Liquid-liquid distribution in reversed micellar systems

The equilibrium distribution of a hydrophilic solute (M^z+ between an aqueous phase and a reversed micellar organic phase (consisting of a surfactant HA with aggregation number x, and dissolved in a hydrocarbon diluent) is analyzed quantitatively by treating the reversed micelles as a pseudophase. It is shown that when the M-A complex is strongly solubilized by the micellar pseudophase, the distribution coefficient (D) has a first-order dependence on the concentration of micellized surfactant (C_s). On the other hand, when the M-A complex is not solubilized by the reversed micelles, a plot of log D versus log C_s has a slope of (z/x); in this case the monomeric species HA is the active extractant and any effect that decreases surfactant aggregation (e.g. low aggregation number, small aggregation equilibrium constant) leads to an increase in the distribution coefficient.