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

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}^{*} $ ρ 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}^{*} $ ρ 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) $ ε γ ( 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} $ ε γ ( M , X ) = b 0 + b 1 ρ 1 ? + b 2 T ? 210 .