Voltammetric detection of micelle formation

Voltammetric Detection of Micelle Formation JOHN TEXTER, *'1 F. RICHARD HORCH,* SYED QUTUBUDDIN,'~ AND E T H I R A J U L U D A Y A L A N t * Photographic Research Laboratories, Eastman Kodak Company, Rochester, New York 14650-2109; and t Department of Chemical Engineering, Case Western Reserve University, Cleveland, Ohio 44106 Received April 3, 1989; accepted June 5, 1989 Voltammetry at the rotating disk electrode of a water- and oil-soluble electroactive probe (N,N-diethyl3-methyl-p-phenylene diamine, PPD) was used to detect the onset of micellization in aqueous sodium dodecyl sulfate (SDS), di-/triisopropyl naphthalene sulfonate (DTINS), and cetyltrimethylammonium bromide (CTAB) systems in 0. t M NaC1 at 25 °C and pH 10. Critical micelle concentrations in agreement with values determined by other methods were obtained. Micellar/aqueous phase distribution coefficients were estimated by modeling the variation of limiting current with amphiphile concentration. Distribution coefficients of 324 and 73 were obtained for the SDS and DTINS systems, respectively. In the CTAB system, sequential distribution coefficients of 272 and 88, respectively, were obtained below and above a "second C M C " (0.001 M ) , which corresponds to the onset of a transition in micellar morphology. © 1990AcademicPress,Inc. INTRODUCTION

Voltammetry in microheterogeneous systems, such as micelles ( 1-23 ), microemulsions ( 24-30 ), and macroemulsions ( 31-34 ) is becoming increasingly popular as a general method for studying heterogeneous redox processes and for characterizing the discontinuous phase in such systems. In this paper we focus on the use of voltammetry at the rotating disk electrode ( R D E ) for studying the onset ofmicelle formation and the distribution of an electroactive probe between continuous and discontinuous phases. We use N , N - d i ethyl-3-methyl-p-phenylene diamine (PPD), which is both water and oil soluble, as the electroactive probe and examine voltammetrically its two-electron oxidation to quinonediimine. The limiting current (6) obtained in this oxidation at the RDE is described by the Levich equation (35) il = 0 . 6 2 n F A D 2 / 3 C o V o l / 6 w l / 2 ,

[1]

1 TO w h o m correspondence~should be addressed.

where n ( = 2 ) is the number of electrons transferred, F is Faraday's constant, A is the electrode area, Co is the concentration, Do is the diffusion coefficient, v0 is the kinematic viscosity, and w is the angular velocity of the electrode. In the cases examined here, an electroactive probe in both the aqueous phase and micelles is susceptible to oxidation at the RDE. The oxidation of the probe in the micelles occurs with a small shift in half-wave potential because of the stabilizing or destabilizing effect of the micelle on the redox potential. In a micellar solution, where the electroactive probe is distributed between the continuous aqueous phase and the micelles, the Levich equation may be written in the pseudophase approximation as il = 0 . 6 2 n F A

{ D ~ / 3 ( Co - Cm) q- D2m/3Cm)F 1/60~1/2,

[21

where Co and Do are the total concentration of the probe and aqueous phase diffusion coefficient, respectively; Cm is the concentration (relative to the total volume) of the probe in 263

Journalof ColloidandInterfaceScience,VoL 135,No. 1, March 1, 1990

0021-9797/90 $3.00 Copyright© 1990by AcademicPress,Inc. All rightsof reproductionin any formreserved.

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TEXTER

the micelles; and Dm is the micelle diffusion coefficient. At kinematic viscosities common to moderately dilute aqueous solutions, the root-angular-velocity dependence of the Levich equation allows one to extract a slope Oil S0 -

il

0(.ol/2 -

[31

wl/2.

