Temperature Dependence of the Parameter of the SIT Model for Activity Coefficients of 1:1 Electrolytes

Abstract

This work analyzes the temperature dependence of the only fitting parameter of the SIT (Specific Ion Interaction Theory) model for activity coefficients. Relations based on the so-called “primitive” model for the inter-ionic potential suggest that the binary interaction parameter is proportional to the pure water density. In order to check this correlation, activity coefficients and related data are tested for twenty 1:1 electrolytes, where the necessary data are available at temperatures up to at least 523 K. A strong correlation with water density is universally observed at temperatures T > 360 K (or water densities, \( \rho_{1}^{*} \), below 950 kg·m−3). In accordance with expectations from theoretical considerations, at these temperatures and water densities, the SIT parameter is constant under isochoric conditions (a constant water density), and decreases with decreases of \( \rho_{1}^{*} \). Over the whole temperature range, usually between 273 and 573 K, the temperature and water density dependence of the SIT model interaction parameters, \( \varepsilon_{\gamma } (M,X) \), where M stands for a cation and X for an anion, is well described (including data at pressures up to 100 MPa) by the following expression \( \varepsilon_{\gamma } (M,X) = b_{0} + b_{1} \rho_{1}^{*} + \frac{{b_{2} }}{T - 210} \).