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 D mo ) D/(1 + C i s /s o ), where D mo and D are the observed and true diffusion coefficients, respectively, C i s is the molar concentration of salt impurity and s o is the maximum miscible water concentration. Addition of CoCl 2 to PVC plasticized with dioctyl adipate (DOA) or nitrophenyl octyl ether (NPOE) showed that D mo followed this expression during initial stages of water uptake. Values of D ) (1.5 ( 0.1) × 10 -6 cm 2 /s and s o ) 51 ( 5 mM, versus D ) (4.3 ( 0.2) × 10 -7 cm 2 /s and s o ) 82 ( 7 mM, were obtained for membranes with 66% DOA or NPOE as plasticizer, respectively. Added CoCl 2 or KB(C 6 H 5 ) 4 reduced D mo . 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% CoCl 2 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 NH 4 + -selective membrane used as an optode, this effect introduced absorbance changes as a function of ionic strength.

A dual-sorption-site model in which both miscible and phase-separated water droplets are present in ion-selective, poly(vinyl chloride) (PVC)-based membranes is applied to data obtained for the H2O concentration profiles during the later stages of water uptake. A finite difference algorithm for calculating the concentration profiles is presented, and the calculated curves are shown to match the experimental data over the entire equilibration period. A relaxation process is identified which is associated with an increasing content of immobilized H2O, C im, as described by C im = C°im(1 − y e-β t ), where y = 0.7, β = 0.27 h-1 for membranes with about 45 mM salt added, and y = 0.6, β = 0.40 h-1 with 5 mM added salt. Calculations are presented showing water equilibration times required for 40−100 μm thick membranes contacted by water on both sides, a solid on one side, or a 10−40 μm thick hydrogel serving as an inner electrolyte reservoir on one side. For membranes thinner than 200 μm that are contacted on both sides by H2O, the slow relaxation phenomenon dominates water uptake rather than the diffusion process. The presence of an internal reservoir, which must be hydrated by transport across the membrane, slows equilibration drastically.

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Liquid-liquid junction membranes prepared from a modified polyvinylchloride (PVC) polymer with 0.6 weight percent (w/o) OH introduced, (PVC-OH) and 0.25 w/o SIC14 added during casting have substantially enhanced adhesion on glass or Si compared to PVC-based membranes. Ion-selective electrodes for NH4 + based on a dioctyladipate/nonactin ion exchanger show enhanced adhesion, as do Ca 2 § membranes using a neutral ion carrier/o-nitrophenyloctylether mixture. Enhanced surface adhesion is demonstrated to result in improved lifetime for K § membranes coated on n-Si electrodes and on ion-sensitive field effect transistors (ISFET's). Electron micrographs show large differences in surface quality for PVC vs. PVC-OH/SiC14-based membranes after aqueous storage.Integrated potentiometric sensors such as the ion-sensitive field effect transistor (ISFET) utilize ion-selective electrode (ISE) technology to produce membrane-coated devices specific to a variety of ions (1). However, the typical polyvinylchloride (PVC) liquid-liquid junction-based membranes used with an ISE are not ideally suited to application to solid-state devices. Problems with membrane lifetime due to leaching of material from the membrane (2) and poor adhesion (3)to the sensor arise. Recently, we reported on a K+-sensitive ISE membrane, based on a chemically modified form of PVC, that exhibits enhanced adhesion on solid surfaces with minimal changes in electrochemical properties (4). Formation of a covalent bond between PVC and a solid surface is accomplished via synthesis of the hydroxyl-substituted derivative, PVC-OH (0.6 weight percent [w/o] OH) and reaction with SIC14 according to Scheme I. In this paper we demonstrate that the ~c,-I ~8i-.o. -o-/-o-..l ~ o,

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