The analogous quantity derived from Eq. [ 2 ], when divided by Eq. [3], gives

× (--u/-1/6,

[4]

\vo/ where effects of electrode area, total probe concentration, and other constants factor out. This slope ratio, as follows from Eq. [2], predicts that no current diminution should be observed below the CMC, and that current should decrease above the CMC as surfactant concentration increases, since the diffusion coefficient of the micelle, Dm, is smaller than that of the probe (Do) in aqueous solution. If we make the simplifying assumption that the density of the micelles equals that of the continuous phase, the distribution coefficient P may be written as p=

Cm -

I - - ~)m

Co - - Cm

(~m

-

Cm C0 - - Cm

• --

1

[5]

,

~m

where 4~r, is the volume fraction of micelles. This expression for P can be used to rewrite Eq. [41 as .S So

1. +

r(om(Dm/Do) . . 2/3 (v / -1/6 1 + Pq9 m

MATERIALS

AND

\ v0 ]

[6]

ET

AL.

proximately 50-50 wt% ) of di- and triisopropyl naphthalene sulfonate (DTINS) (weight average formula weight of 335) was kindly provided to us by Kim Goppert of Kodak's Life Sciences Research Laboratories. Cetyltrimethylammonium bromide (CTAB) was obtained from Kodak Laboratory & Research Products, triturated and washed with diethyl ether, recrystallized from ethanol, and dried before use. N,N-Diethyl-3-methyl-p-phenylene diamine (PPD) was obtained from W. F. Coffey of Kodak's Photographic Research Laboratories as the sulfuric acid salt and used without further purification. Solution densities were measured gravimetrically. Viscosities were measured at 25 °C using a Carri-Med controlled stress rheometer and a Perspex concentric cylinder. Surface tension measurements were carried out at 25°C using a Cahn 2000 microbalance and a platinum Wilhelmy plate. Solutions used for surface tension measurements did not contain the voltammetric probe, PPD. Voltammetric measurements were carried out using a platinum rotating disk electrode as previously described (31, 33). Voltammograms were typically scanned at 20 mV/s from a potential of -0.2 V (versus ESCE) to 0.4 V at rotation speeds of 100-1500 rpm. Limiting currents were read at approximately 150 mV positive of the half-wave potential (El/2 = -- 13- +50 mV). Solutions of approximately 50 ml at pH 10 were examined in 0.1 M NaC1. Probe (PPD) concentrations were typically 0.095 mM. Scans were first made in the absence of surfactant, and then in solutions prepared at various surfactant levels. RESULTS

Kinematic Viscosity

"

METHODS

Sodium dodecyl sulfate (SDS) was obtained as 99.9% pure from BDH and used without further purification. An isomeric mixture (apJournal of Colloid and Interface Science, Vol. 135, No. 1, March 1, 1990

The V/vo ratio in Eq. [61 was determined as a function of surfactant concentration at 25°C. The measured deviations from unity (maximum of 1.4%) were less than or equal to the experimental precision in all cases. The explicit consideration of the v/vo term was

VOLTAMMETRIC CMC DETECTION iii,,i,l,i

therefore dropped, since the weak ( - ~ ) power dependence of this ratio corresponds to less than a 0.3% deviation from unity. There are many micellar systems where the neglect of this kinematic viscosity term would not be appropriate.

Diffusion Coefficients The diffusion coefficient (Do) of PPD in 0.1 M NaC1 (pH 10) at 25°C was determined to be 6.6 × 10 -6 cm2/s. This value compares favorably with the value of 6.9 × 10 -6 cm2/s reported earlier at 40°C in a buffer of higher ionic strength (33). The Dm values were determined voltammetrically, using ferrocene as the probe, in a study to be published separately (Qutubuddin et al. ). For the SDS system, we obtained a Dm of 7.3 × 10 -7 cm2/s at 0.05 M SDS. This value compares favorably with the value of 7.29 X 10 -7 cm2/s deduced (after making a correction for the continuous phase solubility of ferrocene) from the data of Georges and Desmettre (14) at 0.069 M SDS. The Dm for DTINS micelles was found to be 8.0 X l0 -7 cm2/s. In the CTAB system we also found that Din, 6.6 × 10 .7 cm2/s, did not vary significantly over the concentration range of this investigation. This value compares favorably with a Dm of 6.0 × 10 -7 cm2/ s determined by Mackay et al. (30) at higher CTAB concentrations in the absence of added salt.

