Thermodynamics of Aqueous Electrolyte Solutions with Liquid‐Liquid Phase Separation

Initiated by the recent observation of liquid‐liquid phase separation in aqueous solutions of tetra‐n‐butylammonium thiocyanate, a treatment of the thermodynamic properties of aqueous electrolyte solutions in relation to liquid‐liquid phase equilibria is given. For analyzing the thermodynamic data, the ion interaction approach of Pitzer is used in which the osmotic coefficient is described by a long‐range term for Coulombic interaction plus a virial coefficient series to account for short‐range specific interactions. It is shown that the ion‐interaction approach is flexible enough to incorporate liquid‐liquid phase equilibria in a natural and simple way. For 1:1 electrolytes liquid‐liquid coexistence curves are predicted if the second virial coefficients entering into Pitzer's theory are large and negative and the third (or higher) virial coefficients are positive. Experimental data show that largely negative second virial coefficients are generally present if large cations are combined with large anions, so that coexistence curves are predicted for aqueous solutions of tetraalkylammonium salts with large anions. Many of these salts are practically insoluble at room temperature, but at higher temperatures, where crystallization does not interfere, these predictions are experimentally confirmed for tetrabutylammonium iodide, tetrapropylammonium perchlorate and tetrapropylammonium picrate. For 2:2 electrolytes phase separation can result from the behaviour of the long‐range contribution to the osmotic coefficient alone, even if the virial coefficients are positive. Thermodynamic data for aqueous MgSO4 solutions at elevated temperatures indicate a coexistence curve with a lower consolute point at about 239°C, submerged by the crystallization curve. The same approach predicts a coexistence curve for aqueous UO2SO4 solutions, in qualitative agreement with the observation of a lower critical point at 286°C. In earlier interpretations this coexistence curve has been attributed to the peculiar properties of the uranyl ion, but it would appear that to a first approximation the miscibility gap in aqueous UO2SO4 at elevated temperatures represents a general property of 2:2 electrolytes, which for UO2SO4 becomes visible due to the high solubility of this salt.