Ionic Pairing In Binary Liquids Of Charged Hard-Spheres With Nonadditive Diameters

We examine types of short range order that arise in binary liquids from a combination of Coulombic interactions and nonadditivity of excluded volumes, the initial motivation being observations of complex formation by hydrated ions in concentrated aqueous solutions. The model is a fluid of charged hard spheres with contact distances σ+−≠ 1/2 (σ+++σ−−), its structural functions being evaluated in the mean spherical approximation and in the hypernetted chain approximation. Cation–anion pairing is clearly seen in the calculated structural functions for negative deviations from additivity (σ+−<σ++=σ−−), though the absence of true chemical bonding in the model does not allow long-lived complexes. Positive deviations from additivity (σ+−>σ++=σ−−) favor long-wavelength concentration fluctuations and demixing in a neutral mixture: these are suppressed by Coulombic interactions in favor of microscopic intermixing of the two species in the local liquid structure, up to like-ion pairing. Comparison is made with diffraction from concentrated aqueous solutions of cadmium sulphate and other instances of possible applicability of the model are pointed out.