COULOMBIC AND NON-COULOMBIC CONTRIBUTIONS TO THE CRITICALITY OF IONICFLUIDS - AN EXPERIMENTAL APPROACH

The recent discovery of liquid-liquid phase separations in electrolyte solutions with critical points near room temperature enables the syst ematic study of the critical behavior of ionic fluids. Depending on th e nature of the molecular interactions, either sharp mean-field or Isi ng behavior is obtained in the temperature range down to t=(T-T-c)/T-c =10(-4) or less. Mean-field-like criticality is obtained with systems which in the framework of a simple corresponding states model are fair ly close to the critical point of the ''restricted primitive model'' ( RPM) of equally-sized charged spheres in a dielectric continuum. In th ese cases the phase separation is driven by the Coulombic forces (so-c alled Coulombic phase separations). This type of unmixing occurs for 1 :1 electrolytes in solvents of low dielectric constant. Simple mechani sms for unmixing suggested in the literature are discussed in relation to the available data. Some evidence for departures from the simple R PM prediction is found. The presence of additional short-range interac tions leads to sharp Ising behavior. Examples are solutions of tetraal kylammonium salts in water and other highly structured solvents, where phase separation results from the peculiar solvophobic nature of ions (solvophobic phase separations). Previous speculations that this type of unmixing shows the tendency toward closed loops are confirmed by t he first direct observation of a lower consolute point in an aqueous s olution of propyl-tributylammonium iodide. By light scattering studies and measurements of the coexistence curve near the upper and lower co nsolute points Ising criticality is confirmed. A new mechanism for pha se separation is reported for the system ethylammonium nitrate + octan ol, where ion pairs are stabilized by hydrogen bonding beyond what is expected from the RPM. This comparatively subtle additional interactio n (so-called stricky ions) already changes the behavior of otherwise R PM-like systems from mean-field to Ising criticality. The results are discussed with particular emphasis on their implications for possible scenarios for explaining a mean-field critical point or crossover from mean-field to Ising behavior beyond the accessible temperature range.