Enhanced solvent extraction with water-in-oil microemulsions

This paper examines theoretically the conditions which favor enhanced extraction when a microemulsion phase containing a surfactant and a chelating agent is used as the solvent extraction organic phase. A general thermodynamic model of liquid-liquid distribution in reversed micellar systems is presented. The model treats the reversed micellar aggregates of the surfactant HA as a pseudophase and considers (a) the partition of a chelating extractant (HL) between the continuous organic phase and the reversed micellar pseudophase, (b) transfer of the metal ion M^z+into the continuous organic phase via reaction with HA monomers, (c) partition of the M^Z+-HA complex between the continuous organic phase and the reversed micellar pseudophase, (d) reaction of the M^Z+-HA complex with HL in the reversed micellar pseudophase, and (e) partition of the HL-containing complex between the reversed micellar pseudophase and the continuous organic phase. Quantitative expressions are derived that permit one to identify the chemical parameters that influence the liquid-liquid transfer process and therefore permit one to undertake the rational design of microemulsion formulations for specific applications.