Phase Behavior and Salt Partitioning in Two- and Three-Phase Anionic Surfactant Microemulsion Systems: Part I, Phase Behavior as a Function of Temperature.

Microemulsion phase behavior was studied as a function of salinity and temperature. The objectives were to investigate the influence of different electrolytes on optimal salinity and solubilization, and to relate the efficiency of each cation to change in microemulsion phase behavior. Two five-component microemulsion systems using anionic surfactants were studied as a function of type of cations (Na, K, Mg, Ca) and ionic strength. The phase behavior studies were performed at three different temperatures in the region [20 degrees C, 90 degrees C], and at different surfactant concentrations. The optimal salinity, defined as equal water and oil solubilization in the microemulsion phase, was used to quantify changes in phase behavior. Consistently, the divalent ions reached optimal salinity at lower salt concentrations than did monovalent ions. The effect of the different electrolytes on phase behavior was quantified by introducing an efficiency parameter. Knowledge of the efficiency relation between different cations in a microemulsion system provided a tool for predicting optimal salinity for salt mixtures. The microemulsion phase behavior was more sensitive to temperature in monovalent electrolyte solutions compared to divalent ions. At lower surfactant concentration the divalent cations had an even stronger influence on phase behavior compared to monovalent cation electrolytes. Copyright 1999 Academic Press.