocculation behavior of various oxides with PEO.

Thermodynamically, the overall free energy of the polymer adsorption process must be negative. In addition to the enthalpic factors such as the segment/surface and water-surface interactions the overall entropy changes associated with the adsorption process are also important. In fact, the lack of PEO adsorption on alumina and hematite was suggested to be due to the lack of accessibility of PEO molecules to the surface sites [Kok90]. The entropy factor was earlier invoked by Greenland to explain the unreactivity of aluminol and silanol groups with PVA [Gre72a,b]. Thus the adsorption mechanism(s) need to consider








13

the entropy contribution to polymer adsorption. Further, the adsorption mechanism(s) must also be consistent with the observed decrease in adsorption of non-ionic polymers, such as PEO and PVA, with pH [Rub76;Che85;Tad78;Kha88].

In order to further understand the adsorption mechanism of PEO and similar non-ionic polymers, a systematic study involving surface-chemical characterization of various oxides and different types of silicates was undertaken. The reason for selecting a variety of substrates was to examine all the possible mechansims reported in the literature including the accessibility factor, and establish the predominant mecahnism of PEO adsorption on oxides and silicates.

Scope of the Present Study

The specific objectives of the present investigation are as follows:

1 Establish the mechanism(s) of PEO adsorption on oxides. Specifically, it is

proposed to understand the role of different type of surface sites in

interaction with the ether oxygen of PEO.

2. Examine the effect of pH on adsorption of non-ionic polymers such as PEO.

3. Evaluate the role of surface accessibility in polymer adsorption.

4. Examine the adsorption mechanism(s) of PEO on mixed oxides such as

silicates based on better understanding of the same on simple oxides.














CHAPTER 3

EXPERIMENTAL

Materials

Oxide Samples

The oxide samples along with their sources are listed in Table 3.1. These were selected so as to encompass a range of acidic to basic surfaces. The oxide samples were used as received exceptV205and Silica B which were wet ground to obtain -400 mesh (<38 pm) fraction.

Silicate Samples

The silicate samples chosen were representative of the different classes of silicates. The characteristic structural features of these classes are summarized in Table 3.2. The minerals selected and their idealized chemical compositions are listed in Table 3.3. Although the SUO ratio in palygorskite is 2.75, which is characteristic of the amphiboles, the arrangement of the chains is such that the clay is considered to be a pseudo-layered silicate [Gri68]. The crystal structures of the silicate samples are shown in Figure 3. 1. The as received samples from Ward's Natural Establishment Inc., NY, were crushed in a Chipmunk crusher and then pulverized and subsequently sieved to yield different size fractions. The -400 mesh fraction of the silicate samples was used in this study.




14








15
















Table 3.1. Oxide samples and their source. Oxide Sample Code Source
Silica (A) Geltech Inc., FL

(B) IMC- Agrico, FL
Titania Alfa
Hematite Alfa
Alumina (A) Sumitomo

(B) Alcoa
Magnesia Mallingcrockdt
Molybdenum Alfa
Oxide
Vanadium Alfa
Oxide








16















Table 3.2. Structural Units Observed in Crystalline Silicates (After (Kin76]).

Oxygen-Silicon Silicon-Oxygen Structural Units Type
Ratio Groups
2 SiO2 Three dimensional Framework
Network
2.5 Si4010 Sheets Layered
2.75 Si4011 Chains Amphiboles
3.0 SiO3 Chains Pyroxenes
4.0 SiO4 Isolated Orthosilicates
orthosilicate
tetrahedra








17
















Table 3.3. Silicate samples selected and their idealized chemical composition.


Material Type of silicate Chemical Formula
Palygorskite Layer (OH,),(OH),Mg, Si,,O,,. 4HO
Kaolinite Layer A14S'4010(OH)g
Tremolite amphibole Ca2MgjSi.022](OH)2
Augite pyroxene (CaNa)(MgFeAl)(SiAI)20r,
Almandite orthosilicate FeAl2Si,,0,2
Topaz A12(SiO4)(OH)

IL- Olivine (MgFe)2SiO4







18














(i) (ii) (iii)




~A12
-.. 0 (OH)

~03


(iv)


Figure 3.1. Crystal structures of silicate samples: (i) isolated SiO4 tetrahedra as in almandine, olivine and topaz, (ii) SiO4 chain in pyroxene, augite, (iii) double SiO. chains in amphibole, tremolite and (iv) Si04 layer in kaolinite.








19

Polymers

Poly(ethylene oxide) (PEO) of different molecular weights along with the source and calculated radius of gyration is listed in Table 3.4. These samples were used as received.

Other Chemicals

All experiments were conducted in distilled water (Dl) of specific conductivity less than 1 pmho/cm. Potassium hydroxide (KOH) and nitric acid (HNO3) used as pH modifiers were obtained from Fisher Scientific Co. Pyridine, used as a probe molecule for characterization of the surface acidity, was also procured from Fisher Scientific Co.

Methods

Chemical Composition

The as received oxide samples were specified to be of more than 99.9% purity. Silica A was prepared by the sol-gel technique (Stober silica) while silica B was electrostatically separated from the fluorapatite and acid washed to remove the minor phosphate impurity. The P205content of the silica B sample was below the detection limit (0.05 wt.%) of the Perkin Elmer 11 inductively coupled plasma (ICP) spectrometer. Alumina B was determined by ICP to contain, on weight basis, 99.9% A1203, 0.01% SiO2, 0.02% Fe2O3 and 0.07% Na2O.

The silicates samples were characterized for the purity primarily by the hydroxyl band of their DRIFT spectra. No quantitative analysis of the samples was attempted.








20

















Table 3.4. Characteristics of the Polymers used in this study.

Source Molecular Weight Radius of Gyration, R9
(nm)
Polysciences, Inc. 8,000,000 177
Polysciences, Inc. 5,000,000 144
Polysciences, Inc. 4,000,000 126
Aldrich 900,000 52
Polysciences, Inc. 600,000 41
Polysciences, Inc. 1,00,000 14
Polysciences, Inc. 18,500 6








21

Particle Characterization

All the oxide and silicate powders were characterized for particle size distribution and surface area.

Oxides

Particle size distribution. The particle size distribution of the oxide samples was determined using Micromeritics X-ray Sedigraph 5100 and the characteristic diameters are presented in Table 3.5. It is observed that silica A is monodisperse. The silica A particles are spherical in shape, which is a characteristic of the Stober process to synthesize silica. Silica B, MoO3 and MgO particles are coarser than the other oxide samples.

Surface area. The surface area was essential to compare the adsorption density of PEO on different substrate. The size of the high molecular weight PEO is such that the BET method is insensitive to the corresponding pore size, e.g., 288 nm for 5,000,000 MW PEO molecule. The specific surface area of the oxide samples was, therefore, determined by mercury porosimetry (Micromeritics Autopore 111 9420) and the results are presented in Table 3.5.

In mercury porosimetry the raw data generated for a powder sample consists of the intrusion volume of mercury versus the applied pressure. The interparticle pores are filled first and the slope of the graph changes at the point when the intraparticle pores on the surface are intruded by the mercury. The pore radius and corresponding surface area are calculated from the assumption of a cylindrical geometry of the pore. The calculated surface area for non-porous spherical particles of silica A (1 pm) is 2.85 m2/g which is in good agreement with the








22













Table 3.5. Physical Characteristics of the oxide samples


Sample Average Particle Size, pm Specific Surface
Area,
d16 do d84 m2/g
Si02 (A) 1+0.1 3.18
Si02 (B) 2.4 8.5 18.0 1.91
TiO2 0.1 0.3 0.5 11.10
Fe203 0.3 0.6 1.5 8.23
A1203 (A) 0.2 0.5 0.8 7.11
A1203 (B)
MgO 0.8 1.7 6.8 3.38
MoO3 3.1 6.4 8.2 1.33
V_0,_ 0.2 0.6 1.0 7.13








23

experimental value of 3.18 m2/g. Similarly, the size distribution of the other oxide samples and the surface area values indicate that the particles are essentially nonporous.

