LIQUID-LIQUID PHASE SEPARATIONS AND CRITICAL-BEHAVIOR OF ELECTROLYTE-SOLUTIONS DRIVEN BY LONG-RANGE AND SHORT-RANGE INTERACTIONS

The discovery of liquid-liquid phase separations in electrolyte soluti ons with critical points near room temperature enables the systematic study of the critical behavior of ionic fluids. Depending on the detai ls of the molecular interactions mean-field-like criticality, Ising cr iticality or crossover from mean-field to Ising criticality is observe d. Mean-field behavior occurs in systems which in the framework of a s imple corresponding states model are fairly close to the theoretical c ritical point of the ''restricted primitive model (RPM)'' of equally-s ized charged spheres in a dielectric continuum (so-called Coulombic ph ase separations). In these cases the phase transition is driven by the long-range Coulombic forces and may be modeled by a Bjerrum-type theo ry for ion pair association plus an additional term for the interactio n between the ion pairs and free ions. The presence of additional shor t-range interactions shifts the phase transitions towards higher reduc ed temperatures and/or densities in the corresponding states plot. Two major examples are solvophobic unmixings in aqueous solutions of tetr aalkylammonium salts and unmixings of salts with hydrogen-bonds betwee n cations and anions (sticky ion pairs). Such systems exhibit Ising-li ke criticality. In the intermediate range crossover is observed. Possi ble scenarios for explaining the mean-field-Ising dichotomy are discus sed, indicating a major role of the dielectric constant of the solvent in determining critical behavior.