Mixed Micellization of Cetyltrimethylammonium Bromide and

and charge of ionic surfactant micelles from the stepwise thinning of foam films. ... Peter A. Kralchevsky, Kavssery P. Ananthapadmanabhan, Alex L...
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Langmuir 1997, 13, 402-408

Mixed Micellization of Cetyltrimethylammonium Bromide and an Anionic Dimeric (Gemini) Surfactant in Aqueous Solution Raoul Zana* and He´le`ne Le´vy Institut C. Sadron (CNRS), 6 rue Boussingault, 67000 Strasbourg, France

Dganit Danino and Yeshayahu Talmon Technion-Israel Institute of Technology, Haifa 32000, Israel

Klaus Kwetkat Hu¨ ls Aktiengesellschaft, Marl D45764, Germany Received July 15, 1996. In Final Form: October 31, 1996X The mixed micellization between cetyltrimethylammonium bromide (CTAB) and the anionic dimeric surfactant disodium 1,11-didecyl-3,6,9-trioxaundecane)-1,11-disulfate (2) has been investigated by electrical conductivity (Krafft temperature, TK, critical micellization concentration, cmc, and micelle ionization degree), spectrofluorometry (cmc, micelle polarity), time-resolved fluorescence quenching (micelle aggregation numbers), and transmission electron microscopy at cryogenic temperature (microstructure of the system) in water + 50 mM NaBr. Most measurements were performed at a CTAB concentration of 5 mM, a mole fraction of 2 X ) 0.091, and 30 °C. Some measurements were performed in pure water for assessing the effect of NaBr. The presence of 2 enlarged the range of temperature in which a 5 mM CTAB suspension in water + 50 mM NaBr clarifies. The initial micellization step which probably leads to micelles of 2 nearly fully neutralized by CTA+ ions was detected by spectrofluorometry with pyrene as fluorescent probe and took place at a CTAB concentration, C, below 0.1 mM. In both water and water + 50 mM NaBr, the electrical conductivity method was not sensitive to this micellization step. However, it detected a cmc at a much higher value of C which closely corresponds to a free CTA+ concentration equal to the cmc of CTAB alone. This second cmc is not detected by spectrofluorometry. The aggregation numbers of the mixed micelles are larger than those of CTAB micelles even at X as low as 0.02. At X ) 0.091, the apparent aggregation number is very large, with a lower bound value of 410. Electron microscopy shows in this system the presence of vesicles and of very large aggregates, revealing that the system is close to precipitation under the experimental conditions used. Overall the results do not support the existence of the “crosslinked” micelles postulated to explain the large decrease of the NMR relaxation time T2 taking place upon addition of another anionic dimeric surfactant to the same CTAB system (Menger and Eliseev, Langmuir 1995, 11, 1855).

Introduction Dimeric (gemini) surfactants are made up of two identical amphiphilic moieties connected at the level of the head groups by a spacer group which can be hydrophilic or hydrophobic, rigid or flexible.1 These surfactants are currently the subject of increasing interest, as they are superior to the corresponding conventional monomeric surfactants (made up by one head group and one hydrophobic moiety) in all the properties on which surfactant applications rest. Thus, they have much lower critical micelle concentration (cmc), are more efficient in lowering the surface tension of water, have better lime-soap dispersing properties, and are often better wetting agents.2 In addition, some dimeric surfactants have interesting rheological properties at relatively low concentration.3,4 In view of their higher cost, dimeric surfactants are likely to be used mixed with conventional surfactants. Besides, synergistic effects may be found in conventional/ dimeric surfactant mixtures, which may render the use * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, January 1, 1997. (1) Zana, R. Curr. Opin. Colloid Interface Sci. 1996, 1, 566. Specialist Surfactants; Robb, I., Ed.; Chapman Hall Ltd.: New York, in press. (2) Rosen, M. J. Chemtech 1993, 30. (3) Kern, F.; Lequeux, F.; Zana, R.; Candau, S. J. Langmuir 1994, 10, 1714. (4) Schmitt, V.; Schosseler, F.; Lequeux, F. Europhys. Lett. 1995, 30, 31.

