Electrolyte Rejection from Charged Nanoporous Material†

Miha Lukšič, Barbara Hribar-Lee and Vojko Vlachy*
Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva c. 5, SI-1000 Ljubljana, Slovenia
e-mail: vojko.vlachy@fkkt.uni-lj.si

Abstract
RAdsorption of the charge and size symmetric +1 : –1 primitive model electrolyte in disordered media (matrix) with charged (or neutral) obstacles was studied using the Replica Ornstein-Zernike theory and Grand Canonical Monte Carlo computer simulation. The charged matrix was prepared by a rapid quench of the +1 : z⁰ – (z⁰ – = –1, –2, –3, and –4) electrolyte solution being in equilibrium at temperature T₀, and relative permittivity ε₀. Than the positive ions were allowed to anneal and mix with the invading +1 : –1 electrolyte at T, ε₁, while the anions were left quenched and represented the collection of obstacles, called here matrix, to which the external electrolyte was adsorbed. To complement the data for charged adsorbent we also considered the adsorption of the same +1 : –1 electrolyte in the matrix prepared from hard sphere fluid and in the electroneutral matrix formed by quenched +1:–1 electrolyte. In the latter case, the (electroneutral) matrix was represented as an equilibrium distribution (T₀, ε₀) of monovalent cations and anions being quenched during the adsorption of an invading model electrolyte. Special attention was paid to the thermodynamic properties of the adsorbed fluid. We were particularly interested in the mean activity coefficient of the adsorbed electrolyte and in the Donnan exclusion coefficient as a function of the charge density of the matrix. At higher concentrations of the invading electrolyte the adsorption was dominated by the excluded volume effect of the matrix, whereas at low electrolyte concentrations the adsorption was governed by the combined effect of the matrix charge density and the excluded volume. These findings are in good qualitative agreement with those obtained previously for the electrolyte adsorbed in charged cylindrical micropores.

Keywords: Disordered charged matrix, adsorption, Donnan equilibrium, electrolyte, replica Ornstein-Zernike equation, Grand Canonical Monte Carlo simulation.