FTIR-ATR Study of Water Uptake and Diffusion Through Ion-Selective Membranes Based on Plasticized Poly(vinyl chloride)

In this paper, the water uptake and diffusion of water through ion-selective membranes based on plasticized poly(vinyl chloride) (PVC) have been studied by FTIR-ATR spectroscopy. The diffusion of water was modeled by the finite-difference method and the obtained diffusion coefficients are reported for membranes plasticized with bis(2-ethylhexyl) sebacate (DOS) and 2-nitrophenyl octyl ether (oNPOE). The influence of various common membrane additives such as lipophilic salts and ionophores on the water uptake was also determined through representative membrane formulations (potassium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (KTFPB) and/or calcium ionophore IV (ETH 5234)). The best fits of the time dependent water concentration changes in the membrane upon water uptake were obtained with a model consisting of two diffusion coefficients describing fast (ca. 1.5 x 10(-7) cm(2) s(-1)) and slow (ca. 9.5 x 10(-9) cm(2) s(-1)) diffusion of water in the PVC membranes. In contrast to the water uptake of the membranes, the diffusion rates were found to be practically independent of the membrane composition. It is shown that KTFPB and ETH 5234 decrease the water uptake whereas it is facilitated by a higher plasticizer content of the membrane. Monomeric, dimeric, clustered and bulk water could be distinguished by deconvolution of the FTIR-ATR spectra, which is important for understanding the water uptake mechanism.