Langmuir 1997, 13, 5001-5006
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Self-Assembly in Mixtures of Sodium Alkyl Sulfates and Alkyltrimethylammonium Bromides: Aggregation Behavior and Catalytic Properties Reinskje Talhout and Jan B. F. N. Engberts* Department of Organic and Molecular Inorganic Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands Received April 15, 1997. In Final Form: June 23, 1997X Two aqueous mixtures of cationic and anionic surfactants have been studied by means of conductometry, transmission electron microscopy, and microcalorimetry. Their catalytic effects on the decarboxylation of the kinetic probe 6-nitrobenzisoxazole-3-carboxylate (6-NBIC) were also examined in some detail. The mixtures differ profoundly in the hydrophobic match between both surfactant tails, which is perfect for dodecyltrimethylammonium bromide (DTAB) and sodium dodecyl sulfate (SDS) and poor for hexadecyltrimethylammonium bromide (CTAB) and sodium heptyl sulfate (SHS). This difference is reflected in the more pronounced synergism in critical aggregation concentration and catalytic efficiency of the DTAB/SDS mixture and in the phase behavior of the mixtures. CTAB and SHS can be mixed in a 1:1 ratio without precipitation, forming both small, unilamellar and large, multilamellar vesicles. In DTAB/SDS mixtures, however, precipitation of the catanionic surfactant occurs for a mole fraction of DTAB (x) between 0.3 and 0.7, while both vesicles and large bilayer fragments are formed for x ) 0.8. The excess of DTAB in the x ) 0.8 mixture results in the solubilization of the vesicles by DTAB micelles close to the cmc of pure DTAB. A network of interconnected threadlike cylindrical micelles was found as an intermediate stage of aggregation between the vesicles and the mixed micelles. These cylindrical micelles are formed exclusively on further increasing the surfactant concentration.
Introduction Amphiphilic systems containing both cationic and anionic surfactants exhibit several unique properties.1-22 Most importantly, the morphology of the aggregates formed from the mixtures may be radically different from that for the components. For instance, spontaneous vesicle formation has been observed for several mixtures of micelle and/or bilayer-forming components.1,3,9,13,15-18,22 Furthermore, substantial synergism is reflected in the dramatiX Abstract published in Advance ACS Abstracts, September 1, 1997.
(1) Herrington, K. L.; Kaler, E. W.; Miller, D. D.; Zasadzinski, J. A.; Chiruvolu, S. J. Phys. Chem. 1993, 97, 13792. (2) Malliaris, A.; Binana-Limbele, W.; Zana, R. J. Colloid Interface Sci. 1986, 110, 114. (3) Kamenka, N.; Chorro, M.; Talmon, Y.; Zana, R. Colloids Surf. 1992, 67, 213. (4) Lucassen-Reynders, E. H.; Lucassen, J.; Giles, D. J. Colloid Interface Sci. 1981, 81, 150. (5) Chorro, M.; Kamenka, N. J. Chim. Phys. 1991, 88, 515. (6) Hoyer, H. W.; Marmo, A.; Zoellner, M. J. Phys. Chem. 1961, 65, 1804. (7) Scheuing, D. R.; Weers, J. G. Langmuir 1990, 6, 665. (8) Li, G. Z.; Li, F.; Zheng, L. Q.; Wang, H. L. Colloids Surf. 1993, 76, 257. (9) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. A. N. Science 1989, 245, 1371. (10) Stellner, K. L.; Amante, J. C.; Scamehorn, J. F.; Harwell, J. H. J. Colloid Interface Sci. 1988, 123, 186. (11) Kato, T.; Iwai, M.; Seimiya, T. J. Colloid Interface Sci. 1989, 130, 439. (12) Kato, T.; Takeuchi, H.; Seimiya, T. J. Colloid Interface Sci. 1990, 140, 253. (13) Kondo, Y.; Uchiyama, H.; Yoshino, N.; Nishiyama, K.; Abe, M. Langmuir 1995, 11, 2380. (14) Lusvardi, K. M.; Full, A. P.; Kaler, E. W. Langmuir 1995, 11, 487. (15) Marques, E.; Khan, A.; Graca Miguel, M.; Lindman, B. J. Phys. Chem. 1993, 97, 4729. (16) Kaler, E. W.; Herrington, K. L.; Murthy, A. K.; Zasadzinski, J. A. N. J. Phys. Chem. 1992, 96, 6698. (17) Zhao, G. X.; Yu, W. L. J. Colloid Interface Sci. 1995, 173, 159. (18) Fukuda, H.; Kawata, K.; Okuda, H. J. Am. Chem. Soc. 1990, 112, 1635. (19) Zhang, L. H.; Zhao, G. X. J. Colloid Interface Sci. 1989, 127, 353. (20) Li, X. G.; Liu, F. M. Colloids Surf. 1995, 96, 113. (21) Yu, Z. J.; Zhao, G. X. J. Colloid Interface Sci. 1989, 130, 414. (22) Brasher, L. L.; Kaler, E. W. Langmuir 1996, 12, 6270.
