The water activity and its temperature dependence as a function of the amount of water in the L,-phase of the (water/Aerosol OT/isooctane)-system (s. Exper. Part) was determined with the help of a differential pressure gauge and a precision cathetometer. A theoretical approach based essentially upon a solution of the poisson-Boltzmann equation with appropriate boundary conditions led to a description of the activity plot in excellent agreement with experimental results.Introduction. -Stability considerations are central for distinguishing true and apparent so-called microemulsions in the L,-and L,-phases. The frequently discussed transient region between thermodynamically stable water-in-oil (w/o)and oil-in-water (o/w) emulsions depends essentially on their stability domains. In spite of the importance of this fact, there have been only a few attempts [l] [2] to determine the stability region of, for example, a thermodynamically stable water-inoil emulsion.The most direct procedure appears to be the measurement of the partial pressure of the water vapor above an equilibrated water-in-oil microemulsion (L,-phase). This kind of experiment yields straightforwardly the water activity, for example, as a function of the solubilized amount of water. A sensitive experimental set-up should allow one to detect the stability limit of such microemulsions.Parallel to these investigations theoretical considerations with particular emphasis on the electrostatic contributions to the free energy of the system were carried out quite recently by one of us 151 161 on reversed micellar aggregates containing solubilized water which surprisingly well support the present experimental results. Hence, this study is believed to contribute some more fundamental results to our knowledge on water-in-oil thermodynamically stable emulsions formed by ionic surfactants.
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Optical matching phenomena were observed with water/AOT/isooctane (W/O) microemulsions. The phenomenon has been interpreted on the basis of Rayleigh's scattering theory in combination with a quantitative relation between the macroscopic dielectric constant of the microemulsion and the dielectric properties of its constituents. A residual scattering contribution at the [H,O]/[AOT]-ratio, where the system meets the optical matching condition, could be very satisfactorily assigned to density fluctuations of the surfactants in the water/ oil-interface, hence explaining the apparent polydispersity of these microemulsions.
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