Partitioning of Salt in Winsor II Microemulsion Systems with an Anionic Surfactant and the Consequences for the Phase Behavior

Abstract

The distribution of salt between the microemulsion phase and the excess aqueous phase for Winsor II type equilibria is studied experimentally. It has been observed that the salt concentration in the excess phase is significantly higher than the salt concentration in the initial aqueous phase before mixing with the oil phase. This implies that the average salt concentration in the water-rich regions of the microemulsion is smaller than the salt concentration in the excess phase. Under certain conditions the dispersed aqueous region is essentially salt-free. This highly unequal distribution of salt is due to the negative adsorption of coions. It can be quantitatively explained with the Gouy-Chapman-Stern theory adapted for spherical electrical double layers. Because of this unequal distribution of salt the initial solution of salt in water may not be regarded as a pseudocomponent of the system. The consequences of this phenomenon for the phase behavior of microemulsion systems are discussed, and in fact these appear to be of major importance to the ultimate interpretation of the phase behavior.