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 temp erature. The objectives were to investigate the influence of different elec trolytes on optimal salinity and solubilization, and to relate the efficien cy of each cation to change in microemulsion phase behavior. Two five-compo nent microemulsion systems using anionic surfactants were studied as a func tion of type of cations (Na, K, Mg, Ca) and ionic strength. The phase behav ior studies were performed at three different temperatures in the region [2 0 degrees C, 90 degrees C], and at different surfactant concentrations. The optimal salinity, defined as equal water and oil solubilization in the mic roemulsion phase, was used to quantify changes in phase behavior. Consisten tly, the divalent ions reached optimal salinity at loa er salt concentratio ns than did monovalent ions. The effect of the different electrolytes on ph ase behavior was quantified by introducing an efficiency parameter. Knowled ge of the efficiency relation between different cations in a microemulsion system provided a tool for predicting optimal salinity for salt mixtures. T he microemulsion phase behavior was more sensitive to temperature in monova lent electrolyte solutions compared to divalent ions. At lower surfactant c oncentration the divalent cations had an even stronger influence on phase b ehavior compared to monovalent cation electrolytes. (C) 1999 Academic Press .