Electrochemistry in microheterogeneous solutions -- microemulsions

MISSING IMAGE

Material Information

Title:
Electrochemistry in microheterogeneous solutions -- microemulsions
Physical Description:
xi, 138 leaves : ill. ; 28 cm.
Language:
English
Creator:
Myers, Stephanie Ann, 1964-
Publication Date:

Subjects

Genre:
bibliography   ( marcgt )
theses   ( marcgt )
non-fiction   ( marcgt )

Notes

Thesis:
Thesis (Ph. D.)--University of Florida, 1991.
Bibliography:
Includes bibliographical references (leaves 132-137).
Statement of Responsibility:
by Stephanie Ann Myers.
General Note:
Vita.

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 001694775
notis - AJA6880
oclc - 25248356
System ID:
AA00003307:00001

Full Text












ELECTROCHEMISTRY IN MICROHETEROGENEOUS SOLUTIONS--
MICROEMULSIONS

By

STEPHANIE ANN MYERS


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


1991


































This dissertation is dedicated to my grandparents, Cleo

and Dale Myers.















ACKNOWLEDGEMENTS


I would like to thank my committee, Dr. C. Allen, Dr.

V. Young, Dr. Winefordner, and Dr. K. Schanze. I would

especially like to thank Dr. Anna Brajter-Toth, for her help

and encouragement.

I would also like to acknowledge R.A. Mackay for his

collaboration on this project, and U.S. Army Chemical

Research for their financial support.

For all their assistance and friendship, I would like

to thank my colleagues, especially Liakatali Bodalbhai,

Stacey Boyette, Mike Freund and Allan Witkowski.

I would also like to thank my family, especially my

grandparents who were close enough to provide immediate

assistance, when needed.

I would also like to thank God for His support and the

support of His local representatives at University Baptist

Church and in the graduate student Bible study of

InterVarsity Christian Fellowship.


iii

















TABLE OF CONTENTS


ACKNOWLEDGEMENTS . .

LIST OF TABLES . .

LIST OF FIGURES . .

ABSTRACT . .

CHAPTER 1 INTRODUCTION .
Properties and Applications .
Determination of Microstructure
Purpose of this Study .


. iii

. vii


viii


. . 1
. . 1
. . 2
. . 7


CHAPTER 2 EXPERIMENTAL . .. 10
Materials . .... ... 10
Apparatus . . 11
Methods . .. .. .. 11
Preparation of Microemulsions .......... 13
Stability of 1,4-Benzoquinone in Microemulsions 17


CHAPTER 3 DETERMINATION OF MICROEMULSION STRUCTURE
Water Rich Microemulsions . .
Diffusion of Ferrocene (Fc) .
Diffusion of Methyl Viologen (MV2) .
Diffusion of Ferricyanide (Fe(CN)3) .
Partitioning . .
Determination of Droplet Size .
Bicontinuous Microemulsions . .
Structural Changes with Water Dilution
Effect of Oil to Emulsifier Ratio .
Effect of Electrolyte . .
Role of Electroactive Probe in Structure
Determination . .


CHAPTER 4 PROBE REACTIVITY IN MICROEMULSIONS .
Formal Potentials . .
Reactivity of Fc+--Partitioning and
Diffusion Coefficients .
Reactivity of FeCCN)34 .
Reactivity of MV+"I'.. . .
Reactivity of MV/ . .
Kinetics and Adsorption . .


S. 18
18
. 18
21
. 22
22
28
30
30
S. 35
S. 36

S. 38


* 40
* 40

. 40
S. 46
. 46
* 50
. 51











Effect of Surfactant Adsorption on Probe
Kinetics . .
Effect of Surfactant Adsorption on Probe
Adsorption . .


CHAPTER 5 BIOLOGICAL PROBES IN MICROEMULSIONS--
QUINONES .
Introduction to the Biochemistry of Quinones


Spectroscopy of Quinones . .
Electrochemistry of Quinones .
Non-Aqueous Solvents .
Aqueous Solutions . .
Organized Media . .
Electrochemistry of 1,4-Benzoquinone (BQ)
DMF/TEAP . .
Aqueous Solutions . .
Electrochemistry in Microemulsions
Electrochemistry of Ubiquinone 0 .
DMF/TEAP . .
Aqueous Solutions . .
SDS Microemulsions . .
Electrochemistry of Ubiquinone 50 .
Non-Aqueous Solutions .
Aqueous Systems . .
SDS Microemulsions . .
Conclusions . .


CHAPTER 6 MICROHETEROGENEOUS SOLUTION MODEL .
Structure . .
Reactivity . .

CHAPTER 7 MODEL OF ELECTRODE INTERFACE IN
MICROHETEROGENEOUS SOLUTION .
Kinetics . .
Adsorption . .


APPENDIX A




APPENDIX B




APPENDIX C


APPENDIX D


ROLE OF PARTITIONING IN DEPENDENCE OF
PROBE DIFFUSION COEFFICIENT ON THE
CONCENTRATION . .

DEPENDENCE OF FORMAL POTENTIALS ON
DIFFUSION COEFFICIENTS AND PARTITIONING
CONSTANTS . .

EFFECT OF PH AND PKa ON FORMAL POTENTIAL
OF QUINONE . .

QUINONE MECHANISM AND THE EFFECT OF
INDIVIDUAL STEPS ON FORMAL POTENTIAL


51

54


59
59
63
65
65
70
75
75
75
78
85
90
90
93
94
100
100
101
101
105

108
108
111


112
112
114


115




119


124


128


. .
. .
. .
* .*
. .
. .
* .
. .
. .
. .
. .
. .

. .
. .
. .


. .

. .


r

r










REFERENCE LIST . . .. 132

BIOGRAPHICAL SKETCH . .. 138















LIST OF TABLES


Table page

2-1 SDS Microemulsion Compositions Used in This Work 15

3-1 Diffusion Coefficients and Formal Potentials of
Ferrocene (Fc) in SDS Microemulsions .. 19

3-2 Electrochemical Figures of Merit of Ferrocene (Fc)
in SDS Microemulsions . .. 24

3-3 Electrochemical Figures of Merit of Methyl
Viologen (MV+2) in SDS Microemulsions .. 25

3-4 Electrochemical Figures of Merit of Ferricyanide
(Fe(CN)6'3) in SDS Microemulsions .. 26

3-5 Droplet Sizes and Microemulsion Composition 29

5-1 Ubiquinone 50 (UQ50) Absorbance Maxima in
Different Solvents . .. 66

5-2 Ubiquinone 0 (UQO) Absorbance Maxima in Different
Solvents .. . 67

5-3 Formal Potentials of Quinones in Different Media 69

5-4 Acid Dissociation Constants of Quinones 72

5-5 Cyclic Voltammetric Results for 1,4-Benzoquinone
(BQ) in Aqueous Phosphate Buffer ... .81

5-6 Cyclic Voltammetric Results for 1,4-Benzoquinone
(BQ) in Unbuffered 0.1 M NaCl(q .. 84

5-7 Cyclic Voltammetric Results for 1,4-Benzoquinone
(BQ) in SDS Microemulsions . 88

5-8 Cyclic Voltammetic Results for Ubiquinone 0 (UQO)
in SDS Microemulsions . .. 98

5-9 Differential Pulse Voltammetric Results for
Ubiquinone 50 (UQ50) in SDS Microemulsions 104


vii















LIST OF FIGURES


Figure page

1-1 Microstructure of Microemulsions (gEs) 3

2-1 Pseudo Three-Component Phase Diagram of SDS ME 14

3-1 Dependence of DR of Fc on ME Composition .. 32

3-2 Dependence of Do of Fe(CN)6"3 and of MV+2 on gE
Compostion . . 34

3-3 Dependence of DR of Fc on ME Composition for Oil-
to-Emulsifier Ratio of 1:10 and of 2:10 37

4-1 Dependence of E1/2 of Fc on pE Composition 44

4-2 Adsorption of Surfactant on GC Electrode Surface
for both Oil Phase and Aqueous Phase .. 53

4-3 Cyclic Voltammetry of Methyl Viologen 57

5-1 Biologically Significant Quinones .. 60

5-2 Mitochondrial Respiration Cycle . 62

5-3 Cyclic Voltammetry of 3.1 mM 1,4-Benzoquinone in
DMF/TEAP . .... .77

5-4 Cyclic Voltammetry of 1.0 mM 1,4-Benzoquinone in
Phosphate Buffer (pH = 6.9) ... 79

5-5 Cyclic Voltammetry of 1,4-Benzoquinone in
Unbuffered 0.1 M NaCl(aq) . ... 82

5-6 Cyclic Voltammetry of 4.0 mM 1,4-Benzoquinone in
89/1 SDS AE . ... .86

5-7 Cyclic Voltammetry of 3.8 mM Ubiquinone 0 in
TEAP/DMF . .... .91

5-8 Cyclic Voltammetry of 4.5 mM Ubiquinone 0 in 89/1
SDS gE . . .95

viii









5-9 Cyclic Voltammetry of 4.0 mM Ubiquinone 0 in 34/6
SDS gE . . 97

5-10 Cyclic Voltammetry of 1.0 mM Ubiquinone 50 in 45/5
SDS AE . .. 102















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

ELECTROCHEMISTRY IN MICROHETEROGENEOUS SOLUTIONS--
MICROEMULSIONS

By

Stephanie Ann Myers

August 1991

Chairperson: Anna Brajter-Toth
Major Department: Department of Chemistry

Electrochemical methods were used to determine the

effect of microemulsion (AE) composition on the

microstructure of sodium dodecylsulfate (SDS)/1-

pentanol/dodecane/0.1 M NaCl(aq) gEs and the effect of AE

microstructure on probe reactivity.

Chronocoulometry was used to measure diffusion

coefficients (DR or Do) of electroactive probes. Oil phase

structure was reflected by DR of ferrocene (Fc), Do of

ferricyanide (Fe(CN)6 3) reflected the water phase structure

and Do of methyl viologen (MV'2) reflected the surfactant/

membrane structure. Both droplet and bicontinuous micro-

structures were detected. Droplet sizes were determined

from DR of Fc through the Stokes-Einstein equation.

Changes in electrochemical activity with changes in

microstructure were determined by cyclic voltammetry. Oil-









soluble compounds were modeled by Fc0/o. Water soluble

compounds were modeled by Fe(CN)6-3/4. Probe interactions

with the surfactant were modeled by MV+2/'/0. Quinones,

specifically 1,4-benzoquinone, ubiquinone 0 and ubiquinone

50, modeled biological compounds. Formal potential (E*') is

related to partitioning constants and diffusion

coefficients. Since these are a function of ME composition,

E' can be controlled by altering ME composition.

Alternatively, partitioning constants can be determined by

shifts in E'. Kinetics of electron transfer and probe

adsorption are affected by surfactant adsorption on the

electrode. Kinetics are unchanged if the probe easily

penetrates the surfactant, slowed if the probe is repelled

by adsorbed surfactant or enhanced if the probe is attracted

to adsorbed surfactant. Kinetics can be altered by changes

in ME composition which modify the surfactant layer.

Adsorption of hydrophobic probes was eliminated by probe

solubilization in the oil phase and weak adsorption of

probes with both electrostatic and hydrophobic interactions

with the adsorbed surfactant was observed. Quinones in pEs

react from a protic environment (although not necessarily

the aqueous phase). Exchange across the surfactant layer,

both the membrane phase and that adsorbed on the electrode,

was slow for ubiquinones.















CHAPTER 1
INTRODUCTION


Properties and Applications


Microemulsions (AEs) are thermodynamically stable

mixtures of oil, water and surfactant [1]. Often they also

contain a cosurfactant, which is usually a medium chain

alcohol. Inorganic electrolytes, such as NaCI, may also be

included in the mixture. Besides being thermodynamically

stable, jEs are microscopically heterogeneous with distinct

oil and water regions. Other useful properties of gEs

include low interfacial tensions [2,3], high interfacial

areas [4,5], optical transparency [4,6-8] and controllable

microstructure [2,9].

One application of jEs is as an alternative to

conventional solvents [2,8,10]. Being microscopically

heterogeneous, pEs can solubilize significant amounts of

both oil-soluble and water-soluble compounds [8,11]. For

instance, while the solubility of ferrocene in water is only

0.05 mM [12], cetyltrimethylammonium bromide (CTAB) .Es

containing ca. 90% water can dissolve more than 5 mM of

ferrocene [13]. Since reagents in pEs are localized,

effective local concentrations are increased. This can

increase reaction rates [7,8,11,14,15]. The localization of

1











reactant can also limit possible mechanisms for reactions

[4,8] e.g., reducing side reactions, such as isolating a

fluorophor from a quencher [5]. The rate of reaction can be

controlled when gE structure is changed [5,6,16]. Reactions

can be followed spectroscopically, since MEs are optically

isotropic [4,6,7,8].

The heterogeneity of MEs makes them also of interest as

biomimetic systems [4,6,9,16,17]. As in biological systems,

reactions in /Es often occur within or across an interface

[4,18]. Effects of interfacial environment on biologically

significant reactions were shown by Letts and Mackay [18],

who studied the incorporation of copper into tetraphenyl-

porphine in different AE systems. Khmelnitsky et al. [9]

have shown that catalytic activity of trypsin is determined

by microstructure.


Determination of Microstructure


On the microscopic level, gEs organize into specific

structures, with distinct oil and water regions [1,6,19].

In yEs with high water content, i.e. oil-in-water (O/W) .Es,

oil is confined to discrete droplets and water acts as a

continuous phase. The droplets are surrounded by a well-

defined layer of surfactant and cosurfactant [20], which is

often referred to as the membrane phase (Figure 1-1A).