of such mixtures even more attractive. Studies involving nonionic or zwitterionic conventional surfactants and anionic dimeric surfactants focused on the search of synergism in micellization and monolayer formation at the air-water interface.5-7 Other studies dealt with structural aspects and aggregation behavior. It was shown8 that the dimeric dicationic surfactant 12-2-12 2Brforms threadlike micelles which are transformed into spherical micelles upon addition of the corresponding conventional cationic surfactant DTAB (dodecyltrimethylammonium bromide) at a mole fraction of 0.3. DTAB was shown to not accumulate at the hemispherical end caps of the threadlike micelles during the threadlike/ spherical micelle transformation.9 A study10 of the aggregation behavior of a mixture of the cationic surfactant CTAB (cetyltrimethylammonium bromide) and the anionic dimeric surfactant 1 (Figure 1), which includes a rigid hydrophobic spacer, concluded that CTAB-rich micelles cross-linked by dimeric surfactants form at low mole fractions of 1. Cross-linked micelles have been postulated (5) Rosen, M. J.; Gao, T.; Nakatsuji, Y.; Masuyama, A. Colloids Surf. A: Physicochem. Eng. Asp. 1994, 88, 1. (6) Rosen, M. J.; Zhu, Z.; Gao, T. J. Colloid Interface Sci. 1993, 15, 224. (7) Gao, T.; Rosen, M. J. J. Am. Oil Chem. Soc. 1994, 71, 771. (8) Zana, R.; Talmon, Y. Nature 1993, 362, 228. (9) Schosseler, F.; Anthony, O.; Beinert, G.; Zana, R. Langmuir 1995, 11, 3347. (10) Menger, F. M.; Eliseev, A. V. Langmuir 1995, 11, 1855.

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Figure 1. Chemical structure of surfactants 1 (Φ is a phenylene ring; no indication was given on whether the cis or trans isomer or a mixture of isomers was used10), 2, and CTAB.

in the case of aqueous solutions of amphiphilic block copolymers11 and hydrophobically end-capped low molecular weight polyoxyethylenes12 but never thus far for surfactants, whether conventional or dimeric. However, the authors10 did not completely dismiss an interpretation of their results in terms of growth of the CTAB micelles induced by the comicellization with the oppositely charged dimeric surfactant, an effect well investigated in the case of mixtures of anionic and cationic surfactants.13,14 Also they observed the formation of large aggregates by light microscopy. Furthermore, their interpretation of T2 NMR data may not be valid (see Discussion). In view of the importance of the new structure postulated, we felt it worthwhile to undertake a similar study using CTAB at the same concentration (5 mM) and with the same content of NaBr (50 mM). However the anionic dimeric surfactant used (2 in Figure 1) differs from that in ref 10. Indeed we believed that surfactant 2 with its flexible hydrophilic spacer was more likely to give rise to cross-linked micelles than surfactant 1 with its rigid hydrophobic spacer. The studies involved electrical conductivity, spectrofluorometry, time-resolved fluorescence quenching, and transmission electron microscopy at cryogenic temperature (cryo-TEM). The first three methods were used to characterize the mixed systems (cmc, micelle ionization degree, and aggregation numbers). The last method gave a direct visualization of the microstructure of the mixed system. Our results do not support the formation of cross-linked micelles. They show that additions of the disodium disulfate surfactant 2 to the 5 mM CTAB solution result in the growth of the CTAB micelles even at mole fraction X of 2 as low as 0.02. Cryo-TEM showed at X ) 0.091 the presence of vesicles and of other large aggregates similar to those observed with mixtures of conventional cationic and anionic surfactants, revealing that the system is close to precipitation.

