Structure and criticality of ionic fluids*

Two properties render electrolyte theories difficult, namely the long-range nature of the Coulomb interactions and the high figures of the Coulomb energy at small ion separations. In solvents of low dielectric constant, where the Coulomb interactions are particularly strong, electrical conductance and dielectric spectra suggest that the ion distribution involves dipolar ion pairs, which then interact with the free ions and with other dipolar pairs. The dipole–dipole interactions between ion pairs lead to an increase of the dielectric constant, which in turn stabilizes the free ions, thus leading to redissociation at high salt concentrations. An equation of state that accounts for ion pairing, ion–ion pair, and ion pair–ion pair interactions rationalizes the basic features of the ion distribution. It also predicts a fluid-phase transition at low reduced temperatures, which closely corresponds to simulation results and to experimentally observed liquid–liquid phase transitions. The long-range nature of the Coulomb potential driving these transitions raises questions concerning their universality class. Experiments suggest that the Ising universality class applies, but there is cross-over to mean-field behavior rather close to the critical, not yet well explained by theory.