Comparison of Numerical Modeling of Water Uptake in Poly(vinyl chloride)-Based Ion-Selective Membranes with Experiment

Abstract

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.

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.…”

Electrochemical Mechanism of Ferrocene-Based Redox Molecules in Thin Film Membrane Electrodes

“…Harrison and co-workers [23][24][25][26][27] allowed the simultaneous mass transfer of electrolyte (NaCl here) with absorbed membrane water nanodroplets to the buried interfaces of coated-wire electrodes. Accordingly, this is the most probable source of the Cl -anion in the FeCl4 -irreversible reaction product formed at the GC/membrane buried interface (see Fig.…”

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.…”

“…The results were interpreted in terms of the presence of two forms of water, also confirmed by NMR studies, namely of water dissolved in the whole of the membrane and droplets predominantly formed in the waterrich surface region. 11 More recently, experimental data 12,13 were interpreted using a model based on osmotic pressure 19 and assuming the existence of mobile homogeneous and immobile heterogeneous water (dual-sorption-site theory). 20 Thus, true diffusion coefficients of D H2O =(1.5±0.1)×10 -6 and (0.4±0.02)× 10 -6 cm 2 s -1 were obtained for PVC membranes plasticized with dioctyl adipate and 2-nitrophenyl octyl ether (o-NPOE), respectively.…”