Dual-Sorption Model of Water Uptake in Poly(vinyl chloride)-Based Ion-Selective Membranes: Experimental Water Concentration and Transport Parameters

Abstract

An osmotic pressure-driven model for uptake of water in rubber is combined with dual-sorption-site theory to describe the transport of water in highly plasticized, poly(vinyl chloride) (PVC)-based ion-selective electrode membranes, both at equilibrium and during the initial stages of water uptake. During the first hour, mobile, miscible water is predicted to have a diffusion coefficient given by Dmo = D/(1 + Cis/so), where Dmo and D are the observed and true diffusion coefficients, respectively, Cis is the molar concentration of salt impurity and so is the maximum miscible water concentration. Addition of CoCl2 to PVC plasticized with dioctyl adipate (DOA) or nitrophenyl octyl ether (NPOE) showed that Dmo followed this expression during initial stages of water uptake. Values of D = (1.5 ± 0.1) × 10-6 cm2/s and so = 51 ± 5 mM, versus D = (4.3 ± 0.2) × 10-7 cm2/s and so = 82 ± 7 mM, were obtained for membranes with 66% DOA or NPOE as plasticizer, respectively. Added CoCl2 or KB(C6H5)4 reduced Dmo. The dependence of equilibrium water content on added salt and on ionic strength is also predicted by the theory, and qualitative agreement with theory was observed experimentally. Addition of 0.15% CoCl2 to a PVC/DOA membrane increased the equilibrium water content from 0.36 ± 0.05 to 3.04 ± 0.05 M. Light scattering by water droplets formed in the membrane was decreased by increasing the ionic strength of the contacting aqueous solution. For an NH4+-selective membrane used as an optode, this effect introduced absorbance changes as a function of ionic strength.