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cally lowered critical aggregation concentrations (cac’s)4,13 and the enhanced surface activities of the mixtures.4,5,19-21 Efforts have been made to describe this synergism theoretically,23-26 but the results are not unambiguously applicable to mixtures of cationic and anionic surfactants.26 These phenomena arise from the strong electrostatic interaction between the oppositely charged headgroups of the components.9,16-18,22 As a result, the mean effective headgroup area decreases considerably7,27 while the mean hydrophobic volume of the tails remains the same. For two single-chained surfactants, this dynamic ion pairing yields a pseudo-double-tailed catanionic15 (zwitterionic,9,16 ion-pair18) surfactant, which is known to have the preferred geometry of a vesicle-forming surfactant. Since only adsorption of electroneutral combinations of ions can take place at the surface, the surface activity of the mixture is much enhanced due to the presence of this pseudodouble-tailed catanionic surfactant.4 Of course the cac, largely determined by the balance between headgroup repulsion and attraction of the tails, decreases when the headgroup repulsion is diminished. These properties are of both fundamental and commercial interest, since surfactants used in industrial applications (e.g. detergents, tertiary oil recovery, drug carrier systems) are often mixtures. To optimize these applications, it is important to understand the interplay of forces that govern the phase behavior. Microstructures can be tailored by varying the surfactant geometry (governed by branching of the tail groups, the hydrophobic match between the tail groups, and the type of polar headgroups) and the stoichiometry of the mixture. Thus far, most studies on mixed aggregates have focused on the phase diagram1,9-11,13,14,16,22 and related properties (23) Hua, X. Y.; Rosen, M. J. J. Colloid Interface Sci. 1982, 90, 212. (24) Rosen, M. J.; Murphy, D. S. J. Colloid Interface Sci. 1986, 110, 224. (25) Puvvada, S.; Blankschtein, D. J. Phys. Chem. 1992, 96, 5567. (26) Sarmoria, C.; Puvvada, S.; Blankschtein, D. Langmuir 1992, 8, 2690. (27) Zhao, G. X.; Ou, J. G.; Tian, B. M.; Huang, Z. M. Acta Chim. Sin. 1980, 38, 409.
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like cac,1,2,4-6,8,11,13 aggregation number,1,2,5,7,8,11 and composition of the aggregate.2,12,14,22 To the best of our knowledge, the effect of mixtures of cationic and anionic surfactants on organic reactions has not been investigated. In this paper, we explore the kinetic behavior of two types of mixtures which differ only in the hydrophobic match of their tails. Firstly, we examined equimolar mixtures of hexadecyltrimethylammonium bromide (CTAB) and sodium heptyl sulfate (SHS), surfactants with linear tails of unequal lengths. Secondly, we studied mixtures of dodecyltrimethylammonium bromide (DTAB) and sodium dodecyl sulfate (SDS), also surfactants with linear tails but of equal length; here the influence of the stoichiometry of the mixtures has also been tested. As a model reaction we employed the highly solventdependent, unimolecular decarboxylation of 6-nitrobenzisoxazole-3-carboxylate (1; 6-NBIC), one of the most extensively studied kinetic probes for micellar28-30 and vesicular31,32 media.