Silicates

Particle size distribution. The particle size distribution of the ground -400 mesh fraction (< 38 pm) of tremolite and augite determined by Micromeritics X-ray Sedigraph 5100 is shown in Table 3.6. It is revealed that the silicate particles are coarser than the oxide samples.

Surface area. The specific surface area of the silicate minerals was determined by Autosorb Micromeritics ASAP 2000 system. The surface area was estimated by the BET method using adsorption and desorption of nitrogen and is presented in Table 3.6. The relatively higher specific surface areas for tremolite and augite indicate that the samples have a significant amount of porosity associated with them. The Hg-porosimetry, as in the case of the oxides, was, therefore, used to determine the effective surface area for PEO adsorption for tremolite and augite.

PEO Characterization

The PEO samples received were granular in appearance and the molecular weight specified by the manufacturer is an average value. In order to determine the polydispersity of the high molecular weight fractions light scattering on PEO solutions was performed using Brookhaven BI 90. The size measured is the hydrodynamic diameter (2xRgz) of the polymer molecules in solution. The results presented in Figure 3.2 reveal that the polymers are polydisperse in nature.








24















Table 3.6. Physical characteristics of the silicate samples.

Sample Average Particle Size, pm Specific Surface Area,
m2/g
d16 ds50 d84 Hg-P" BET
Tremolite 6.0 11.0 32.0 8.81 13.88
Augite 5.0 15.0 30.0 3.51 4.11
Kaolinite 15.52
Palygorskite < 38.0 117.0

Almandite 0.79
Olivine 1.33
Topaz 4.54

Hg-P" = mercury porosimetry








25












100



80


.C
4 60



75 40
E
40

0 8,000,000 20 11 5,000,000
A 4,000,000 V 900,000





50 100 150 200 250 300
Hydrodynamic Diameter (nm) Figure 3.2. Size distribution of PEO samples. The molecular weights are indicated in the legend.








26

Surface Chemical Characterization

The determination of the adsorption mechanism of PEO involved a knowledge of the surface chemical groups and charge characteristics of the samples.

Infrared studies

Samples used for FT-IR analysis were prepared by mixing a 0.05g of vacuum dried sample with about 0.75 g of potassium bromide (KBr). The mixture was then filled in a sample cup mounted on a diffuse reflectance stage. The IR spectra was taken using a Nicolet 740 spectrometer. The beam was aligned for every sample to yield a maximum signal and the baseline was adjusted using the software. The evolution of the DRIFT spectra as a function of temperature was performed on Nicolet 60SX spectrometer using the heating stage.

The acidity of the surface chemical groups on the oxides was probed by pyridine adsorption which involved soaking 1 g of the sample in 25 ml liquid pyridine for 1 hour. The samples were air-dried under a fumehood and subsequently kept in a vaccum oven at 60'C for 12 hours. The samples were then mixed with KBr prior to obtaining the DRIFT spectra.

Electrokinetic studies

Zeta potential of the oxide samples was measured using the Laser Zee Meter (Pen Kemn Model 501) to determine their isoelectric point. 50 mg of the powder sample was suspended in 100 ml of .03 kmol/M3 KNO3 solution except for MoO3 and V20, which exhibited ionic strength in excess of .03 kmol/m3.








27

For these two oxides, the centrifuged suspension from a 2 wt.% slurry was used as such to determine the zeta potential of the suspended solids. The Laser Zee Meter was also used to determine the zeta potential of PEO coated silica samples. AFM Studies

The Atomic Force Microscope (AFM) studies were conducted to characterize the conformation of the adsorbed polymer layer at the solid/solution interface. The samples were imaged in a liquid cell where the suspension conditions such as the pH and ionic strength can be simulated. The samples used for the AFM study were as-received fused silica plates from Herasil Amereus and polished samples of augite and tremolite rocks which were received from Ward's Natural Establishment Inc., NY. The final polishing of the silicate samples was done using 0.03 pm iron oxide suspension from Buehler.

The AFMV was used in both the contact and tapping modes to image the microstructure at the solid solution interface. The interpretation of the images was facilitated by the image analyzer software of the AFM.

The contact mode was also used to obtain force/distance profiles between a glass sphere of 10-40 pm diameter attached to the cantilever and the fused silica plate with and without the adsorbed polymer. These profiles were obtained by suspending the x-y raster motion of the piezoelctric and measuring the deflection of the cantilever as it approached the surface. The cantilever used had a force constant of 0.30 N/in. Thus the measured deflection values were converted to the corresponding force values as a function of the separation distance.








28

Flocculation Studies

These were conducted to macroscopically evaluate the effect of PEO adsorption on the particles.

Flocculation apparatus

The flocculation of particulate suspensions has been shown to be sensitive to the type of agitation used [Hog85]. Hence it is necessary to maintain uniform, hydrodynamic conditions in all the flocculation experiments. The mixing unit employed in this study is based on the standard tank design [Dir8l]. The dimensions of the mixing tank are listed in Table 3.7. A 150 ml beaker fitted with removable plexiglas baffles of appropriate dimensions was used for flocculation tests. A stainless steel turbine impeller with four blades mounted on a variable speed motor was employed to agitate the sample. Flocculation procedure

Material suspension of pulp density 2g/100 ml was prepared in DI water and aged for one hour. The maximum pH variation after aging was determined to be 0.2 pH units. After aging, the suspension was agitated at 1100 rpm, for 240 seconds while the pH was adjusted to the desired value. The suspension was sonicated for 30 seconds at a setting of 50 W to ensure complete dispersion, and a predetermined amount of PEO was added. The agitation was continued at 1100 rpm for 120 seconds within which the flocculation was observed to be complete. The formation of flocs was evaluated in a sedimentation column while the quantification of floc formation was obtained via floc separation over a 400 mesh screen.








29


















Table 3.7. Dimensions of the flocculation cell.

Flocculation Cell Part Dimension, cm
Tank diameter 6.2
Impeller height from tank bottom 0.5
Impeller blade width 1.6
Liquid height 4.6
Baffle width 4.0








30
Polymer Solution Preparation

Polymer solutions were prepared by mixing 0.25 g granular polymer with 500 ml Dl water to yield a 500 ppm solution. The solution was stirred for 16h at 500 rpm and covered to avoid exposure to ultraviolet radiation which decomposes the polymer. The polymer solution was prepared fresh every day for the experiments since changes were observed in PEO adsorption when stored for more than one day [Mou92].

Adsorption Studies

These were conducted to measure the affinity of the surface for the PEO molecules. The adsorption isotherms, in conjunction with the knowledge of the surface chemical groups, provided insight into the adsorption mechanism of PEO on oxide and silicate surface.

Adsorption was carried out by contacting the polymer solution with 2 wt.% solids in 100 ml solution in 150 ml beakers (same as the flocculation tank). Sixteen beakers were simultaneously stirred on a 16-pad magnetic stirrer at 500 rpm. The equilibrium time for adsorption was first determined by studying the kinetics of polymer adsorption with a high polymer dosage (1lOmg/g solids or 200 ppm). After equilibration the sample was centrifuged at 15,000 rpm for 10 minutes and the supernatant withdrawn. The residual PEO in the solution was determined by Total Organic Carbon (TOO) analyzer. Adsorption was determined by the solution depletion method. The saturation adsorption density, which is the maximum possible amount of polymer on the surface, was obtained by fitting Langmuir equation for high concentrations [At9l, Beh93].








