Modelling the ion-exchange equilibrium in nanoporous materials

TL;DR

This study models ion-exchange equilibrium in nanoporous materials using theoretical and simulation methods, revealing insights into ion selectivity and activity coefficients in electrolyte mixtures within charged matrices.

Contribution

It introduces a combined theoretical and computational approach to model ion-exchange in nanoporous materials, including a simple model for ion-exchange resin and selectivity predictions.

Findings

Good agreement between replica Ornstein-Zernike and Monte Carlo results.

Selectivity increases with adsorbent capacity.

Higher charge density ions are more selectively adsorbed at low concentrations.

Abstract

Distribution of a two component electrolyte mixture between the model adsorbent and a bulk aqueous electrolyte solution was studied using the replica Ornstein-Zernike theory and the grand canonical Monte Carlo method. The electrolyte components were modelled to mimic the HCl/NaCl and HCl/CaCl_2 mixtures, respectively. The matrix, invaded by the primitive model electrolyte mixture, was formed from monovalent negatively charged spherical obstacles. The solution was treated as a continuous dielectric with the properties of pure water. Comparison of the pair distribution functions (obtained by the two methods) between the various ionic species indicated a good agreement between the replica Ornstein-Zernike results and machine calculations. Among thermodynamic properties, the mean activity coefficient of the invaded electrolyte components was calculated. Simple model for the ion-exchange resin was proposed. The selectivity calculations yielded qualitative agreement with the following experimental observations: (i) selectivity increases with the increasing capacity of the adsorbent (matrix concentration), (ii) the adsorbent is more selective for the ion having higher charge density if its fraction in mixture is smaller.