Diffusive permeability of solutes in poly(vinyl alcohol) membranes as a function of the degree of hydration

Sorption and permeation of sodium chloride, Congo Red (a direct dye), and Sunset Yellow (an acid dye) in PVA membranes were measured to discuss the relation among the degree of hydration H , permeability P , and diffusion coefficient D . Above H of 0.9, the permeability and diffusion coefficient of various solutes could be described by free volume theory provided that the value of V * s / V fw is estimated properly, where V * s is the ratio of critical volume of permeating molecule to vander Waals volume of water and V fw is the free volume fraction of water. Below H of 0.9, however, Ogston's relation which predicts a linear relation between (1‐ H ) and the logarithm of P or D fits better than the free volume relation. It seemed to indicate that the polymer networks simply had a geometrical obstruction effect in the diffusion of solutes. The diffusion coefficient extrapolated at H = 1 in Ogston's plot decreased with the increase in the interaction between solute and polymer as suggested by the partition coefficient. The hydration dependence of the partition coefficient K changed at H = 0.8 corresponding to the maximum amount of the bound water.