Langmuir, Vol. 13, No. 3, 1997 403 asymmetric carbon atoms (See Appendix 1). It was synthesized in two steps starting with the addition of 2 equivs of 1,2epoxydecane to di(ethylene glycol) catalyzed by Al2O3, followed by a sulfatation of the resulting compound with chlorosulfonic acid in the presence of acetic acid as promotor. The surfactant was purified by column chromatography on Kieselgel 60 with ethyl acetate/methanol (4:1 v/v) as eluent. Mass spectrometry (using matrix-assisted laser desorption ionization and time of flight) of the purified compound gave satisfactory results. More details on the synthesis are given in Appendix 1. Suprapure grade NaBr was from Merck (Germany). Methods. The electrical conductivity was used to determine the Krafft temperatures of CTAB and of the CTAB/surfactant 2 mixture at X ) 0.091 in water + 50 mM NaBr. At the Krafft temperature the suspension of solid CTAB clarifies and the conductivity, K, versus the CTAB concentration, C, plot shows a break.15 The electrical conductivity was also used for cmc determinations and the evaluation of an approximate value of the micelle ionization degree, R, taken as the ratio of the slopes of the K versus C plot above and below the break corresponding to the cmc.15 The conductivities were measured as previously described.15,16 It is shown below that the electrical conductivity method did not permit the detection of the cmc of the mixed CTAB/2 systems. This result calls for caution when using electrical conductivty for studying the micellization of mixed systems. Spectrofluorometry with pyrene as fluorescent probe was used to determine cmc's, seen as a break in the variation of the fluorescence intensity ratio I1/I3 of the first and third vibronic peaks in the emission spectra of pyrene, with the CTAB concentration, in the absence and presence of surfactant 2.15,16 The value of I1/I3 gave an estimate of the polarity of the mixed micelles relative to those of pure CTAB micelles.15-17 The fluorescence emission spectra were recorded using a Hitachi 4010 spectrofluorometer.15,16 The micelle aggregation numbers N were obtained using the time-resolved fluorescence quenching (TRFQ) method.16-22 The apparatus was the same as in previous studies.16,22 The fluorescent probe used was the dichloride of Ru(bpy)2bpy2C17, which is a Ru2+ ion chelated by three bipyridines, one of them bearing two heptadecyl chains which anchor the probe to the micellar structure.16 This probe has a longer lifetime than pyrene (over 650 ns versus 180 ns for pyrene, in CTAB solution), which extends the range of N values that can be obtained by TRFQ. It was obtained as a gift from Pr. M. Gra¨tzel (EPFL, Lausanne, Switzerland). The quencher was 9-methylanthracene.23 The microstructure of the CTAB/surfactant 2 mixed solution at X ) 0.091 was investigated by cryo-TEM. Thin samples of vitrified solution were prepared, and their microstructure was visualized as previously described.14,24-26

Results Cmc of Surfactant 2. This surfactant had not been investigated before. Its cmc was found to be 0.20 mM by electrical conductivity, and 0.24 mM by spectrofluorometry (see Figure 2). The conductivity plot yields a micelle ionization degree of about 0.55. The value of I1/I3 in the micellar range is about 1.20 at 25 °C, that is equal to that for sodium dodecyl sulfate

Experimental Section Materials. The sample of CTAB was purchased from Aldrich and purified by three recrystallizations from an ethyl acetate/ ethanol mixture. The dimeric surfactant 2 was in fact a 4:1 mixture of surfactants 2a and 2b (disodium 1,11-didecyl-3,6,9trioxaundecane-1,11-disulfate and disodium 1,10-didecyl-3,6,9trioxaundecane-1,11-disulfate, respectively) and possibly of optical isomers of these two compounds, since 2a and 2b contain (11) Yang, Y. W.; Yang, Z.; Zhou, Z. K.; Attwood, D.; Booth, C. Macromolecules 1996, 29, 670 and references therein. (12) Yekta, A.; Xu, B.; Duhamel, J.; Adiwidjaja, H.; Winnik, M. A. Macromolecules 1995, 28, 956 and references therein. (13) Malliaris, A.; Binana-Limbe´le´, W.; Zana, Z. J. Colloid Interface Sci. 1986, 110, 114. (14) Kamenka, N.; Chorro, M.; Talmon, Y.; Zana, R. Colloids Surf. 1992, 67, 213.

(15) Kamenka, N.; Burgaud, I.; Treiner, C.; Zana, R. Langmuir 1994, 10, 3455 and references therein. (16) Zana, R.; Le´vy, H.; Papoutsi, D.; Beinert, G. Langmuir, 1995, 11, 3694. (17) Zana, R. In Surfactant Solutions. New Methods of Investigation; Zana, R., Ed.; Marcel Dekker Inc.: New York, 1987; Chapter 5, p 241. (18) Infelta, P. Chem. Phys. Lett. 1979, 61, 88. (19) Tachiya, M. Chem. Phys. Lett. 1975, 33, 289. (20) Almgren, M. Adv. Colloid Interface Sci. 1992, 41, 9. (21) Gehlen, M.; De Schryver, F. C. Chem. Rev. 1993, 93, 199. (22) Malliaris, A.; Lang, J.; Zana, R. J. Chem. Soc., Faraday Trans. 1 1986, 82, 109. (23) Turro, N. J.; Yekta, A. J. Am. Chem. Soc. 1978, 100, 5951. (24) Claussen, T. M.; Vinson, P. K.; Minter, J. R.; Davis, H. T.; Talmon, Y.; Miller, W. G. J. Phys. Chem. 1992, 96, 474 (25) Bellare, J. R.; Davis, H. T.; Scriven, L.; Talmon, Y. J. Electron. Microsc. Technol. 1988, 10, 87. (26) Talmon, Y. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 364.

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Figure 2. Surfactant 2: variation of the electrical conductivity K (O) and the ratio I1/I3 (b) with the surfactant concentration at 25 °C.

