Counterion Association in Mixed Micelles of Cationic and Nonionic Detergents M A U R I C I O M E Y E R AND LUIS SEPI[ILVEDA Departamento de Qulrnica, Facultad de Ciencias Bdsicas y Farrnacduticas, Universidad de Chile, Las Palmeras 3425, Casilla 653, Santiago, Chile
Received August 3, 1983; accepted November 1, 1983 A study of a mixed micellar system formedby mixing a cationicsurfactant(cetyltrimethylammonium bromide) with a nonionic surfactant (polioxiethylene (23) Lauryl Ether, Brij 35) is presented. The variation of the micellar surface charge density with the composition of the mixed micelles and its influence upon some micellar surface properties has been obtained. These effectsare observed for the decomposition of uncharged substrates such as 2,4-dinitrochlorobenzene and p-nitrophenyldiphenyl phosphate. The measured quenching produced by bromide ions upon the fluorescence of biphenyl adsorbed by micelles was used to obtain the concentration of the bromide cotmterions. The variation of the specific conductance and the critical micelle concentration as a function of the composition of the surfaetant mixtures are also reported. The results are discussed in terms of a minimum ionic surfactant composition necessaryto form a Stern layer in the mixed micelle. INTRODUCTION Mixed micelles are those composed by more than one amphiphilic species, each species being able to form micelles separately. This definition does not consider as mixed micelles those micellar solutions in which a simple solute molecule has been only sorbed by existing micelles. The definition also implies that in a solution with two different amphiphiles the composition may be varied from a pure micellar solution of one component to a pure micellar solution of the other passing through all possible mixed micelles compositions. In this context an ionic detergent micellar solution having different counterions should be considered a mixed micellar solution since any counterion may form in principle a different surfactant molecule. Previous work in nonionic mixed micelles indicates that some properties such as the critical micelle concentration (CMC) does not vary in direct relation to the composition and CMC of the pure components (1-3). Several theories which assume a uniform micellar composition have been proposed in order to account for these results (1, 2, 4).
Mixed micelles of anionic,and nonionic detergents have also been studied (5-9) and one of the conclusions obtained is that the degree of ionization (~) increased as the nonionic detergent composition of the mixed micelle increased. These kinds of results are expected but they have not been conveniently studied and discussed. A mixed micelle formed with an ionic and a nonionic surfactant might require a minim u m composition in the ionic surfactant to give the micelle the surface charge necessary to adsorb counterions and form the Stern layer. This statement is the basis of the present work which shows that in order that counterions associate with micelles a certain degree of charge density must be reached. In this work the micellar charge density was varied by changing the composition o f mixed miceIles formed by a cationic and a nonionic surfactam. The transition from a nonionic to an ionic micelle might be reflected in properties such as conductivity, quenching of the fluorescence of a probe by the counterions, catalysis of a chemical reaction in which counterions also participate, change in CMC, etc.