Similarly, water-in-oil (W/O) yEs have high oil content and

water droplets, surrounded by the membrane phase, exist in a



















water


A







C


Figure 1-1


Microstructures of Microemulsions


O/W droplet ME
W/O droplet AE
Bicontinuous ME


oil

- membrane


D











continuous phase of oil (Figure 1-1B). If both oil and

water content of the solution are significant, the so-called

bicontinuous ME is formed. In such systems, both oil and

water act as continuous phases, with a sponge-like

organization (Figure 1-1C) [21]. As with droplet AEs, the

membrane phase separates the oil from the water [6].

As described above, AE structure is generally

determined by its relative oil and water content. However,

other factors also influence structure. Of these,

cosurfactant chain length has the most significant effect

[22-24]. Medium chain length alcohols, such as butanol and

pentanol, promote bicontinuous structures [22,24]. On the

other hand, longer chain alcohols promote a droplet

structure [22,24]. Other composition effects are observed

primarily in droplet pEs. In W/O PEs, shorter chain oils

are more likely to penetrate into the membrane layer. This

causes a more rigid and curved membrane phase and,

therefore, smaller droplets [23]. The nature of the

surfactant, including headgroup size and charge, counterion

and chain length have only a slight influence on structure

[24]. However, W/O MEs can be perturbed toward bicontinuous

structures with reduction of surfactant chain length [24].

In W/O EEs, droplet size can be increased by increasing the

cosurfactant/surfactant ratio or decreasing electrolyte

concentration [2,23]. Similarly, the droplet size of a O/W

ME can be increased by decreasing the cosurfactant/









5
surfactant ratio or increasing the electrolyte concentration

[2,3,23,25,26].

Many techniques have been used to determine the

microstructure of yEs. Quasielastic light scattering (QELS)

[17,19,25,26,27] and small angle neutron scattering (SANS)

[21,28] have been used to determine droplet size in both O/W

and W/O systems. Fluorescence quenching [29-31] has been

used to determine surfactant aggregation number or the

number of surfactant monomers attached to the droplet. From

this number, droplet size can be determined. Typical

droplet radii are 30-300 A [4,5,17].

The techniques mentioned above are used only to

determine droplet size. Consequently, they provide little

useful information about bicontinuous yEs. Fourier

transform pulsed gradient spin echo nuclear magnetic

resonance (NMR) may be used to determine the microstructure

of both droplet and bicontinuous gEs [1,22,32] through

measurement of the self-diffusion coefficient of each gE

component. If the diffusing species is attached to the jE

droplet, it cannot diffuse over macroscopic distances and,

therefore, the diffusion coefficient measured is the

diffusion coefficient of the droplet (Ddrop) [19,26,33]. For

example, in an O/W ME, the species attached to the droplet

would be either oil or surfactant. The Stokes-Einstein

equation relates Ddrop to droplet size. Typical self-

diffusion coefficients of species confined to AE droplets











are on the order of 10'7 cm2/s [24,32]. On the other hand,

typical self-diffusion coefficients of the continuous phase

species in a droplet MEs are slightly lower than in neat

liquid (DO) due to the obstruction effect [20,32,34-36].

Typical values of D" = 10-5 cm2/s [22,32]. The obstruction

effect slows the self-diffusion coefficient of a component

in a continuous phase by lengthening its diffusion path.

For example, the volume excluded by oil and surfactant in an

O/W pE will lengthen the diffusion path of water, since

water will not penetrate the oil droplets [20]. In

bicontinuous jEs, the volume of excluded phase is larger

than in droplet MEs and the self-diffusion coefficients of

both oil (Doi) and water (Dwate) are lower than in the

continuous phase of a droplet pE. Typical Doi and Dwater

values in bicontinuous AEs are 2-10 X 10-6 cm2/s [24,32].

These self-diffusion coefficients are lower than the self-

diffusion coefficient values of the same component in a neat

liquid and higher than the values for the same component in

droplets. Since the diffusion path of the surfactant is

restricted by both oil and water, its self-diffusion

coefficient (Dsuf) in bicontinuous AEs is lower than either

DOi or Dater, typically ca. 1 X 106 cm2/s [3,24,32].

Electrochemical methods offer a simple and convenient

method to determine pE structure with the same versatility

as NMR. With electrochemical techniques, the diffusion

coefficient of an electroactive probe (D') will measure











diffusion in the phase in which the probe resides

[12,13,19]. Consequently, D' determines gE structure in the

same manner as self-diffusion coefficients from NMR. For

example, a probe which resides in droplets typically has D'

= 10'7 cm2/s [13,19,26] and a probe in a bicontinuous phase

has D' on the order of 10"6 cm2/s [13]. Electrochemical

methods can also be used to determine droplet size from the

D' of a probe attached to the droplet. Like self-diffusion

coefficient values from NMR, D' is related to droplet size

by the Stokes-Einstein equation. Droplet sizes determined

electrochemically compare well with those from other

techniques, such as QELS [13,19,26]. In addition,

electrochemical techniques can be used to study redox and

related chemical reactivity in the microheterogeneous

environment of .Es [11-13,37,38].


Purpose of this Study


In this work, electrochemical methods were used to

determine the structure of an anionic gE of sodium

dodecylsulfate (SDS)/l-pentanol/dodecane/0.1 M NaCl(aq) and

to determine the parameters which control the structure. In

addition, reactivity as a function of AE structure was

evaluated. Since many phospholipids, which are components

of biological membranes, carry a negative charge, an anionic

AE should provide a better model for biological systems than

cationic or nonionic MEs. The large AE region [39] allows











changes in structure to be observed over a wide range of

compositions. Since this AE has been well-characterized in

the literature, there is sufficient data for comparison of

electrochemical results to those obtained by other methods.

Diffusion coefficients of well-characterized probes

were measured and used to determine AE structure at

different compositions. Ferrocene (Fc) was used to probe

the oil phase of the AE. Because of its low solubility in

water [12] compared to dodecane [40], Fc resides almost

completely in the oil phase. Water-soluble ferricyanide

(Fe(CN)6 3) was used to probe the aqueous phase. Since

Fe(CN)6"3 is negatively charged, it does not interact with

the anionic surfactant. Methyl viologen (MVf2) was used to

probe the membrane phase. Since MV+2 is a cationic, water-

soluble probe, it associates with the anionic surfactant

layer.

The effect of AE structure/composition on the

reactivity of these probes was also investigated. Both

formal potential (reactivity) and electrochemical

reversibility (kinetics) were affected by gE composition.

1,4-Benzoquinone (BQ), ubiquinone 0 (UQO) and ubiquinone 50

(UQ50) were used to model changes in reactivity of simple

biological molecules with gE composition.

Effects of AE composition on structure and reactivity

were also shown by comparison of the results of this study

to the results of a study of a CTAB/1-butanol/hexadecane/









9

water AE [13]. Differences in structure of EEs were

demonstrated by a comparison of diffusion coefficients of

probes residing in similar phases. In the CTAB pE, Fc

probed the oil phase, MV+2 probed the aqueous phase and

Fe(CN)6"3 probed the membrane phase. Reactivity and kinetics

of these probes were shown to be different in CTAB than in

SDS AEs.















CHAPTER 2
EXPERIMENTAL


Materials

Sodium lauryl sulfate (sodium dodecylsulfate, SDS),

sodium chloride (NaCl), potassium chloride (KC1), 1,4-

benzoquinone (BQ), n,n'-dimethylformamide (DMF), HPLC grade

acetonitrile (ACN) and potassium ferricyanide (Fe(CN)6-3)

were obtained from Fisher. 1-Pentanol, 2,3-dimethoxy-5-

methyl-l,4-benzoquinone (ubiquinone 0, UQO) and methyl

viologen dichloride hydrate (MV+2) were obtained from

Aldrich. Ferrocene (Fc) was from Arapahoe Chemicals.

Tetraethylammonium perchlorate (TEAP) and tetraethylammonium

chloride (TEAC) were obtained from Kodak. n-Dodecane was

from Alfa products. Water used was deionized and then

distilled. Both DMF and ACN were dried over 4A molecular

sieves (Fisher) before use. All other chemicals were used

without further purification.

Phosphate buffer of ionic strength 1.0 M was prepared

from anhydrous dibasic sodium phosphate, NA2HPO4

(Mallinckrodt), and monobasic sodium phosphate, NAH2PO4'H2O

(Mallinckrodt, Fisher), in deionized distilled water.

Adjustments of pH of the buffer solutions were made by











adding a small amount of either phosphoric acid, H3PO4

(Mallinckrodt), or sodium hydroxide, NaOH (Fisher).


Apparatus


For cyclic voltammetry (CV) and chronocoulometry a

Bioanalytical Systems Electrochemical Analyzer (BAS-100) was

used. In the electrochemical measurements, which were

conducted in a three electrode configuration, the working

electrode was glassy carbon (GC) from High Performance,

Englewood, CA, or Electrosynthesis. Glassy carbon

electrodes were prepared by sealing a glassy carbon rod (3

mm in diameter) in a glass tube with epoxy cement (Dexter).

Mercury was used to electrically connect GC to a copper wire

lead. The auxiliary electrode was a platinum wire and the

reference was a saturated calomel electrode (SCE). Before

each measurement, GC working electrodes were polished with

Gamal gamma alumina/water slurry (Fisher) on a microcloth

using Ecomet 1 polishing wheel (Beuhler). After polishing,

the electrodes were ultrasonicated in deionized distilled

water for about five minutes immediately before use.

Ultraviolet spectra were recorded using a Tracor

Northern TN-6500 diode array spectrophotometer.


Methods


Working electrode areas were determined by

chronocoulometry using 3.1 X 10-3 M Fe(CN)6-3 in 0.5 M KC1 (aq)











Using the diffusion coefficient, Do = 7.6 x 10.6 cm2/s [41],

GC electrode areas were determined to be 0.070.01 cm2. In

the measurements of electrode area, the pulse width was 250

ms and the potential was stepped from +0.400 to -0.100 V.

Typical resistances in pEs were between 100 to 500 n

before compensation and were compensated to less than 50 0

using the BAS-100. Peak potentials (Ep) and peak currents

(ip) were measured after iR compensation. The separation of

anodic (Epa) and cathodic (Epc) peak potentials, AEp, was

used to estimate the kinetics of electron transfer. For

reversible (fast) systems, AEp = 60/n mV, where n = number

of electrons transferred. Systems with slower kinetics

(i.e. quasi-reversible or irreversible) have AEp > 60/n mV.

Peak current (ip) depends on scan rate (v). For diffusion

controlled systems, ip 0 v1/2 and for adsorption controlled

systems, ip v. Thus slopes of 0.5 of a log ip vs. log v

plot indicate diffusion controlled behavior. Slopes > 0.5

indicate adsorption effects. Slopes < 0.5 occur when slow

kinetics decrease ip. Reactivity of an electroactive probe

in solution is related to its formal potential. Formal

potential, E"', was estimated as E' = E1/2 = (Epa + Epc)/2.

As kinetics become slower, E1/2 becomes a less accurate

estimate of E0'.

The potential step window for chronocoulometry was

chosen following CV. The potential was stepped from a

potential where no electrochemical reaction occurs to a











potential where the electrochemical reaction is diffusion

limited. Pulse widths were 250 ms. Diffusion coefficients

for the reduced, Dg, or the oxidized, Do, form of a probe

were calculated from slopes of plots of Q vs t'12 [42]. All

measurements were carried out at 252C and all potentials

(including values from the literature) are cited vs. SCE.


Preparation of Microemulsions



The phase diagram of the SDS AE used in this study

(Figure 2-1) has been reported [39]. The diagram of the

pseudo three-component system represents aqueous (0.1 M

NaCl( ), oil (dodecane) and emulsifier (1:2 ratio of

surfactant (SDS) to cosurfactant (1-pentanol)) phases in

weight percent (w/w). The composition of the gE was varied

along the two straight, solid lines shown in Figure 2-1.

These lines correspond to keeping the ratio of oil to

emulsifier constant at 1:10 (A) or at 2:10 (B), while

changing the ratio of oil to water. The range of

compositions which was investigated corresponds to a

relatively low oil content (< 10% oil) and a range of water

content from 89% to 34%. Specific compositions are

summarized in Table 2-1. In the text, gEs are referred to

by their ratio of water to oil. For example, a SDS gE with

89% water, 1% oil and 10% emulsifier is an 89/1 SDS gE.

Since % water was varied over a wide range, results in the

figures are plotted versus % water. It is apparent that in


















4N



00
cb.
U .
A

C8 O/


80 60 40 20
Water + NaCI 0.1 mol/I


Figure 2-1 Pseudo Three-Component Phase Diagram

From reference 39. Aqueous phase is 0.1 M NaC1i ,
oil phase is dodecane, and emulsifier is a 2:1
ratio of 1-pentanol to SDS. Units are in weight
percent (w/w). The two straight lines indicate
dilution lines used in lE preparation, with
constant oil to emulsifier ratio of 1:10 (A) and
2:10 (B). Exact compositions are listed in Table
2-1. Points on these lines correspond to specific
compositions.