Figure 3. Variation of the electrical conductivity of a 5 mM CTAB solution in water + 50 mM NaBr with temperature, in the absence of surfactant 2 (O) and in the presence of 2 at mole fraction X ) 0.091 (b). The shift between the two plots is due to the presence of 2. The arrows indicate the temperature of clarification of the systems.

(SDS),27 which can be considered as the approximate monomer of 2. Thus pyrene senses nearly the same polarity in micelles of 2 and of SDS. This suggests that the hydrophilic spacer of 2 is located in the aqueous phase, leaving pyrene in the same microenvironment in micelles of 2 and of SDS. Krafft Temperature of the CTAB/Surfactant 2 mixture. CTAB has a Krafft temperature TK ) 26 °C in water.28 This value may be increased in the presence of NaBr and surfactant 2. The solubility behavior of CTAB was therefore investigated by conductivity15 to determine the temperature above which measurements of cmc and N would be performed. Figure 3 shows the variation of the conductivity K of a 5 mM CTAB solution in water + 50 mM NaBr, in the absence and in the presence of surfactant 2 at mole fraction X ) 0.091. The measurements were performed upon increasing temperature, and the values of K were measured 10 min at least after the selected temperature was reached. The Krafft temperature corresponds to the complete clarification of the surfactant suspension and to the upper break in the conductivity plots, indicated by an arrow in Figure 3. The value of TK in water + 50 mM (27) Anthony, O.; Zana, R. Langmuir 1996, 12, 3590. (28) Rico, I.; Lattes, A. J. Phys. Chem. 1986, 90, 5870.

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Figure 4. Variations of the ratio I1/I3 with the CTAB concentration for CTAB + 2 at X ) 0.091 in water (O) and water + 50 mM NaBr (b) at 30 °C.

NaBr is 25 °C, i.e., very close to that reported in pure water. Also the clarification temperature of the mixture CTAB + 2 at a mole fraction X ) 0.091 is close to that of CTAB. However, the T-range of rapid increase of K is now significantly widened, stretching over about 3 °C instead of 0.3 °C in the absence of 2, and its magnitude is smaller by a factor of about 5. The concentration of 2 used in these experiments was of 0.5 mM, that is larger than the cmc of 2 alone. It is thus likely that the free cetyltrimethylammonium ions (CTA+) become incorporated into the micelles of 2 present in the system, in an amount increasing with temperature but nevertheless lower than when reaching the full solubilization of CTAB at the clarification temperature. This explains the widening of the range of rapid increase of the conductivity. It also suggests that the mixed CTAB + surfactant 2 micelles at X ) 0.091 are more ionized than the pure CTAB micelles.29 The measurements below were therefore performed at 30 °C in order to avoid the problems which may arise from a slow precipitation of the system at 25 °C. Cmc Determinations. Figure 4 shows the variations of I1/I3 with the CTAB concentration C at X ) 0.091 in water and in water + 50 mM NaBr. The results show the usual sigmoidal decrease of I1/I3, and the cmc, referred to as cmc1, was taken as the intercept of the extrapolations of the rapidly varying part of the plot and of the nearly horizontal part of the plot, at high C.30 The values of cmc1 are thus about 0.091 and 0.065 mM in water and water + 50 mM NaBr. The value of I1/I3 at C >> cmc1 is about 1.30. As expected on the basis of the composition of the mixed micelles, this value is closer to that for CTAB (1.38 at 30 °C) than to that for 2 (1.18 at 30 °C). Figures 5 and 6 show the variations of the electrical conductivity of CTAB and CTAB + surfactant 2 at X ) 0.091 solutions with C in water and in water + 50 mM NaBr, respectively. The plots for the various systems show no break at C close to cmc1. However, for CTAB, both in the absence and in the presence of NaBr, a clear break is seen at about 0.95 and 0.27 mM, respectively, and the conductivity plots both below and above the break are quite linear. The plots for the CTAB + surfactant 2 mixture also show breaks at concentrations slightly larger than that for pure CTAB, 1.1 and 0.30 mM, respectively. The concentrations corresponding to these breaks, referred to as cmc2, are not well defined, particularly in the presence of NaBr, owing to an extended curvature of the conduc(29) Gaboriaud, R.; Charbit, G.; Dorion, F. J. Chim. Phys. 1984, 81, 497. (30) Frindi, M.; Michels, B.; Zana, R. J. Phys. Chem. 1992, 96, 8137.