Critical Micelle Concentrations The CMC for SDS in 0.1 MNaC1 at 25°C has been reported to be 1.39-1.7 m M (3639). The CMC for the DTINS system was determined from surface tension measurements. These data are illustrated in Fig. 1. The intersection of the asymptotes indicates a CMC of 3.0 m M . The depression immediately following the CMC is due to the mixture of isomers in the system. The CMC for CTAB in 0.1 M NaC1 does not appear to have been reported. Figure 2 shows relative surface tension measurements obtained at 25 °C. The intersection of the asymptotes (dotted lines) indicates a

265 i

i

i

,

I

'

'

i

,

I

0"... .9 .8 0

~.

.7 O.

o..

.6

.5 .........! ~ ' ~ I t I

.4

IlI l i t

.,3

-5

-4

log

-3

-2

-1

[DTINS]

FIG. 1. Relativesurfacetension of DTINS solutions at 25°C as a function of concentration.The intercept of the dotted lines indicates a CMC of 3.0 raM.

CMC of 0.089 m M . This value is considerably lower than values of 0.8-0.98 m M obtained in the absence of supporting electrolyte by surface tension (40) and conductance (41) measurements, and greater than values of 0.05-0.06 m M o b t a i n e d in 1 MKC1 by surface tension and polarographic maximum methods (1). The subsequent lowering of the surface tension at concentrations higher than the CMC appears related to morphological transitions in the micelles, as discussed later. VOLTAMMETRY

SDS System The onset of micellization in aqueous SDS was studied first. Limiting current was monitored 150 mV positive of the half-wave potential; the half-wave potential decreased from 20 to - 8 mV as the SDS concentration was increased. The root-angular-velocity dependence of thislimiting current, where PPD was present at 0.0945 m M , is shown in Fig. 3 for a series of solutions 0-9.6 m M in SDS. All solutions were at pH 10 and 0.1 M in NaC1, Journal of Colloid and Interface Science, Vol. 135, No. 1, March 1, 1990

266

TEXTER ET AL. I

. . . .

, , , , l ~ , , , i t , ~ , l , , ~ , l

1 .9 .8 0

~"

.7 e

.6

o

":.l O.O . O l l o ~ '... ~"qll

.5 .4 .5

I

-6

,

,

,

,

I

-5

,

,

,

,

i

,

-4

,

,

I

,

i

,

-5

,

[

,

-2

,

,

,

obtained for a series of solutions 0-100 m M in DTINS. Half-wave potentials shifted from 20 to - 1 3 m V as the D T I N S concentration was increased. The relative slopes for this system are illustrated in Fig. 5. Linear leastsquares fits (dotted lines) to the asymptotic components of the data indicate a CMC of 3.9 m M , which compares favorably with the 3.0 m M determined in Fig. 1. This CMC value (3.9 m M ) , a D o value of 6.6 × 10 -6 cm2/s, and a Dm value of 8.0 X 10-7 c m 2 / s were used to fit the data of Fig. 5 to the model of Eq. [6]. A distribution coefficient P of 73 was obtained, which yielded the solid line illustrated.

CTAB System

I

-1

log [CTAB] FIG. 2. Relative surface tension of CTAB solutions at 25°C as a function of concentration. The intercept of the dotted lines indicates a CMC of 0.089 mM. A "second CMC" is suggested in the neighborhood of 1 mM.

and measurements were recorded at a temperature of 25 °C. The data show that at some SDS concentration the limiting current decreases below that obtained in the absence of SDS. The excellent linearity exhibited shows that the redox behavior of the PPD is well behaved and diffusion controlled over the range of SDS concentrations investigated. The relative slope ratio, S/So, obtained from these curves and others is plotted in Fig. 4 as a function of SDS concentration. The intersection of the dotted lines in Fig. 4 indicates a CMC value of 1.58 m M , which is in excellent agreement with values of 1.39-1.7 m M reported in the literature (36-39). The solid line in Fig. 4 is a fit of the experimental S/So data, using the Do and Dm values described above, to the model of Eq. [ 6 ]. A distribution coefficient of 324 was obtained in this fit, using a C M C of 1.58 m M .

