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Langmuir 2001, 17, 6794-6803
Microemulsions in the Didodecyldimethylammonium Sulfate (Bromide)/Hydrocarbon/Water System. Microstructure and Specific Counterion Effects Magnus Nyde´n,*,† Olle So¨derman,† and Per Hansson‡ Physical Chemistry 1, Centre for Chemistry and Chemical Engineering, Box 124, University of Lund, S-221 00 Lund, Sweden, and Physical Chemistry, Uppsala University, Box 532, S-752 21 Uppsala, Sweden Received December 26, 2000. In Final Form: March 15, 2001 The microemulsion phase in the didodecyldimethylammonium sulfate/hydrocarbon/water system was investigated with NMR self-diffusion, 2H NMR relaxation, and the time-resolved fluorescence quenching technique. From the NMR self-diffusion experiments, it was found that the diffusion of the surfactant and the hydrocarbon are equal for a certain range of surfactant to oil concentration ratios, indicating that in this range of composition the structure is one of discrete aggregates. Upon an increase of the surfactant to oil volume ratio, there is an onset of a network formation, as indicated by the fact that the surfactant self-diffusion is smaller than that of the hydrocarbon. Close to the lower values of the oil to surfactant concentration ratio for which the microemulsion exists, a bicontinuous micellar network is formed. Phase studies and self-diffusion experiments in mixtures of a microemulsion with normal curvature, that is, with sulfate as counterion, with a microemulsion with reversed curvature, that is, with bromide as counterion, showed that the ratio of the two counterions can be used to tune the curvature of the surfactant film. Reasons for this behavior are discussed. Finally, based on NMR relaxation and NMR diffusion experiments, a value for the lateral diffusion of surfactant along the surfactant film is derived.
Introduction The structure and dynamics in the complex solutions formed when surfactant, hydrocarbon, and water are mixed have been the topics of numerous investigations during the past decades. When the mixtures form a nonbirefringent one-phase solution, one often refers to them as microemulsions. For an overview of the properties of such systems, see refs 1 and 2. Microemulsions can be found in a wide range of different surfactant systems, both of the ionic and nonionic varieties. The structure in these systems is determined by a hierarchy of different free energy contributions, of which the striving of the surfactant film to obtain an optimal mean curvature is thought to be the most important contribution. In the case of nonionic surfactants of the CmEn type, this optimal curvature is to a large extent determined by temperature.2 This is due to the fact that the solubility of ethylene oxide in water is mainly determined by temperature.3,4 For ionic surfactants, electrostatic effects dominate the curvature. One example of this state of affairs is found in the DDAX system (X being Br-, Cl-, OH-, SO42-), where the ternary phase diagrams with different hydrocarbons have been thoroughly investigated.5-10 One interesting * To whom correspondence should be addressed. Present address: Applied Surface Chemistry, Chalmers University of Technology, 412 96 Gothenburg, Sweden. † University of Lund. ‡ Uppsala University. (1) Jo¨nsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. In Surfactants and Polymers in Aqueous Solution; John Wiley & Sons: Chichester, 1998. (2) Olsson, U.; Wennerstro¨m, H. Adv. Colloid Interface Sci. 1994, 49, 113. (3) Andersson, M.; Karlstro¨m, G. J. Phys. Chem. 1985, 89, 4957. (4) Karlstro¨m, G.; Carlsson, A.; Lindman, B. J. Phys. Chem. 1990, 94, 5005. (5) Blum, F. D.; Pickup, S.; Ninham, B. W.; Chen, S. J.; Evans, D. F. J. Phys. Chem. 1985, 89, 711.
