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.

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