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 water phase and a microemulsion phase were studied for two five-component anionic microemulsion systems at 20 degrees 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. Copyright 1999 Academic Press.