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Figure 5. Variation of the electrical conductivity of CTAB solutions in water in the absence of surfactant 2 (O) and in the presence of 2 at mole fraction X ) 0.091 (b), with the CTAB concentration at 30 °C. The (b) plot has been shifted upward by +10 µS for the sake of clarity.

Figure 6. Variation of the electrical conductivity of CTAB solutions in water + 50 mM NaBr in the absence of surfactant 2 (O) and in the presence of 2 at mole fraction X ) 0.091 (b), with the CTAB concentration at 30 °C.

tivity plots in the cmc range. This curvature reveals the formation of mixed micelles at concentrations well below those for the breaks, but the conductivity method fails to reveal cmc1 because the contributions of the micellized CTA+ and 22- ions to the conductivity are small with respect to those of Br- and Na+ counterions, which remain free (see part 1 of Appendix 2). This is also why the slopes of the K versus C plots below the break in the absence and in the presence of 2 differ little, irrespective of the presence of NaBr (see part 1 of Appendix 2). However, above cmc2, these slopes differ much. We show in part 2 of Appendix 2 that cmc2 for the CTAB + 2 system probably corresponds to the CTAB concentration where the concentration of free CTA+ ions is equal to the cmc of CTAB alone, both in the absence and in the presence of NaBr. The large difference in the slopes of the conductivity plots above the breaks is explained nearly quantitatively on the assumption that the CTA+ ions added to the system above cmc2 are incorporated into the mixed micelles, enriching them with CTA+ (see part 3 of Appendix 2). However the resulting mixed CTAB + 2 micelles are more ionized than the pure CTAB micelles, owing to the presence of the 22ions, which neutralize CTA+ ions, confirming the conclusion reached on the basis of the solubilization behavior (see above). Micelle Aggregation Numbers. All measurements were performed with solutions in water + 50 mM NaBr. The first experiments concerned the CTAB + surfactant 2 system at X ) 0.091. The decay curve was nearly linear

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Figure 7. Variation of the aggregation number of CTAB in the mixed CTAB + 2 systems as a function of the mole fraction of 2 at 30 °C (b); 35 °C (0); and 45 °C (2) in water + 50 mM NaBr.

at 30 °C. Only little curvature was apparent at 45 °C, and the decay curve was rather difficult to analyze. This behavior indicated a small quenching rate constant and thus very large aggregates.20-22 A lower bound value of 420 was obtained for the aggregation number at 45 °C by using the standard fluorescence decay equations. One cannot be sure however that the solution contained micelles as the only aggregated species. These results led us to undertake measurements on the 5 mM CTAB system at lower mole fraction of 2. Two systems with X ) 0.0212 and 0.0552 were investigated in the range between 30 and 45 °C. For these systems the long time part of the decay curves always showed the same slope in the absence and in the presence of quencher, at all temperatures, indicating frozen probe and quencher distributions on the fluorescence time scale. Also, the decay plots were sufficiently curved for a meaningful analysis and in all instances the value of the product of the fluorescence lifetime the intramicellar quenching rate constant was well above 1. The aggregation number of CTAB in the absence of 2 was also measured. The aggregation number values for CTAB and 2, N and N2, respectively, in the mixed aggregates were obtained from the values of the fitting parameter A316-22 using the equations

N ) [C - cmc2(1- 2p)]A3/[Q] and

N2 ) (pC - cmc1)A3/[Q] where the molar concentration of 2 is pC with X ) p/(1 + p) and [Q] is the concentration of quencher. The total aggregation number (number of decyl and hexadecyl chains) is Nt ) N + 2N2. Figure 7 shows the variations of the CTA+ aggregation number, N, with X and T in the mixed CTAB + 2 systems. Figure 7 shows a considerable micelle growth with X above 0.02 at the three temperatures used. The represented results show that N decreases as T is increased, the more so the larger N, a result similar to that found for other ionic surfactants.31 It is obvious that values of N g 150 are inconsistent with a spherical micelle shape. Indeed for a surfactant with a hexadecyl chain, the aggregation number of the minimum spherical micelle is about 100. In view of the (31) Malliaris, A.; Le Moigne, J.; Sturm, J.; Zana, R. J. Phys. Chem. 1985, 89, 2709.

