Phase behavior and salt partitioning in two- and three-phase anionic surfactant microemulsion systems: Part II, partitioning of salt

The partitioning of salt (chlorides of Na, K, Ca, Mg) between an excess wat er phase and a microemulsion phase were studied for two five-component anio nic microemulsion systems at 20 degrees C. Three-phase microemulsions conta in two phases that include inorganic salts, that is, excess brine and the m icroemulsion phase. The aim of the investigation was to describe the partit ioning of Na+, K+, Ca++, Mg++, and Cl- between water in the microemulsion p hase and water in the excess water phase. Both surfactant systems were anio nic 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 p reference for the microemulsion phase. The validity of considering (NaCl water) as pseudo-component brine has been examined. Including a chloride de pleted region, caused by electrostatic repulsion from the negatively charge d surfactant layer, in the surfactant pseudocomponent, 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 e lectrolyte. The preferential partitioning toward the microemulsion phase fo r K+, Ca++, and Mg++ seems to be caused by an ion exchange of Na+ in the su rfactant layer. (C) 1999 Academic Press.