Criticality and Corresponding States in Ionic Systems

Ionic liquids, which are molten salts with melting points below 100 °C, down to – 80°C, are a hot research topic at present. Many applications in chemical engineering and preparative chemistry are envisaged for this new fascinating group of compounds. The interplay of Coulomb interaction and with van der Waals interactions provides a challenge for the theoretical understanding of the special properties of the ionic liquids and of their solutions. Some ionic liquids are soluble in non-polar solvents as hydrocarbons others in polar solvents like water. Vice versa some are insoluble in non-polar others insoluble in polar solvents.

Liquid-liquid phase transitions are observable at ambient temperatures enabling investigations of the critical properties (coexistence, critical fluctuations, critical dynamics) with mK accuracy. Such research is of fundamental interest: While in nonionic systems the liquid-gas as well as liquid-liquid phase transitions are driven by short range van der Waals interactions with an r-6 -range dependence, the phase transitions in the ionic systems are driven by long-range r-1 -Coulomb interactions. The universality hypothesis that liquid-gas as well as liquid-liquid phase transitions all belong to the Ising universality class has been theoretically proven for r-n interactions with n>4.97, while the nature of the critical point in Coulomb systems was unknown.

Some experiments reported mean-field behavior for such systems. Meanwhile, experiments as well as simulations support the conclusion that Coulomb systems also belong to the Ising universality class. The simulations concern the so called restricted primitive model (RPM), which considers equal sized charged hard spheres in a dielectric continuum. The critical points of the liquid-liquid phase transitions in ionic solutions in non-polar solvents are in agreement with the prediction of the RPM. Corresponding state analysis based on the reduced variables of the RPM reveals different behavior, when comparing phase separation in aprotic solvents (hydrocarbons) with that in protic solvents (alcohols , water). In terms of the RPM- variables the phase separation in aprotic solvents, which is driven by Coulomb interactions, have an upper critical solution point, while the coexistence curves in protic solvents have a lower critical solution point, typically for phase separation caused by hydrophopic interactions.