DTINS System Linear limiting current versus root-angularvelocity curves, similar to those of Fig, 3, were Journal of Colloid and Interface Science,

Vol.

135, No.

1, M a r c h

1, 1990

Limiting currents in CTAB solutions ( 0 100 m M ) were also diffusion controlled. In this system, however, the half-wave potentials shifted positively, 20 to 50 mV, as the CTAB concentration was increased. Relative slopes

50

'

'

'

'

I

'

'

'

'

I

/

'

'

'

'

I

4O

<

30

/ _

"~~ 20

10 0

0

, , , , I 5 10 15 ~1/2 ( r a d l / 2 $-1/2)

FIG. 3. Variation of limiting current (150 mV positive of the half-wavepotential) with root-angular-velocityof the RDE for PPD in the SDS systemat 25°C. (SDS concentrations: O, 0 M; a, 0.95 mM; O, 4,8 mM; l, 7.7 raM; 0, 9.6 mM. PPD concentration: 0.0945 raM.)

VOLTAMMETRIC CMC DETECTION I

'

'

'

'

I

'

'

'

'

I

267

culty in fitting the relative slope data to the model of Eq. [6 ]. A two-stage model, incorporating such a morphological transition, can be described by the equation 1 + (PlCml + P2Om2)(Dm/Do) 2/3 S/So

=

1 + Pi~bml + P2q~m2

O

(£)

[7]

v)

o [

-4

i

i

i

i

[

I

i

-3

i

i

-2

log [sDs] FIG. 4. Relative slopes as a function of SDS concentration. The intemept of the dotted lines indicates a CMC of 1.58 mM. The solid line represents a fit to the data according to the model of Eq. [6] with a distribution coefficient of 324 and a CMC of 1.58 mM.

are depicted in Fig. 6. There appear to be three regions which can be fitted satisfactorily to straight (dotted) lines, with break points at 0.14 m M ( t h e CMC) and 2.5 raM. Using Do and Dm described above and a CMC value of 0.14 m M for CTAB, the data of Fig. 6 were fitted to the model of Eq. [ 6 ] to yield a distribution coefficient of 89. This fit is illustrated by the dashed curve and is problematic because of the overestimation of the relative slopes below 10 raM. Relative surface tensions for this system, illustrated in Fig. 2, indicate a CMC of 0.089 m M . However, the surface tension data show that subsequent, small but reproducible, stepwise drops occur in the surface tension at 1.0 m M and above 2.5 m M . These decreases, corresponding to subsequent CMC, most likely occur because of morphological transformations in the CTAB micelles. For instance, if the transition at 1.0 m M w e r e due to a sphere-to-rod transition, the distribution coefficient would likely change significantly, and this would account for the diffi-

where PI is the distribution coefficient operative immediately above the first C M C ; / ' 2 is operative above the second CMC; and ~nl and q~m2are volume fractions, respectively, of micelles below and above the second CMC. In this model, the transition from PI to P2 does not occur by a first-order process. The amount of CTAB present in micelles characterized by P~ saturates at the second CMC (CMC2) and that in micelles characterized by P2 increase linearly with [CTAB ] above CMC2. A fit using such a scheme is illustrated by the solid line in Fig. 6 and was obtained with a P1 of 272 (below 1 r a M ) and a P2 of 88 above 1 m M .

I ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1 ' ' ' ' 1

~.

Ih

"..

T

0

(/3

-5

-4-

-3

-2

-1

log [DTINS] FIG. 5. Relative slopesas a function of DTINS concentration. The intercept of the dotted lines indicates a CMC of 3.9 raM. The solidline is a fit of the distribution model of Eq. [6 ] usinga distribution coefficientof 73 and a CMC of 3.9 mM. Journal of Colloid andInterfaee Science, V o l .

135, N o . 1, M a r c h

1, 1990

268

TEXTER ET AL.

',....

