Interaction with micelles as a driving force for uphill transport of organic cations across ion exchange membranes

Uphill transport of L-Dopa and phenylalanine (Phe) across cation-exchange membranes into micelle-containing receiver solutions is reported. With L-Dopa as the analyte in a sample solution at a pH where it is it a zwitterion, preconcentration by a factor of 3.2 +/- 0.2 (n = 5) is observed when 0.10M sodium dodecyl sulfate (SDS) is the receiver. When the SDS concentration is varied, preconcentration of L-Dopa is not observed until the critical micelle concentration is reached. Similar results were obtained with Phe as the analyte under conditions where it is protonated in both the sample and receiver. The transport is demonstrated to obey the assumptions required to quantify the results by the fixed-time kinetic method. That is, the amount of Phe transferred from a 200-ml sample across a 10-cm(2) membrane into a 5-ml receiver was directly proportional both to the dialysis time for up to 90-min and to the initial concentration of Phe in the sample when a 60-min dialysis time was used. The latter yielded a constant enrichment factor, 4.8 +/- 0.2 (n = 6), when the sample concentration of Phe was in the range 0.61 mM-6.0 muM. Means to increase the enrichments to practical values are discussed.