Liquid Membrane Transport of Amino Acids Mediated by Reverse Micelles

Abstract

The potentialities of reverse micelles of AOT (bis(2-ethylhexyl) sodium sulfosuccinate) for the selective transport of tryptophane (Trp) and p-iodophenylalanine (pIPhe) are investigated using two different experimental devices. Winsor II systems were obtained from mixtures of AOT/isooctane/water (0.1 M phosphate buffer). The microemulsion phase was used as a liquid membrane in which the reverse micelles play the part of mobile carriers. In the first device there was a direct liquid-liquid contact between the different phases whereas in the second one semi-permeable membranes were introduced at the interfaces. The transfer rates measured for Trp and pIPhe at different AOT concentrations show very different behaviors, which are only moderately altered by the presence of the semi-permeable membranes. Single uptake and release experiments, which were independently undertaken show that the rate-determining step is the release of the amino acids. A transport mechanism assuming a competitive release by two processes (droplet opening at the interface and ion pair association/dissociation) is proposed. The best separation between the two amino acids is obtained at very low AOT concentrations.