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Journal of Colloid and Interface Science, Vol. 99, No. 2, June 1984
Accordingly, we have examined catalytic, quenching, conductivity and CMC properties of mixed micellar solutions formed by cetyltrimethylammonium bromide (CTAB) and
polyoxyethylene (23) lauryl ether (Brij 35). The micellar catalyzed reactions were the alkalyne hydrolysis of p-nitrophenyldiphenyl phosphate (PNPDP)
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described (11, 12) at 25 and 35°C for Reactions [I] and [II], respectively. f'YX OConductivities. The conductivities o f difNO 2 - O ~ 2 - Cl + O H - - - ~ NO 2 - ~ ferent mixtures having an (XcrAB) of 0.2, 0.4, ' \ [II] NO 2 NO 2 0.6, 0.8, and 1.0 were measured as a function The fluorescence quenching of biphenyl pro- of total surfactant concentration at 25 duced by bromide counterions was used to _+ 0.01 °C. A Schott Gerate CG 851 conducmeasure the ionization degree of the mixed timeter coupled with a cell having a cell constant of 0.1 was used. micelles. Fluorescence. The quenching of the fluoThe interest in the properties o f ionic-nonrescence of byphenyl solubilized in C T A B ionic mixed micelles as compared to pure miBrij 35 solution with 0.01 total surfactant concelles mainly stems in that they may be more centration and at several XCTAB values was closely related to biological systems such as measured in a Perkin Elmer 2045 spectroenzymes and membranes where the surface fluorimeter. The wavelength of excitation was charged groups might be 'separated by non260 n m and the emission spectrum was regionic components. istered between 280 and 350 nm. The quenching values of maximum intensity were EXPERIMENTAL considered proportional to the disappearance Reagents. CTAB, Matheson C. was recrys- of the band since this agrees with the area tallized twice from ethanol ether mixtures. Brij under the emission spectrum. 35, Atlas C.I. was used without further puCritical micelle concentrations (CMCs). The rification after finding that the rate of Reaction CMCs of the different CTAB-Brij 35 mixtures [I] in the presence of unpurified Brij 35 was were measured by the solubilization method not significantly different from the rate using Orange OT (1-(O-tolylazo)-2-naphthol). with Brij 35 purified in water-t-butanol mix- For every value of XCXAB several solutions of tures (10). total detergent concentration ranging between Orange OT, PNPDP, and 2,4-DNCB were 0.0 and 0.01 M were prepared and saturated samples kindly supplied b y Dr. C. A. Bunton with Orange OT. After 3 days at 25 + 0.1 °C from the University of California. the solutions were filtered to remove the excess Kinetics. Mixtures of CTAB and Brij 35 of dye and the solubilized Orange O T detercontaining a/micellar molar fraction XCTAB mined by measuring the absorbance at 495 (X = [CTAB]/[CTAB] + [Brij 35]) of 0.0, 0.2, n m in a Shimadzu UV-150-2 spectrophotom0.6, 0.8, and 1.0 were prepared. The total con- eter. centration ([CTAB] + [Brij 35]) in all of the above mixtures was varied from 0 to 0.08 M RESULTS AND DISCUSSION to study the effect of mieelles on the rate of Reaction [I] and from 0 to 0.003 M to study Kinetics. Second-order rate constants (k2) the rate of Reaction [iI]. The rates were fol- for Reaction [II] in 0.05 M N a O H as a funclowed spectrophotometrically as previously tion of total surfactant concentration and at and 2,4-dinitrochlorobenzene (2,4-DNCB).
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different XCTABmicellar molar fractions are shown in Fig. 1 where it is observed that as 250 800 the micellar nonionic surfactant content increases, k2 drastically decreases. If it is assumed 1 200[ that the substrate is always completely bound 600 to any kind of micelles independently of their detergent compositions (12-14), the decrease in k2 can be attributed to a decrease in the 400 amount of hydroxide ions bound to the miceUes. This could be a consequence of the lowering in surface charge density as the mi20O ceUes become poorer in the ionic surfactant 5C component. It may also be noted in Fig. 1 that the maximum k2 (k~ ax) for mixed miceUes containing 0.2 0,4 0.6 0,8 1,0 an 80% of CTAB is practically the same as X CTAB that found in pure CTAB, which suggests that FIG. 2. Maximum second-order rates constants for the at this micellar compositions the micelles still hydrolysis of 2,4-DNCB (e) and PNPDP ((3) as