A SELF-DIFFUSION STUDY OF MICROEMULSION STRUCTURE USING A POLAR-SOLVENT MIXTURE

The diffusion processes underlying the observed transport properties o f the polar solvent in some reverse aggregated, L(2) phase microemulsi ons has been studied. Three different surfactant-oil-polar solvent sys tems with the surfactants sodium bis(2-ethylhexyl) sulfosuccinate (AOT ), didodecyldimethylammonium bromide (DDAB), and tetraethylene glycol dodecyl ether (C(12)E(4)), respectively, were-investigated. The polar solvent was dilute aqueous solutions of either N-methylformamide or te tramethylammonium chloride. These additives used have essentially no i nfluence on the phase behavior compared to pure water. Following the a pproach of a previous publication (J. Phys. Chem. 1988, 92, 6675) we h ave measured the self-diffusion coefficients of the two polar species in the microemulsions, the ratio being sensitive to the nature of the diffusion process. In the AOT system a dominating aggregate diffusion process was found at lower temperatures indicating closed isolated rev ersed micellar aggregates. With increasing temperature, structural cha nges cause a crossover to a dominating molecular diffusion in a medium similar to the neat aqueous solution. This implies a crossover to a b icontinuous network at higher temperatures. In contrast, we found no r egion of dominating aggregate diffusion process in the DDAB and C(12)E (4) systems. A dominating or almost dominating molecular diffusion pro cess was detected even when the absolute diffusion coefficients of the polar solvent species were low.