Diffusion processes in synthetic hydrogel membranes.

Water transport through fully swollen poly(2-hydroxyethyl methacrylate) (pHEMA) hydrogels, containing varying concentrations of ethylene glycol dimethacrylate (EGDMA) and tetraethylene glycol dimethacrylate (TEGDMA) cross-linkers, was investigated using tritiated water. Diffusion coefficients obtained in these experiments decrease as the concentration of cross-linker increases. This decrease is very sharp at cross-linker concentrations of 0-2.5 mole-%. At higher cross-linker concentrations (2.5 - 7.5 mole-%), the diffusion coefficients appear to reach a limiting value. This study indicates that a cross-linker pHEMA membrane may provide both "partition" and "pore" mechanisms of solute transport based on cross-linker concentration. Hydrogels or varing water content was prepared to investigate the relationship between water content and diffusion coefficient. Poly (methoxyethyl methacrylate) (pMEMA), poly(methoxyethoxythl methacrylate) (pMEEMA) and copolymers of MEMA and MEEMA with HEMA were formulated to give hydrogels with water content ranging from approximately 3% to 63%. The results of the diffusion experiments were examined in light of a free-volume model of diffusive transport. Excellent theoretical-experimental correlation was obtained for the hydrogels and temperatures used in this study. This mode was used also to correct for the effects of temperature on gel swelling. Arrhenius activation energies obtained using this correction, approach the activation energy for tritiated water in pure water as gel hydration increases. A free volume approach has been used to describe the effect of apparent solute molecular size on solute membrane diffusion coefficient in pHEMA and PHEMA cross-linked with 1 mole % EGDMA. Analysis of experimental results indicates that for hydrophilic nonelectrolytes and inorganic chloride salts in the molecular weight range 20 - 500, the membrane diffusion coefficient decreases exponentially with increasing molecular size. Solute partition coefficients have been calculated for pHEMA and pHEMA cross-linked with 1 mole % EGDMA using a three state model for gel water. Comparison of these calculations with experimental partition coefficient indicates that hydrophilic nonelectrolytes and inorganic chloride salts partition primarily into bulk water regions. From this result it may be implied that solute permeation occurs in gel bulk water. The Flory-Huggins interaction parameter was related to the solubility parameter. The solubility parameter was used to describe swelling in a pHEMA-urea-water system. This approach was unable to provide an adequate description of the swelling process in the pHEMA-urea-water system. Swelling of pHEMA in inorganic salt solutions indicated that the anion plays a major role in gel swelling at low concentrations. At high concentrations, association of water with cations of high charge density (Li+, Ca+2) is of major importance in determining gel swelling. At high concentrations for cations of low charge density (Na+, K+) the anion retains its significance in determining gel swelling.