a function have a Stern layer similar to pure CTAB mi- of the molar fraction of CTAB in the whole system, at celles. The general pattern of k~ ~x is shown 0.01 M NaOH and 35 and 25°C, respectively. in Fig. 2 which indicates that for values o f XCTABbelow 0.2 and above 0.8 the mixed mi- a minimum surface charge is required in order celles behave as pure nonionic or cationic mi- that O H - counterions can be adsorbed by the celles, respectively. These results indicate that mixed micelles and participate in Reaction [II]. On the other hand, an XCTABvalue of 0.8 should be sufficient to give the mixed micelles a surface charge large enough to allow the 2.5~ / o adsorption of O H - counterions in similar O amounts as in pure CTAB miceUes, k2 and ~o k~ aax of Reaction [I] in the presence of mixed 2.C ~.o micelles show a similar pattern as Reaction [II] (Figs. 2 and 3). The differences observed can be attributed to the fact that the rate of E 1.5 o Reaction [I] is inhibited by nonionic micelles 0.8 while the rate of Reaction [II] is not affected. The effect of nonionic micelles on the rates of reactions [I] and [II] has been discussed by Bunton et al. (12). They concluded that the inhibition upon the rate of reaction [I] is due 0.5 / • to the inclusion of the substrate in the miceUar core and that the small or null effect of nonionic micelles on the rates of decomposition of nitrohalobenzenes is due to the fact that 20 io ab go " these substrates tend to stay in the exterior [CD] total (mM) water rich region of the miceUe. FIG. 1. Second-order rate constants for the hydrolysis Both experimental and theoretical works of 2,4-DNCB in 0.01 M N a O H and 35°C as a function of the total surfactant concentration and at the given molar indicate that k2 for Reactions [I] and [II] go fractions of CTAB. through a m a x i m u m as the concentration of ,-
Journal of Colloid and Interface Science, Vol. 99, No. 2, June 1984
for the quenching process, Tv is the time of half life of the fluorescence, and [Q] is the concentration of the quencher (Br- ions in this work). When [Q] corresponds to the concentration of Br- ions associate with pure CTAB micelles (BrcrAB) or to Br- ions associate with mixed micelles (Brmix) the following relation is obtained according to Eq. :
1 Io/Imix- 1
4.0 6.0 [CD] total (raM)
FIG. 3. Second-order rates constants for the hydrolysis of PNPDP in 0.0! M N a O H and 25°C as a function of total suffactant concentration and at the ~ven molar fractions of CRAB.
cationic micelle increase (13-16). The mixed cationic-nonionic micelles here studied show the same behavior. However, for both reactions the decrease of k2 with total detergent concentration is much more pronounced than for pure CTAB (Figs. 1 and 3). Romsted's treatment (16) predicts that the approach to and away from the rate m a x i m u m becomes more rapid when the substrate binding constants or the micellar rate constant is increased. However, none of these parameters can be expected to be larger i n a Brij 35 or a Brij 35CTAB mixed micelle than in CTAB micelles due to the strong interactions of the benzene rings of the substrates molecules with the positive charge of CTAB micelles (12, 17). This effect may be explained on the basis that O H - counterions are removed by Brcounterions more easily from mixed micelles than from pure CTAB micelles. Fluorescence. To account for the fluorescence results the quantum yield in the SternVolmer equation can be replaced by fluorescence intensities, giving
- - = 1 + kQTF[Q] 1
where lo and I stand for the fluorescence intensity in the absence and in presence of a quencher, respectively, kQ is the rate constant
Equation  assumes that kQTF for pure CTAB has the same value as in the mixtures with Brij 35. The concentration of bromide ions associated to pure CTAB micelles is related to the degree of dissociation (a) through the relation Br~TAB = (Co - CMC) × (1 - a)
where the total concentration of surfactant (Cb) was 10 m M a n d a and the CMC of pure CTAB was taken as 0.2 and 8 × 10 -4 M, respectively (16, 18). In this way Brmi~ is easily obtained from Eq. . This Brmix concentration divided by the total concentration of bromide ions present in a given solution will give the fraction 13 = (1 - a) of the bromide ions associated with the different mixed micelle systems. Figure 4 shows the quenching of biphenyl expressed as 1/lo and the binding degree of 6.0
~ 0.6 0.4