31

PEO Analysis

The amount of residual polymer in the supernatant was determined using the TOO (Shimadzu). Calibration curves were obtained for PEO of MW 5,000,000 and 8,000,000 by using standard solutions of PEO (see Figure 3.3) and correcting the measured values for the initial carbon content in the supernatant. The inorganic carbon was minimized by acidifying the PEG solutions with 85 wt.% phosphoric acid and sparging the solution with carbon-free air from gas cylinder. The concentration value measured had a cumulative variance of less than 1 %.








32









35000 I


30000


25000


S20000


CU)

<5 10000


1000


o MW = 5,000,000
MW = 8,000,000
0



0 50 100 150 200 250
PEO concentration, ppm Figure 3.3. Calibration curves for analysis of PEO in solution.














CHAPTER 4

FLOCCULATION AND ADSORPTION STUDIES ON OXIDES Introduction

In this Chapter the flocculation behavior of common oxides such as silica, titania, hematite, alumina and magnesia with PEO and the related adsorption studies are discussed to identify the underlying adsorption mechanism(s).

Flocculation Studies

Effect of Polymer Molecular Weight

The adsorption of a relatively high molecular weight polymer on the substrate generally implies the possibility of floc formation. However, there exists a critical molecular weight beyond which one can detect flocculation [Beh93a]. At lower molecular weight the polymer may act as a dispersant.

The flocculation behavior of all the oxides as a function of the molecu r weight of PEO at 0.5 mg/g dosage is shown in Figure 4.1. It is observed tht irrespective of the molecular weight of PEO no flocculation of any oxide except silica A is observed. The critical molecular weight of PEO at which flocculation of silica A occurred was found to be 8,000,000.

Effect of Dosage

It is observed from Figure 4.2 that silica flocculation exhibits a maximum as a function of PEO dosage, whereas the other oxides did not flocculate in the dosage



33








34






100 I



80 -0- silica A



0
o 60O




o 40
IL

0
E
20



0 G O

Silica B, Titania, Hematite, Alumina, Magnesia I I I I I
Oe+0 2e+6 4e+6 6e+6 8e+6 le+7

Polymer Molecular Weight Figure 4.1. Flocculation behavior of oxide samples as a function of
the molecular weight of PEO (dosage = 0.5 mg/g; pH = 9.5).








35












100



80 --0- Silica A

600

0 4
0 6




0



Silica B, Titania, Hematite, Alumina, Magnesia
II I I I I
0 2 4 6 8 10 12
Flocculant Dosage, mg/g Figure 4.2. Flocculation behavior of oxide samples as a function of dosage of PEO of MW 8,000,000 at pH 9.5.








36
range examined. The existence of a maximum in flocculation followed by restabilization is in agreement with the bridging mechanism of flocculation. In silicaPEO system a similar flocculation behavior with dosage has been reported in the past by Rubio and Kitchener [Rub76] and Cheng [Che 85] with PEO of 5,000,000 MW.

Effect of Floc Detection Technique

It must be noted that silica particles are 1 pm in size and flocs are separated using a 400 mesh (37pm) screen. It is, therefore, likely that smaller flocs formed with a lower molecular weight flocculant may not be detected by sieving. Thus, determination of the critical molecular weight is strongly influenced by the floc detection technique.

In order to observe flocculation with 5,000,000 MW PEO settling tests were conducted on silica A and the results are shown in Figure 4.3. It is seen that the trend of bed volume with dosage is similar to that shown in Figure 4.2 indicating floc formation in the system. Koksal et al [Kok9O] and Shah [Sha86] reported no measurable flocculation of hematite and alumina with PEO using settling tests to detect the onset of flocculation.

The lack of flocculation of silica B at pH 9.5 is not unexpected. Koksal et al [Kok90] observed the flocculation of quartzite only near the isoelectric point of 2.5 while no flocculation was detected beyond pH 4.0. Rubio and Kitchener [Rub76] observed that the precipitated silica was virtually non-flocculable at higher than pH 8.0 while the heat treated silica was only slightly flocculated when the pH was raised to 9.5 from 2.0. Cheng [Che85] also observed an enhanced flocculation of silica








37










7

-0-- 60s
6 --- 120s
--600 s
-7- 1200s 5 --0- 3600 s
--0-- 7200 s o--0- 10800s 4-- 14400s

0
>3
"0


2

1


0 I I
0 1 2 3 4
Polymer Dosage (mg/g) Figure 4.3. Bed volume of silica sediment as a function of PEO dosage (MW = 5,000,000) at pH = 9.5.








38

at pH 3.7. Thus flocculation of silica B is expected near its isoelectric point and this may be true for the other oxides.

Electrokinetic Studies

The electrokinetic behavior of the oxide samples is summarized in Figure 4.4 The measured isoelectric point (iep) values summarized in Table 4.1 are in agreement with the literature values [Ree92]. Effect of pH on Flocculation of Oxides with PEO

The effect of pH on flocculation of the silica samples with PEO of 5000,000 MW is shown in Figure 4.5. In accordance with previous work flocculation of both the silica samples in the acidic pH range was observed. Further, a sharp decrease in flocculation beyond pH 3.0 was noticed indicating a decrease in the adsorption of PEO on negatively charged surfaces. In fact, when the pH of the flocced slurry of silica B was raised to 9.5 the flocs begin to disappear and a dispersed suspension of silica particles was obtained.

Flocculation of oxides other than silica was not observed in the pH range 2.510.0. The non-flocculation of hematite and alumina in this pH range was also reported by Koksal et al [Kok90]. This implies that neither neutral sites such as MOH (which are a maximum at the isoelectric point) nor positively charged sites such as MOH2'(which predominate below the isoelectric point) lead to adsorption of PEO on these oxides. Further, silica A flocculated when it exhibited a high negative charge, and both silica samples flocculated near the isoelectric point. Thus electrokinetic characteristics of an oxide sample do not necessarily govern the adsorption of PEO.








39









80

O silica B 60 0 silica A
0A titania
V hematite K0 alumina A 40 alumina B


20 -
0
-
C 0
N

-20



-40


-60 I I
0 2 4 6 8 10 12

pH
Figure 4.4. Electrokinetic behavior of oxides as a function of pH
(KNO3= 0.03 kmol/m3).








40















Table 4.1. Isoelectric Point of Oxides determined by Electrokinetic Studies

Oxide Isoelectric Point
Silica A 2.0
Silica B 2.0
Titania 3.8
Hematite 8.4
Alumina A 8.8
Alumina B 7.4
Magnesia








41









100 I I I

--0-- silica A 80 ----- silica B



o- 60
-)
0
o 40 4
C
0
E
< 20



0




0 2 4 6 8 10

pH
Figure 4.5. Flocculation behavior of silica samples as a function of
pH (PEO MW = 5,000,000; dosage = 0.5 mg/g).








42

The critical molecular weight of PEO for flocculation of silica A shifted to 900,000 from 8,000,000 when the pH was decreased from 9.5 to 3.0 (see Figure 4.6). Assuming the adsorption behavior of PEO to be similar at both pH values the increase in flocculation of silica particles with decrease in pH may be attributed to the reduction in the zeta potential of the silica surface allowing a closer approach between the particles. In such a case it is only with PEO of 8,000,000 MW that the zeta potential is expected to be significantly low at pH 9.5 due to the adsorbed layer exceeding a critical thickness of the order of the thickness of the order of the thickness of the electrical double layer (110 nm at 1=1lO-kmol/m').