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showed threadlike micelles, absent with the CTAB + 2 systems.) The presence of these large aggregates probably signals that the CTAB + surfactant 2 system is very close to its precipitation composition at X ) 0.091. Similarly, Menger and Eliseev10 observed for the CTAB + surfactant 1 system at X ) 0.094 the presence of particles of sizes up to 500 nm, using light microscopy, at a temperature around 25 °C. This system is also probably very close to, if not below, the precipitation temperature. An important observation regarding the micrographs is the absence of cross-linked micelles, which should have shown as clusters of black dots. Discussion

Figure 8. Electron micrographs of thin vitrified samples of the CTAB + 2 mixture at X ) 0.091, quenched from a temperature of 30 °C ([CTAB] ) 5 mM; [NaBr] ) 50 mM). (a and b) Vesicles, some of them with defects; the black dots represent some microcrystals of ice. (c and d) Larger structures. Bar ) 0.1 µm. The scale is the same in all micrographs.

known tendency of CTAB to form elongated32 and thus polydisperse micelles, it can be anticipated that systems with N > 150 contain such micelles. The micelle polydispersity was not investigated, as our primary purpose was to check whether micelle growth or micelle crosslinking takes place upon addition of 2 to a 5 mM CTAB solution in the presence of 50 mM NaBr. The results above unambiguously demonstrate significant micelle growth. However, they do not permit us to discard micelle cross-linking. Indeed, the time-resolved fluorescence quenching method measures the concentration of hydrophobic microdomains among which probe and quencher are distributed. If these microdomains are linked, as in the postulated model, the method will still measure the true number of microdomains per unit volume, provided the time required for the probe and/or quencher to migrate from one microdomain to another remains much larger than the time required for quenching to take place. As pointed out above, this condition is well fulfilled. Cryo-TEM. Only the system at X ) 0.091 (largest value of X in Figure 7), with [CTAB] ) 5 mM and [NaBr] ) 50 mM, was investigated, and thin vitrified samples were quenched from a temperature of 30 °C. Vesicles of various sizes are clearly seen in Figure 8a and b. Most of them have defects and look “leaky”. Micrographs in Figures 8c and d show other types of structures of larger dimensions, in addition to vesicles. These large aggregates look like distorted multibilayered vesicles with many defects. Somewhat similar structures have been visualized by cryoTEM in DTAB + SDS mixtures at DTAB mole fractions between 0.32 and 0.38.14 (However this last system also (32) Reiss-Husson, F.; Luzzati, V. J. Phys. Chem. 1964, 68, 3504.

The present study failed to find evidence in the CTAB + surfactant 2 system for the presence of the cross-linked micelles postulated10 to interpret the variations of the 1H NMR T2 relaxation times for the CTAB + surfactant 1 system, under identical conditions of CTAB concentration and salt (NaBr) content. The main differences between the two studies concern the nature of the dimeric surfactant, 2 instead of 1 (see Figure 1) added to the CTAB solution, and the temperature: 30 °C instead of 23 °C. The Krafft temperature of the CTAB + 2 system at X ) 0.091 being 25 °C (Figure 2), that of the CTAB + 1 system at the same composition and NaBr content is probably very close to that value. This raises questions concerning the stability of the CTAB + 1 systems particularly during NMR relaxation time measurements which involved data accumulation for 2-4 h.10 A slow precipitation occurring in the systems and leading to the aggregates observed by light microscopy cannot be dismissed. The presence of such large aggregates would explain the reported decrease of T2.10 Note that micelle cross-linking as represented in Figure 1 of ref 10 cannot be used to explain this decrease, contrary to the authors’ claim. Recall that, in the twostep process widely used for the interpretation of the NMR relaxation times data for surfactant-containing systems,33-35 the reorientational motion of surfactant molecules in surfactant aggregates is divided into fast local motions involving the surfactant alkyl chains (rotational isomerism) and slow motions associated with aggregate tumbling and lateral diffusion of surfactant molecules within the micellar interface. In most surfactantcontaining systems, including the ones investigated here and in ref 10, these slow motions are the main ones determining the changes of T2. The expressions of their characteristic times contain the micelle radius. It results that T2 will be quite insensitive to association of micelles under the form of cross-linked micelles,10 remaining relatively small owing to the rapid lateral diffusion of the surfactant over the surface of the individual micelles. On the contrary, T2 decreases upon micelle growth as both tumbling and lateral diffusion are slowed down. Thus, the observations reported in ref 10 give in fact evidence for the growth of CTAB micelles upon addition of the oppositely charged dimeric surfactant 1 (or 2). Micelle growth followed at higher mole fraction of dimeric surfactant by the formation of very large aggregates10 and finally by precipitation of a neutral salt of the cationic and anionic surfactants are the same steps as for mixtures of conventional anionic and cationic surfactants. However, growth is much more important with dimeric than with monomeric surfactants, on a charge per charge basis, (33) Wennerstro¨m, H.; Lindman, B.; So¨derman, O.; Drakenberg, T.; Rosenholm, J. B. J. Am. Chem. Soc. 1979, 101, 6860. (34) Halle, B.;Wennerstro¨m, H. J. Chem. Phys. 1981, 75, 1928. (35) Wennerstro¨m, H.; Lindblom, G.; Lindman, B. Chem. Scr. 1974, 6, 97.

