The liquid-liquid phase transitions in ionic solutions: criticality, crossover and complete scaling
W. Schröer
The critical properties of phase transitions of fluids and fluid mixtures are known to agree with that of the 3d-Ising model. Although mean-field criticality was presumed because of the long-range nature of the Coulomb potential and stated by some measurements of the liquid-liquid phase transition of ionic solutions, it is now almost certain that the phase transitions in ionic systems also belong to the Ising universality class. This is stated by experiments investigating the liquid-liquid phase transition of ionic solutions and simulations on the model systems of charged hard spheres.
In this paper we report measurements of the viscosity, of the coexistence curve and the static and dynamic light scattering of various solutions of ionic liquids, including such in alkanes alcohols, arenes, arenhalides and water. All data show Ising critical behaviour with indications of crossover to mean-field criticality. Is in variance to theoretical estimates, the crossover is shifted towards larger separations from the critical temperature with increasing dielectric permittivity of the solvent, thus stating Pitzer´s observation.
In variance to the Ising model the phase diagrams are highly asymmetric. Furthermore, the diameter, which is the mean value of the compositions in the coexisting phases, is non-linear in variance to the predictions of mean-field theories but also to the observations on noble gases.
The asymmetry, which is analysed using complete scaling theory, is found to decrease with increasing dielectric permittivity of the solvent.