The electrokinetic data for silica A with and without adsorbed PEO of 18,500 MW is plotted in Figure 4.7. It is indicated that the adsorbed layer thickness of the 18,500 MW PEO is sufficient to result in a zero zeta potential. Thus a decrease in the zeta potential is not the reason for a decrease in the critical molecular weight of PEO for flocculation of silica suspensions with increase in pH. Rubio and Kitchener [Rub76] and Cheng [Che85] showed that the saturation adsorption density of PEO on silica decreases with increase in pH. A direct correlation between flocculation and saturation adsorption density was suggested by BehI and Moudgil [Beh93J for adsorption of PEO on several dolomite samples. Adsorption studies were therefore undertaken to explain the flocculation behavior of the different oxide samples and identify the PEO adsorption mechanism(s).








43















100 -- pH 3.0
-A-- pH 9.5
,80


--i 60
0
I
~-40
0
E
< 20


0A



10000 100000 1000000 10000000

Polymer Molecular Weight

Figure 4.6. Flocculation behavior of silica A as a function of PEO molecular weight at different pH (PEO dosage = 0.5 mg/g).








44











0



-10



E -20

4
0
CO
a -30

N

-40

O PEO MW 18,500
0 without PEO
-50


-60 II I I I
1 2 3 4 5 6 7 8 9 10

pH Figure 4.7. Eelctrokinetic behavior of silica A with and without PEO
(1=0.03 kmol/m3).








45

Adsorption Studies

Adsorption Kinetics of PEO on Oxides

The adsorption kinetics of PEO of 8,000,000 MW on the different oxide samples were examined and the results are presented in Figure 4.8. It is observed that equilibration is achieved in about 4 hours for silica A while the remaining samples did not exhibit any adsorption of PEO even after 24 hours. Thus further adsorption tests were performed with an equilibration time of 4 hours. Adsorption Isotherms of PEO on Oxides

The adsorption isotherms for all the oxide samples at pH 9.5 and 3.0 with PEO of 5,000,000 MW are plotted in Figures 4.9 and 4.10 respectively. The saturation adsorption density of PEO on various oxides under the two pH levels is summarized in Table 4.2. Considering that an equivalent monolayer of PEO corresponds to a saturation adsorption density of 0.4 Mg/ M2 it is clear that oxides other than silica did not exhibit significant surface coverage by PEO.

It is observed from Figures 4.9 and 4.10 that in contrast to silica A PEO adsorption on silica B was significantly affected by pH. The notable effect of pH on adsorption of PEO on silica is indicative of the flocculation behavior of the silica suspensions described in Figures 4.2 and 4.5. Although the saturation adsorption density of PEO on silica A is significant with 5,000,000 MW it is 25% lesser at the higher pH indicating a lesser number of the active sites and hence a lower probability for flocculation at pH 9.5 according to the equivalent site concept proposed by Behl and Moudgil [Beh93c]. Thus larger flocs were obtained only in the acidic pH range and not at pH 9.5 (see Figure 4.5). However, with 8,000,000








46






2.0 1 1 1






1.5

- 0 silica A
E


S1.0

CL
0
I'




0.5






0.0 I I I I
0 5 10 15 20 25 30

Time (h) Figure 4.8. Equilibrium adsorption time for PEO on oxides (PEO MW =
8,000,000; pH 9.5).








47








2.0


o silica B
0 alumina A 1.6 Ak alumina B
E V titania
") < hematite
E 0 magnesia

. 1.2
0


E
0
C. 0.8
0

E
0.4



0.0
0 20 40 60 80 100 120 140 160 180
Residual Concentration, ppm Figure 4.9. Adsorption isotherms for oxide-PEO system at pH 9.5
(PEO MW = 5,000,000).








48







2.0 I I

O silicaA
O silica B
A alumina A 1.6 7 alumina B
E 0 titania
"0 hematite
E O magnesia

1.2
0


E
- 6 o
a. 0.8 0 0
.4.
0 L
OO
o [
0
E LI
< []
0.4




0.0
0 20 40 60 80 100 120 140 160
Residual Concentration, ppm


Figure 4.10. Adsorption isotherms for oxide-PEO system at pH 3.0
(PEO MW = 5,000,000).








49














Table 4.2. Saturation adsorption density of PEO (MW = 5,000,000) on oxide samples at different pH.

Oxide Saturation Adsorption Density,
Mg/M2
pH 3.0 pH 9.5
Silica A 0.80 0.65
Silica B 0.63 0.17
Titania
Hematite < 0.1
Alumina A Alumina B Magnesia








50

MW PEO large flocs were observed at pH 9.5 indicating the adsorption to be similar to that obtained at pH 3.0 with 5,000,000 MW PEO. The saturation adsorption density of 8,000,000 MW PEO at pH 9.5 from the isotherm in Figure 4.11 was determined to be 0.8 mg/m2 which is similar to that obtained for 5,000,000 MW PEO at pH 3.0 (see Table 4.2).

The saturation adsorption density of 0.8 mg/m2 of PEO on silica A is twice that of the equivalent monolayer adsorption density. Fleer et al [Fle83] and Blaakmeer [Bla9O] have suggested that the saturation adsorption is about 2 to 5 times the equivalent monolayer, primarily due to compaction of the polymer molecule at high adsorption densities. The maximum in flocculation of silica A at pH 9.5 is observed at about 0.16 mg/m2 adsorption density ( 0.4 mg/g dosage) with both 5,000,000 and 8,000,000 MW PEO (see Figures 4.2 and 4.3). The occurrence of maximum in floc formation at less than half the surface coverage (0.2 mg/m2) may be attributed to the polydispersity of the polymer [Beh93b].

Adsorption studies for PEO/oxide system besides silica have been reported only for alumina. The negligible adsorption of 5,000,000 MW PEO on alumina was earlier shown by Shah [Sha86]. In a recent study of adsorption behavior of 8000 PEG on oxides and silicates it was shown that the adsorption of PEO on alumina was negligible [Wa1961. The presence of impurity ions such as sodium on alumina B also did not influence the adsorption results when it is well known that sodium ions complex with PEO in solution [Ana87;Bai76; Pra95] indicating that complexation with adsorbed surface ions is not a mechanism for PEO adsorption.








51





1.0


0.9
0

0.8


'3'0.7
E)
p0.6
0
(D)

E
0
a. 0.4


E



0.1


0 20 40 60 80 100

Residual Concentration of Polymer, ppm Figure 4.11. Adsorption isotherm of PEO (MW = 8,000,000) on
silica A at pH 9.5.








52

Adsorption Mechanism of PEO

It has been shown so far that positively charged sites (MOH2') or adsorbed ions which complex with ether oxygen in solution, and electrostatic considerations do not play a major role in the PEO adsorption process. Rubio and Kitchener [Rub76I suggested the following possible adsorption mechanisms of PEC on silica to explain the effect of pH.

1 Increased repulsion of PEO from an increasingly negatively charged

interface as the pH is increased. This is because the ether oxygen with the two lone pair of electrons is considered to be slightly negatively charged.

Thus the effect of surface ionization is through a general double-layer phenomenon rather than the loss of binding sites. The evidence in support of this explanation was the slight increase in PEO adsorption at a given pH

with electrolyte addition.

2. The binding sites for the ether oxygen of PEO are the surface hydroxyls on

the solid surface. It has been shown by infra-red studies that hydroxyls of different acid strength are present on the silica surface. The chemical characteristics of the surface hydroxyls have been shown to vary and it is possible that the most readily dissociable silanol group, i.e., the most acidic group are the most important one for adsorption of PEG. These sites are thus ionized first as the pH is increased leading to decreased adsorption of

PEG.

