CRITICAL PHENOMENA IN IONIC FLUIDS - A SYSTEMATIC INVESTIGATION OF THE CROSSOVER-BEHAVIOR

The critical behavior of turbidity for an array of ionic fluids is reported. The ionic fluid is comprised of a low melting organic salt in an organic solvent, and the critical temperature (Tc) refers to the liquid-liquid coexistence. The phase separation is driven, predominantly, by Coulombic interactions in the low dielectric constant (ε) solvents and, primarily, by solvophobic effects in the high ε solvents. For the moderately Coulombic case, the critical exponents of osmotic compressibility and correlation length (γ and ν, respectively), which are deduced from turbidity, exhibit a crossover from their Ising to mean-field values when moved away from the Tc. The crossover region moves closer to Tc as the effective ε of the solvent is decreased. For the solvophobic case, the critical behavior of turbidity can be adequately described by Ising critical exponents without correction-to-scaling terms. In addition, several specific features of this class of ionic fluids are presented. Some potential factors that influence the values of the critical exponents are also discussed. © 1995 American Institute of Physics.