feature found in these diagrams is that the phase behavior depends strongly on salt content and on the type of counterions as well as on the type of hydrocarbon used, whereas the appearance of the phase diagram shows only a slight dependence on temperature. In the context of this report, we note that the change from bromide to sulfate changes the curvature from being of the normal type with sulfate to reversed with bromide, counting curvature toward oil as normal. In a previous report,11 we discussed the phase behavior and microstructure, on the basis of NMR self-diffusion data for the water and oil components, in the DDAS/ hydrocarbon/water system. This paper is an extension of that work and provides a more detailed picture of the structures and how the structures evolve with changes in composition in the microemulsions region. In addition to oil and water self-diffusion, we also present and discuss surfactant self-diffusion data. Moreover, NMR relaxation and fluorescence quenching data are also presented. To shed some light on the specific ion effects for the DDAX systems, referred to above, we have designed a system based on an effective surfactant composed of a mixture of DDAS and DDAB. By variation of the ratio of the counterions, the curvature of the surfactant film is tuned. At a certain mixing ratio of the two counterions, a balanced microemulsion is formed. By means of selfdiffusion measurements of water, oil, and surfactant, the microstructure evolution is monitored as the mixing ratio is varied. (6) Brady, J. E.; Evans, D. F.; Warr, G. G.; Grieser, F.; Ninham, B. W. J. Phys. Chem. 1986, 90, 1853. (7) Ninham, B. W.; Evans, D. F.; Wel, G. J. J. Phys. Chem. 1983, 87, 5020. (8) Sjo¨blom, J.; Skurtveit, R.; Saeten, J. O.; Gestblom, B. J. Colloid Interface Sci. 1990, 141, 329. (9) Kang, C.; Kahn, A. J. Colloid Interface Sci. 1993, 156, 218. (10) Fontell, K.; Ceglie, A.; Lindman, B.; Ninham, B. W. Acta Chem. Scand. 1986, A40, 247. (11) Nyde´n, M.; So¨derman, O. Langmuir 1995, 11, 1537.
10.1021/la001804v CCC: $20.00 © 2001 American Chemical Society Published on Web 09/29/2001
Microemulsions in DDAS/Hydrocarbon/Water
Langmuir, Vol. 17, No. 22, 2001 6795 Table 1. Compositions of the Samples in the Mixed Microemulsionsa sample number
fBrb
R ) Φo/Φo+wc
Φoc
Φsc
1 2 3 4 5 6 7 8 9 10 11
0 0.080 0.164 0.252 0.343 0.440 0.541 0.647 0.758 0.876 1
0.249 0.295 0.344 0.395 0.449 0.506 0.566 0.630 0.698 0.770 0.847
0.1236 0.1897 0.2528 0.3130 0.3706 0.4256 0.4782 0.5285 0.5767 0.6229 0.6671
0.197 0.190 0.183 0.176 0.168 0.159 0.151 0.142 0.132 0.122 0.112
a All samples contained a microemulsion phase, which in some cases was in equilibrium with a small fraction of excess oil, amounting to not more than 5 vol % of the total sample volume. b f c Br is the mole fraction of bromide: nBr/(nBr + nSO4). Φo is the volume fraction of oil, Φs is the volume fraction of surfactant, and Φo+w is the total volume fraction of oil plus water.
Figure 1. The ternary phase diagrams of DDAS/water/C12 and DDAB/water/C12 reproduced from ref 9. The different phases are as follows: L1 and L2, normal and reversed microemulsions; D, D1, and D2, lamellar phases; E and F, hexagonal phases; I, cubic phase. Also included is a schematic drawing of the different dilution lines. Lines 1 and 2 represent water dilution lines when going from the right down to the water corner. Lines 3, 4, and 5 represent oil dilution lines when going from line 2 to line 1, that is, toward the oil corner. Copyright 1993 Academic Press.
Table 2. Compositions of the Stock Solutions Used in Preparing the Samples along Lines 3-5 in Figure 1 line in phase diagram (see Figure 1) ub
3 3l 4u 4l 5u 5l
Experimental Section Materials. DDAB was obtained from Tokyo Kasei and was used without further purification. The ion exchange procedure from DDAB to DDAS was according to ref 9. For the 2H relaxation measurements, DDAS-d6 surfactant, which was deuterated in the methyl headgroup, was synthesized according to a previously reported synthesis path in ref 30. The degree of isotopic substitution of 1H for 2H was >99%, as judged by 1H NMR. Deuterium depleted water was supplied from Fluka, with an isotopic purity of HDO of
