Permeability of ionic salts through poly(γ-methyl L-glutamate) membrane

Abstract

The permeability of various ionic salts through poly(γ-methyl L-glutamate) (PMLG) membrane, which consists of noncharged α-helical molecules, was studied. Based on the results, the mechanism of ion transport and the degree of hydration of ions in the membrane and their relation to salt rejection is discussed. The permeability coefficient of salt, PS, is almost independent of salt concentration over a wide range of concentration in aqueous solution, which contrasts with the case of charged membranes. The activation energy for PS is about 15 kcal/mole, which is almost half of that for cellulose acetate membranes. The ionic diffusion coefficient Di. decreases with increase of ionic radius, but it does not obey Stokes' law. The behavior is analyzed by the free volume theory, and it is found that the degree of ion hydration in PMLG membranes is depressed especially in the case of strongly hydrated ions in aqueous solution. This fact also explains the good permeability of salt through the rather hydrophobic polypeptide membranes. According to the analysis of the results of separation experiment, the low rejection of salt by PMLG membranes is also due to a coupling effect, i. e., to ionic flow accompanied by water flow. In this case, too, the hydration tendency of the ions in the membrane has a significant influence on the behavior.