Table 2-1
SDS Microemulsion Compositions Used

brineb %dodecane %SDS

34.02 5.94 20.09

45.00 5.03 16.64

52.02 8.00 13.32

56.12 4.01 13.27

64.00 6.00 10.00

67.02 3.00 10.01

76.00 4.00 6.66

88.02 1.98 3.32

89.00 1.00 3.35


in This Work

%l-pentanol

39.96

33.33

26.66

26.60

20.00

19.96

13.34

6.68

6.66


units are weight percent (w/w)

b0.1 M NaCl(,q), unbuffered, pH z 5.6

coil to emulsifier ratio = 1:10

doil to emulsifier ratio = 2:10


AEa%

34/6c

45/5C

52/8d

56/4c

64/6d

67/3c

76/4d

88/2d

89/1c









16
this investigation changes in pE composition show the effect

of dilution by water.

In preparing yEs, each component was added by weight

and the solution was mechanically stirred until clear and

homogeneous. The AEs were stable for several months and

could be frozen and thawed. Ultrasonication was used to aid

in dissolving the electroactive probes in gEs.

The reduction of oxygen is observed at ca. -0.550 V

in MEs. With the exception of MV+2, the potential window in

which CV was conducted did not overlap with the potential

window for oxygen reduction.

In the case of MV+2, however, the reduction of oxygen

interferes with the first reduction peak of MV2.

Therefore, solutions of MV+2 were deaerated with nitrogen

(N2) before measurements. The solution was purged with N2

using a bubbler containing AE which was first deaerated for

at least 30 minutes. The composition of pE used in the

bubbler was the same as that used to make the solution of

MV+2. Deoxygenation was confirmed by CV which showed

disappearance of the oxygen peak at -0.550 V. A positive

pressure of N2 was maintained throughout CV and

chronocoulometic experiments when MV+2 solutions were

studied.










Stability of 1,4-Benzoquinone in Microemulsions


1,4-Benzoquinone (BQ) reacts in the presence of light

to form hydroquinone (QH2) and 2-hydroxy-l,4-benzoquinone

(QOH) [43].


2BQ L> QH2 + QOH (2-1)

The disappearance of BQ can by monitored by ultraviolet (uv)

spectroscopy, where the absorbance maximum (Ax) of BQ is

255 nm in both aqueous solutions and in gEs. In aqueous

solutions after one hour, the absorbance at Ax, and,

therefore, [BQ], is 80% of the original value. After one

hour, a decrease in [BQ] is also detected by a decrease of

i in CV.

In SDS gEs, BQ is less stable than in aqueous

solutions. The rate of BQ decomposition depends on gE

composition and [BQ]. After one hour, uv spectra show that

[BQ] is 75% of the original concentration in an 89/1 SDS pE

and 70% of the original concentration in a 45/5 SDS pE. The

solutions used in CV were more concentrated than those used

in uv. Therefore, decomposition of BQ in solutions used for

CV is more rapid than in solutions used for uv spectroscopy.

For example, ip of BQ in a 45/5 SDS gE decreases to 20% of

its original value after one hour. Therefore,

electrochemical experiments were conducted immediately

following preparation of BQ solutions and ip errors are

larger than for the other probes.














CHAPTER 3
DETERMINATION OF MICROEMULSION STRUCTURE


Water Rich Microemulsions

Diffusion of Ferrocene (Fc)


Ferrocene (Fc) is a hydrophobic probe with a reported

solubility in water of 0.05 mM [12] and a solubility in

dodecane of 0.15 M [40]. Since Fc concentrations in gEs

were typically 3 mM, i.e. significantly greater than its

water solubility, the probe is expected to reside primarily

in the oil phase. The concentration was limited to 3 mM by

solubility in the pE. In SDS gEs with Fc concentrations

from 1-3 mM, diffusion coefficients (DR) and formal

potentials (E1/2 = (Epa + Epc)/2 E01) were not dependent on

the concentration of the probe (Table 3-1). In CTAB gEs,

dependence of D, on probe concentration was observed at [Fc]

< 5 mM [13]. The oxidation product of Fc, ferricinium

cation (Fc+), is water soluble. Water solubility of Fc' has

been demonstrated in micellar solutions, where Do ; Daq = 6.7

X 10-6 cm2/s [12,38], rather than a lower Do (=10"7 cm2/s)

typical of the electroactive probe interacting with

micelles.











Table 3-1
Diffusion Coefficients and Formal Potentials of Ferrocene
(Fc) in SDS Microemulsions



pEa [Fc] Db X 106 E1/2

(mM) (cm2/s) (mV)

89/1 0.77 0.810.02 +2254
1.84 0.530.03 +2213
2.25 0.580.03 +2193

67/3 0.94 3.230.06 +2755
1.97 2.840.04 +2672
2.88 2.730.04 +2739

45/5 0.83 4.310.04 +29716
1.52 4.060.13 +2913
2.44 3.360.02 +29911
3.15 3.460.05 +2914


aSee Table 2-1 for exact SDS jE compositions.

bD, = D' used in equations 3-4 and 3-5.










In water rich SDS yEs, oil-soluble Fc was expected to

reside in droplets, with DR reflecting the diffusion

coefficient of droplets (Ddrop) In 89/1 SDS AE, Fc DR = 5.8

X 10'7 cm2/s, ca. an order of magnitude lower than in aqueous

solutions (DR, = 6.7 X 10'6 cm2/s [12]) or in dodecane (DR,oit

= 4.4 X 10.6 cm2/s [44]). Therefore, DR does not reflect Fc

diffusion through a continuous phase but is consistent with

the expected diffusion with the droplet and is a measure of

drop [45]. The self-diffusion coefficient of oil in O/W MEs

measured by NMR is typically Doi 10'7 cm2/s [32], in

agreement with this model [22,33,36]. Measurement of

droplet diffusion by QELS also gives Ddr 10-7 cm2/s [26].

Diffusion coefficient is related to droplet size by

the Stokes-Einstein equation:


D = kT/67rlr (3-1)

where D is the diffusion coefficient (cm2/s), k is the

Boltzman constant (J/K), T is the temperature (K), n is the

viscosity of the solvent (P), and r is the radius of the

diffusing species (cm). Since DR of Fc reflects Ddrop, it can

be used to calculate droplet size from equation 3-1.

Typical droplet sizes which were obtained from such

calculations correspond well to droplet sizes determined by

techniques such as QELS [25] and SANS [28]. Calculations of

droplet sizes from electrochemical DR are discussed in

detail in a later section of this chapter.










Diffusion of Methyl VioloQen (MV+2)


Water soluble MV+2 is reduced through a stable cation

radical, MV', to water insoluble MV:


MV+2 + e" MV+" (3-2)


MV"* + e" MV (3-3)


The MV+2 is expected to associate with the anionic SDS in

the membrane layer. Typical MV+2 concentrations were 1 mM,

since at higher concentrations the pE became turbid. This

demonstrates that SDS ME structure is sensitive to

electrolyte concentration and changes in electrolyte

concentration can result in a solution which does not form a

.E.

In water rich MEs, MV+2 should be electrostatically

attached to the anionic membrane layer of the droplet and,

like Fc, can be used to measure droplet diffusion. In all

SDS iEs tested, Do is significantly lower than in aqueous

solutions, where Doaq = 6.56 X 10-6 cm2/s [46]. The low Do

indicates that the probe moves with the aggregates.

However, DO of MV*2 is not as low as Ddrop [3,13,19,26,32],

nor Dsurf [32], both of which are ca. 10'7 cm2/s.

Partitioning of MV+2 between the membrane and aqueous phases

may contribute to higher electrochemical Do. Effects of

partitioning on electrochemical Do will be discussed in

detail in a later section of this chapter.










Diffusion of Ferricyanide (Fe(CN),3)


Due to electrostatic repulsion from the membrane, the

water soluble anions, Fe(CN)6'3 and its reduction product,

Fe(CN)6"4, reside in the aqueous phase of SDS yEs.

Concentrations of Fe(CN)63 were typically 1 mM; similar to

the effect of MV*2, higher concentrations were found to

cause turbidity.

Since Fe(CN)6"3 resides in the aqueous phase, its Do

should be a measure of probe diffusion in water. In an 89/1

SDS ME, the diffusion coefficient of Fe(CN)6"3, Do = 5.9 X

106 cm2/s, is only slightly lower than D ,,ac(aq) = 6.7(0.1) X

10-6 cm2/s and DKCLaq) = 7.6 X 10.6 cm2/s [41]. The small

decrease in Do can be attributed to the obstruction effect.

In NMR studies, a similar obstruction effect was shown to

reduce Dwater. For example, in a O/W SDS/1-butanol/toluene/

water AE, Dwate = 1.5 X 10-5 cm2/s, compared to neat water,

D"water = 2.27 X 10'5 cm2/s [32].

Partitioning


As is apparent from Do values obtained for MV+2 in a

water rich ME, probe partitioning can affect diffusion

coefficients. Partitioning will also occur with other

probes, as demonstrated with Fc in CTAB yEs [13]. In a

multicomponent system, such as a ME, all electrochemical

diffusion coefficients are affected by probe partitioning

[46] because of the resulting multiple diffusional paths.










These paths occur because of place (mass) or electron

exchange across phases. Low concentrations and large

differences in E'' decrease the driving force of an

electron-exchange reaction [47]. In droplet pEs,

partitioning can contribute to higher diffusion coefficient

values than expected for Ddrop. In the iEs studied in this

work, electron exchange is unlikely since it requires a

similar formal potential (E'0) in both hydrophobic and

hydrophilic phases [47]. As shown in Tables 3-2 and 3-3,

E1/2 of Fc and of MV*2 in gEs are significantly different from

the E''aq. (Table 3-4 containing the electrochemical

figures of merit for Fe(CN)6-3 is included following Table

3-2 and 3-3 for comparison.) However, residence times of

probes in pEs are typically ca. 10'5 to 10'3 s [8]. This is

faster than the time scale of chronocoulometry, which was

used to measure diffusion coefficients. Therefore, cross-

phase place exchange may be considered rapid.

The relationship between apparent diffusion

coefficient (D') measured electrochemically and diffusion

coefficient of the droplet (Ddrop) depends on the rate of

exchange across the phases [46]. When exchange between

phases is fast, the D' may be expressed as


D' = D1,f + D2f2 (3-4)

where, for a droplet ME, D1 = Daq, and f, is the fraction of

probe in the aqueous phase; D2 = Ddro and f2 is the fraction












Table 3-2
Electrochemical Figures of Merit of Ferrocene (Fc) in SDS
Microemulsions


[Fc ] MEa

(mM)


4.98 ACNc

2.11 34/6

3.15 45/5

3.13 52/8

2.80 56/4

3.06 64/6

2.88 67/3

2.98 76/4

2.87 88/2

1.54 89/1

-- SDSd

-- aqe


E b
E1/2b

(mV)


+4194

+3113

+2914

+2926

+2821

+2876

+2739

+2687

+2204

+2193

+195

+160


AEpb

(mV)


618

646

678

756

622

686

7418

7214

599

555


iab

(AA)


1605

24.30.1

33.61.7

34.40.6

30.10.6

34.10.4

28.80.9

27.10.7

9.30.1

9.40.2


iWib


0.990.09

1.290.08

1.160.09

1.310.06

1.260.04

1.300.02

1.140.13

1.310.06

1.150.05

1.080.05

--


DR X 106

(cm2/s)


3.87.11

3.46.05

3.30.03

3.04.06

3.26.03

2.73.03

2.38.07

0.32.01

0.58.03

0.61

6.7


aSee Table 2-1 for exact SDS ME compositions.

by = 100 mV/s, iR compensation to < 50 n

C0.5 M TEAC in ACN

d0.28 M SDS in 0.1 M NaCl(aq) [37]

e0.1 M NaCl(aq) [38]





























































































r- 01 H 0 l m r


10
0
x




N


(4
1 U





*0
N
-,

.9-


0

o



a)
0

.r1


M
I



Cm











0
*4
N









Cl

.1



- 4J


44I
QC











C4
0


U)
9-1
r4











cu
0
r-I








04.
aQ)
rz
Sl
(M


.0
n
04>
N






0 ->
ia
JI




.




















N
I *
i,
*^(


O on

0 VD


0 0o
.sr M


n a0






CO
** *
co






o o
'. .o















Table 3-4
Electrochemical Figures of Merit of Ferricyanide (Fe(CN)63)
in SDS Microemulsions


[Fe(CN) 63]

(mM)

1.04

1.37

1.19

1.05

1.14

1.09

1.05

1.10

1.17

2.68

2.46


MEa



34/6

45/5

52/8

56/4

64/6

67/3

76/4

88/2

89/1

SDSc

aqd


E b
E1/2

(mV)

+22211

+20014

+2007

+18013

+17114

+17315

+15436

+15420

+1515

+14023

+17225


AEpb

(mV)

8122

10828

15716

26926

38127

36430

60772

30840

84610

88545

15260


ib

(MA)

4.40.2

6.00.6

4.80.2

2.50.3

5.90.3

6.10.2

4.20.1

8.90.1

9.50.2

22.1.5

31.42.3


i /ib Do' X 106

(cm2/s)

1.180.06 0.540.01

1.130.24 0.320.01

1.090.07 0.920.04

1.000.16 1.00.1

1.590.42 1.80.1

1.220.06 1.60.1

1.760.16 4.00.1

2.020.21 5.30.2

2.370.29 5.90.1

2.210.20 5.20.3

0.980.13 6.7+0.1


aSee Table 2-1 for exact


SDS ME compositions.


by = 100 mV/s, iR compensation to < 50 n

c2.97 mM SDS in 0.1 M NaCl(q)

d0.1 M NaCl(q)
(aq)












27
of the probe in the droplet [46]. For bicontinuous pEs, D1

= Dg and D2 is a diffusion coefficient for a probe in the

continuous oil phase.