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because the tendency to comicellize is likely to be much stronger with dimeric than with monomeric surfactants. This report did not confirm the existence of cross-linked micelles in the mixed system made up of CTAB and the anionic surfactant 2 at low mole fraction. Nevertheless studies of systems of oppositely charged dimeric and conventional surfactants should be continued, as they may prove of practical interest. Acknowledgment. The authors gratefully acknowledge Prof. B. Lindman (University of Lund, Sweden) for furthering their understanding of the variations of the NMR relaxation times in surfactant-containing systems. Appendix 1: Synthesis of Surfactant 236 Under inert gas, 0.4 mol of 1,2-epoxydecane is added to 0.2 mol of di(ethylene glycol) and 5% (w/w relative to reaction mixture) aluminum oxide. The reaction mixture is heated at 160 °C for about 4 h with the reaction monitored by gas chromatography. The reacted mixture is then cooled, diluted with 150 mL of dichloromethane, washed with a saturated sodium hydrogen carbonate solution, and dried with sodium sulfate. After solvent evaporation the crude diol is distilled under vacuum. The purity of the resulting product (1,11-didecyl-1,11-dihydroxy-3,6,9-trioxaundecane) is checked by gas (or gel) chromatography, and it is characterized by 13C NMR. A solution of 0.12 mol diol in 250 mL of dichloromethane is slowly added to a mixture of 0.4 mol of chlorosulfonic acid and 0.4 mol acetic acid at 0 °C, in such a way that the temperature does not exceed 5 °C. The mixture is then allowed to warm up to room temperature and stirred for 1 h. The reaction progress is monitored using thin layer chromatography (Kieselgel plates; Rf ) 0.475; eluent ) dichloromethane/methanol (10/1, v/v); spots visible after treatment with a solution of 0.2 g of vaniline in 340 mL of methanol, 50 mL of water, and 10 mL of concentrated sulfuric acid solution). The reaction mixture is neutralized with a sodium carbonate solution while cooling with ice, and the raw surfactant is extracted with n-butanol. The solvent is evaporated and the surfactant purified as described (see Materials). The final product was characterized by 13C NMR at 25 °C, using DMSO-d6 as solvent and tetramethylsilane as reference. Characteristic chemical shift values for the main product were as follows: 74.2 ppm (C4); 71.5 ppm (C3); 69.6 ppm (C2); 69.2 ppm (C1); 31.1 ppm (C5); 28.5 (2×), 28.1, 24.2, and 23.9 ppm (C6-C10); 21.5 ppm (C11); and 13.2 ppm (C12). The detailed analysis of the chemical shifts of the carbon atoms attached to the oxygen atom indicated the presence of about 0.2 mol fraction of primary sulfate groups in the final product, e.g. as shown in 2b. Other structures are also possible. The NMR spectra showed no hint of different diastereomers of 2 even though these surfactants contain asymmetric carbon atoms. It was thus assumed that the method did not distinguish between these optical isomers or that most of them were eliminated in the purification procedure. Appendix 2: Interpretation of the Conductivity Results The system considered is made of CTAB at concentration C and 2 at molar concentration pC, which corresponds to a mole fraction of 2, X ) p/(p + 1). The system is characterized by cmc1, the cmc seen by spectrofluorometry but not detected by conductivity, and cmc2, which shows as a break in the conductivity plot but is not seen by (36) Kwetkat, K.; Schro¨der, W. Patent WO96/160033 (30/05/96).

