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Langmuir 1988, 4, 360-364
Dielectric Properties of Some Microemulsion Systems Containing Electrolytes Bo Gestblom Department of Physics, Uppsala University, Box 530, S - 751 21 Uppsala, Sweden
Johan Sjoblom* Institute for Surface Chemistry, Box 5607, S-114 86 Stockholm, Sweden Received December 29, 1986. I n Final Form: July 2, 1987 The influence of electrolytes (KC1and CaC12)on microemulsions in the systems Aerosol OT-toluenewater (salt), sodium octanoate-decanol-water (salt), and tetraethylene glycol nonylphenol ether-dodecane-water (salt) has been investigated by means of time domain spectroscopy (TDS). No significant influence on the dielectric parameters of the chosen compositions in the first two model systems was observed. The nonionic microemulsions containing electrolyte show a pronounced dispersion in the megahertz range, for which both the dielectric increment and the relaxation time are reduced on high electrolyte content. This dispersion has its origin in a Maxwell-Wagner interfacial polarization, the high value of tSindicating the presence of elongated aggregates.
Introduction The concept microemulsion covers isotropic and thermodynamically stable solutions consisting of oil, water, and one or several surfactants.’ The name introduced by Schulman et a1.2 is as such an extrapolation from traditional (macro) emulsion toward unknown structures, though characterized by considerably smaller dimensions of the droplets as deduced from the lack of turbidity. The existence of ordered structures, preferentially spherical droplets, has been the basis for the interpretation of a large number of different experimental ~ t u d i e s . ~ - ~ In the microemulsions where the amount of oil and water is comparable, the spherical structure outlined above seems to be an oversimplification. Valuable structural information of these solutions has recently been obtained by means of Fourier transform pulse gradient spin echo NMR diffusion measurements.’&l2 These measurements indicate in many cases an effective bicontinuity for water and oil concentrated micromulsions. Hence these solutions contain water and oil domains that are characterized by properties similar to those of “pure aqueous solution” and “pure apolar solution”. However, irrespective of the detailed microstructure of the microemulsion, its thermodynamic properties can in many cases be conveniently described in terms of the so-called “pseudophase“ mode1.13-16
(1) Danielsson, I.; Lindman, B. Colloids. Surf. 1981, 3, 391. (2) Hoar, T. P.; Schulman, J. H. Nature (London) 1943, 152, 102. (3) Schulman, J. H.; Friend, J. A. J. Colloid. Sci. 1949, 4, 497. (4) Schulman, J. H.; Bowcott, J. F. L. 2.Electrochem. 1959,59,283. (5) Zulauf, M.; Eicke, H. F., J. Phys. Chem. 1979, 83, 48. (6) Eicke, H. F.; Rehak, J. Helu. Chem. Acta 1976, 59, 2883. (7) Cazabat, A. M.; Langevin, D.; Pouchelon, A,, J. Colloid Interface Sci. 1986, 73, 1. (8) Cebula, D. J.; Ottewill, R. H.; Ralston, J. J . Chem. Soc., Faraday Trans. 1981, 77, 2585. (9) Cole, R. H.; Delbos, G.;Winsor, P., IV; Bose, T. K.; Moreau, J. M. J. Phys. Chem. 1986,89, 3338.
(10)Lindman, B.: Stilbs, P.: Moselev, M. E., J. Colloid Interface Sci.
1981, 83, 569. (11) Lindman, B.; Stilbs, P., In Microemulsions; Friberg, S., Bothorel, P.. Eds.: CRC: Cleveland. 1987. ‘(12) Wlrnheim, T.; Sjoblom, E.; Henriksson, U.; Stilbs, P. J . Phys. Chem. 1984,88,5420. (13) Biais, J.; Bothorel, P.; Clin, B.; Lalanne, P. J. Colloid. Interface Sci. 1981. 80. 136. (14) Biais; J.; Bothorel, P.; Clin, B.; Lalanne, P. J. Dispersion Sci. Technol. 1981, 2,,67. (15) Biais, J.; Odberg, L.; Stenius, P. J . Colloid Interface Sci. 1982, 86. - - , 360. ~ - -
(16) Sjoblom, E.; Jonsson, B.; Jonsson, A,; Stenius, P.; Saris P.; Odberg, L. J. Phys. Chem. 1986, 90, 119.
