Langmuir 1998, 14, 6599-6602
6599
Notes On the Formation of Vesicles by Mixtures of Anionic and Cationic Surfactants in Ethanol R. Zana*,† and B. Michels‡ Institut C. Sadron (CNRS), 6 rue Boussingault, 67000 Strasbourg, France, and LDFC, CNRS-ULP, 4 rue B. Pascal, 67000 Strasbourg, France Received June 17, 1998. In Final Form: July 30, 1998
Introduction The formation of vesicles in mixtures of anionic and cationic surfactants of sufficient chain length in aqueous solution is well documented.1-6 It has been advanced on both experimental2-5 and theoretical grounds7 that some of these vesicular systems are thermodynamically stable, but this is still under discussion.8,9 Recently, Huang et al.10 have examined similar surfactant mixtures in ethanol/ water mixtures and also in absolute ethanol. They reported the presence of vesicles at all compositions of the ethanol/water mixtures, from pure water to absolute ethanol when using equimolar mixtures of (i) dodecyltrimethylammonium bromide (DTAB) and sodium dodecanoate (NaC12) and of (ii) nonyltrimethylammonium bromide and sodium nonanoate. For the DTAB/NaC12 mixtures, vesicles were reported to occur at the fairly low total surfactant concentration of 5 mM, as indicated by differential scanning calorimetry (DSC) and freezefracture electron microscopy. That vesicles were observed in absolute ethanol was very surprising to us. Indeed ethanol, as well as methanol or propanol for that matter, is often used to prevent the self-association of amphiphiles. The intuitive argument to explain this behavior is that methanol and ethanol are half like water because of their hydroxyl group and half like oil because of their methyl or ethyl group. This argument is supported by light scattering, gel filtration, and electrical conductivity investigations, which showed that the micelle molecular weight11-13 and ionization degree14 decreased and in* Author to whom all correspondence should be sent. † Institut C. Sadron (CNRS). ‡ LDFC. (1) Hargreaves, W. R.; Dreamer, D. W. Biochemistry 1978, 17, 3759. (2) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. R.; Zasadzinski, J. A. N. Science 1989, 245, 1371. (3) Fukuda, H.; Kawata, K.; Okuda, H.; Regen, S. L. J. Am. Chem. Soc. 1990, 112, 1635. (4) Kaler, E. W.; Herrington, K. L.; Murthy, A. K.; Zasadzinski, J. A. N. J. Phys. Chem. 1992, 96, 6698. (5) Herrington, K. L.; Kaler, E. W.; Milller, D. D.; Zasadzinski, J. A. N.; Chiruvolu, S. J. Phys. Chem. 1993, 97, 13792. (6) Kamenka, N.; Chorro, M.; Talmon, Y.; Zana, R. Colloids Surf. 1992, 67, 213. (7) Safran, S. A.; Pincus, P.; Andelman, D. Science 1990, 248, 354. (8) Lasic, D. J. Colloid Interface Sci. 1990, 140, 302. (9) Laughlin, R. G. Colloids Surf., A. Physicochem. Eng. Asp. 1997, 128, 27. (10) Huang, J. B.; Zhu, B. Y.; Zhao, G. X.; Zhang, Z. Y. Langmuir 1997, 13, 5759. (11) Becher, P. J. Colloid Sci. 1965, 20, 728. (12) Parfitt, G.; Wood, J. Koll. Z. Z. Polym. 1969, 229, 55. (13) Suzuki, H. Bull. Chem. Soc. Jpn. 1976, 49, 1470. (14) Zana, R. Adv. Colloid Interface Sci. 1995, 57, 1 and references therein.
