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 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.

Pre-Polarized Hydrophobic Conducting Polymer Solid-Contact Ion-Selective Electrodes with Improved Potential Reproducibility

“…[18][19][20][21] The role of the SC is to provide a stable inner phase boundary potential 20,22,23 at the substrate/SC and SC/ISM interfaces, and to prevent the aqueous layer formation at the inner interface, which coupled to diffusion of ionic species within the ISM may lead to potential instability. 24 As all ISMs exhibit water uptake [25][26][27][28][29][30][31] , the prevention of the aqueous layer formation 32,33 with ill-defined ion activity has proven to be a major challenge.…”

Chromoionophore-Mediated Imaging of Water Transport in Ion-Selective Membranes

“…[1][2][3] Consequently, they always contain a certain amount of water, a fact recognized already in the seventies [4][5][6] and studied intensively by the group of Harrison in the nineties. [7][8][9][10][11][12][13] It is well known that clear plasticized PVC membranes may become cloudy when soaked in water and transparent again when the process is reversed through drying. The uptake of water may affect the performance of polymer membranes in ionselective electrodes (ISEs) and optodes in several ways.…”

“…14 In addition, long-term potential drifts observed with miniaturized sensors were attributed to slow changes in the equilibrium water content of their membranes. 12 The presence of water droplets (so-called heterogeneous water), on the other hand, can give rise to light scattering and bias the absorbance measurements. 9,12,15 Moreover, the swelling or shrinking of membranes with changes in their water content affects optode measurements, especially when a layer of defined thickness is a prerequisite (as for ATR).…”

“…12 The presence of water droplets (so-called heterogeneous water), on the other hand, can give rise to light scattering and bias the absorbance measurements. 9,12,15 Moreover, the swelling or shrinking of membranes with changes in their water content affects optode measurements, especially when a layer of defined thickness is a prerequisite (as for ATR). 16,17 Also, the adhesion of such membranes to a solid support is influenced by the water transported through them.…”

Multispectral imaging of ion transport in neutral carrier‐based cation‐selective membranes

“…The stacked membrane approach in conjunction with pH-sensitive chromoionophores has been used later for the optical determination of the distribution of free and complexed ionophore across the membrane (18). The first high-resolution true imaging technique for studying transport processes in ISE membranes was described by Harrison and coworkers (19,20). The authors used CoCl 2 as a water sensitive dye in combination with a spatial imaging photometer to study the water uptake of potassium selective PVC membranes.…”