If more than one probe is bound to a droplet, D' may

depend on probe concentration, Cx (See Appendix A). The

dependence of D' on Cx has been described by Rusling et al.

[48] where


D' = D1/(l + CK"Cxn-1) + D2CMK"Cxn-1/(1 + CMK"Cxn-1) (3-5)


In equation 3-5, CM = total droplet concentration, n = the

number of probe molecules bound to a droplet, and Kn is the

equilibrium constant for the binding of n solute molecules

to a droplet:


M + nX MXn (3-6)


where M = droplet and X = probe. More specifically, K =

nK'n, where K', is


K'n = [MXn]/([M][X]") (3-7)


In SDS AEs, D' = DR of Fc does not depend on Cx (Table

3-1). According to equation 3-5, this will occur if n = 1.

However, since high concentrations of Fc were used in this

work, it is reasonable that a droplet will contain more than

one probe (n > 1). Thus, n = 1 cannot explain the

independence of DR on [Fc]. If Kn, Cm, or Cxn'" is very low,













D' will be independent of Cx and D' = D, = D Since DR of

Fc << D&, this does also not explain the independence of DR

on [Fc]. However, if K", C, or Cxn"1 is high, D' = D2 = Ddrop.

This behavior is consistent with the observed DR of Fc,

where DR does not change with [Fc] and DR zDdrop. Unlike in

SDS AEs, D' depends on [Fc] in CTAB AEs [13]. Since Cx and

CM are similar in SDS and CTAB jEs, differences in the

dependence of D' on Cx must be due to differences in K". A

higher K" in SDS AEs leads to D' approaching D2 at lower Cx

than in CTAB AEs, resulting in the observed independence of

D' on Cx in SDS pEs in the same concentration range.

Determination of Droplet Size


As discussed previously, DR of Fc Ddrp in SDS AEs.

However, to accurately determine Ddrop partitioning must be

considered. In this work, [Fc] is close to its solubility

limit in SDS EEs. Therefore, the solubility of Fc in water

(5 X 10-3 mM [12]) is used to calculate fl and f2. Using

equation 3-4, these fractions, DR of Fc = D' and Daq = D,,

Drop (D2) is calculated [12,38]. For an 89/1 SDS AE, Ddrop
3.6 X 10"7 cm2/s is calculated. Using the calculated Ddrop and

assuming the viscosity of the continuous phase to be water =

0.0089 P, the droplet radius (r) is 68 A from equation 3-1.

Table 3-5 lists droplet sizes (r) for several O/W iEs.






















o c N



00


0 0
ocU



O -
L 0 0
0 '. U *







So
4.3
U 0 M -



o
0
QM
0)





a)
r-l
a4



E 3 4
0 -






U)
00




g- S ^
t, (0*


Ln
NCN 0 1
r- Ln I" I* tn


r-

ON


0)
U





0
r-


n 0
-44.4


0 0 %0

co rN cO
c c m
o CE
Q Q)


4
0









C4






cw


*
'-4

















co
CQ
10










0


tn
Q A
o a.

>*

a I

O


1 ) II
>4 14

0 II
S II --

)0







-4 0
43 o H







0 0 0 D



4C) 1
H1 V











uc
-4 r-4
C "^
>1 to



















) II4J-I
= r4
+




C 0


.-4

do o

0
oU1





O 0
4J








.H
R *o1


0



as a

4J .0



3 "
004 4








CN
24


I






0



a) 0

-4 C"
go m








o
r- 0 0

0 -









-4 a)
C C
C, M -










aO.




4e r0

o a


- + 0U)

44 m4-)
(,0 i40


wM 0 U
u s-u 0












30
Values in Table 3-5 are consistent with the values obtained

by other methods which are also listed in Table 3-5.

From QELS it has been shown that if ME oil content

increases, droplet size increases [25]. This is confirmed

by electrochemical results obtained here. For example, r =

123 A in an 88/2 SDS AE, when oil content is twice that of

the 89/1 jE, where r = 68 A (Table 3-5). Increasing the

surfactant to cosurfactant ratio (s:c) decreases the

hydrophilicity of the pE and as a result droplet size, but

to a lesser extent than the changes in oil content. For

example, the 89/1 SDS ME, where r = 68 A, has s:c = 1:2, but

if s:c = 2:1, r = 72 A (Table 3-5).


Bicontinuous Microemulsions


Structural Changes with Water Dilution


Diffusion coefficients of electroactive probes in

bicontinuous EEs correspond to values of diffusion

coefficients in neat liquids. However, diffusion

coefficients in AEs are lower because of obstruction due to

the presence of the membrane phase and the phase in which

the probe does not reside. The larger the volume of these

phases, the more obstruction which will occur and the lower

the diffusion coefficient. Since Fc resides in the oil

phase, as the volume of the obstructing water phase













decreases, DR of Fc will increase. Results in Figure 3-1

show this increase in DR as water content decreases.

According to equation 3-1, the observed increasing DR

of Fc will correspond to a decrease in droplet size if Fc

continues to diffuse with the oil droplets. However, with a

decrease in gE water content, oil content increases and,

therefore, size and/or number of oil droplets will increase

[2,25,31,39,49]. Therefore, changes in droplet size cannot

explain the increase in DR. However, with an increase in

the size and number of droplets, coalescence of droplets is

more likely. Thus, a continuous path through the oil phase

is formed and diffusion through this pseudo-continuous phase

becomes important. This new diffusion path leads to an

increase in DR. From results in Figure 3-1, it is concluded

that with decreasing water content, this form of diffusion

becomes more important.

When continuous diffusion paths through both the oil

and water phase exist, the structure is considered

bicontinuous. The transition from droplet to bicontinuous

structures is confirmed by comparing electrochemical and NMR

diffusion coefficients where changes in DR of Fc with

composition follow Doit values from NMR for both SDS and CTAB

MEs. For example, when water content is high, Doit and DR of

Fc are ca. 10-7 cm2/s [22,32,36,50-53], consistent with a pE

droplet structure. From NMR, as water content decreases,

















5.00 -




4.00


CO
13.00

E


o 2.00
0


X
1.00

0.00

0.00 z


Figure 3-1


"aE

r i "


*.*.. 3 mM Fc in SDS A.E
ooooo 5 mM Fc in CTAB A.E
e Fc in dodecone


I l l lll, t 1,1,1 1 Il l l l l l l l l lTllm i l l
0 20 40 60 80
% water in microemulsion


100-
100


Dependence of DR of Fc on AE Composition

Microemulsion composition is expressed in
weight percent water. See Table 2-1 for exact
compositions.


C



O


I













Don increases, approaching D"oi (neat liquid) but not

achieving it [50-52]. This is consistent with a transition

from discrete oil droplets to a bicontinuous structure with

oil as a pseudo-continuous phase. Specifically, in a

bicontinuous SDS/1-butanol/toluene/water AE, Dtotuene = 8 X

10-6 cm2/s, while D*toLne = 2.4 X 105 cm2/s [32]. Similarly,

electrochemically determined DR of Fc increases as the water

content decreases, without reaching DRoit = 4.4 X 10-6 cm2/s

[44] (Figure 3-1).

While D, of Fc corresponds to DolL from NMR, Do of

Fe(CN)6-3 in SDS AEs corresponds to Dater. In AEs with high

water content, Do is slightly less than Do,,act(). Similarly

from NMR, Dwater is slightly less than Dwater [32] in these

systems. Like Dwater, Do is reduced because of obstruction of

aqueous diffusion by oil droplets [33,36,52-54]. The

changes in Do follow changes in Dwater (Figure 3-2). As water

content decreases, Do decreases due to increasing

obstruction of Fe(CN)6-3 diffusion. The increased

obstruction is a result of increasing amount of the

obstructing oil phase as water content decreases. In CTAB

MEs, Do of MV*2 reflects diffusion in the aqueous phase,

since MV*2 is repelled from the CTAB membrane. Like Do of

Fe(CN)6"3 in SDS IEs, D of MV+2 in CTAB AEs decreases as

water content decreases [13] due to obstruction (Figure

3-2).


