O

03 03

I

,

-5

,

L

,

I

. . . .

I

. . . .

I

-4 -5 -2 log [CTAB]

i

I

I

t

I

-1

FIG. 6. Relative slopes as a function of CTAB concentration. The leftmost intersection of the dotted lines at S~ So = 1 indicatesa CMC of 0.14 mM. The rightmostdotted line indicates a subsequent asymptotic regime, following the breakpoint at 2 mM. The dashed line is a fit of the distribution model of Eq. [6] using a distribution coefficient of 89. The solid line is a fit to the modified model of Eq. [7], using a distribution coefficientof 272 below 1 mM, and a distribution coefficientof 88 above 1 mM.

DISCUSSION

C M C Determinations The accuracy of this voltammetric method for determining C M C appears competitive with that of other methods used in systems containing a supporting electrolyte. There is a 26% deviation in the D T I N S system (3.9 versus 3 m M by surface tension). The surface tension technique is more sensitive to impurities, and here there is a significant (nearly equimolar) mixture of di- and triisopropyl isomers and trace amounts of mono- and tetraisopropyl isomers. The m i n i m u m in Fig. 1 can be attributed to this isomeric heterogeneity. There is a 37% deviation in the CTAB system (0.14 versus 0.089 m M ) . Inspection of the error bars on the respective S/So plots shows that all of these relative deviations are within the experimental precision of the VO1Journal of Colloid and Interface Science, Vol. 135, No. 1, March 1, 1990

tammetric measurements. The use of a probe which has appreciable solubility in both the continuous and the micellar phases makes this approach similar to the approaches of other solubilization methods, such as spectral techniques (42). Since the probe is uncharged, it appears to function equally reliably in anionic and cationic micellar systems. It will be useful to extend this study to nonionic micellar systems and to investigate the limits of the technique as supporting electrolyte levels are diminished. A variety of other electrochemical methods have been applied to CMC determinations. Colichman (1) found that polarographic m a x i m a were suppressed at surfactant concentrations corresponding to the CMC. This m a x i m u m suppression method was pursued by T a m a m u s h i and Y a m a n a k a (43) and by Malik and co-workers (44, 45 ). Shinozuka and Hayano (4) do not recommend the m a x i m u m suppression method but do not elaborate on the reasons. The modeling of the heterogeneous nature of the m a x i m u m suppression phenomenon, and the relationship of this suppression to micelle formation, is not yet well defined. However, C M C determinations by the m a x i m u m suppression method are in satisfactory agreement with values determined by other techniques in systems having a supporting electrolyte. The technique of electrosorption or tensammetry for determining CMC has been reviewed by Shinozuka and Hayano (4) and applies to systems without supporting electrolyte (46) and to nonionic micellar systems (47, 48). Kaifer and Bard (16), in a cyclovoltammetric (CV) study of methylviologen in micellar systems, noted that the CMC of SDS could be estimated from changes in adsorption-desorption patterns of the cation radical. Mandal et al. (23) have more recently shown that CV peak currents of ferrocyanide can be used to detect CMC in a variety of micellar systems. The voltammetric approach described here is useful in that it also provides a basis for modeling the partitioning or binding of the probe to the micelles. Several earlier polarographic studies

VOLTAMMETRIC

have shown that diffusion currents decrease with increasing surfactant concentration (18, 49). These studies focused on the analysis of apparent diffusion coefficients and did not infer the detection of CMC from the limiting behavior as surfactant level was decreased.