creased rapidly, respectively, while the critical micelle concentration (cmc) increased,14 upon increasing alcohol content. At concentrations higher than ∼20-40% (v/v) alcohol, the solutions no longer show evidence of selfassociation of the solubilized surfactant.11-13 This situation led us to reexamine the results for the DTAB/NaC12 equimolar mixtures in ethanol and water/ethanol mixed solvents by light scattering, electrical conductivity, and DSC methods. Indeed, light scattering is one of the most powerful techniques for detecting the formation of organized assemblies in surfactant solutions.15 Electrical conductivity is also very efficient for the same purpose in the case of ionic surfactants.16 The formation of vesicles from micelles or molecularly dispersed solutions results in a decrease of the experimental value of the surfactant diffusion coefficient,6,17 and thus should show as a decrease of slope in the variation of the conductivity. The results described next do not confirm the existence of vesicles in equimolar mixtures of DTAB and NaC12 in pure ethanol, even at a concentration higher than that used by Huang et al.10 At a total surfactant concentration of 6 mM, our results suggest that vesicles persist up to an ethanol content of ∼19% v/v but disappear at higher ethanol contents. Our results also indicate the absence of any significant self-association in solutions of DTAB, NaC12, their equimolar mixture (EQMM), and of the dodecyltrimethylammonium dodecanoate (DTA-C12) in absolute ethanol. Experimental Section Materials. DTAB (Aldrich) and NaC12 (Fluka) were purified by recrystallizations from ethanol or ethanol/ethyl acetate mixtures. The catanionic surfactant DTA-C12 was prepared by mixing equimolar amounts of DTAB and potassium dodecanoate in aqueous solutions, leaving the milky mixture for 2 weeks at 5 °C until a white precipitate separated well from the solution. The white solid obtained by filtration was washed several times with cold water and dried at 40 °C under reduced pressure (1 mm Hg) in the presence of P2O5. The elemental analysis was very satisfactory and showed the complete absence of residual KBr. The sample of dioctadecyldimethylammonium chloride (DODAC) was a gift from Kao Soap Company (Japan). It was used as received. Didodecyldimethylammonium bromide was prepared by reacting dodecyldimethylamine with dodecylbromide. This surfactant was purified by several recristallizations from ethyl acetate. Methods. The electrical conductivities were measured using a Wayne-Kerr conductivity bridge B905, operated at a frequency of 1 kHz. The solutions were contained in a double-walled glass container that maintained the temperature of the solution constant to within (0.01 °C. The intensity of light scattered by the solutions was measured at a 90° angle using a spectrometer MM1 from Amtec (France) operating at 632.8 nm. The solutions were cooled at 10 °C and centrifuged at 12 000 rpm for 2 h prior to measurements to remove dust. The refractive index increments were measured at 25 °C (15) Candau, s. J. In Surfactant Solutions. New Methods of Investigation; Zana, R., Ed.; M. Dekker: New York, 1987; Chapter 3 and references therein. (16) Zana, R. J. Colloid Interface Sci. 1980, 78, 330. (17) Chorro, M.; Kamenka, N. J. Chim. Phys. 1991, 88, 515.
10.1021/la980712r CCC: $15.00 © 1998 American Chemical Society Published on Web 09/30/1998
6600 Langmuir, Vol. 14, No. 22, 1998
Notes
Table 1. Intensity (I) of the Light Scattered by Various Surfactant Solutions in Absolute Ethanol at 25 °C and at a Scattering Angle of 90°a
b
system
I (a.u.)b
ethanol 4.97 mM NaC12 5.15 mM DTAB 5.58 mM EQMM 5.35 mM DTA-C12 benzene
3141 ( 50 3126 ( 50 3201 ( 50 3153 ( 50 3149 ( 50 9635 ( 100
a The listed values represent averages over 6-8 measurements. a.u., Arbitrary units.
using a Brice Phoenix differential refractometer and found to be 0.101 and 0.127 cm3/g for ethanol solutions of DTA-C12 and DTAB, respectively. The DSC measurements were performed using a very sensitive microcalorimeter, the DSAM-4 with matched platinum cells of 460 µL capacity. This apparatus only permits runs at increasing temperatures. The apparatus was interfaced to a Bull Micral microcomputer for data storage.18 Data analysis and integrations used the software Origin from Microcal Inc.19
Results Qualitative Observations of the Systems. NaC12, DTAB, their EQMM, and DTA-C12 were all found to give clear and low viscous solutions in ethanol up to a concentration of ∼10 mM, with NaC12 showing the slowest rate of solubilization. More concentrated solutions were not investigated but can probably be prepared for DTAB and DTA-C12 in view of their extremely high rates of solubilization in ethanol. DTAB and NaC12 are readily soluble in water. However the EQMM gave milky systems at very low concentration, due to vesicle formation. The systems became almost clear upon sonication for 1 h using a sonicator Bransonic type 2200, operated at 47 kHz. At room temperature, a sonicated EQMM solution at an alkyl chain concentration of 6 mM showed no visual appearance of a precipitate over a period of weeks. The nonsonicated EQMM system clarified somewhat upon heating at 80 °C and stirring, but remained more turbid than the sonicated system. The situation is very different with the DTA-C12 surfactant, which gave very turbid systems in water at an alkyl chain concentration of 6 mM. The turbidity decreased somewhat upon heating without complete clarification. When maintained at 25 °C for 2 days, the system showed phase separation with a creamy upper phase of very small volume (∼1% of the total volume) and a milky lower phase. The behavior of these systems was not further investigated because this was beyond the scope of this work. Nevertheless, the stability of the sonicated EQMM vesicular system is noteworthy. Also, the observations in water clearly emphasize the importance of the NaBr present in the EQMM as regards the vesicle stability. Light Scattering. The measurements were performed on solutions of DTAB, NaC12, DTA-C12, and EQMM in pure ethanol. The values of the scattered intensities are listed in Table 1. No difference is seen between the values for pure ethanol and the surfactant solutions, within the experimental error. The intensities were also measured at scattering angles of 45° and 135° for DTAB and DTAC12. The intensities were larger than at 90° but were the same for the two solutions. Note the much larger value of the intensity of light scattered by pure benzene. These results indicate no excess scattering with respect to ethanol (18) Privalov, P. L. Pure Appl. Chem. 1980, 52, 479. (19) Faetibold, E.; Michels, B.; Waton, G. J. Phys. Chem. 1996, 100, 20063.