10.00
-



8.00


n:
S6.00
N-



E


o 4.00
0


X
2.00
0
0.00



0.00


1 mM Fe(9CN)63 in SDS AE
ooooo 5 mM MV" in CTAB u.E


'If


20
% water


~~~~~~~~I Ilr I I l ii i.........


I I I I II II I .i ....... ... 1 0I
40 60 80 100
in microemulsion


Figure 3-2


Dependence of Do of Fe(CN)6'3 and of MV+2 on IE
Composition

Microemulsion Composition is expressed in
weight percent water. See Table 2-1 for exact
composition.


--:,
f#













The Do of MV*2 in SDS yEs is a measure of the

diffusion of the membrane or surfactant phase. In SDS pEs,

Do is significantly lower than D0,aq and changes little with

composition (Table 3-3). Similarly, the low Dsurf from NMR

does not change significantly with composition [22,32],

since the movement of surfactant is restricted at all

compositions. In droplet yEs, surfactant, like oil,

diffuses with the droplets. In bicontinuous systems,

diffusion of surfactant is confined to the interface and is

thus slow, typically Dsurf = 1 X 10-6 cm2/s [22,50,51]. The Do

of MV*2, which is ca. 2 X 10"6 cm2/s, is larger than Dsrf,

probably due to a contribution to Do from diffusion of MV*2

in the aqueous phase as a result of MV+2 partitioning

between the membrane and the aqueous phase. In CTAB gEs, Do

of Fe(CN)6-3 reflects Dsf and is lower than Do of MV*2 in SDS

gE [13]. This is due to greater partitioning of Fe(CN)6"3

into the membrane phase.


Effect of Oil to Emulsifier Ratio


As shown for Fc, diffusion of the probe through the

oil phase is significant in bicontinuous systems. Both

electrochemical and NMR results show that diffusion of Fc in

bicontinuous AEs is slower than in neat liquid due to the

obstruction by the surfactant and water phases [50-52].

Consequently, bicontinuous AEs with higher oil to emulsifier













(o:e) ratio will cause less obstruction and, therefore,

higher diffusion coefficients will be measured. This is

shown for Fc in a system with a ratio of o:e = 2:10, where

DR of Fc is higher than in comparable systems where the o:e

= 1:10 (Figure 3-3).


Effect of Electrolyte


The transition from a droplet to a bicontinuous

structure will occur at different compositions, depending on

jE components. For example, Figure 3-1 shows that DR of Fc

in SDS pEs reaches the high (> 1 X 10-6 cm2/s) values which

are characteristic of a bicontinuous AE at higher water

content than in CTAB MEs. Shielding of the droplet charge

by an inorganic electrolyte may lead to more facile droplet

merging [30]. Thus SDS AEs, which contain NaC1, favor a

bicontinuous microstructure at lower oil content than CTAB

gEs which do not contain NaCI. This electrolyte effect can

also be seen when an SDS pE lacks electrolyte. In a 79/5

SDS ME in the absence of 0.1 M NaCIl ), Fc DR = 2.2 X 10.7

cm2/s, typical of DdrI. However, for the same 79/5 system in

the presence of 0.1 M NaC1(a, Fc DR = 1.2 X 10.6 cm2/s. The

higher DR in the presence of NaC1 must reflect bicontinuous

structure since the value is typical of diffusion in a

continuous phase and is significantly larger than Ddrop,

which is typically ca. 10'7 cm2/s. Other studies have shown

















4.00





3.00
e--1~-
C~


E
0 2.00

(0 -
CD


0
X 1.00


0.00


0.00 -


oil:emulsifier = 1:10 4
o000o oil:emulsifier = 2:10
I 1 111i 1 I 1 1' 1 1 1 1 1 11"l I I 1 1 11 1 I I I 1 I 11 i I I 1 i 1 1 I
0 20 40 60 80 100
% water in microemulsion


Figure 3-3 Dependence of DR of Fc on p.E Composition for
Oil-to-Emulsifier Ratio of 1:10 and of 2:10

Microemulsion Composition expressed in weight
percent water. See Table 2-1 for exact
compositions.












38
that droplet size increases when electrolyte is added to the

aqueous phase of an O/W gE [2,23]. An increase in droplet

size should correspond to a decrease in DR (equation 3-1).

Since experimentally, DR with added NaCl is larger, it must

reflect a facile transition of large droplets to a

bicontinuous structure. As ME composition changes, neither

DR of Fc nor Do of Fe(CN)63 show an abrupt change, which

would be expected if the change from droplets to

bicontinuous structures occurred at a specific composition.

Results from NMR also do not show such a change from

droplet to bicontinuous structures [55].


Role of Electroactive Probe in Structure Determination


As is clear from the results, electrochemically

measured diffusion coefficients can be used to determine ME

microstructure by appropriate choice of probes. In both

CTAB and SDS AEs, electrochemically determined DR of Fc is a

measure of diffusion of oil and is comparable to Doi from

NMR (Figure 3-1). Diffusion in the aqueous phase is

measured by Do of Fe(CN)6-3 in SDS AEs and Do of MV+2 in CTAB

MEs, since both are water soluble and do not associate with

the surfactant in the membrane due to electrostatic

repulsion. For this reason, Do values for MV*2 in CTAB IEs

are comparable to values of Do for Fe(CN) 63 in SDS iEs and

Dwater from NMR (Figure 3-2). As with changes in Do of













Fe(CN)63 and Dwater with jE composition, the decrease in MV`2

Do with decreasing water content [13] is due to obstruction.

In SDS LEs, Do of MV+2, like Do of Fe(CN)6 3 in CTAB AEs,

correlates well with Dsurf [32] showing little change in Do

with composition.

In bicontinuous SDS AEs, DR of Fc is larger than in
CTAB yEs of similar compositions (Figure 3-1). This is

probably due to the differences in the viscosity of the oil

phase for each IE and to differences in Fc solubility in

each jE. Viscosity (q) of the hexadecane oil phase in CTAB

AEs is ca. twice that of the dodecane oil phase in SDS gEs

(7hexadecane = 3.34 cP [44], 7odecane = 1.35 cP [44]). It follows
from equation 3-1 that diffusion coefficient will be lower

in hexadecane and consequently DR of Fc will be lower in

CTAB yEs.














CHAPTER 4
PROBE REACTIVITY IN MICROEMULSIONS


Formal Potentials

Reactivity of Fc0+--Partitioning and Diffusion Coefficients


The standard potential, E, is the potential at 25C

when all species in solution are at unit activity. The

potential when the concentration of the oxidized form of the

probe (Ox) and the reduced form (Red) are 1.0 M and pH = 7

is called the standard redox potential (Eo') by biochemists.

In this work, reactivity is measured using the formal

potential, E', where [Ox] = [Red] for specific solutions.

For a reversible system, E0' z E1/2 = (Epa + Epc)/2. As the

system becomes less reversible, the approximation of E0' =

E1/2 becomes less accurate. However, E1/2 can still be used

to observe trends in reactivity.

In droplet 89/1 SDS AE, E1/2 of Fc = +0.2190.003 V,

which is 59 mV more positive than its aqueous formal

potential, E'q = +0.160 V [38]. Since Fc is solubilized

primarily in the oil, and ferricinium ion (Fc*) is

solubilized in water [12,38], their diffusion coefficients,

D, and Do respectively, are not equal. The difference










between DR and Do will cause a shift in E1/2 vs. E'aq [42]

where


E1/ = E 'q + (RT/nF) In (D,/DO)1/2 (4-1)
aq

In this system, Fc DR = 5.8 X 10-7 cm2/s from chrono-

coulometry, and Fc+ Do = Do,aq = 6.7 X 10-6 cm2/s [12,38].

With these values, a negative 31 mV shift vs. E"'a is

predicted from equation 4-1. Since the experimental E1/2 is

more positive than the E" q' differences between DR and Do

cannot account for the observed shift in E1/2. The

partitioning of Fc and Fc* between gE phases directly

affects E1/2 as well. Considering the membrane and oil phase

as one, the relevant equilibria can be expressed as [12,37]


Fc + e" Fc
til tI 1 (4-2)
Fc (oi + e" Fc(oi


where aq and oil represent the aqueous and oil/membrane

phases, respectively. The partitioning constants K, (for

the oxidized form of the probe) and KR (for the reduced form

of the probe) are defined as


Ko = [OX(aq)]/[OX(oiL] = [Fc+(aq)]/[Fc+(oit] (4-3)

and


KR = [Red(q)]/[Red(oit)] = [Fc(aq)]/[Fc(oil)]


(4-4)










In aggregate systems such as droplet PEs, partitioning

constants measured experimentally (K1' and K%') will be


Ko' = [Ox(aq)C/[[Ox(oi1)] = KoC, (4-5)

and


K' = [Red(,q]CM/[Red(Oi,)] = KC (4-6)

where C, is the concentration of aggregates, i.e. droplets

in a droplet AE. In micellar solutions containing

aggregates, typical C, values are in the mM range [29,48]

and ME droplet concentrations should be similar [29]. In a

strict definition of K%' and K' for the equilibria between

aggregates and the probe, C, must be defined as the

concentration of aggregates without a probe. However, if

Poisson distribution is assumed, i.e. that the solubil-

ization of one probe in an aggregate does not affect

solubilization of the next probe, then C, = total droplet

concentration [56].

Ohsawa and Aoyagui [37] have shown that for systems

such as the one described in equation 4-2, E1/2 dependence on

% and Ko can be described by (See Appendix B)


El/2 = E'aq + RT/nF In (D,/DO)1/2
+ RT/nF In (Ko(l+KR)/KR(1+Ko)) (4-7)

Since Fc* is water soluble, K>>1. Therefore, equation 4-7

reduces to










E1/2 = E' + RT/nF In (DR/Do)1/2
+ RT/nF In {(1+KY) /K} (4-8)

Using in equation 4-8, Fc DR = 5.8 X 10-7 cm2/s determined

from chronocoulometry, Do,, for Fc* = Do = 6.7 X 10-6 cm2/s

[12,38], E"'@ = +0.160 V [38] and E1/2 = +0.219 V from CV, KR
is estimated to be 3 X 102. This KR value is consistent

with literature values for binding of hydrophobic probes

[57] to micelles. The KR calculated from the shift in E1/2

is also consistent with Kg calculated from the water

solubility of Fc. Using equation 4-4, [Fc ] = 0.05 mM [38]
and [Fcoit] = [Fctotat] [Fcaq] = 1.49 mM, then KR = 3 X 102.

With decreasing AE water content, E1/2 of Fc becomes

more positive (Figure 4-1). In a 34/6 SDS ME, Fc E/2 =

+0.311 V, 151 mV more positive than E01 (Table 3-2). In

this system, DR has increased to 3.9 X 10-6 cm2/s (Table 3-2)

and Do of Fc+ will be lower than in the 89/1 system because

of the increased obstruction effect. Like Fc, Fe(CN)6'3 and

hydroquinone are water soluble and in pEs of similar

composition, Do = 5.4 X 10'7 cm2/s for Fe(CN)63 and D, = 3 X

10'7 cm2/s for hydroquinone [39]. From these values, the

minimum Do must be 3 X 10-7 cm2/s. Using these values for DR,

Do and K, = 3 X 10-2 in equation 4-8, the maximum E1/2 shift,

due solely to changes in diffusion coefficient with

composition, is +123 mV. While the direction of the shift

is correct, the magnitude is not sufficient to account for

















400-



350-



E 300

LJ
O 250
Cn


> 200
CN%
r-


150-



100-


Figure 4-1


*_*.* 3 mM
0oooo 5 mM
eeeee Fc in


Fc in SDS /JE
Fc in CTAB pE
aqueous solution


IliI I l II I T I I I I Tl l l lT I I I I I I F I I I I I I I I I I I I 1
20 40 60 80 100
% water in microemulsion



Dependence of El/z of Fc on IE Composition

Microemulsion composition is expressed in
weight percent water. See Table 2-1 for
exact compositions.


%%
\.


I5- I:-










the observed +151 mV shift. Therefore, Kg must be a

function of tE composition. Using the values above for DR

and Do in equation 4-8, it follows that for the 34/6 SDS AE,

Kg 5 1 X 10-2, which is a decrease from the KR value in the

89/1 SDS ME. According to equation 4-6, KR will decrease

when C, increases. In the bicontinuous 34/6 SDS ME, C,

obviously cannot be a measure of droplet concentration, as

defined for the 89/1 AE. However, since the probe

partitions into the oil and membrane phases of the ME, CM

must be related to their concentration in bicontinuous yEs.

Oil and surfactant content, thus C,, is greater in the 34/6

AE than in 89/1 pE. Therefore, the observed decrease in Kg

is consistent with equation 4-6.

As shown in Figure 4-1, E1/2 of Fc in CTAB MEs is more

positive than in SDS MEs of similar composition.

Differences in DR do not account for the differences in E1/2

in droplet yEs. Since, DR is about the same in droplet SDS

(Table 3-2) and CTAB MEs [13] and to obtain the more

positive E112, DO would have to be much lower in CTAB than in

the SDS system. This is unlikely since electrostatic

attraction of Fc+ to the SDS predicts lower DO in SDS MEs.

It is more likely that KR is smaller in the CTAB ME, leading

to more positive E1/2 shifts. Smaller KR in CTAB ME is

consistent with larger solubility of Fc in the hexadecane

oil phase of this ME than in the dodecane oil phase of the

SDS ME. However, in bicontinuous MEs, DR of Fc is greater










in SDS jEs than in CTAB MEs. This can also lead to a more

positive E/2.


Reactivity of Fe(CN)-3/-4


For Fe(CN)6"3/' in SDS LEs, there should be little

difference between K, and K1, since neither form is

associated with either oil or surfactant phase. As a result

partitioning constants and diffusion coefficients of both

forms must be approximately equal at any composition (KR z

K% and DR z Do), and, therefore, based on equation 4-7, no

change in E1/2 is expected. In fact, E1/2 of Fe(CN)6-3 in SDS

LEs is within experimental error of E"' (Table 3-4) for
aq
many ME compositions. Differences in partitioning between

Fe(CN)6"3 and Fe(CN)6'4 can shift E1/2.

In CTAB MEs, both forms of the Fe(CN)6'3/'4 couple are

strongly bound to the surfactant interface [13]. As a

result, both forms of the probe are equally affected by

composition and KR Ko and DR z Do. Consequently, E1/2 does

not change with composition in CTAB MEs [13] because the

probe is in a different environment. Therefore, it is the

difference in the environment of Ox and Red, rather than the

environment itself which shifts E/2.


Reactivity of MV+2/+"


Interactions of MV+2/+' with SDS surfactant in a ME

should be similar to interactions with SDS micelles. The










redox couple associates with SDS micelles through a

combination of electrostatic and hydrophobic interactions

[58]. In SDS micelles, K (MV*2) = 1.15 X 10'3 and K, (MV') =

2.4 X 10'4 [58], so it is reasonable that Ko and K << 1 in

SDS EEs. Consequently, equation 4-7 reduces to


E1/2 = E'aq + RT/nF In (DR/Do)'2 + RT/nF In Kg/K (4-9)


From values of K1 and KR in micelles, both MV+2 and MV+'