3. Hydrated counter-ions prevent PEG from approaching the surface as the

negative charge increases with the pH. This hypothesis was also suggested








53
by Her [11e751 who showed that incorporation of aluminosilicate anions into the silica surface enabling it to retain a negative charge in the acidic solution led to a decrease in flocculation.

Effect of Negatively Charged Surface

The non-adsorption of PEO on oxides other than silica at their isoelectric points indicates that explanation (1) above probably plays a minor role in adsorption of PEO. The repulsion of PEC molecule from a negatively charged silica surface resulting in decreased adsorption upon increase in pH implies that a positively charged surface should attract PEO which was found not to be the case as oxides below their iep did not exhibit significant adsorption of PEO. Also, electrokinetic studies showed that silica A possesses a similar charge as silica B at any pH yet their adsorption behavior is very different.

The evidence provided by Rubio and Kitchener [Rub76] in favor of the electrostatic interactions is the slight increase in PEO adsorption with electrolyte addition. There is about 10% increase in saturation adsorption for precipitated silica in the presence of 0.02 M NaCI. Similarly, Cheng [Che85] also observed a 10% increase in adsorption of PEO with 0.02 M NaCI. Addition of NaCI up to 1 M strength to non-flocculating oxide suspensions, however, did not result in any measurable adsorption of PEC indicating that repulsion from a highly negative charged surface is not a major reason for non-adsorption of PEO. Effect of Hydrated Counter-Ions

The hypothesis of hydrated counter-ions preventing the approach of PEO molecules to the surface at higher pH assumes that the binding sites remain intact.








54
This implies that the adsorption of PEO at higher pH can be realized by contacting silica particles at a lower pH close to the isoelectric point and then increasing the pH. This hypothesis was experimentally observed at the solid/solution interface with the aid of AFM.

AFM Studies

Imaging of Adsorbed PEO Layer using Tapping Mode. The tapping mode of the AFM was used to image the adsorbed PEO molecules at i) pH 3.0 and (ii) pH 9.5 by injection of 50 ppm of 5,000,000 MW PEO. In a separate experiment the initial pH was first maintained at 3.0 then changed to 9.5 and the desorption process observed. The AFM image of the bare silica plate is shown in Figure 4.12 with the average surface roughness determined to be less than 2 nm.

The AFM image of the adsorbed polymer layer at pH 3.0 is presented in Figure 4.13. The thickness of the molecules was determined to vary from 25-40 nm which can be attributed to the polydisperse nature of the PEO molecules. The hydrodynamic thickness of adsorbed homopolymers on a saturated surface in a good solvent has been predicted to be between 2-3 Rg [deG 81; deG 82; Sch82]. In the present case it thus seems that the probe did not detect the hydrodynamic thickness of the PEO molecules since the R. of the PEO molecule of 5,000,000 MW is 144nm.

Photon Correlation Spectroscopy (PCS) measurements for the latex-PEO system has shown that for PEO molecular weight greater than 280,000 the hydrodynamic thickness is more than 2 Rg [Coh84;Cos84]. In the same system, Cosgrove et al [Cos84], however, found that Small Angle Neutron Scattering











55






























:3 cv)






J








56












































Figure 4.13. AFM Tapping Mode topographic image of adsorbed PEO (MW= 5,000,000 at pH 3.0)








57
(SANS) grossly underestimated the adsorbed layer thickness. The thickness of PEO molecule of 660,000 detected by SANS was found to be only 15 nrn whereas with PCS a thickness of 95 nm was calculated. They attributed this discrepancy to detection of the segment density distribution of only the trains and loops and not of the tails by SANS. The segment density distribution of the adsorbed polymer has been shown to decrease exponentially with distance from the solid/solution interface [Cos84]. Thus a significantly lower thickness may be detected if the probe is not sensitive to the periphery of the adsorbed layer.

The image of the adsorbed polymer molecules at pH 9.5 after an hour of pH change is shown in Figure 4.14. A much lower adsorption density of polymer molecules on the surface is observed. The image after another hour is similar to that of the virgin surface except a few isolated patches (see Figure 4.15). This, is the first reported direct proof of desorption of polymer molecules of high molecular weight upon pH change.

It has been argued that at any given instant the probability that all the attached segments detach from the surface is so low that the adsorption for all practical purposes can be considered to be irreversible. However, de Gennes has pointed out the fallacy in this argument and showed that desorption is possible [deG87]. The desorption in the present study cannot be attributed to dilution since the polymer concentration was always maintained at 50 ppm. Further, the polymer was already adsorbed so that the possibility of inaccessibility to the surface due to the presence of hydrated counter-ions does not arises. The only reason which can then account for PEO desorption with increasing pH is the loss of binding sites for the ether oxygen of PEO.











58











A 0
0



cq

x rq
oi

CL


0
w


a)
-0
2
0 ca
-0
m
0
a) CD
m
E
0 !E
CL cc
L
CY)
0
CL co 0
a)
-0
0

cm
c
7a
CL
m
F
0
U_ CL
L
< 0
&n
a)

40

:3 0 CD








59







































Figure 4.15. AFM Tapping Mode topographic image of adsorbed PEG at pH 9.5 after two hours of desorption.








60
Force/Distance Profilies with Contact Mode. The adsorption of PEC on the silica plate at pH 3.0 and its desorption at pH 9.5 was corroborated in the AFM studies by obtaining force/distance profiles using a glass sphere attached to the AFM cantilever. The results presented in Figure 4.14 indicate that at pH 3.0, in the absence of PEO, the net interaction profile does not show the presence of a repulsive force, as expected from the electrokinetic considerations and the DLVO theory. However, in the presence of PEO a steric repulsive force is observed revealing the adsorption of the polymer. On the other hand, at pH 9.5, the force/distance profiles remain virtually unchanged with and without the PEO indicating the absence of an adsorbed layer.

The onset of the steric repulsion was observed at about 100 nm (see Figure 4.16). Thus the thickness of the adsorbed layer is estimated be about 50 nm on each surface which is an underestimate with respect to the predicted and observed values in latex/PEO system [deG8l; deG82; Sch82; Coh84; 0os84]. However, as mentioned above, the detection of the hydrodynamic thickness is dependent upon the sensitivity of the measuring probe to the peripheral layers of the adsorbed polymer. The measurement of force distance profiles to estimate the adsorbed layer thickness has been shown to be insensitive to the outer regions of the adsorbed polymer layer [Luc9O]. Luckham and Klein [Luc9O] measured the onset of repulsive interactions between adsorbed PEC layers (MW = 1,200,000) on mica in Surface Force Apparatus (SFA) at 190 nm. However, the thickness of the adsorbed PEO layer (95 nm) is about the Rg (86 nm) of the PEO molecule. Further, these investigators have shown the effect of incubation time on the measured force/distance profiles. Measurements after only 1 hour of polymer







61









..- pH 3.0 None
z
E
6 pH 3.0 PEO
0
2 pH 9.5 None
0"0.4
LL
.o 0.2 9.5 PEO

*" 0
4-I

-0.2
0 20 40 60 80 100 120
Separation Distance (nm)


Fig 4.16. Effect of pH on interparticle forces between silica sphere and a flat plate with and without PEO(MW = 5,000,000).








62
contact time with the mica surfaces indicated the onset of interaction at 20 nm while after 16 h this value was found to be 190 nm. This behavior is not clear at present and further studies are needed to resolve this issue. An incubation time of 1 hour in the present case may, therefore, be responsible for a lower thickness of the adsorbed layer.

Role of Srecific Surface Binding Sites in PEC Adsorption on Silica

It is clear from the AFM studies that there are specific PEO binding sites present on the silica plate at pH 3.0 which are lost when the pH is increased to 9.5. These are probably the most acidic silanol groups which dissociate at higher pH leading to the ionization of the site to a negatively charged species and loss of the bond with the ether oxygen. The ionization of the binding site to a negatively charged species is corroborated by the electrokinetic data (see Figure 4.4) and the force/distance profiles (see Figure 4.16).