Figure 1. Variation of the electrical conductance of solutions of (9) NaC12 (2), DTAB, and (b) EQMM in ethanol at 25 °C, with the surfactant concentration. The broken curve 1 and the dotted curve 2 have been calculated as indicated in the text.
for the surfactant solutions investigated. Because the refractive index increment of the solutions was >0.1, the scattering data indicate the absence of surfactant aggregates in the ethanol solutions that were investigated. Electrical Conductivity. Figure 1 shows the variation of K - K0 (difference between the conductances of solution and solvent) with the surfactant concentration C for solutions of NaC12 and DTAB, and the variation with Ctot for EQMM in ethanol (Ctot represents the sum of the concentrations of NaC12 and DTAB in the EQMM solution). As expected, the plot for EQMM falls between the plots for the individual surfactants. This plot is to be compared with the limiting curves 1 and 2. Curve 1 was obtained by assuming conductance additivity: the conductance of the EQMM solution at a given Ctot was assumed equal to the sum of the conductances of solutions of C12Na and DTAB, each at a concentration Ctot/2. This procedure did not account for the nonlinearity of the K versus C plots for DTAB and NaC12, which reveal that the measured conductances are dependent on the ionic strength of the system. Curve 2 was obtained by assuming the conductance of an EQMM solution at a concentration Ctot equal to half the sum of the conductances of solutions of NaC12 and DTAB, each at a concentration Ctot. This procedure largely accounts for the effects responsible for the curvature of the K versus C plots and compares the conductance of the EQMM solution with the sum of the conductances of the individual surfactant solutions at constant ionic strength (Ctot). The assumption leading to curve 2 is more reasonable than that leading to curve 1. The experimental plot for EQMM is seen to fall above curve 2, whereas the presence of vesicles in the EEQM solution should have resulted in an experimental plot below curve 2 (decrease of conductivity). Very similar results and plots have been obtained for two closely related systems: tetradecyltrimethylammonium bromide/potassium dodecanoate and DTAB/potassium dodecanoate (not shown). The same experiments were repeated in water and the results are shown in Figure 2. The various plots are now nearly linear in the concentration range covered and, as a result, curves 1 and 2 calculated as before are very nearly coincident. The EQMM plot is located below curve (1,2),
Notes
Figure 2. Variation of the electrical conductance of solutions of (9) NaC12, (2) DTAB, and (b) EQMM in water at 25 °C, with the surfactant concentration. The dotted curve labeled 1,2 has been calculated as indicated in the text.
Figure 3. Effect of the surfactant concentration on the electrical conductance of solutions of (9) NaC12 and (2) DTAB in water and variation of the conductance of (b) a 7.32 mM NaC12 solution upon addition of DTAB at 25 °C.
because of vesicle formation, as indicated by the bluishness of the EQMM solutions even at the lowest concentration investigated (i.e., 0.79 mM). The presence of vesicles results in a lowering of the conductance because of the reduced mobility of the surfactant ions making up the vesicles and the trapping of counterions within the vesicles. The lowering of conductance associated with the presence of the vesicles is shown more clearly in the plots of Figure 3, which represent the results of additions of DTAB to water and to a NaC12 solution. The last addition to the NaC12 solution corresponds to equimolarity. Bluishness appeared at the second addition, corresponding to a DTAB mole fraction of 0.21. The difference between the slopes of the plots for DTAB in water and in water + NaC12 is clearly seen and is associated with the formation of vesicles in the system. Conductivity measurements have also been used to determine how much ethanol the vesicles present in the EQMM system in water can sustain. At a total surfactant concentration of 6 mM, the bluishness of the systems disappeared between the ethanol volume fractions 0.19 (bluish system) and 0.20 (clear solution). This visual observation is supported by the results in Figure 4, which show a rather sharp change of slope in the plot of the conductance versus ethanol volume fraction at about the same volume fraction. Micelles may still be present above this volume fraction. Indeed other studies showed that micelles disappear at larger volume fraction of ethanol (0.25 to 0.35).11-13
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Figure 4. Variation of the conductance of EEQM in water/ ethanol mixtures with the ethanol volume fraction, at Ctot ) 6 mM and 25 °C.