should reside predominately in the oil/membrane phase.

Therefore, DR D Do, and equation 4-9 reduces even further to


E,1/ = E'aq + RT/nF In KO/IK (4-10)

From equation 4-10, experimental E1/2 = -0.664 V and
E*'/ = -0.690 V for MV2/+*, K /K z 2.8 in the 89/1 SDS AE.

This indicates that the partitioning of MV** into the

oil/membrane phase of the jE is more favorable than the

partitioning of MV+2. This, in turn, suggests that hydro-

phobic interactions between the probe and the oil/membrane

phase may be more important than the electrostatic

interactions, since based solely on electrostatics, MV+2

should be more strongly associated with the surfactant

layer. The predominance of hydrophobic interactions of MV+'

was also observed by Kaifer and Bard in SDS micelles where

K. is about 5 times greater than Kg [58].

The effect of a possible difference between DR and Do on

E1/2 can be analyzed with equation 4-9. For the 89/1 SDS AE,










DR will reach a maximum value when MV* is not associated

with the oil/membrane phase, DRx = D,aq = 6.6 X 10-6 cm2/s

[58]. A minimum value of DR occurs when MV** is strongly

associated with the oil phase. This value can be estimated

as DRmin = Ddrop 6 X 10"7 cm2/s DR of Fc in 89/1 SDS pE.
With experimental Do = 3.3 X 10'6 cm2/s, E,/2 = -0.664 V, E'

= -0.690 V [58] and assuming DRX = 6.6 X 10'6 cm2/s, K/KR "

2 from equation 4-9. Using the same values of Do, E1/2 and

E*'aq and DRmin in equation 4-9, K/IY = 7. Consequently,

regardless of possible differences between DR and Do, the

association of MV*+ with the oil phase is always stronger

than that of MV+2 (Ko>KR) Therefore, DR,min must reflect DR of

MV+" better than DRma and the estimation of KI/KR from

equation 4-10, i.e. assuming Do = DR, will have <60% error.

As the water content of the jE decreases, E1/2 becomes

less negative but Do remains relatively constant (Table

3-3). Since as shown above, MV+" associates more strongly

with the oil/membrane phase than MV+2, its DRx will follow

the D, of Fc. In a 34/6 SDS ME, MV+2/' E1/2 = -0.632 V, Do =

2 X 10"6 cm2/s and DR,mx = Fc DR = 3.9 X 10.6 cm2/s, thus Ko/KR

= 7 from equation 4-9. If MV" is not much more associated

with the oil phase than MV+2 then DRin z Do = 2 X 10'6 cm2/s

and KI/K = 10 from equation 4-9. If DR, like Do, does not

change with composition, the shift in E1/2 from the 89/1 to

the 34/6 ME can be attributed to increasing KG/KR. However,

changes in DR alone may be sufficient to account for the










observed shift in E1/2. For E1/2 shifts to be solely

dependent on diffusion coefficient, MV*' must be associated

with the oil/membrane phase of the PE, so that in the 89/1

SDS fEm DR of MV* = DR of Fc = 6.7 X 10-7 cm2/s and in the

34/6 AE, DR of MV+ = DR of Fc = 3.9 X 10-6 cm2/s.

Association of MV*' with the oil/membrane phase is not

inconsistent with calculated KI/KY values which show that

MV" partitions more into the oil phase than MV*2. However,

since the difference between Kg and KR is less than an order

of magnitude, DR should not be considerably different from

Do. Therefore, the E1/2 shift is probably due to changes in

both DR and K/KR.

The dependence of K/KR on composition can be explained

by considering the solubilization of probe (X) in a gE

droplet (M), which can be described as


nX + M Xn-M (4-11)

where n = number of probes per droplet (equation 4-11 is the

same as equation 3-6). Equations 4-5 and 4-6 assume either

Poisson distribution, where solubilization of a probe in a

droplet does not affect solubilzation of the next probe

(therefore, M = total droplet concentration = C,) or that n

= 1. If neither of these assumptions are valid, and n when

X = Ox is not the same as the n when X = Red, then K/KR

will be a function of AE composition. This is probably the

case for the MV*2/* couple and MV+2 and MV*' may be in










different environments and, therefore, be affected

differently by composition.

The small changes in KI/k with jE composition indicate

little change in partitioning. Since both MV+2 and MV+' are

electrostatically attracted to the surfactant head groups,

both should reside near the membrane/water interface. Since

MV' is more hydrophobic, it will also associate with the

hydrophobic portion of the membrane, thus having a lower

partitioning constant (KR) than MV+2 (1%). However, with

both forms near the interface, effects of ME composition on

partitioning of both forms are similar. In CTAB gEs, MV+'

hydrophobically associates with the oil/membrane phase, MV+2

resides in the water and neither are electrostatically

attracted to the CTAB head groups. With the each form of

the probe in a different environment, the effect of

composition on partitioning is very different for each form.

This is indicated by greater shifts in El/ (due to

differences in /KR) with CTAB ME composition than in SDS

yEs, where the probes were in similar environments.

Reactivity of MV+'/O

In SDS MEs, E1/2 for the MV+'/ couple is less negative

than its E' = -1.020 V. Since both forms of the probe

must reside primarily in the oil phase, DR = Do and equation

4-10 can be used to describe shifts in E1/2. For an 89/1 SDS

ME, E1/2 = -1.015 V, and according to equation 4-10, KI/K










1.2. In a 34/6 SDS ME, E1/2 = -0.944 V and so KI/K, z 19.

Therefore, MVO is more strongly associated with the oil

phase than MV+ for all ME compositions, which is consistent

with their relative solubilities. The K/K, of the MV+*'/

couple is more affected by composition than the MV*2/+'

couple, having a smaller value in 89/1 SDS pE and a larger

value in 34/6 AE.

The reason is that with this couple, MV+" is attracted

to the interface as described before. However, hydrophobic

MVo will partition into the oil phase. According to

equation 4-11, both K, and K should decrease with

decreasing water content (M increases). However, KQ/KR

increases as ME water content decreases. Therefore,

partitioning of MVo must be more affected by pE composition

than that of MV*'.


Kinetics and Adsorption

Effect of Surfactant Adsorption on Probe Kinetics


Since pEs are optically transparent, the distance

between water/oil interfaces in bicontinuous EEs and droplet

size in droplet MEs cannot be more than 500 nm [59]. Since

electrodes are mm in diameter, part of the electrode will be

in contact with the aqueous phase and part with the oil

phase. Regardless of the solution, surfactant adsorbs tail

first onto hydrophobic surfaces, such as GC [59]. For the

fraction of the electrode in contact with the aqueous phase,










the polar head groups extend into the aqueous solution

(Figure 4-2). For the fraction of the electrode in contact

with the oil phase, the polar head groups interact with

other surfactant head groups rather than the non-polar oil

phase. Thus, reverse hemimicelles are formed on that

portion of the electrode surface (Figure 4-2). The

structure on the electrode surface will affect the

electrochemical response by partially blocking the surface

and through electrostatic and hydrophobic interactions

[8,59]. Addition of other compounds to the solution, such

as salt or alcohol, will affect the structure of the

surfactant layer (59].

Since hydrophilic Fe(CN)6'3 resides in the aqueous phase

of the ME, only electrostatic interactions between the probe

and the adsorbed surfactant occur. Consistent with this,

repulsion of Fe(CN)6"3 from like-charged SDS results in

slower electrochemical kinetics. Slow kinetics are shown by

AEp = 846 mV in 89/1 SDS ME and AEp = 885 mV in SDS micelles

compared to AEp = 152 mV in 0.1 M NaC(1~) (Table 3-4). In

CTAB iEs, adsorbed cationic surfactant attracts Fe(CN)6"3 to

the electrode, and the kinetics improve over 0.1 M NaCl(aq)

with AEp = 60 mV [13].

As the water content of the SDS ME decreases, AEp of

Fe(CN)63 decreases (Table 3-4). It has been shown that the

alcohol content in the oil/water interface increases as ME

water content decreases [20]. Changes in solution should












OI


LUI


0
0 '


C


water


Figure 4-2


Adsorption of Surfactant on GC Electrode
Surface for both Oil Phase and Aqueous Phase


O
11.
0 1


O

1










affect the electrode/solution interface in the same way as

the oil/water interface since the surfactant organizes in a

similar manner at both interfaces. With more alcohol at the

electrode/solution interface, the surfactant head groups are

further apart, decreasing charge per area and, thus, the

electrostatic repulsion of the electrode surface toward

Fe(CN)-3/'-. The neutral polar group of the alcohol also

increases the hydrophilicity of the surface [59], which may

also improve the kinetics of the Fe(CN)6'3/'4 couple.

In SDS MEs, AEp of Fc is about 60 mV and does not

change significantly with composition (Table 3-2),

indicating reversible electron transfer. Small changes in

AEp with AE composition may indicate some weak adsorption

effects. Hydrophobic Fc must easily penetrate the

surfactant layer allowing facile electron transfer. A AEp

of 60 mV for Fc is also observed in CTAB jEs [13], SDS

micelles [12] and CTAB micelles [38] where hydrophobic

interactions also allow Fc to easily penetrate the

surfactant layer.

In SDS pEs, AEp of MV+2/* is ca. 60 mV (Table 3-3). The

MV+2 can interact both hydrophobically and electrostatically

with the adsorbed surfactant, allowing for facile electron

transfer. In CTAB AEs, MV+2 will be electrostatically

repelled from CTAB but will still interact hydrophobically.

Since AEp in CTAB AEs is ca. 60 mV [13], hydrophobic









55
interactions must predominate. This is consistent with the

conclusions from E1/2 shifts and calculated KO/K .


Effect of Surfactant Adsorption on Probe Adsorption


In SDS AEs, the electrochemical behavior of Fc on

glassy carbon electrodes is diffusion controlled. This is

indicated by plots of log anodic peak current (i ) vs. log

scan rate (v), where slope is ca. 0.5. Similar plots for

Fe(CN)6"3 show slopes lower than 0.5. The lower slopes are

attributed to kinetic effects [42]. Kinetic effects on the

reduction of Fe(CN)63 are confirmed by the AEp which

increases with v. These results show that no difference in

adsorption behavior occurs in AEs for probes which do not

adsorb on GC in aqueous solutions.

Plots of log i_ vs. log v for the MV+2/+* couple in SDS

AEs have a slope of ca. 0.5 in all AE compositions used,

indicating that MV+2 is not adsorbed in SDS yEs. However,

the ratio (ic/i.)i < 1 and decreases with increasing v.

Since AEp is ca. 60 mV throughout the range of v, this

behavior indicates weak adsorption of MV+'. Kaifer and Bard

also observed weak adsorption of MV+' in SDS solutions which

was eliminated when SDS exceeded its critical micelle

concentration [58].

For the MV+'/ couple in all SDS yEs tested, plots of

log ip versus log v have a slope of ca. 0.5, indicating

that the system is primarily diffusion controlled. The










ratio (ip/ipa)2 > 1 and increases slightly at high (= 500

mV/s) scan rates. This is consistent with greater

adsorption of MV+' than MV. This is unlike aqueous

solutions where significant adsorption of MVO is observed

(Figure 4-3). The decrease of adsorption of MV was

attributed to the solubilization of MVO in the oil phase of

the AE. No adsorption of MV was observed in SDS micelles

[58], which also solubilize MV.

In MEs, strong probe adsorption is eliminated, as shown

for MV. This is attributed to solubilization of the

hydrophobic probe and blocking of the electrode surface by

surfactant. However, weak probe adsorption may occur. If

the adsorption depended soley on electrostatic attraction

between probe and SDS, adsorption of MV+2 would be greater

than adsorption of MV+'. If this adsorption were determined

soley by hydrophobic interactions, adsorption of MVo would

be greater than adsorption of MV". Since greater

adsorption of MV' is observed for both MV+2/*+ and MV+'0

couples, both electrostatic and hydrophobic interactions

must affect adsorption. Blocking of the surface by

surfactant also affects adsorption. Enhanced kinetics of

Fe(CN)6'3 with decreasing AE water content showed that the

adsorbed surfactant layer became more disordered as jE water

content decreased. In SDS micelles, where adsorbed

surfactant efficiently blocks the surface, no adsorption of

MV+" was observed [58]. However, in SDS solutions below
















30-






10
2 ', 2c





22a



-200-






-30-

...... 1.0 MM MV in 0.1 M NaCl(,q)
20 -0 -1400 -1600















Figure 4-3 Cyclic Voltammetry of Methyl Viologen
v = 100 m/s
lo \ '

0 -20- p \


-30-
1.2 mM MV+2 in 67/3 SDS /E
..-... 1.0 mM MV+2 in 0.1 M NaCI(oq)
-- 4 0 I I II1 1 1 11 11 I 1 1 1 1 1 1 1 1 1 I I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 I I I I I I I I I I I I 1
-200 -400 -600 -800 -1000 -1200 -1400 -1600
potential (mV)



Figure 4-3 Cyclic Voltammetry of Methyl Viologen
v = 100 mV/s
on GC electrode, area = 0.071








58
cmc, the surface was less blocked and adsorption of MV+' was

observed [58]. In SDS pEs, as the adsorbed surfactant

becomes more disordered (with decreasing AE water content),

coadsorption of MV+" increases. This is indicated by

increasing (ip/ip)2 with decreasing AE water content (Table

3-3).

Shifts in E1/2 indicate that MV*" interacts more with the

oil/membrane phase than MV*2. This is consistent with the

preferential adsorption of MV+' by interaction with the

adsorbed SDS. However, shifts in E1/2 for MV*'/o couple

indicate that MVo is more strongly associated with the

oil/membrane phase than MV+". Since observed adsorption, as

indicated by (ip/iP)2 > 1 (Table 3-3), suggests that MV*'

preferentially interacts with the adsorbed SDS, MVO must be

primarily solubilized in the oil rather than in the membrane

phase. As the oil content increases, the solubilization of

MVo in the oil phase increases, thus less MVo interacts with

the adsorbed surfactant at the electrode and (ip/ipa)2