0.2 0~2 ' 0,4
FIG. 4. Fluorescence quenching (Io/I=ix) of biphenyl and association degree (/3) of bromide counterions as a function of the molar fraction of CTAB in the whole system. Journal of Colloid and Interface Science, Vol. 99, No. 2, June 1984
MEYER AND SEP(ILVEDA
counterions (/3) as a function of mixed micellar composition at 10 m M total surfactant concentration. The values o f bound (Br~,ix), aqueous (Bra) and total (Br~-) bromide counterions and/3 values are also shown in Table I calculated according to Eqs.  through . In both Fig. 4 and Table I it is observed that the quenching Io/I and also/3 increase as the cationic composition of the mixed miceUes increases but the increment begins approximately after a 20% of cationic surfactant composition has been reached. However at higher XCrAB, I0/I and /3 do not seem to reach a plateau as it would be expected from kinetic data. The quenching of the fluorescence is due to the bromide counterions existing in the Stem layer (19) which, according to the data in Fig. 4 and Table I, is just formed when micelles have reached a surface charge density given by a 20% of the cationic component CTAB. The value of 13 obtained by the fluorescence method for pure CTAB is slightly higher than the 0.22 value currently accepted (16). A classical electrostatic model allows one to figure out the surface potential o f a mixed micelle with an XCTAB= 0.2 through the equation (20) ( 134 ] ~P0 = 50.4 sinh -1 ~Ax(Ci)l/2 ]
where A is the area available in the surface to
TABLE I Variation of Micellar (BrOnx) Aqueous Br~-) and Total (Brt) Molar Bromide Concentration, and Association Degrees (~) in Terms of the XCrAB Molar Fraction of CTAB at a 0.01 M Total Surfactant Concentration Xcr~
Br~ X 103
Bra × 103
0.1 0.3 0.5 0.5 0,6 0.8 0,8 0,9 1,0
0.01 0.22 1.3 1.1 2.1 3.4 3.3 5.2 7.4
1.0 3.0 5.0 5.0 6.0 8.0 8.0 9.0 10.0
0.09 2.8 3.7 3.9 3.9 4.6 4.7 3.8 2.6
Journalof Colloidand InterfaceScience, Vol. 99, No, 2, June 1984
0.01 0.07 0.25 0.22 0.34 0.42 0.41 0.67 0.74
each surface charge and Ci is the ionic strength in moles/liter. If the radius o f the spherical micelle is taken as the length o f the dodecyl groups of the nonionic detergent (16.5 A) and an aggregation number of 100 is assumed, fro comes to be 78 mV. This value o f ~bo should correspond to the minimum surface potential large enough to associate counterions. The same calculation performed for a micelle of pure CTAB with a radius o f 22 A and an aggregation number of 100 gives a surface potential of 105 mV. It is then concluded that counterion association is extremely dependent on the surface potential of the miceUes. The surface charge density and the counteflon binding degree (13) might also be refleeted in the electrical conductance properties of the mixed micelles. Figure 5 shows the conductivities of the different mixtures as a function of total surfactant concentration. It may be seen that the different mixtures do not present a clear break in the slopes when micelles are supposed to be formed as it may be seen in the curve for pure CTAB where the CMC is clearly defined by the intersection of the lines before and after the CMC. Since the conducting species are CTA +, Br-, and the charge micelles, it is interesting to see how the conductivity changes as a function of the actual concentration of CTAB which is shown in Fig. 6 where several facts can be noted. First, the conductivities of the mixtures with higher nonionic surfactant composition are greater than those having the poorest content. This behavior has also been observed in anionic-nonionie mixed micelles (21, 22) and can be explained considering that an increase in the nonionic surfactant composition tends to increase the ionization degree a which results in a higher concentration of free counterions and in a higher electrical micellar mobility due to an increase in the miceUar charge density. Second, and as a consequence of the first considerations, at low CTAB concentrations when CTAB exists only as monomers or micelles are fully ionized, the conductivity is almost independent of the nonionic composition. Finally, the absence of a sharp break
0.E 20 0.6
0.4 "~ 1oo
012'0.4 016 018 £0 ~ X CTAB
U 120 210 310 410 510 OJO 710 8:0 9J~" [CD] total (raM) FIG. 5. Specific conductivities (X) of the mixtures of CTAB and Brij 35 at the given molar fractions of CTAB as a function of the total surfactant concentration.