Figure 5. Response of the DASM-4 microcalorimeter upon increasing the temperature of a 6 mM vesicular DODAC system at a rate of 1 °C/min.
DSC. To check the sensitivity of the DASM-4 microcalorimeter, DSC runs were performed on a 6 mM dispersion of DODAC. This amphiphile gives rise to vesicles in water20,21 and a phase transition has been evidenced between 43 and 48 °C.21 The DSC runs performed at a scan rate of 0.25 and 1 °C/min showed a large endothermal effect at 46.7 °C (see Figure 5), with an associated enthalpy of 24 ( 4 J/g. Further runs on the same solution yielded the same effect. However, an aqueous dispersion of didodecyldimethylammonium bromide (concentration 6 mM), which is also known to form vesicles,22 showed no transition. The 6 mM EQMM system in water showed a very small effect that was not reproduced in a second run. Because these two aqueous systems contained vesicles, the absence of any thermal effect suggests that the melting of the C12 chains in these systems must take place at 0.20, contrary to the findings of a recent study.10 The results obtained in the present study are in accord with previous findings that showed that surfactants do not self-associate in ethanol. Indeed, the driving force for amphiphile self-association in water, the hydrophobic interaction, is absent in ethanol because of the nature of this solvent. Our results nevertheless showed that the vesicles formed in water by the DTAB/NaC12 equimolar mixture are very stable at room temperature. Another point of interest is the fact that the electrical conductivity of the EQMM in water and in water/ethanol
Notes
mixtures depended very little on whether the system was sonicated or not sonicated. However, the turbidity of the aqueous solution as judged from the naked eye was much lower for the sonicated than for the nonsonicated system, indicating smaller vesicles. A similar effect has been reported for other vesicular systems upon addition of vesicle-breaking surfactants.23,24 The change of turbidity of the aqueous equimolar mixture of DTAB and NaC12 upon addition of ethanol showed two interesting features. First, the turbidity of the sonicated system first increased then decreased upon addition of ethanol, with a maximum of turbidity occurring before complete clearing of the solution. For the nonsonicated system, the turbidity remained somewhat unchanged until clearing occurred upon ethanol additions. These results suggest that the smaller vesicles produced upon sonication are less stable than the larger ones, and that these small vesicles first grow upon addition of alcohol (this work) or surfactant23,24 before breaking up. The second point concerns the variation of the electrical conductivity upon addition of ethanol, as represented in Figure 4. These experiments were performed by adding a 6 mM equimolar DTAB/NaC12 solution in ethanol to a 6 mM equimolar system in water or water + ethanol. The thermal equilibrium of the system was reached in ∼2 min under the experimental conditions used. However, we observed that after 2 min the conductivity kept decreasing and reached a stable value only after several minutes. This slow variation may reflect complex structural changes taking place in the system upon addition of alcohol. This result suggests the use of fast mixing experiments to study the kinetics of these processes that may be related to the vesicle-to-micelle transformation. The kinetics of this process has been recently investigated and characteristic times ranging from tenth of seconds to hours were found, depending on the way the transformation was achieved.25-29 Our results fall within this large range. Acknowledgment. The authors thank Dr. J. Selb for performing the light scattering experiments. LA980712R (23) Edwards, K.; Gustafsson, J.; Almgren, M.; Karlsson, G. J. Colloid Interface Sci. 1993, 161, 299. (24) Walter, A.; Vinson, P. K.; Kaplun, A.; Talmon, Y. Biophys. J. 1991, 60, 1315. (25) Friberg, S.; Campbell, S.; Fei, L.; Yang, H.; Patel, R.; Aikens, P. Colloids Surf., A. Physicochem. Eng. Asp. 1997, 129-130, 167. (26) Campbell, S.; Yang, H.; Patel, R.; Friberg, S.; Aikens, P. Colloid Polym. Sci. 1997, 275, 303. (27) Campbell, S.; Yang, H.; Friberg, S.; Patel, R. Langmuir 1998, 14, 590. (28) Farquhar, K. D.; Misran, M.; Robinson, B. H.; Steytler, D.; Morini, P.; Garrett, P. R.; Holzwarth, J. F. J. Phys.: Condens. Matter 1996, 8, 9397. (29) O’Connor, A.; Hatton, A. T.; Bose, A. Langmuir 1997, 13, 6931.