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spectrofluorometry. Below cmc1 the conductivity is given by

KC cmc1 micelles of 2 form in the system. It is assumed that these micelles are fully neutralized by CTA+ ions and thus do not contribute to the conductivity of the solution. It is further assumed that the composition of these micelles does not change in the concentration range between cmc1 and cmc2. The conductivity in this range is made up of a constant term K1, obtained by making C ) cmc1 in eq A1, and C-dependent terms and is given by

Kcmc1,cmc2 ) K1 + (C - cmc1)λ(Br) + 2p(C - cmc1)λ(Na) + (1 - 2p)(C - cmc1)λ(CTA) (A2) As is customary, it is assumed that all ions under micellar form and having their charge neutralized by a counterion do not contribute to the conductivity.37 Under this assumption the ratio R1 of the slopes of the conductivity plot above and below cmc1 is

R1 ) [λ(Br) + 2pλ(Na) + (1 - 2p)λ(CTA)]/[λ(Br) + λ(CTA) + 2pλ(Na) + pλ(2)] (A3) This ratio can be evaluated, using the reported values λ(Na) ) 50.1 S cm2 mol-1 ,37 λ(Br) ) 78.1 S cm2 mol-1,38 and λ(CTA) ) 20 S cm2 mol-1 39 and taking 2λ(2) ) 53 S cm2 mol-1 on the basis of the value reported for the dimer of the dodecyl sulfate ion.40 Thus, for p ) 0.1, i.e., X ) 0.091, R1 ) 0.92. This value is close to 1, and the break corresponding to cmc1 will not show in the conductivity plot, particularly if cmc1 is low. If normal micelles of 2 with an ionization degree R1 ) 0.55 had formed in the system at C > cmc1, the ratio of the slopes of the conductivity plots above and below cmc1 would be 0.76. A break would then be detected in the conductivity plot at cmc1, the method being sufficiently sensitive. That no break was observed lends support to the assumption that the micelles of 2 forming above cmc1 are fully neutralized by CTA+ ions. The ratio R1 between the slopes of the conductivity plots for CTAB alone and CTAB + 2 below cmc2 is

R1 ) [λ(Br) + λ(CTA)]/[λ(Br) + 2pλ(Na) + (1 - 2p)λ(CTA)] (A4) For p ) 0.1, R1 ) 0.94, a value close the experimental values of 0.97 and 0.93 found in water and in water + 50 mM NaBr. The above estimations have been performed using λ(X) values at 25 °C instead of 30 °C, but this approximation introduces a negligible error as all λ(X)’s increase nearly in proportion with temperature. 2. Consider now the system at C > cmc2. To a first approximation, cmc2 should correspond to a concentration of free CTA+, Cf ) cmc2(1 - 2p), equal to 0.95 mM, the cmc of CTAB in the absence of 2. With p ) 0.1 and cmc2 ) 1.1 (37) Zana, R. J. Colloid Interface Sci. 1980, 78, 330. (38) Robinson, R. A.; Stokes, R. H. Electrolyte Solutions, 2nd ed.; Butterworths: London, 1959; p 465. (39) Sepulveda, S. L.; Cortes, J. J. Phys. Chem. 1985, 89, 5322. (40) Mukerjee, P. J. Phys. Chem. 1958, 62, 1390.

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mM (see Figure 4), we obtain Cf ) 0.88 mM, a value close to 0.95 mM. 3. At C > cmc2, the solution is assumed to contain only mixed micelles. Neglecting cmc1 with respect to C, the concentrations of 22- and CTA+ under micellar form are respectively pC and C - cmc2(1 - 2p). The concentration of micellized CTA+ ions not neutralized by 2 ions is thus (C - cmc2)(1 - 2p). We assume that Br- ions bind to micellized CTA+ ions to the same extent as in pure CTAB micelles (association degree 1 - R). This last approximation holds best at low values of p, as is the case here. The expression of the conductivity at C > cmc2 includes C-dependent terms for Br-, Na+, and the micellized but unneutralized CTA+ ions, and a constant term, K2, obtained by making C ) cmc2 in eq A3 and is

KC>cmc2 ) K2 + [2pC + R(C - cmc2)(1 - 2p)]λ(Br) + R(C - cmc2)(1 - 2p)λ(CTA)] + 2pCλ(Na) (A5) whereas the conductivity of micellar CTAB solution is

Zana et al.

KC>cmc ) [λ(Br) + λ(CTA)]cmc + R(C - cmc)[λ(Br) + λ(CTA)] (A6) The ratio R2 of the slopes of the K versus C plots above the cmc’s for pure CTAB and the CTAB + 2 mixture is obtained as

R2 ) R[λ(Br) + λ(CTA)]/{[2p + R(1 - 2p)]λ(Br) + 2pλ(Na) + R(1 - 2p)λ(CTA)} (A7) With R ) 0.25, and p ) 0.1, we obtain R2 ) 0.56 as compared to the experimental values of 0.64 and 0.65 in water and water + 50 mM NaBr, respectively. The agreement is satisfactory in view of the approximations involved. It would be even better if the value of R in the mixed CTAB + 2 micelles was larger than that in pure CTAB micelles, which is probably the case. LA9606963