Partitioning The use of relative slopes, S/So, decreases errors from probe concentration fluctuations in cases (as in the present situation) where the probe is susceptible to aerial oxidation. The models of Eqs. [6] and [7] enable the distribution coefficient to be estimated, subject to the assumptions made about micellar density. It is recognized that an alternative approach, involving probe binding and one or more binding sites, can be used to analyze probemicellar interactions (22, 50). The magnitude of the distribution coefficients obtained in this study for PPD can be compared with those obtained for PPD in oilin-water emulsions and in neat oils (33) where the buffered-aqueous phase was at a considerably higher ionic strength (I = 0.75 M). The relatively high distribution coefficients of 324 and 272 obtained in the SDS and (initially in) CTAB systems suggests that the micellar core is similar to a variety of amphiphilic oils such as primary alcohols and N,N-diethyldodecanamide. These oils are not "dry" when equilibrated with water, and contain 2-4% water at saturation. The secondary distribution coefficient of 88 obtained above the second CMC in the CTAB system suggests the onset of ordering of the alkyl chains and a lowering of hydration forces, making the micellar interior less attractive to the PPD molecule. A sixfold decrease in the distribution coefficient for PPD has been observed on changing from 1-undecanol to dodecane in both macroscopic oil/ water systems and in corresponding emulsions (33). The lower distribution coefficient, 73, obtained in the DTINS system suggests that DTINS micelles are more hydrophilic than the SDS and CTAB micelles. DTINS micelles have an aggregation number of approximately

CMC

DETECTION

269

210 and an apparent hydrodynamic diameter of about 6 nm (Qutubuddin et al., to be published). These micelles cannot have a significant volume fraction ofhydrophobic core and are necessarily much more hydrophilic than the other systems studied here. Most voltammetric studies in miceUar systems have used probes with extremely low water solubility ( 18, 22, 51 ). The use of the watersoluble PPD is not suitable for particle diffusion coefficient determinations, and Zana and Mackay (18) have recommended using probes that essentially are completely solubilized in the micellar pseudophase for such studies. However, the mixed solubility of PPD facilitates the observation of the onset of micelle formation. The present approach relies on the measurement of limiting currents rather than half-wave potentials. The major kinetic assumption is that probe molecules present in both the continuous phase and micelles are susceptible to redox chemistry at the electrode surface.

CTAB Morphology The transition observed at 1 m M in the CTAB surface tension data, and that inferred in the relative slope data, has been tentatively assigned to a morphological transition in micellar structure. Sphere-to-rod transitions have been documented as a function of salt level (52, 53 ). However, the assignment of the observations in this study to a sphere-to-rod transition is problematic because the salt-induced transition occurs at a much higher salt level (1.18 M NaC1) in CTAC1/NaC1 and at 0.06 M NaBr in CTAB/NaBr. Less work has been carried out on the sphere-to-rod transition induced by surfactant level increases. It appears accepted that 0.1 M CTAB in 0.1 M NaC1 has both spherical and rod-like micelles present (22), and it is reasonable to assume that the spherical morphology forms first and that the rods form as the CTAB level increases. The onset of rod formation in this system most likely occurs above a level of 0.01 M CTAB. There is evidence for a morphological transiJournal of Colloid and Interface Science, Vol. 135, No. 1, March 1, 1990

270

TEXTER ET AL.

