Kinetics and equilibria associated with the absorption and desorption of water and lithium chloride in an ethylene—vinyl alcohol copolymer

Abstract

Sorption and desorption equilibria and kinetics for LiCl and H 2O in an ethylene—vinyl alcohol copolymer film containing 70 mole percent vinyl alcohol were investigated at 25°C. The swelling behavior of water in the polymer was characterized by vapor and liquid sorption experiments over a range of water activities. p]The effects of LiCl content on the water sorption kinetics and equilibria in the films are presented and discussed. The kinetics and mechanism of LiCl sorption have also been studied. The amount of salt sorbed into the polymeric films increases linearly with the salt concentration in the external aqueous solutions. Both the rate and the amount of sorbed water increase significantly as the LiCl content increases. p]The desorption of LiCl, previously sorbed into the polymer, was characterized for different salt loadings. The rate of fractional salt release is independent of LiCl concentration in the film. Initially, the salt release is controlled by the nearly constant-rate absorption of water. The salt release, at long times, lags behind the swelling-controlled water uptake, indicating that the salt release is not completely controlled by the water sorption and that diffusion in the swollen polymer matrix contributes significantly to the long term elution of LiCl. Independent thermal analysis experiments suggest the formation of a metal salt—poly(ethylene—vinyl alcohol) complex.