Phase-behavior-based surfactant–contaminant separation of middle phase microemulsions

Abstract

This research studied separation of contaminants solubilized in middle phase microemulsions by shifting the contaminant-rich microemulsions from Winsor type III to type I systems. Precipitation-based exchange of polyvalent cations (Al3+ and Ca2+) with equivalent amounts of a monovalent cation (Na+) broke the middle phase microemulsions. Most of the contaminants solubilized in the middle phase microemulsion separated as free phase contaminant due to the much lower solubilization capacity of the resulting type I system. The phase transitions between Winsor type III and type I systems were reversible with the precipitation and re-dissolution of the polyvalent cations, allowing subsequent reuse of the surfactant solution. The phase transitions brought about by deprotonation/protonation of oleic acid, an organic additive used to promote formation of middle phase microemulsions, were also studied. The neutral form of oleic acid was more effective than its dissociated anion for promoting the formation of middle phase microemulsions, but oleic acid in the middle phase microemulsions could not be deprotonated by adding aqueous OH−. On the other hand, the oleate anion could be protonated even when it was in middle phase microemulsions because of its relative distribution in the systems. While exchange of surfactant counterions could separate contaminant solubilized in middle phase microemulsions, transformation of oleic acid in middle phase microemulsion systems did not produce the desired result.