Kemp et al.33-35 showed that the decarboxylation rate is smallest in water and is dramatically enhanced in dipolar, aprotic solvents. They related the observed rate enhancement to transition state stabilization by dispersion interactions with the solvent. Grate et al.,36 however, are sceptical about dispersion interactions and provided strong evidence that hydrogen-bonding effects and ion pairing dominate the variation in reaction rates. For surfactant aggregates this would mean that the extent of water penetration into the aggregate is reflected in the rate constant. Indeed, the catalytic efficiency increases with decreasing fluidlike character of the aggregate surface (micelles e bilayers (above Tc) e bilayers (below Tc)).37 In addition, we have used several other techniques for further characterization of the mixtures: conductivity measurements to determine the cac of the mixtures, electron microscopy (freeze-fracture and negative staining) for characterizing the morphologies of the aggregates, and microcalorimetry to determine the heats of micellization (only of the DTAB/SDS mixtures). Experimental Section Materials and Preparation of Mixtures. CTAB was purchased from Merck, DTAB from Sigma, and SDS from BHD Chemicals. These surfactants (purity > 99%) were used as received. SHS was prepared according to a literature procedure.38 NaBr was purchased from Merck and dried before use over P2O5 (28) Bunton, C. A.; Minch, M. J.; Hidalgo, J.; Sepulveda, L. J. Am. Chem. Soc. 1973, 95, 3262. (29) Bunton, C. A.; De Buzzaccarini, F. J. Phys. Chem. 1981, 85, 3139. (30) Nusselder, J. J. H.; Engberts, J. B. F. N. Langmuir 1991, 7, 2089. (31) Kunitake, T.; Okahata, Y.; Ando, R.; Shinkai, S.; Hirakawa, S. J. Am. Chem. Soc. 1980, 102, 7877. (32) Germani, R.; Ponti, P. P.; Savelli, G. N.; Cipiciani, A.; Cerichelli, G.; Bunton, C. A. J. Am. Chem. Soc., Perkin Trans. 2 1989, 1767. (33) Kemp, D. S.; Paul, K. G. J. Am. Chem. Soc. 1970, 92, 2553. (34) Kemp, D. S.; Paul, K. G. J. Am. Chem. Soc. 1975, 97, 7305. (35) Kemp, D. S.; Cox, D. D.; Paul, K. G. J. Am. Chem. Soc. 1975, 97, 7312. (36) Grate, J. W.; McGill, R. A.; Hilvert, D. J. Am. Chem. Soc. 1993, 115, 8577. (37) Patel, M. S.; Bijma, K.; Engberts, J. B. F. N. Langmuir 1994, 10, 2491. (38) Noordman, W. H. M.Sc. Thesis, University of Groningen, 1993.
Talhout and Engberts in vacuo at 100-150 °C for 24 h. The water used was demineralized and distilled twice in an all-quartz distillation unit. CTAB/SHS mixtures were prepared by adding water (pure, in the case of conductometry and microcalorimetry or at pH 11.3, NaOH, in the case of kinetics and electron microscopy) to the surfactants. DTAB/SDS mixtures were prepared by adding a stock solution of SDS in water to a stock solution of DTAB in water while stirring at 30 °C; the solutions were equilibrated for at least 6 h. Analytical Data of the Catanionic Surfactant DTA:DS. The precipitated crystals of DTA:DS were tested for the percentage of the elements N (found, 2.91; calculated, 2.84) and S (found, 6.61; calculated, 6.49). Kinetic Measurements. 6-Nitrobenzisoxazole-3-carboxylate (1; 6-NBIC) was prepared according to a literature procedure.39,40 First-order rate constants for the decarboxylation of 1 were measured at 30.0 ( 0.1 °C by monitoring the increase in absorption at 410 nm for at least 4 half-lives, using a Perkin Elmer λ5 or Perkin Elmer λ2 spectrophotometer. The rate constants (kobs, reproducible to within 3%) were calculated using the Guggenheim method for the PE λ5 and Enzfitter (a fitting program) for the PE λ2. In a typical experiment 7 µL of a freshly prepared stock solution of 1 in methanol (8 × 10-2 M) was added to 2.5 mL of the surfactant solution (pH 11.3, NaOH) in the