BUFFER EFFECTS ON AQUEOUS SWELLING KINETICS OF POLYELECTROLYTE GELS

Electrolytes are often added to a gel-swelling medium under the assumption that the important conditions which characterize swelling rates are the solution pH and ionic strength, with little emphasis on the nature of the electrolyte. Previous research by Siegel et al. has indicated that the presence of the un-ionized acidic form of an electrolyte buffer is a primary rate determinant for swelling of a polybase gel. A systematic swelling study on two separate gels, 2-hydroxyethyl methacrylate copolymerized with methacrylic acid (HEMA/MAA) and N,N-dimethylaminoethyl methacrylate (HEMA/DMA), has been performed to investigate the influence of the concentration of the un-ionized buffer by three principal factors: (1) total buffer concentration, (2) solution pH, and (3) buffer pK(a). Swelling and deswelling kinetics were obtained. In the presence of an electrolyte buffer, a dramatic swelling rate increase is observed for the HEMA gels, with substantial gains in rate obtained as total buffer concentration rises. Results also emphasize that to enhance swelling kinetics, the pH must be such that the buffer is essentially un-ionized.