The influence of cosurfactants on the solubilization of phenylalanine in water-in-oil microemulsion

Abstract

The influence was studied of cosurfactants (a series of n-alkanols) on the equilibria of phenylalanine (Phe) solubilization in a water-in-oil (w/o) microemulsion (Winsor II system) as well as on the kinetics of phenylalanine mass transfer from an aqueous phase to a micellar phase. The distribution coefficient of phenylalanine was independent of the alcohol type and concentration. The resulting values of the water ratio and the interfacial tension as well as alcohol partitioning between the microemulsion and solvent phases were measured, also. The kinetics results (obtained from stirred cell measurements) are discussed from the view point of the three-step bud mechanism previously proposed for interfacial solubilization in w/o microemulsions. The observed “catalytic” effect of the added alcohol is discussed with respect to its influencing the incorporation of Phe molecules into the Aerosol OT (sodium bis(2-ethylhexyl) sulfosuccinate) layer as well as influencing the dynamics of the fusion of monolayers in the neck of the buds. Generally, the short-chain alkanols (pentanol, hexanol) accelerate the mass transfer and enable the easy reorganization of Aerosol OT molecules during the Phe incorporation, while the long-chain alkanols ( n >/ 9) retard the mass transfer from the aqueous to the micellar phase. These effects seem to be connected with the mechanical state of the surfactant/cosurfactant interfacial layer: rigid or fluid.