Phase Behavior and Salt Partitioning in Two- and Three-Phase Anionic Surfactant Microemulsion Systems: Part II, Partitioning of Salt

Abstract

The partitioning of salt (chlorides of Na, K, Ca, Mg) between an excess water phase and a microemulsion phase were studied for two five-component anionic microemulsion systems at 20°C. Three-phase microemulsions contain two phases that include inorganic salts, that is, excess brine and the microemulsion phase. The aim of the investigation was to describe the partitioning of Na+, K+, Ca++, Mg++, and Cl− between water in the microemulsion phase and water in the excess water phase. Both surfactant systems were anionic with Na+ as counterion. It was found that Na+ and Cl− partitioned more strongly toward the excess water phase while K+, Ca++, and Mg++ show more preference for the microemulsion phase. The validity of considering (NaCl + water) as pseudo-component brine has been examined. Including a chloride depleted region, caused by electrostatic repulsion from the negatively charged surfactant layer, in the surfactant pseudo-component, is found to describe the NaCl partitioning. When discussing partitioning of K+, Ca++, and Mg++ between the microemulsion phase and the excess water phase in these systems, it is important to consider that there are two cations in the solutions, Na+ as counterion for the surfactant and either K+, Ca++, or Mg++ for the electrolyte. The preferential partitioning toward the microemulsion phase for K+, Ca++, and Mg++ seems to be caused by an ion exchange of Na+ in the surfactant layer.