Enrichment of an amino acid by micellar-enhanced ultrafiltration

Abstract

Abstract It is difficult to enrich low molecular weight solutes by membrane processes with sufficient speed under low applied pressure. However, combination of a solute with micelles of a surfactant enables enrichment of the solute into the retentate solution by ultrafiltration. The micellar-enhanced ultrafiltration of an amino acid, l -tryptophan, was carried out in a batch-stirred cell. The characteristics of the ultrafiltration of the micelles were examined with respect to the permeation flux and micelle rejection. As the concentration of the micelle-forming substance, cetylpyridinium chloride monohydrate (CPC; MW 358), in the retentate solution increased, the permeation flux decreased but the concentration in the permeate solution was limited. The effect of membrane molecular weight cut-off (MWCO) on permeation flux was obvious at low CPC concentrations but only slight at high concentrations. l -Tryptophan (MW 204) combined with CPC micelles to be retained by membranes of MWCO 20,000 as well as MWCO 10,000; there was no difference in rejection between the two membranes. The rejection coefficient was estimated as a function of the solubilization constant and the micelle concentration. The rejection coefficient increased with increasing CPC concentration but too high CPC concentrations resulted in the leakage of CPC into the permeate solution.