increases. Thus, the increasing (ip/ip)2 with decreasing

water content supports the model of MVO solubilization in

the oil phase.















CHAPTER 5
BIOLOGICAL PROBES IN MICROEMULSIONS--QUINONES


Introduction to the Biochemistry of Quinones


Quinones are of interest in many biological

systems where reduction of quinone (Q) to quinol (QH2) is an

important step in electron transport. Throughout this work,

"Q" refers to any quinone. More specific symbols are used

for particular quinones. An example of a biologically

important quinone is ubiquinone 50, 2,3-dimethoxy-5-methyl-

1,4-benzoquinone with a sidechain of 10 methylbutenyl units

(Figure 5-1A). It is one of the most studied of biological

quinones due to its importance in mitochondrial respiration

[60].

In a cell, the mitochondria is the organelle

responsible for the production of energy, in the form of

ATP, from carbohydrates, lipids and amino acids [61]. The

mitochondria has two membranes, a smooth, somewhat elastic

outer membrane and an inner membrane with many inward folds

called cristae [61]. It is in the inner membrane that

mitochondrial respiration takes place, transferring

electrons from the NADH/succinate dehydrogenase system

through ubiquinone 50 to molecular oxygen, while producing
59




















































Figure 5-1


Biologically Significant Quinones

A) Ubiquinone; for ubiquinone 50, n = 10
B) Plastoquinone
C) Vitamin K2


CJ
1 C
""2Cn










ATP (Figure 5-2) [61]. The inner membrane of the

mitochondria consists of proteins in a phospholipid bilayer,

where the negatively charged phosphate head groups of the

phospholipid compose the exterior of the bilayer and the

hydrocarbon tails (with typical chain lengths of eighteen

carbons) form a non-polar interior [61].

Ubiquinone 50 (UQ50) resides in the inner membrane of

the mitochondria, where it transfers electrons from NADH and

succinate dehydrogenase to cytochrome b (Figure 5-2) as part

of the respiration cycle [60]. The UQ50 is restricted to a

pool in the middle of the bilayer of the inner membrane.

Diffusion of UQ50 within the hydrophobic pool in the center

of the bilayer is fast but there is little or no diffusion

through the phospholipid head groups into the hydrophilic

exterior [62]. Unlike UQ50, the reduced form, ubiquinol

(UQ50Hg), may penetrate between the phospholipids of the

bilayer [62].

Ubiquinones are commonly referred to by the number of

carbons in the sidechain. However, other names for UQ50

include ubiquinone 10 and coenzyme Q10, where ten refers to

the number of methylbutenyl units ("n" in Figure 5-1A). For

consistency in this work, the first method of nomenclature

is used. While UQ50 is the form of ubiquinone most

frequently found in mammals, ubiquinones with fewer

methylbutenyl units exist in other biological systems [63].

For instance, ubiquinone 30 (UQ30) participates in the


























2-Ketoglutarate
Pyruvate Succinate
\FP, \
FPs FP2

Malate
ATP
\ ATP ATP ATP
Isocitrate \
NAD-- FPi(4Fe-S)---Q -* (2Fe-S) cyt b(Fe-S)cyt ci- cyt c---cyt aa3 --.0
Glutamate Rotenone. Antimycin A Cynnide
amytl

3-Hydroxyacyl-CoA FPi/
Fatty acyl-CoA- 'FP
Glycerol phosphate










Figure 5-2 Mitochondrial Respiration Cycle

FP = flavoprotein, e.g., FP1 = NADH
dehydrogenase; Fe'S = iron-sulfur center; Q =
ubiquinone 50; cyt = cytochrome










respiration of microorganisms such as yeasts [63], and

ubiquinone 0 (UQO) is used as a chemical defense in African

millipedes [64]. Another use of UQ50 is as a cardiovascular

drug. Administration of the drug is thought to correct

deficiencies of natural UQ50, allowing the body to produce

ATP more efficiently [63]. Other important biological

quinones include plastoquinone, a 2,3-dimethyl-1,4-

benzoquinone with up to 9 methylbutenyl units on the 6

carbon (Figure 5-1B), which participates in photosynthetic

electron transport [60] and vitamin K2, a 2-methyl-l,4-

naphthoquinone with a methylbutenyl sidechain containing up

to 12 units (Figure 5-1C), which promotes blood clotting

[60].


Spectroscopy of Quinones

Ultraviolet (uv) spectra of ubiquinones have a strong

absorbance maximum (Ax) between 270 and 280 nm, with a

typical absorptivity (e) of ca. 14 mM-cm'' [62]. This A

has been attributed to a r~r* transition of the enone ring.

As the polarity of the solvent increases, Amx shifts to

longer wavelengths [62]. The reduced form (UQH2) has a weak

max z 290 nm, with typical e = 4 mM-1cm''. The UQH2 ax is
attributed to an n-r* transition and is only slightly

affected by the polarity of the solvent.

Mitochondrial membranes have been modeled by

phospholipid vesicles in uv studies. The uv spectra of










ubiquinones in phospholipid vesicles show that the solvent

environment of the ubiquinone becomes less polar as the

length of the ubiquinone sidechain increases [62]. For

instance, for ubiquinone 5 (UQ5) Amx in lecithin vesicles

Am in water [62]. However, for UQ50 M in vesicles max

in non-polar petroleum ether [62]. Ubiquinones with

sidechain lengths between 5 and 50 carbons have Amx values

in vesicles which are between water and petroleum ether

values, and are approximately equal to MX in ethanol [62].

Changes in Amx with sidechain length indicate that

ubiquinone penetrates deeper into the non-polar center of

the phospholipid bilayer as its sidechain length increases.

The uv spectra of ubiquinones in vesicles have not been

directly compared to spectra in mitochondrial membranes,

since spectra in mitochondrial membranes are complicated by

turbidity, cytochrome absorbance, light scattering and other

interference [62]. However, calorimetry studies of

ubiquinone in the mitochondria suggest the same changes in

microenvironment with ubiquinone sidechain length [65].

Specifically, ubiquinones with shorter sidechains reside in

the more polar region of the phospholipid, causing the

structure to become more rigid and raising the melting

point. On the other hand, UQ50 does not alter the melting

point, indicating that it does not affect the bilayer

structure [66].










Because of the hydrophobicity of UQ50, it must reside

in the oil phase of the yE. However, A.x of UQ50 in SDS MEs

is 275 nm for all ME compositions (Table 5-1). This Imx

indicates that UQ50 resides in a more polar environment than

that of petroleum ether (which should correspond to pure

dodecane), where Am = 270 nm [62]. This can be attributed

to partitioning of alcohol and water into the oil phase [1].

Thus, despite being in the oil phase, UQ50 is in a

relatively polar microenvironment.

The microenvironment of UQO must change with ME

composition, since A.x of UQO is a function of pE

composition (Table 5-2). For UQO in an 89/1 SDS ME, ax -

270 nm = AM in water. Therefore, in yEs with high water

content, UQO exists primarily in the aqueous phase.

However, as water content decreases, the microenvironment

becomes less polar, as indicated by decreasing AIx. This

can be attributed to increased partitioning of UQO into the

oil phase, with increasing ME oil content.


Electrochemistry of Quinones

Non-Aqueous Solvents


Since biological quinones are usually found in the

non-aqueous environment of the phospholipid bilayer

interior, the electrochemistry of many types of quinones in

non-aqueous systems has been studied extensively. In non-

aqueous systems, the quinone (Q) is reduced in two steps,











Table 5-1
Ubiquinone 50 (UQ50) Absorbance Maxima in Different Solvents


solvent A E
(nm) (cm mM)'


watera 287 7

ethanol 275 15

petroleum ethera 270 15

76/4 SDS ME 275 14

52/8 SDS PE 275 13

45/5 SDS ME 275 10


reference 62












Table 5-2
Ubiquinone 0 (UQO) Absorbance Maxima in Different Solvents


solvent


water"

0.1 M NaC1)

ethanol

1-pentanol

petroleum ethera

dodecane

89/1 SDS AE

52/8 SDS tE

45/5 SDS AE


max
(nm)

268

270

263

263

259

260

270

268

265


E
(cm- mM) -1

15

17

15

12

13

15

15

16

14


a reference 62









68
first to the semiquinone anion radical (Q") and then to the

dianion (Q-2) [60,67,68].


Q + e- Q- (5-1)


Q" + e'" Q-2 (5-2)

Experimentally, E' of both reactions is solvent dependent,

becoming less negative as the polarity of the solvent

increases [60,69]. Table 5-3 shows E'' values for several

Q/solvent systems. Adding a weak acid to the solution

(e.g., diethyl malonate at twice the concentration of UQ5

[70]) protonates Q-2 (equation 5-3) and shifts E' of the

second peak to more positive potentials.


Q-2 + H+ QH' (5-3)

Stronger acids (e.g., benzenethiol at ten times the

concentration of UQ5 [70]) protonate Q" (equation 5-4).

Since QH' is easier to reduce than Q, QH' is then reduced

simultaneously with Q (equation 5-5).


Q" + H+ QH" (5-4)


Q + H+ + 2e" e QH' (5-5)

The reaction in Equation 5-5 produces one reduction peak in

CV which is twice the height of the peak for the reaction

in equation 5-1. With a sufficiently high [H*], this peak











Table 5-3
Formal Potentials of Quinones in Different Media


quinone eqna solvent E'' (V) reference


BQ 5-8 pH=0, aqueous +0.457 66

BQ 5-1 ACN/TEAP -0.51 55
5-2 -1.14

BQ 5-1 DMF/TEABF4 -0.401 71
5-2 -1.155

UQO 5-8 pH=6.9, aqueous -0.143 83

UQ5 5-1 DMF/TEABF4 -0.622 71

UQ15 5-1 DMF/TEABF4 -0.611 71

UQ50 5-1 DMF/TEABF4 -0.602 61

UQ50 5-8 pH=7.5, aqueous -0.08 62
SDS micelles


aEquation for redox reaction









70
may occur at more positive potentials than the peak for the

equation 5-1 process [60,70].

In solutions with very strong acids (e.g., perchloric

acid at twice the concentration of UQ5 [70]), Q is

protonated (equation 5-6) and the protonated quinone (QH*)

is the species reduced (equation 5-7).


Q + H+ QH+ (5-6)


QH* + 2e" + H* QH2 (5-7)

If the concentration of acid is not sufficient to completely

protonate the available Q (e.g., 1:1 ratio of perchloric

acid to UQ5 [70]), it is possible to see reduction peaks for

both reactions 5-5 and 5-7 [60,70,71].

Electrochemical behavior is affected by interactions

other than acid/base reactions. For example, Q" or Q-2 may

complex with electrolyte cations. The formation of a

complex by the dianion shifts the potential of the second

peak (equation 5-2) to considerably more positive values.

The shift may be sufficiently large so that CV peaks due to

reactions in equations 5-1 and 5-2 merge so that only one CV

peak is observed [60,72].


Aqueous Solutions


In aqueous solutions, Q undergoes a two electron, two

proton reaction to hydroquinone (QH2):









71
Q + 2H+ + 2e" QH2 (5-8)

According to the Nernst equation, the potential of reaction

5-8 depends on the pH of the solution:


( [Q] [H']2 )
E = E' + (RT/2F) In I---------I
S[QH2]
= E'' + RT/F In [H+] (5-9)

where E is the standard potential, E' is the formal

potential when the concentration of the oxidized form (Q)

equals the concentration of the reduced form (QH2) and the

other symbols have their usual meaning. Since the

dissociation of QH2 (equations 5-10 and 5-11) changes its

concentration, these dissociations,


QH2 QH' + H (5-10)

QH' Q- + H (5-11)

can affect E. The acid dissociation constants for reactions

5-10 and 5-11 are K1 and K2, respectively. Table 5-4 shows

pKas for different acid/base forms of different quinones.

For Q, the E' is when


[Q] = [QH2] + [QH] + [Q-2] (5-12)

At E=EE', E'' may be related to KI and K2 in a straight-

forward fashion (See Appendix C):


E"' = E + (RT/2F) In ([H]2 + [H]KI + K1K2)


(5-13)











Table 5-4
Acid Dissociation Constants of Quinones


acid form

QH2

QH-

QH'

QH*

QH2

QH2

QH2


eqn8

5-10

5-11

5-4

5-6

5-10

5-10

5-10


pKa

10.35

11.4

4.0

-1.0

10.2

10.0

9.9


reference

54

54

54

54

61

61

61


aEquation for acid dissociation reaction


quinone

BQ

BQ

BQ

BQ

UQ30

UQ45

UQ50









73
If pH << pK1, the second and third term under In in equation

5-13 are negligible and the dependence of E' on pH is -60

mV/pH. This means that acid/base reactions have no effect

on E'' and equation 5-13 simplifies to equation 5-9.

Similarly for pK, < pH < pK2, the dependence of E' on pH is

-30 mV/pH, and for pH >> pK2, the dependence is 0 mV/pH.

This dependence of E"' on pH has been experimentally

confirmed by Bailey and Ritchie with 1,4-benzoquinone on

gold electrodes [73].

The exact sequence of electron and proton transfers

for reduction of quinone in aqueous solutions is still under

dispute. The most commonly accepted mechanism was proposed

by Vetter in the fifties. Vetter obtained Tafel plots on

platinum electrodes for benzoquinone (BQ)/benzohydroquinone

(BQH2) in solutions with pH between 0.2 and 7.2 [74].

Between pH 5 and 6, changes in the Tafel slope indicated a

change of mechanism. Specifically, electrochemical reaction

orders indicate the species which undergoes oxidation or

reduction, eg. QH* or Q. The reaction order is determined

by the change in the current as the concentration of one

reactant is varied while the ratio of the other two

reactants is held constant. This determines the

stoichiometry of that reactant involved in the overall

reaction. For example, when [H'] is changed while the ratio

[BQ]/[BQH2] is held constant, the stoichiometric coefficient

of H is determined. Using this approach, Vetter proposed











that two different consecutive charge-transfer reactions

occur within the pH range of 0.2 to 7.2. Below pH 5, the

order of electron and proton transfer was proposed to be

HeHe. For pH greater than 6, the order proposed was eHeH.

Recently Laviron presented a more specific analysis of

the mechanism for the reaction of quinone in aqueous

solutions [75]. Laviron described the possible mechanistic

steps of quinone, including nine possible chemical species

and six possible electron transfers, using the 9-square box

scheme [77]:


Q Q Q- Q-2
tf t4 tf
QH+ QH' QH' (5-14)
ti t4 t4
QH2 +2 QH2 H QH2


The protonations in 5-14 are assumed to be at equilibrium

and the rates of the forward and reverse electrochemical