tion in this c o n c e n t r a t i o n range in the surface t e n s i o n d a t a o f Fig. 2. W e observed in o u r viscosity m e a s u r e m e n t s t h a t the viscosity rem a i n e d relatively c o n s t a n t as C T A B c o n c e n t r a t i o n was increased to 0.01 M, a n d t h e n rose 13% as the c o n c e n t r a t i o n was increased to 0.1 M . A n alternative e x p l a n a t i o n for the transition at 1 m M C T A B is t h a t chloride b i n d i n g to the micelles is s u p p l a n t e d by b r o m i d e b i n d ing, as the C T A B c o n c e n t r a t i o n is increased. B r o m i d e b i n d i n g is stronger t h a n chloride b i n d i n g ( 5 4 ) ; the resulting micelles with brom i d e c o u n t e r i o n s have a lower degree o f ionization ( 5 4 ) a n d higher aggregation n u m b e r s ( 9 0 - 9 5 (53, 55) versus 80 ( 5 6 ) ) t h a n with chloride counterions. Such a m o r p h o l o g i c a l t r a n s i t i o n is m o r e subtle t h a n a s p h e r e - t o - r o d transition. It is likely a c c o m p a n i e d b y a m o r e tightly p a c k e d a r r a n g e m e n t o f the a m p h i p h i l e s because o f decreased head-group repulsion a n d lowered h y d r a t i o n forces as discussed above. It has b e e n observed (57, 58) t h a t N,N-dimethylaniline, a molecule very similar to PPD, preferentially distributes close to the surface o f C T A B micelles ( h a v i n g b r o m i d e counteri o n s ) as a result o f r e d u c e d h y d r a t i o n forces as discussed above. This o b s e r v a t i o n is consistent with the m o r p h o l o g i c a l t r a n s i t i o n proposed a n d the c o n c o m i t a n t change in effective d i s t r i b u t i o n coefficient observed in the m o d eling o f the partitioning. ACKNOWLEDGMENTS We thank Dr. R. A. Mackay and Dr. K. Chaff for discussions, criticisms, and suggestions. REFERENCES 1. Colichman, E. L., J. Amer. Chem. Soc. 72, 4036 (1950). 2. Yeh, P., and Kuwana, T., J. Electrochem. Soc. 123, 1334 (1976). 3. Vieil, E., "Etude th6orique de m6canismes 61ectrochimiques en m6thode stationnaire." Th6se, L'Universit~ Scienfifique M~dicale de Grenoble, 1979. 4. Shinozuka, N., and Hayano, S., "Solution Chemistry of Surfactants," Vol. 2 (K. L. Mittal, Ed.), pp. 599623. Plenum, New York, 1979. 5. Ohsawa, Y., Shimazaki, Y., and Aoyaguf, S., J. Electroanal. Chem. 108, 385 (1980). Journal of Colloid and Interface Science, Vol. 135, No. 1, March 1, 1990

6. Ohsawa, Y., Shimazaki, Y,, Suga, K., and Aoyagui, S., J. Electroanal. Chem. 123, 409 ( 1981 ). 7. Genies, M., and Thomalla, M., Electrochim. Acta 26, 829 ( 1981 ). 8. Muller, E., and Dorfler, H.-D., Z. Chem. 21, 28 (1981). 9. Mclntire, G. L., Cbiappardi, D. M., Casselbery, R. L., and Blount, H. N., J. Phys. Chem. 86, 2632 (1982). 10. Ohsawa, Y., and Aoyagui, S., J. Electroanal. Chem. 136, 353 (1982). 11. Bertbod, A., and Georges, J., Anal, Chim. Acta 147, 41 (1983). 12. Ohsawa, Y., and Aoyagui, S., J. Electroanal. Chem. 145, 109 (1983). 13. Eddowes, M. J., and Gratzel, M., J. Electroanal. Chem. 163, 31 (1984). 14. Georges, J., and Desmettre, S., Electrochim. Acta 29, 521 (1984). 15. Rusting, J. F., and Kamau, G. N., J. Electroanal. Chem. 187, 355 (1985). 16. Kaifer, A. E., and Bard, A. J., J. Phys. Chem. 89, 4876 (1985). 17. Mousty, C., Pouillen, P., Maitre, A.-M., and Mousset, G., J. Colloid Interface Sci. 113, 521 (1986). 18. Zana, R., and Mackay, R. A., Langmuir 2, 109 (1986). 19. Quintela, P. A., and Kaifer, A. E., Langmuir 3, 769 (1987). 20. Rusting, J. F., Shi, C.-N., Gosser, D. K., and Stiukla, S. S., J. Electroanal. Chem. 240, 201 (1988). 21. Kamau, G. N., and Rusling, J. F., J. Electroanal. Chem. 240, 217 (1988). 22. Rusting, J. F., Shi, C.-N., and Kumosinski, T. F., AnaL Chem. 60, 1260 (1988). 23. Mandal, A. B., Nair, B. U., and Ramaswamy, D., Langmuir 4, 736 (1988). 24. Mackay, R. A., Dixit, N. S., and Agarwal, R., in "Inorganic Reactions in Organized Media" (S. L. Holt, Ed.), pp. 179-194. Amer. Chem. Soc. Syrup. Ser., Vol. 177, Amer. Chem. Soc., Washington, DC, 1982. 25. Georges, J., and Berthod, A., Electrochim. Acta 28, 735 (1983). 26. Mackay, R. A., Dixit, N. S., Agarwal, R., and Seiders, R. P., J. Dispersion Sci. TechnoL 4, 397 (1983). 27. Berthod, A., and Georges, J., J. Colloid Interface Sci. 106, 194 (1985). 28. Chokshi, K. R., Qutubuddin, S., and Hussam, A., J. Colloid Interface Sci. 129, 315 (1989). 29. Chen, J.-W., and Georges, J., J. Electroanal. Chem. 210, 205 (1986). 30. Mackay, R. A., Dixit, N. S., Hermansky, C., and Kertes, A. S., Colloids Surf 21, 27 (1986). 31. Matsubara, T., and Texter, J., J. Colloid Interface Sci. 112, 42 (1986). 32. Georges, J., and Desmettre, S., J. Dispersion Sci. Technol. 7, 21 (1986).