A similar mechanism was proposed by Evans and Napper [Eva73] to explain the decrease in PEO adsorption with increasing pH for the latex/PEO system. The binding sites on the latex were carboxylic acid groups which at low pH participated in hydrogen bonding with the ether oxygen of PEO. The ionization of the acid sites led to the loss of PEC adsorption at higher pH.

It is also observed that the adsorption behavior of PEO as a function of pH on silica plate used in the AFM resembles that of silica B. The relative insensitivity of PEO adsorption on silica A to pH is in contrast to the behavior on silica B and silica plate and may be related to a different distribution of acidic sites and will be examined in Chapter 6. The lack of PEO adsorption on certain oxides which is not clear yet is discussed first in Chapter 05.













CHAPTER 5

ROLE OF SURFACE ACIDITY OF OXIDES IN PEO ADSORPTION Introduction

In the last Chapter it was shown that no oxide other than silica exhibited significant adsorption of PEO. The mechanism of PEO adsorption on silica, consistent with the effect of pH, was indicated to involve the acidity of surface sites. In this Chapter this concept is discussed further to understand the reasons for lack of adsorption of PEG on other oxides.

Koksal et al [Kok9Ol suggested that the lack of adsorption of PEG on alumina and hematite was due to the inaccessibility of the surface adsorption sites to PEG molecules. This explanation was found to be inconsistent with the effect of pH on PEG adsorption on silica in Chapter 4. The reason advanced by Koksal et al [Kok9O] for the inaccessibility to the surface sites of PEG molecules, however, was not the hydrated counter ions but entropic factors which are discussed next.

Accessibility of Surface Sites to PEG Molecules

The net free energy change associated with the polymer adsorption must be negative and involves changes in both enthalpic and entropic factors. In the case of the oxides the processes contributing to the entropy changes on PEG adsorption are: i) the loss of water from the oxide surface, ii) loss of conformational entropy of the PEG molecule due to attachment of segments to the surface which were otherwise mobile and the iii) the entropy of dilution of the bulk phase.


63








64

The ether oxygen itself has three water molecules attached to it and they have been shown not to detach in solution when PEO complexes with various ions [Pra95]. The presence of the hydration layer has been demonstrated on Si02, TiO2, and A1203 through measurement of hydration forces attributed to this layer [Gra93]. The disintegration of the hydration layer will be favored since it will result in more degrees of freedom for the water molecules along with dilution of bulk water. However, if the loss of conformational entropy of the polymer exceeds the gain in entropy by the loss of hydrated water molecules then the adsorption process is not favorable. In such a case, the surface sites will remain inaccessible to the polymer molecules. The replacement of water molecules on the surface is related to the concentration of the hydroxyl groups since these provide the adsorption sites for water molecules.

Concentration of Surface Hydroxyls

The concentration of surface hydroxyl groups on oxides has been determined by several investigators and is summarized in Table 5.1 [And82J. It is clear that alumina, hematite and titania are more hydrated than the silica surface and therefore release of water molecules is more favored from their surfaces. Thus the lack of PEO adsorption on alumina, hematite and titania does not seem to be due to entropic reasons. Koksal et al [Kok9O], however, suggested that stronger hydrogen bonding of water molecules to the surface hydroxyls on hematite and alumina prevents the interaction of PEO with the latter. In such a case favorable entropy changes for PEO adsorption are expected to be affected the most for








65


















Table 5.1. Concentration of surface hydroxyl groups [And82].

Oxide Number of OH/nm2
Silica 4.2-5.1
Titania 4.9-6.2
Hematite 4.6-9.1
Alumina 15








66
hydrated oxides such as alumina and hematite. The argument of Koksal et al [Kok90] for stronger hydrogen bonding between the water molecules and surface hydroxyls was based on the number of surface hydroxyls per unit area and not on the energetics of water-surface hydroxyl interaction. Further, some overlap of the hydroxyl concentrations between the first three oxides indicates that the inaccessibility to the surface sites may not be the major reason for the nonadsorption of the PEO.

Heat of Wetting of Oxides

The enthalpy of interaction of water molecules with the surface hydroxyls is measured as the heat of wetting of the oxides (see Table 5.2)[Che59;Hea65]. It is observed that the heat of wetting of oxides follow the same trend as the concentration of surface hydroxyls. However, the interaction of water per surface hydroxyl show a considerable overlap for the oxides under consideration. This indicates that the water molecules are bonded with a similar strength to the surface hydroxyls on any oxide. The nature of surface hydroxyls, however, on different oxides may not be the same and this aspect is discussed next.

Nature of Surface Hydroxyls

Point of Zero Charge of Oxides

For oxides with a hydrated surface, the surface chemistry in water is dominated by the chemical reactions

MOH 2(surface) = MOHsuace + Hsolution+

K1= [MOHsurface] [Hsolution] / [MOH2(sudace) ] pK, = pH + Iog{[MOH2(,urace)*]/[MOHsufcl]







67



















Table 5.2. Heat of Wetting values for oxides in water [Che59; Hea65].

Solid Heat of wetting

(ergs/cm2)
Quartz 260-370
Amorphous silica 165-220
Rutile (TiO,) 550, 550 Fe20,3 530
ALO. 650-900








68



MOHsuriace = MO-sufac + Hsolution

K2 = [MOsurface] [H+ soution]/[ MOHsur]a
pK2 = pH + log{[MOHs,8 ]/[MOsurface]} where M represents a metal ion at the surface. From these relations the point of zero charge (PZC) of the surface may be defined in terms of the pK's of reactions, i.e.,

PZC = 0.5 [pK,+ pK2]

and indicates the average acid-base characteristic of the surface. At any given pH, if an oxide surface donates relatively more protons than other it is more acidic and hence will have a lower pzc. In other words, the point of zero charge of an oxide is directly related to the acidity of the surface hydroxyl groups. The acidic surface hydroxyls are referred to as Bronsted acid sites (see Figure 5.1). Correlation between Heat of Wetting and pzc of Oxides

Healy and Fuerstenau [Hea65] showed a linear correlation between the heat of wetting of oxides in water and their pzc (see Figure 5.2). This substantiates the assumption made in the preceding section that although the nature of the surface hydroxyls varies on different oxides the interaction strength with water molecules per surface hydroxyl is similar. Thus the role of solvent is not expected to be significantly different in PEO adsorption on the various oxides in aqueous medium.












69



























0 co


)0 0I 0 C C

U) 0 )2> o -Crn



o LFnD







70







1000
o Amorphous silica
900 C Quartz
A Tin oxide V Hematite
800 0 Titania
o Chromia NE 700 0 Alumina

2) 0
S600

500
0)
CO

400

300

200

100 I I I I I I
1 2 3 4 5 6 7 8 9
pHpzc

Figure 5.2. Heat of wetting of oxides as a function of their point of zero charge
(after [Hea65]).








71

Role of Bronsted Acidity in PEO Adsorption Strength of Bronsted Acid Site and Electronegativity of Cation

It is shown above that the acidity of the Bronsted sites is the strongest on silica and the weakest on magnesia among the oxides under consideration. This can be understood qualitatively in terms of the electronegativity of the metal ion to which the surface hydroxyls are attached. The electronegativity of the surface metal atom governs the extent to which the electron pair shared between the metal and oxygen atoms is displaced towards the oxygen end.