in the conductivity curves makes impossible the determination of the CMCs of the mixed micelles by a conductimetric method. However, this information was obtained by solubilization of Orange OT and the results are
/ 04/" z ,8, // n/
FIG. 7. Critical micelle concentrations of the mixtures of CTAB and Brij 35 as a function of the molar fraction of CTAB in the whole system. The broken curve stands for an ideal mixture (20, 21) and (0) correspond to literature values (18).
shown in Fig. 7, where it may be seen that the CMC gradually changes from its value in pure CTAB to the value in pure Brij 35. Since the abrupt change in slope of the conductivity curves beyond the CMC is due to counterion adsorption, the absence of such a change in mixtures of low ionic surfactant content is a further indication that micelles in those mixtures are completely dissociated. The ideal CMCs predicted by the treatment of Lange and Beck (23) and Clint (24) are also shown in Fig. 7. The experimental CMCs are well above the predicted values suggesting that these mixed micelles systems are less stable than the corresponding pure micellar solutions.
ACKNOWLEDGMENTS Support of this work by the Departamento de Desarrollo de la Investigaci6n de la Universidad de Chile is gratefully acknowledged. We also thank Dr. Carlos Andrade for helpful discussions and Miss Eliana Villagra and Miss Maria Luz Pefia for typing and drawing.
110 2'.0 3'.0 4'.0 510 6'.0 7',0 8'.0 0~.~ [CTAB] total (raM)
FIG. 6. Specific conductivities (X) of the mixtures of CTAB and Brij 35 at the given molar fractions of CTAB as a function of CTAB concentration.
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2. Moroi, Y., Nishilddo, N., and Matuara, R., J. Colloid Interface Sci. 46, 111 (1974). 3. Shinoda, K., J. Phys. Chem. 54, 541 (1954), 4. Moroi, Y., Nishilddo, N., and Matuara, R., J. Colloid Interface Sci. 52, 356 (1975). 5. Lange, H., and Beck, K. H., Kolloid Z-Z Polym. 251, 424 (1973), 6. Shick, M. J., and Manning, D. J., J. Amer, Oil. Chem. Soc. 43, 133 (1966). 7. Shick, M. J., J. Amer. Oil Chem. Soc. 43, 681 (1966). 8. Corkill, J. M., Goodman, J. F., and Tare, J. R., Trans. Faraday Soc. 60, 986 (1964). 9. Toldwa, F., and Moriyama, M., J. Colloid Interface Sci. 30, 338 (1969). 10. Shinoda, K., "Colloidal Surfactants." Academic Press, New York, 1962. 11. Bunton, C. A., and Robinson, L., J. Amer. Chem. Soc. 90, 5965 (1968). 12. Bunton, C. A,, and Robinson, L., J. Org. Chem. 34, 773 (1969). 13. Bunton, C. A., Catal. Rev. Sci. Eng. 20, 1 (1979).
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14. Bunton, C. A., Prog. SolidState Chem. 8, 239 (1973). 15. Fendler, J. H., and Fender, E. J., "Catalysis in MiceUar and Macromolecular Systems." Academic Press, New York, 1975. 16. Romsted, L. S. Ph.D. Thesis, Indiana University (1975). 17. Sepflveda, L., and Hirose, C., J. Phys. Chem. 85, 3689 (1981). 18. Mukerjee, P., and Mysels, K. J., Natl. Stand. Ref Data Serv., Natl. Bur. Stand. 36, 107 (1971). 19. Abuin, E., Lissi, E., Quina, F. H., and Sepfilveda, L., J. Phys. Chem., in press. 20. Davies, J. T., and Rideal, E. K., "Interracial Phenomena." Academic Press, New York, 1961. 21. Biswas, A. K., and Mukherji, B. K., J. Phys. Chem, 64, 1 (1960). 22. Corkill, J. M., Goodman, J. F., and Tate, J. R., Trans. Faraday Soc. 60, 986 (1964). 23. Lange, H., and Beck, K. H., Kolloid Z-Z. Polym. 251, 424 (1973). 24. Clint, J., J. Chem. Soc. 71, 1327 (1975).