VOLTAMMETRIC CMC DETECTION 33. Texter, J., Beverly, T., Templar, S. R., and Matsubara, T., J. Colloid Interface Sci. 120, 389 (1987). 34. Rusling, J. F., Connors, T. F., and Owlia, A., Anal Chem. 59, 2123 (1987). 35. Levich, V. G., "Physieochemical Hydrodynamics." Prentice-Hall, Englewood Cliffs, NJ, 1962. 36. Phillips, J. N., Trans. Faraday Soc. 51, 561 (1955). 37. Williams, R. J., Phillips, J. N., and Mysels, K. J., Trans. Faraday Soc. 51, 728 (1955). 38. Anacker, E. W., Rush, R. M., and Johnson, J. S., J. Phys. Chem. 68, 81 (1964). 39. Schott, H., J. Phys. Chem. 70, 2966 (1966). 40. Schick, M. J., J. Amer. Oil Chem. Soc. 43, 681 (1966). 41. Scott, A. B., and Tarter, H. V., J. Amer. Chem. Soc. 65, 692 (1943). 42. Ross, S., and Oliver, J. P., J. Phys. Chem. 63, 1671 (1959). 43. Tamamushi, R., and Yamanaka, T., Bull. Chem. Soc. Japan 28, 673 (1955). 44. Malik, W. U., and Chand, P., J. Amer. Oil Chem. Soc. 43, 446 (1966). 45. Malik, W. U., and Thamb, O. P., J. Amer. Oil Chem. Soc. 49, 170 (1972). 46. Shinozuka, N., Suzuki, H., and Hayano, S., Kolloid Z. Z. Polym. 248, 959 (1971).

271

47. Vollhardt, D., TensideDeterg. 12, 225 (1975). 48. Vollhardt, D., Colloid Polym. Sci. 254, 64 (1976 ). 49. Hayano, S., and Shinozuka, N., Bull. Chem. Soc. Japan 44, 1503 (1971). 50. Melo, E. C. C., and Costa, S. M. B., J. Phys. Chem. 91, 5635 (1987). 51. Mclntire, G. L., and Blount, H. N., J. Amer. Chem. Soc. 101, 7720 (1979). 52. Imae, T., and Ikeda, S., ColloidPolym. Sci. 256, 1090 (1987). 53. Imae, T., Kamiya, R., and Ikeda, S., J. Colloid Interface Sci. 108, 215 (1985). 54. Fabre, H., Kamenka, N., Khan, A., Lindblom, G., Lindman, B., and Tiddy, G. J. T., J. Phys. Chem. 84, 3426 (1980). 55. Ekwall, P., Mandell, L., and Solyom, P., Z Colloid Interface Sci. 35, 519 ( 1971 ). 56. Roelants, E., Gelade, E., van der Auweraer, M., Croonen, Y., and de Schryver, F. C., Z Colloid Interface Sci. 96, 288 (1983). 57. Eriksson, J. C., and Gillberg, G., Acta Chem. Scand. 20, 2019 (1966). 58. Lindblom, G., Lindman, B., and Mandell, L., ar. Colloid Interface Sci. 42, 400 (1973).

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