In case of a predominantly ionic bond such as in MgO the electron pair is close to the oxygen atom which results in a stronger attraction for the proton. Frequency shifts in the IR spectra observed during water and benzene adsorption on MgO indicated that these hydroxyl groups are more basic than those on silica surface [And65]. However, for an oxide with a significant character of covalent bond e.g. SiO2 the proton will not be strongly bound to the oxygen and therefore this type of hydroxyl is expected to be acidic. A1203 has a more covalent character of AI-O bond than Mg-O but at the same time it is more ionic compared to Si02. Thus, acidity of Bronsted sites is stronger on silica than on alumina.

The dependence of electronegativity on the type of bonding and electronic environment was shown by Pauling [Pau63]. This implies that strict comparisons between oxides as regards to their surface acidity should be made only for a particular type. Thus for MO2 type of oxide the pzc should increase in the order Si02 < TiO2< ZrO2

because the electronegativity differences increase from 1.7 for silica to 2.0 for zirconia. The higher acidity of Bronsted sites on silica than titania thus explains the








72

adsorption behavior of PEO for the two oxides. This also illustrates the sensitivity of the interaction of the ether oxygen of PEO to the acid strength of the Bronsted sites.

Relation Between Type of Oxide and its Point Of Zero Charge

Parks [Par65] has summarized the broad probable ranges of the pzc characteristic of the cation oxidation state from the known literature values which are reproduced in Table 5.3. The relation between the pzc and the cationic size and charge was explained by an electrostatic model involving the coordination number with crystal field and hydration corrections. It is noted that the isoelectric points determined for the oxides under consideration are in close agreement with the predicted values given in Table 5.3.

It is predicted from Table 5.3 that MO3 and M2O5type of oxides should exhibit stronger Bronsted acidity than the other oxide types. The validation of Parks model [Par65] has been corroborated for MoO3 and V205 by spectroscopic investigations using probe molecules of known acidity or basicity.

It has been established through infra-red studies of adsorbed probe molecules that proton acid centers in simple oxides are essentially different in strength [Hai67;Dav9O]. For instance, pyridine, a molecule only slightly less basic than ammonia, is not protonated on the surface of alumina which indicates that alumina has weak proton-donating properties [Cha63]. On the other hand, MO3 and M205 type of oxides such as MoO 3and V p respectively protonate not only pyridine but such weak bases as propene and ethene (Dav90]. Experimental data indicate







73




















Table 5.3. Probable ranges of pzc of different types of oxides [Par65]. Oxide M20 MO M203 MO2 M03,M20s
pzc, pH >11.5 8.5-12.5 6.5-10.4 0 7.5 < 0.5








74

that the strongest Bronsted centers may be associated with the presence of Mo6+ ions. Thus oxides such as MoO3 and V Q are expected to exhibit stronger Bronsted acid sites than silica and should adsorb and flocculate with PEO. Adsorption and Flocculation Behavior of MoO, and V20, with PEO

Assuming that it is the presence of Bronsted acid sites with a strength comparable to or more than the acidic hydroxyls on surface of silica which is required to facilitate adsorption of PEO, MoO3 and V20, should exhibit adsorption of PEO and flocculate.

Electrokinetic studies

It is observed from the electrokinetic behavior of MoO3 and V205 shown in Figure 5.3 that both the materials exhibit high negative zeta potentials in the pH range studied. According to the prediction of pzc in Table 6.2, both the oxides have a pzc of less than pH 2.0. The electrokinetic studies on MoO3 and V20, are also in agreement with the reported pzc values of 0.5 and 1.5 respectively [Par65]. The suspensions for both the samples drifted back to the natural pH of about 2.7 within an hour of measuring the zeta potentials. Thus, adsorption and flocculation tests with these oxides were attempted only at pH 3.0. Adsorption of PEO on MoO, and V20Adsorption isotherms for both MoO3 and V20s with PEO of MW = 5,000,000 are shown in Figure 5.4. Both the isotherms are of high affinity type similar to that for silica (see Figures 4.10 and 4.11). The saturation adsorption densities of all the oxides as a function of their acidic strength, which is measured by the pzc, are summarized in Figure 5.5. The saturation adsorption densities of PEO on all the








75











0

O MoO3
-10 V205


> -20
E

( -30
O
0O
13.3 N -40


-50
~O

-60 I I I I I
1 2 3 4 5 6 7 8 9

pH

Figure 5.3. Electrokinetic behavior of MoO3 and V20s suspensions as function of pH.







76










1.0 I


0 0
E 0 .8 ._ __-0_

E

0 .
I




0


E 0.2- t



0.0

0 20 40 60 80 100
Residual Concentration, ppm Figure 5.4. Adsorption isotherms of PEO on MoO3 and V205 suspensions (PEO MW = 5,000,000; pH 3.0).








77








1 .0 I I I I I I I I I


O 0 V205

0.8 [M0O3
E / SiO2
0) 0
E v TiO2
> "Fe203
0.6 O A1203
OMgO
0

"o0.4
CO



00
ol 0.2



0.0 I I I I I I I I I I I I I

0 1 2 3 4 5 6 7 8 9 10 11 12 pHpzc

Figure 5.5. Saturation adsorption density of PEO (MW = 5,000,000) at pH 3.0 as a function of the point of zero charge of the oxides.








78

three oxides are similar indicating a similar adsorption mechanism. The insignificant adsorption of PEO on TiO2 indicates the sensitivity of the interaction of the ether oxygen with the surface Bronsted acid sites. Only those MO2 type oxides with a pzc lesser than that of silica are expected to adsorb PEO. Flocculation behavior of MoO3 and V20, with PEO

The flocculation behavior of MoO3 and V1205 as a function of the molecular weight of PEO is plotted in Figure 5.6. It is observed that for both the oxides the critical molecular weight for flocculation is 5,000,000 PEO. Large flocs which were retained over 400 mesh screen, as in the case of silica, were formed. This, to our knowledge, is the first report of flocculation of any oxide other than silica with PEO.

The flocculation of the two oxides with PEO as a function of dosage is plotted in Figure 5.7. It is observed that the breadth of flocculation decreases with increase in the molecular weight of the flocculant. The breadth of flocculation is larger for MoO3 and V120, than Si02 with PEO of 8,000,000 MW (see Figures 4.2 and 5.7). Silica flocculation is reduced to 20% at 8 mg/g while the other two oxides flocculate to the extent of more than 60% at dosage of 15 mg/g. The critical molecular weight of PEO for flocculation of silica is 8,000,000 while that for MoO3 and V20., is 5,000,000. It appears from these observations that the breadth of flocculation increases with the flocculant molecular weight greater than the critical value. Surface charge and PEO adsorption

It was shown in Chapter 4 that a negatively charged surface repelling PEO molecules is not a valid mechanism to explain the decrease of PEO adsorption with increasing pH. Further evidence that the PEO adsorption is not affected by the








79






100 i I I I


-C-- MoO3

80 -- V205



o 60



0
0 40
U
0
E
< 20




0


I I I I
Oe+0 2e+6 4e+6 6e+6 8e+6 le+7
Polymer Molecular Weight


Figure 5.6. Flocculation behavior of MoO3 and V205 as a function
of PEO molecular weight (dosage = 0.5 mg/g; pH = 3.0).








80







100
O


# 60
80




00 60



0o 40
U

0
E
< 20
V 0 MoO3-8M PEO

SMoO3- 5M PEO
0 A V205- 8M PEO ]
v V205-5MPEO

I I I III I
0 2 4 6 8 10 12 14 16
Flocculant Dosage, mg/g

Figure 5.7. Flocculation of MoO3 and V205 as a function of PEO dosage at pH 3.0.







81
negatively charged surface has been provided by adsorption and flocculation of MoO3 and V20s with PEO at pH 3.0 (> -40 mV zeta potential).