cell. Conductivity Measurements. Cac’s were measured conductometrically using a Wayne-Kerr Autobalance Universal Bridge B642 fitted with a Philips electrode PW 9512101 with a cell constant of 0.71 cm-1. The conductivity cell was thermostated at 30.0 ( 0.1 °C and equipped with a magnetic stirring device. Titration Microcalorimetry. Enthalpograms were recorded using a Microcal Omega titration microcalorimeter (Microcal, Northhampton, MA). The sample cell was stirred (350 rpm) and thermostated at 30.0 ( 0.1 °C. The raw data were analyzed using Omega software (Origin 2.9). Freeze Fracture Electron Microscopy. Freeze-fractured replicas were made by placing a small drop of the sample between two small holder plates (cleaned with nitric acid) and immersing them rapidly into a nitrogen slush (a mixture of solid and liquid nitrogen). This vitrified sample was broken at -176 °C under low pressure ( 0.7 could be studied kinetically. The rate profiles at 30 °C for different x values and for pure DTAB are shown in Figure 3; again, lines are to guide the eye. The rate profile of DTAB is of the typical form, as discussed in the previous section. On the other (47) Szleifer, I.; Ben-Shaul, A.; Gelbart, W. M. J. Chem. Phys. 1987, 86, 7094. (48) Yuet, P. K.; Blankschtein, D. Langmuir 1996, 12, 3819.
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Figure 3. Plot of rate profiles for the decarboxylation of 6-NBIC for DTAB/SDS surfactant mixtures at 30 °C, showing atypical behavior for fractions with x e 0.95: ([) x ) 0.8; (O) x ) 0.9; (+) 0.95; (0) DTAB. Table 2. Rate Constants for the Decarboxylation of 6-NBIC, Catalyzed by Aggregate 1 and by Aggregate 2, for DTAB/SDS Mixtures at 30 °C surfactant
104kagg1 (s-1)
104kagg2 (s-1)
DTAB DTAB/NaBr 1/1 DTAB/SDS 95/5 DTAB/SDS 9/1 DTAB/SDS 8/2
4.62 5.07
4.52 4.98 4.95 5.00 6.89
12.7 16.6
hand, the rate profiles for the mixtures are atypical: catalysis starts at very low concentrations, and after a kinetic maximum is reached, the catalytic efficiency decreases and eventually levels off to a plateau value. Therefore, at very low concentrations aggregates (aggregate 1) must be formed, presumably by the pseudodouble-tailed catanionic surfactant. We contend that, at the concentration corresponding to the maximum rate constant, a transition occurs to another, less catalytically efficient, aggregate (aggregate 2). A decrease in catalytic efficiency after transition to another aggregate at higher concentrations is very unusual; normally, aggregates formed at higher concentrations are less fluidlike49 and are thus more effective catalysts.37 However, the present picture is consistent with previous results: Lucassen-Reynders et al.4 showed by surface tension measurements that the first cac of the mixtures is in the region 10-3 to 10-2 mM. Kaler et al.1 found a second transition for the x ) 0.9 and x ) 0.95 mixtures at approximately 15 mM (25 °C) using conductometry. In the atypical rate profile of the mixtures, the plateau value is equal to kagg2; kagg1 was calculated using the Menger-Portnoy enzyme model50 (Table 2). In the case of a normal rate profile, both rate constants should be the same; indeed, for DTAB and DTAB/NaBr, both values are the same within experimental error. The rate constant kagg1 increases with decreasing x, which supports the hypothesis that the aggregate formed by the pseudodouble-tailed catanionic surfactant is causing the catalysis. The second cac at 30 °C was probed with conductometry (Table 3) and corresponds to the concentration at the kinetic maximum. Assuming that the aggregates below (49) Davis, H. T. Colloids Surf., A: Physicochem. Eng. Aspects 1994, 9. (50) Menger, F. M.; Portnoy, C. E. J. Am. Chem. Soc. 1967, 89, 4698.