reactions are assumed to be the same (charge transfer

coefficient, a=0.5). Under these conditions, the apparent

standard potentials for the first and second electron

transfers are simply related to standard potentials (E) and

proton dissociation constants (Ka) of each individual step

in 5-14 (See Appendix D). Individual apparent E and Ka

values were calculated for BQ from experimental results of

Bailey and Ritchie [76]. Using these values and the scheme

in 5-14, Laviron concluded that the primary mechanism of

electron transfer changed from HeHe to eHHe to eHeH as the










pH of the solution increased. The contribution of each

mechanism to current changes with pH. At pH 3.5, half the

current was due to the HeHe mechanism and half due to the

eHHe mechanism. At pH 5.5, half of the current was due to

the eHHe mechanism and half due to the eHeH mechanism. The

change in mechanism changes the apparent rate constants and

apparent E* values, which affect overall potential.


Organized Media


Electrochemical activity of quinones in organized

media has not been extensively studied. In micelles and

phospholipid vesicles, E' of UQ50 is shifted to values more

negative than in aqueous solutions [60,77-79]. The

dependence of E' on pH for UQ50 in buffered solutions of

SDS micelles = -8 mV/pH [78]. This change from -60 mV/pH in

ethanol/water solutions was attributed to a lowering of the

charge transfer coefficient by the surfactant. The E'

dependence on pH of UQ30 in lecithin vesicles [77] and of

UQ50 in asolectin vesicles [79] was the same (-60 mV/pH) as

in aqueous solutions.


Electrochemistry of 1.4-Benzoquinone (BQ)


DMF/TEAP

The solvent system DMF/TEAP was chosen because it gave

good electrochemical results. Unlike in ACN and in DMF with

0.1 M NaCO04, redox couples for the processes described in










equations 5-1 and 5-2 could be observed. Also, peaks in

DMF/TEAP systems were not complicated by adsorption which

occurred in ACN.

Figure 5-3 shows CV of 1,4-benzoquinone (BQ) on a GC

electrode in DMF, with 0.10 M TEAP as electrolyte. Peaks

Ic/Ia are attributed to the reaction described by equation

5-1. The reduction is diffusion controlled, as determined

from a plot of log peak Ic current (i ,,) vs. log scan rate

(v), where the slope = 0.450.01. The kinetics are quasi-

reversible with AEp = 8633 mV. The ratio of peak currents

(iP/i ), = 1.00.1, which is consistent with quasi-

reversible behavior. Experimental E1/2,1 = -0.420.02 V and

corresponds to reported E/2,, = -0.400.02 V [80], in DMF

with tetrabutylammonium tetrafluoroborate as the electrolyte

(DMF/TBABF4) .

Peaks IIc/IIa correspond to the process described by

equation 5-2. This process is also diffusion controlled, as

indicated by a slope of 0.420.03 of a log iP,, vs. log v

plot. This reaction also has quasi-reversible kinetics,

indicated by AEp = 84145 mV and (ip/ipa)I = 0.70.2. The

magnitude of iC11 is about 50% of ipc,, which is attributed

to the disproportionation of Q"[60]:


2Q'" Q + Q-2


(5-15)
















80



40



0



-40-



-80-



-120-
0


-1500 -2000 -2500

(mV)


Figure 5-3


Cyclic Voltammetry of 3.1 mM 1,4-Benzoquinone
in DMF/TEAP


v = 100 mV/s
on GC electrode, area = 0.070 cm2


2


0,

C-


background
BQ
I I 1 I1 I 1 11 I II r 1 '111ii
-500 -1000

potential










Since disproportionation increases [Q-2] as well as

decreasing [Q"], it is also consistent with (ipc/ip),I < 1.

Experimental E/2,11 = -1.020.05 V is more positive than

E1/2,1 = -1.150.04 V reported for DMF/TBABF4 [80]. The more
positive experimental potentials may result from the

reaction of Q-2 with protons in solution (equation 5-3)

[60,68]. Since Q-2 is the most basic species in solution

(Table 5-4), it is the most likely species to be protonated.

In solutions containing only 0.10 M TEAP in DMF

(TEAP/DMF), a cathodic peak at -0.870.04 V, with a reverse

peak at -0.820.1 V, was observed (Figure 5-3). These peaks

show diffusion controlled behavior, with slope = 0.520.05

from log ip vs. log v plot. Since peak IIIc in CV of BQ

(Figure 5-3) occurs at the same potential with approximately

the same magnitude ip (at same v) as the peak in the

DMF/TEAP solution, peak IIIc is not attributed to the

reaction of BQ. The i,1 was measured using the decaying

current of peak IIIc as baseline.


Aqueous Solutions


Figure 5-4 shows CV of BQ on GC in pH 6.9 aqueous

phosphate buffer. The results are consistent with the

mechanisms proposed by Laviron and Vetter. In aqueous

solutions, the apparent formal potential for the second

electron transfer is more positive than for the first.

Thus, only a single, two electron transfer described by









79






40


30


20


10

C
D 0


-10


-20
-30




600 400 200 0 -200 -400 -600

potential (mV)

Figure 5-4 Cyclic Voltammetry of 1.0 mM 1,4-Benzoquinone
in Phosphate Buffer (pH = 6.9)

v = 100 mV/s
on GC electrode, area = 0.070 cm2










equation 5-8 is observed. The process is diffusion

controlled, as determined by the slopes of 0.460.01 of log

ip vs. log v plots. Slow electron transfer kinetics are

indicated by AEp z 360 mV. For the same process on carbon

paste electrodes in the pH range of 2-8, Adams also observed

slow electron transfer, with AEp = 300 mV [81]. The ratio

ipc/ipa changes from 1.5 to 3.0 as the pH increases from 2 to
8 (Table 5-5). As the pH of the solution increases, [QH2]

decreases, due to the deprotonations described in equations

5-10 and 5-11. The decrease in [QH2] is reflected in

decreasing ipS and, consequently, increasing ipc/ip. At pH

6.9, experimental E12 = +0.110.02 V and correlates well

with E1/2 = +0.13 V at pH 6 reported by Adams [81]. The

dependence of E1/2 on pH is -465 mV/pH over the pH window of

2-8. This deviation from the theoretical value of -60 mV/pH

may be due to the change in mechanism over this pH range.

As shown by Laviron [75], a change in mechanism will affect

the apparent formal potential. For reduction of BQ on

carbon paste electrodes, Adams observed a similar dependence

of E12 on pH of ca. -50 mV/pH. From the intercept of E1/2

vs. pH, E" is estimated to be +0.460.02 V which corresponds

to reported E = +0.46 V [67,68,81].

Since yEs used in this work have an unbuffered aqueous

phase of 0.1 M NaC1~q), the electrochemical behavior of BQ

in unbuffered aqueous solutions was studied. Figure 5-5

shows CVs of BQ in unbuffered 0.1 M NaCI ,) at different pH











Table 5-5
Cyclic Voltammetric Results for 1,4-Benzoquinone (BQ) in
Aqueous Phosphate Buffer


AEpb

(mV)


36324

41218

36039

29841


ipC/[BQ]b

(AA/mM)


255

304

343

355


1.520.39

2.250.25

2.870.39

3.100.55


aionic strength = 1.0
by = 100 mV/s, electrode area = 0.075 cm2


pHe


(mE)
(mV)


[BQ]

(mM)


1.42

2.40

1.27

1.75


2.4

5.0

6.9

7.9


+30612

+2249

+11220

+4921
































ci)

U


150-



100



S50



0-



-50-



-100-



-150-
1200


I


' I
'I


3.05
3.41
3.54


mM
mM
mM


BQ, pH 2.5
BO, pH 3.4
BQ, pH 6.6


800 400

potent


TII vI.. . . .


0 -400 -800

al (mV)


Figure 5-5


Cyclic Voltammetry of 1,4-Benzoquinone in
Unbuffered 0.1 M NaCl(q)

v = 100 mV/s
on GC electrode, area = 0.075 cm2










values. At pH = 2.5, E1/2 = +0.26 0.04 V and AEp = 48580

mV. The AEp indicates slow electron transfer. The reaction

of BQ in all unbuffered 0.1 M NaCl,) solutions with pH < 3

was irreversible with AEp z 400 mV. At pH 6.5, E/ =

-0.150.01 V and AEp = 6711 mV. This AEp indicates quasi-

reversible kinetics for a two electron transfer. In all 0.1

M NaCi1) solutions with pH > 5, the reaction of BQ has E1/2

= -0.150.01 V and AEp = 6614 mV. At pH = 3.4, two redox

couples were observed. One couple corresponds to the

kinetically slow process described for unbuffered solutions

at pH < 3. The other couple corresponds to the quasi-

reversible process described for unbuffered solutions at pH

> 5. As pH increases from 3 to 5, i and ip for the

irreversible process decrease, with a corresponding increase

in ip and ip for the quasi-reversible process. Table 5-6

shows the results from cyclic voltammetry in unbuffered 0.1

M NaCl(q).

The behavior of BQ in unbuffered solutions can be

explained by considering that the pH of the reaction layer

rather than the pH of the solution determines reaction

potential. At pH = 2.5, proton concentration must be

sufficient in the reaction layer for the reaction to proceed

as in buffered solutions at pH z 4 (this pH was estimated

from E,/2). At pH = 6.5, the reduction of BQ consumes the

available protons, leading to a high effective pH in the

reaction layer. According to equation 5-13, the reaction











Table 5-6
Cyclic Voltammetric Results for 1,4-Benzoquinone (BQ) in
Unbuffered 0.1 M NaCIl(a
(aq)


AEpa

(mV)


ipc/[BQ]a ip/ipa

(AA/mM)


3.05 2.51 +25440

3.41 3.39b +26715
-1483

2.45 4.02b +24721
-1466

3.34 5.20 -14710

3.54 6.56 -1486

3.54 7.35 -1478


63780

40729
615

37542
7711

5620

6711

7116


256

51
282

31
312

212

181

161


1.380.48

2.281.07
1.010.13

7.2 1.6
1.100.17

1.100.22

1.070.07

1.10f0.11


"v = 100 mV/s, electrode area = 0.075 cm2

intermediate pH range shows two redox couples


[BQ]

(mM)


E1/2

(mV)










layer pH must be greater than 11.5 in unbuffered solutions

with pH > 5, since E1l2 is no longer dependent on pH.

Because E1,2 is independent of pH in unbuffered 0.1 M NaCl(q

with pH > 5 and is equal to -0.15 V, E1/2 values more

negative than -0.15 V (observed for the reaction of BQ in

gEs) cannot be attributed to pH effects. At an intermediate

pH (i.e. 3 < pH < 5), the reduction of BQ consumes available

protons of the reaction layer. After the reaction layer has

been depleted of protons, the remaining BQ must be reduced

at a high effective pH. Thus, both the irreversible process

typical of BQ in low pH 0.1 M NaCl(aq) and the quasi-

reversible process typical of BQ at high pH are observed.

The ratio of cathodic peak currents of these processes

depends on [BQ] and solution pH. Similar changes in CV with

pH were observed by Bailey and Ritchie in unbuffered 0.1 M

NaC104(aq) [73].


Electrochemistry in Microemulsions


Figure 5-6 shows CV of BQ in 89/1 SDS pE.

Electrochemical behavior in pEs was compared to that in 0.1

M NaCl(, since 0.1 M NaClq) (with pH z 5.6) forms the gE

aqueous phase. In 89/1 SDS AE, plots of log i vs. log v

have a slope = 0.430.10, indicating a diffusion controlled

process. Quasi-reversible kinetics of the two electron

process are indicated by AEp = 574 mV and i /ipa =

1.350.35 at v = 100 mV/s. As described in the previous
















80


60


40


20


00


-20


-40-


-60 11111111111111111111 ll llll llll 111 11 ll n,,liI
100 0 -100 -200 -300 -400 -500 -600

potential (mV)

Figure 5-6 Cyclic Voltammetry of 4.0 mM 1,4-Benzoquinone
in 89/1 SDS gE

v= 25 mV/s
on GC electrode, area = 0.071 cm2
For exact ME composition see Table 2-1.










section, in 0.1 M NaCl( q similar to those used to prepare

the aqueous phase of the .E, the reduction is also diffusion

controlled and quasi-reversible. In the ME, the relative

peak current ip/[BQ] = 17.6 AA/mM at v = 25 mV/s. This is

lower than in unbuffered 0.1 M NaCl(a) where ip/[BQ] = 20.3

AA/mM, at the same scan rate. The lower iP/[BQ] in the AE

is attributed to lower Do of BQ. For BQ in an 89/1 SDS AE,

Do = 8.3 X 10.6 cm2/s compared to 0.1 M KNO3q where D,a =

8.6 X 10-6 cm2/s [81]. As shown in Chapter 3, a probe

residing in the continuous phase of a droplet gE will have a

diffusion coefficient slightly lower than in an aqueous

solution due to obstruction by the droplets. Based on the

Do, BQ must reside in the aqueous continuous phase. Since

the reduced form (BQH2) is more water soluble than BQ [82],

it must also reside in the aqueous phase. Thus, DR z D0.

In this system, E1/2 = -0.1500.002 V, which is about the

same as E'aq = -0.148 0.002 V in 0.1 M NaCl No shift

in E1/2 will be observed if both BQ and BQH2 reside in the

aqueous phase of the pE (See Appendix B). Thus, both

diffusion coefficent and the value of E1/2 are consistent

with BQ and BQH2 residing primarily in the aqueous phase of

the LE.

As gE water content decreases, ip/[BQ] decreases

(Table 5-7). This can be attributed to decreasing Do of BQ.

As shown in Chapter 3 for Fe(CN)63, the diffusion












Cyclic Voltammetric


Table 5-7
Results for 1,4-Benzoquinone (BQ) in SDS
Microemulsions


[BQ] MEa

(mM)


3.07 DMFc


5.98

4.69

4.57

3.98

1.87


34/6

45/5

67/3

89/1

NaCld


E1/2

(mV)


-42317
-102973

-2098

-2023

-1722

-1502

-1454


AEpb

(mV)


8633
84145

416

474

634

574

626


ib

(AA)


282
153

564

527

635

705

384


b/i


1.000.14
0.680.23

1.600.21

1.370.36

1.400.37

1.350.35

1.220.21


"See Table 2-1 for exact SDS ME composition.
by = 25 mV/s, electrode area = 0.071 cm2

CO.1 M TEAP supporting electrolyte. Two redox couples were
observed.

d0.1 M NaCl()
(aq)


iPb/[BQ]

(IA/mM)


9.4

11.1

14.4

17.6

20.3









89
coefficient of a probe residing in the aqueous phase of a ME

decreases as water content decreases.

However, E1/2 becomes more negative as ME water content

decreases (Table 5-7). As shown for unbuffered solutions of

0.1 M NaCl(,q), changes in pH will not shift E1/2 to values

more negative than -0.15 V (Table 5-6). Consequently,

changes in pH cannot account for the observed E1/2 shift.

Thus, partitioning of the probe must change with

composition. For a bicontinuous ME, it is reasonable to

assume that DgR Do, since both probes are in a continuous

microenvironment, even if those microenvironments are

different. Shifts in E1/2 due to partitioning can be

expressed by the following equation (Appendix B):


E12/ = E' + RT/nF ln (DR/D) 1/2

+ RT/nF In (K%(l+KR)/KR(l+%K) (5-15)


For a 34/6 SDS ME, E1/2 = -0.209 V. Substituting DR = Do, E1/2

and E' = -0.15 V into equation 5-15 results in


0.09K Ko = 0.91KRKo (5-16)

Since Kg and K% are positive by definition


0.09KR K, > 0 (5-17)

and


0.09 > K/KR


(5-18)