It was also shown in Chapter 4 that the presence of adsorbed ions, which in solution state are capable of complexing with the ether oxygen, does not significantly affect PEO adsorption. It is, however, possible that complexation of the dissolved ions from MoO3 and V20s with ether oxygen may occur in solution and subsequent precipitation of the complex on the surface may lead to PEO adsorption. The complexation of Mo ions with PEO in aqueous solution was shown by Vassilev et al [Vas86]. Tests were therefore conducted to isolate the role of dissolved ions in the PEO adsorption process. Effect of dissolved ions

Dissolution studies. MoO3 suspension was aged for 1 hour and the supernatant used to condition A1203 and TiO2 powders to determine the adsorption of dissolved ions on the A1203 and TiO2 surfaces respectively (see Table 6.4). It is observed that the maximum adsorption occurs on AI203 while the least on Sig This is expected since at pH 3.0 A1203 is positively charged while SiO2 is slightly negatively charged and the dissolved Mo is present as negatively charged species MoO42[Kun89]. The formation of crystalline MoO3 on the underlying substrate occurs only when more than 4.5 Molybdenum atoms/nm2 of the surface are present [Kun89]. In the present case, monomeric ions are expected on SiO2 (< 1 Molybdenum/nm2) while heptameric species, Mo7024', and an octahedrally coordinated polymeric surface species are present on A1203 and Tp (1-4.5 Molybdenum/nm2) [Oka88;Kun89].








82













Table 5.4. Dissolution behavior of MoO3 and V20, powders and the adsorption
of dissolved ions on other oxides.

pH = 3.0
Solids loading 2g/1 00 ml
Aging time = 3600 s
Amount of Molybdenum dissolved = 330.6 ppm (16.5 mg/g solids)
Amount of Vanadium dissolved = 275.1 ppm (13.7 mg/g solids)

Oxide Residual in solution Amount of Molybdenum Number of
after 6h adsorbed Molybdenum ions
(ppm) (mg/m2) per nm2 of oxide
surface
TiO2 280.5 0.23 1.4
A1203 268.5 0.44 2.7
SiO;, 320.5 0.16 1.0








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PEO adsorption and flocculation of Mo coated oxides. The non-flocculation behavior of the molybdenum coated alumina and titania was corroborated by adsorption studies wherein no measurable adsorption of 8,000,000 MW PEO was detected indicating that adsorbed ions do not cause adsorption of PEO. Thus it is shown that adsorbed ions do not cause adsorption of PEO. It is expected though that formation of crystalline MoO on the surface of a non-flocculating oxide may lead to PEO adsorption. In another study surface modification of A120 3 by surfactant coating was found to result in PEO adsorption [Ram88]. However, PEO adsorption occurred only after the initiation of hemi-micellization of the surfactant on the alumina surface [Ram88]. This may be due to the interaction between the induced hydrophobic sites on alumina through hemi-micelle formation and the hydrophobic (CH2-CH2)- moiety of the PEO. Thus surface chemical modification through formation of either crystalline MoO3/V2O5 or hemi-micelles of oxides not amenable to flocculation with PEO present alternative routes to induce PEO adsorption.

Role of Lewis Acid Sites

It has been shown so far that the adsorption of PEO on oxides is sensitive to the acidity of the surface Bronsted sites. The other type of acid sites present on the oxide surfaces are Lewis sites which are exposed cations with unsaturated valence as illustrated schematically in Figure 5.8. It is known that the strongest Lewis acid sites (AI3 ions) are found on alumina surface [Dav90]. In principle, an acid-base interaction should be expected between the exposed Al3" ions and ether oxygen of PEO. The A13' Lewis acid site on exposure to water does not convert to







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Figure 5.8. Schematic showing a Lewis acid site (exposed cation).
Large sphere = exposed cation Small spheres = oxygen atoms.








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a Bronsted site [Parr63] and thus there exists a possibility of Al"~ ion and ether oxygen interaction.

Oxide/PEO/CCI, system

van der Beek [Van9l I has shown that inCC4 solvent the adsorption energies for various polymers are larger on silica than on alumina. The strength of the segment-surface interaction for different polymers varied in the same fashion for both alumina and silica. The adsorption energy of PEO was determined to be 1 kT higher on silica than on alumina and this was the largest energy difference between adsorption on silica and alumina for the polymers examined. This energy difference on silica and alumina may indicate that OH groups are more accessible for ether groups than Lewis acid sites. Ether groups in the main chain of adsorbate molecules are less exposed than functional end groups present on other polymers and are therefore more sterically hindered to acquire optimal orientations on the substrate. This steric hindrance will be more for Lewis acid sites than for hydroxyl groups because Lewis acid sites have more rigidly fixed positions on the surface than the protons taking part in hydrogen bonding. The hydroxyl groups have the ability to rotate and may therefore adjust their direction to the adsorbing groups for optimal interactions. Hence, the adsorption energy is significantly affected by stenic hindrance between adsorbate molecules and Lewis acid sites compared to interaction between the ether groups and Bronsted acid sites. Hematite/Starch/Water System

The interaction of Lewis acid sites with the adsorbate polymer molecules has recently been illustrated in the hematite-starch system [Pra9l ;Wei95I. Hydrogen








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bonding between starch hydroxyl groups and mineral surface hydroxyl groups has been the favoured mechanism for many years mainly because of the presence of a large number of hydroxyl groups on the starch and mineral surfaces [1wa82]. Later, evidence was obtained to suggest that a chemical interaction between the polysaccharide and the mineral is the likely mechanism for adsorption [Kho84]. Pradip postulated this interaction as a molecular recognition mechanism of Fe surface sites and the ether oxygen groups in starch molecule [Pra9lI. Recently, Weissenhorn and co-workers [Wei95] have shown through DRIFT studies that Fe sites on the surface of hematite participate in interaction with amylopectin and that hydrogen bonding plays a minor role in starch adsorption.

An important conclusion from the hematite-starch system is that hydrogen bonding cannot be postulated as an interaction mechanism just because seemingly appropriate functional groups are present on the adsorbate and the adsorbent. It is the chemical nature of the surface sites that determines the bonding to the polymer functional groups. It must be noted that the functional groups in starch are not in the backbone chain like the ether oxygen in PEO and hence interaction with the Lewis acid sites is more probable for the former. The PEO molecule, however, also shows specificity of hydrogen bonding to oxide surfaces by interacting only with strong Bronsted sites as shown in the preceding sections.














CHAPTER 6

CHARACTERIZATION OF PEO BINDING SITES ON OXIDE SURFACES Introduction

It is generally accepted that the hydroxyl coverage on oxide surfaces occurs as a result of water dissociation, assuming that every surface oxygen joins the hydrogen atom and the water OH groups are bound to metal atoms (see schematic in Fig 6.1). The evidence for chemical surface hydration of oxides has been provided by infrared absorption studies, heat of wetting measurements, and the thermal behavior of the adsorption-desorption kinetics of water. These studies have also shown that the surface hydroxyls on a given oxide are not equivalent in their chemical nature [You58;Hai67;Roc75].

It was observed that although silica A and silica B show similar affinity for PEO at pH 3.0, in Chapter 4, their behavior was significantly different at pH 9.5. Previous work on silica-PEO/PVA system identified the isolated silanols as the principal binding sites for the ether oxygen of PEO [Rub76;Che85;Kha87]. In this Chapter surface characterization of the silica samples as well as other oxides via DRIFT studies is described to further elucidate the role of isolated hydroxyls in PEO adsorption. Additionally, it was shown in Chapter 5 that the acidity of the surface hydroxyls on oxides determines the interaction with the ether oxygen of PEC. The surface acidity of the oxides is also characterized by DRIFT studies using pyndine as the probe molecule.

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