Counterion-specific swelling behaviour of crosslinked poly( l-glutamic acid) membranes in aqueous alcohols

Abstract

Poly( l-glutamic acid) (PLGA) membranes were prepared by crosslinking PLGA with a polyether diisocyanate. Counterion species were then exchanged with alkali metal cations (Li +, Na +, K +, Cs +) by immersing the PLGA membranes in aqueous solutions of the respective alkali hydroxides. Equilibrium swelling ratios in aqueous alcohols (methanol, ethanol, 2-propanol) and ethanol distributions were measured for the membranes as a function of the alcohol composition. An appreciable counterion specificity was observed for swelling ratios in a medium alcohol content range (40–70 vol.%); Cs + ≧ Li + > K + > Na +. This sequence has been interpreted to be the result of specific counterion binding to the polymer charges, which reduces the solvent osmotic pressure and the membrane hydrophilicity depending upon the effectiveness of the charge neutralization. The difference in counterion specificity with that observed for helix stabilization suggests that specific counterion binding consists not only of contact ion-pairing but also of solvent-shared ion-pairing.