0743-7463/88/2404-0360$01.50/0
In this model the components of the microemulsion are considered to be distributed in equilibrium among three different types of domains in the solution: an aqueous domain containing some surfactant and cosurfactant; a hydrocarbon domain containing all of the nonpolar hydrocarbon, some surfactant, and water + cosurfactant in certain proportions; and a membrane domain that separates the aqueous and hydrocarbon domains and contains most of the surfactant and some cosurfactant. It has been shown experimentally that such a model can approximately predict microemulsion proper tie^.'^-'^ NMR-self-diffusion measurements can provide valuable information about the degree of ordered structures or disorder in the microemulsions. But it is also obvious that, irrespective of what kind of microstructure will predominate, the microemulsion will have a charged (or polar) interface, the properties of which are difficult to determine by means of conventional techniques. In this respect dielectric spectroscopy provides a direct tool for this kind of study. With a pronounced interfacial polarization present in our solutions one obtains a specific relaxation in the dielectric spectrum. In order to amplify the effect from the interface, addition of electrolytes may be necessary. In this article we present dielectric data for three model systems containing electrolyte. The ionic systems consist of sodium octanoate-decanol-water (salt) and sodium bis(2-ethylhexyl)sulfosuccinate (Aerosol 0T)-toluenewater (salt), while the nonionic microemulsion system is tetraethylene glycol nonylphenol ether-dodecane-water (salt). The nonionic surfactant is also known under the commercial name Triton N-42.
Experimental Section Chemicals and Phase Equilibria. The sodium octanoate (NaCs) was prepared by neutralization of octanoic acid (special pure, BDH, England) with sodium hydroxide (Titrisole, Merck AG, Germany). The salt was recrystallized from absolute ethanol and dried under vacuum at 110 O C . The molar mass was checked by titration with perchloric acid in glacial acetic acid, using crystal violet or oracet blue B as indicator, and differed less than 0.5% from the theoretical value. The decanol (C,,OH) (Zur Synthese, Merck AG, Germany) was vacuum-distilled, and its water content was less than 0.003 g cm-3 according to Karl Fisher titrations.
Sodium bis(2-ethylhexyl)sulfosuccinate (Aerosol OT) (Fluka, puriss) was dissolved in methanol and recrystallized 3 times before use. The tetraethylene glycol nonylphenol ether (commercial name Triton N-42, Rohm and Haas), the dodecane (C,,HZ6,pract, Fluka AG), and the toluene (C7HI4,pract, Fluka AG) were used
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Langmuir, Vol. 4, No. 2, 1988 361
Dielectric Properties of Microemulsion Systems as supplied. The water was double-distilled and ion-exchanged immediately before use. Phase diagrams (complete or partial) have been determined previously and can be found in ref 17-20. In this context it should be mentioned that all solutions studied are located within the organic solution phase (Lz)of each model system. In the system sodium octanoate-decanol-water (salt)the mixtures have a mole ratio water/sodium octanoate = 18, while Aerosol OT/toluene and Triton N-42/dodecane = 1:l by weight. Measurements. The dielectric spectra were obtained by the total transmission TDS technique. This method is based on the study of the influence of the dielectric sample on the shape of a step pulse propagating in a coaxial line, Fourier transformation of incident ( u ( t ) V(w))and transmitted ( r ( t ) R(w))pulse shapes gives a transmission coefficient spectrum T(w) = R(w)/ V(w). From transmission line theory T(w)can be expressed as a function of permittivity, and the dielectric spectrum, e*(o) = ( ( w ) - id'(@), is obtained by solution of the corresponding equation. A reduction in measurement errors due to unwanted reflections in the coaxial lines can be achieved by using as reference a pulse, rret(t),transmitted through a sample with known dielectric parameters. As indicated below, measurements were done both relative to an empty cell as well as relative to appropriatereference liquids. Details of the measurement system and the total transmission method have been published previously.21-22 In a sample showing dc conductivity, u, a term -iu/weo should be added to the total permittivity. Here eo is the permittivity of free space. In a total transmission TDS measurement, u is obtained from the ratio of the fiial levels of the pulses transmitted through the liquid-filled sample cell and the empty respectively. However, in fitting the spectra to definite model functions as indicated below the conductivity was treated as a variable parameter. In the study of the sodium octanoate-decanol-water (salt) system, a cell length of 40 mm with a time window of 20 ns was used. With a sampling interval of 20 ps accurate Fourier transforms of the pulse shapes can be calculated up to frequencies