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Table 3. Second Cac for DTAB/SDS mixtures at 30 °C, Expressed in Total Concentration Surfactant or Concentration of Catanionic Surfactant and Concentration of Free DTAB DTAB/SDS
2nd cac (mM)
100/0 95/5 9/1 8/2 0/100
16.0a 15.3 16.3 20.3 8.39a
a
2nd cac in [DTAB]free and [DS] (mM) 16.0; 0a 13.8; 0.8 13.0; 3.2 12.2; 4.1
In this case: 1st cac.
the second cac are formed by the pseudo-double-tailed catanionic surfactant, the second cac can be expressed in the concentration of catanionic surfactant and the concentration of free DTAB. For every mixture, the concentration of free DTAB is approximately equal to the cmc of pure DTAB. The lowering of this concentration for smaller x is most probably caused by the larger amount of pseudo-double-tailed catanionic surfactant present, which induces micellization of DTAB. We therefore contend that the second cac is caused by micellization of DTAB. This hypothesis is supported by optical inspection of the solutions; at concentrations above the second cac the solutions are clear. On the other hand, solutions at concentrations below the second cac are turbid. An electron microscopy study was therefore performed on the x ) 0.8 mixture at 18 mM, the concentration of the kinetic maximum, which corresponds to the second cac. Negativestaining EM measurements were consistent with the presence of large bilayer fragments (500-800 nm) and small, unilamellar vesicles (30-100 nm) in the turbid solutions (Figure 4a). Also, a network of interconnected threadlike cylindrical micelles was observed (Figure 4b). This process closely resembles the solubilization of lipid vesicles by conventional surfactants, which has been observed to proceed through a region of coexistence between open vesicles and/or lamellar sheets and long threadlike cylindrical micelles.51-55 As the surfactant concentration is increased further, the lipid becomes completely solubilized into mixed micelles which gradually decrease in size with decreasing lipid/surfactant ratio. The enthalpograms at 30 °C of the mixtures and pure DTAB are quite revealing (Figure 5). The enthalpogram of DTAB is a typical example of curve type B, which reflects thermodynamical nonideality.56 On the other hand, the enthalpograms of the mixtures are atypical. After the decrease of the enthalpy of dilution (∆dilH), in the region of the cac, ∆dilH increases again and, after a maximum value, levels off to a plateau value. These lumps are broader and higher for smaller x. Enthalpies of micellization are taken as the difference between the top of the curve and the baseline (infinite injection number)56 and are listed in Table 4. Assuming that the measured values are the enthalpies of micellization of DTAB, the values must be corrected for the amount of free DTAB in the solution (by dividing by the mole fraction of free DTAB, Table 4). It can be seen that the corrected values are approximately equal to the value (51) Inque, T.; Kawamura, H.; Okukado, S.; Shimozawa, R. J. Colloid Interface Sci. 1994, 168, 94. (52) De la Maza, A.; Parra, J. L. Langmuir 1995, 11, 2435. (53) Silvander, M.; Karlsson, G.; Edwards, K. J. Colloid Interface Sci. 1996, 179, 104. (54) Inque, T.; Yamahata, T.; Shimozawa, R. J. Colloid Interface Sci. 1992, 149, 345. (55) Edwards, K.; Gustafsson, J.; Almgren, M.; Karlsson, G. J. Colloid Interface Sci. 1993, 161, 299. (56) Bijma, K. Ph.D. Thesis, University of Groningen, 1995.
Figure 4. Negative staining electron micrographs of the x ) 0.8 mixture at 18 mM, showing (top, a) large bilayer fragments and small, unilamellar vesicles and (bottom, b) a network of interconnected threadlike micelles. The bar represents 100 nm.
Figure 5. Enthalpograms for DTAB/SDS mixtures at 30 °C, showing atypical behavior for fractions with x e 0.95: (0) DTAB; (+) x ) 0.99; (+) x ) 0.95; (O) x ) 0.9; ([) x ) 0.8; (]) SDS.
of pure DTAB; the lowering for smaller x is due to the formation of vesicles by the pseudo-double-tailed catanionic surfactant, an exothermic process. The onset of the lumps in the vicinity of the cac, when expressed in the amount of free DTAB, is at approximately the same concentration for each mixture.
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Table 4. Enthalpies of Micellization for DTAB/SDS Mixtures at 30 °C, Uncorrected and Corrected for the Concentration of free DTAB DTAB/SDS
∆Hmic (kJ/mol)
∆Hmiccorr (kJ/mol)
DTAB 99/1 95/5 9/1 8/2 SDS
-4.4 -4.1 -3.6 -3.1 -2.3 -3.7
-4.4 -4.2 -4.0 -3.9 -3.8 -3.7
We therefore propose that the vesicles formed at concentrations below the second cac are solubilized by DTAB micelles to form mixed micelles; this kind of solubilization is a common phenomenon in biochemistry.52-55 Presumably, the first lowering of ∆dilH and the cac observed with conductometry is caused by micellization of DTAB. The DTAB micelles solubilize the vesicles formed by the pseudo-double-tailed catanionic surfactants into mixed micelles, which are less structured compared with bilayers. This endothermic process causes the lumps in the enthalpograms and the decrease in catalytic efficiency. As anticipated, kagg1, the rate constant of the catalysis by the mixture of bilayers and vesicles, is larger than kagg2, the rate constant for catalysis by the mixed micelles. The increase of kagg2 with decreasing x can be explained by the significant structure-enhancing headgroup interactions in the mixed micelles. For DTAC/SDS mixtures at a total concentration of 300 mM, Weers et al.7 showed with FT-IR spectroscopy that ordering increases as equimolar ratios are approached. This ordering is accompanied by a large increase in size of the aggregate: Malliaris et al.2 reported an aggregation number N of 60 for pure DTAC and 190 for the x ) 0.8 mixture at a total concentration of 300-350 mM. Kaler et al.1 observed the same trend at 70 mM for DTAB/SDS mixtures: N is 74 for pure DTAB and 111 for the x ) 0.9 mixture. Regarding the nature of the enhanced structuring, it was found that, in mixtures of poly(oxyethylene)-type nonionic surfactants and diacylphosphatidylcholines, the lipid bilayer structure is partly retained within the interior of the mixed micelle after solubilization.51
Conclusions In the present study, two mixtures of a cationic and an anionic surfactant differing in the hydrophobic match of their tails have been characterized by several techniques. Both mixtures show large synergism in aggregation and in their catalytic effect on the unimolecular decarboxylation reaction of 6-NBIC; this synergism is more pronounced in the DTAB/SDS mixtures due to the higher hydrophobic match compared to that of the CTAB/SHS mixture. Furthermore, the phase behavior of the mixtures is profoundly influenced by the hydrophobic match: CTAB and SHS can be mixed in a one to one ratio without precipitation, forming both small, unilamellar and large, multilamellar vesicles. In DTAB/SDS mixtures, however, precipitation of the catanionic surfactant occurs for mixing ratios 0.3 < x < 0.7, since crystalline lattice formation is energetically favored by the large London dispersion interactions between the tails. Both vesicles and large bilayer fragments are formed for x ) 0.8. Close to the cmc of pure DTAB, the excess of DTAB causes the solubilization of the vesicles by micelles of DTAB, which results in the formation of mixed micelles. Analogous to the solubilization of lipid vesicles by conventional surfactants, a network of interconnected threadlike cylindrical micelles is formed as an intermediate stage of aggregation between the vesicles and the mixed micelles. The vesicle-micelle transition was evident from optical inspection and electron micrographs, conductivity measurements, and the atypical curve types of the rate profiles and the enthalpograms. There is also strong kinetic evidence for enhanced structuring in the mixed micelles as compared to pure cationic micelles. Acknowledgment. We wish to thank Dr. Arjen Sein, Mr. Jan van Breemen, and Dr. Jan Kevelam for the preparation of the electron microscopy samples and their skillful work at the microscope. LA970388K
