Investigation Of The Critical Crossover Phenomena In Ionic Solutions

The critical concentrations and the critical temperatures of binary solutions of the room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium tetrafluoroborate (C4mimBF4) in a series of alcohols (1-propanol,2-propanol,1,3-propanediol, and 1,4-butanediol) were measured by the technique of the "equal volumes". It was found that the lager the value of the dielectric constant in the solvent, the lower the critical temperature of the RTIL solution in the solvent, which was in accordance with the prediction of the RPM model. The precise coexistence curves were determined by measuring the temperature dependence of the refractive index in the two phases of above solutions with the critical compositions. The coexistence curves of (T, n) were converted to the coexistence curves of (T, x) and (T,0), (x and (?) are mole fraction and volume fraction, respectively). The critical exponentsβ, the critical amplitudes B, the amplitudes of Wegner correction terms B1 and the diameters of coexistence curves pd were deduced from the coexistence curves. Near the critical points, the determined effective exponentsβeff for the four systems were close to the universal Ising value; while the deviations of the values of the effective exponentsβeff form the Ising one were clearly detected as the temperatures were far away from the critical points. It was found to be negative when the refractive indexes or the volume fractions was chosen as the concentration variable, but the deviations were found to be positive when the mole fraction was chosen as the concentration variable. The coexistence curves were analyzed by the crossover model proposed by Anisimov and Sengers, which was found to be valid. All the values of the exponents calculated from the crossover model for the three order parameters exhibited monotonic crossovers from 0.327 to 0.45 after the regular terms were subtracted. The effects of dielectric constantεof the solvents on the crossover temperature were not observed.The viscosities and densities were measured for the binary solutions of the room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium tetrafluoroborate (C4mimBF4) in 1,3-propanediol at different temperatures and concentrations. The regular background of the viscosity was obtained by the interpolation of the results of measurements at non-critical concentrations. For the critical concentration, the viscosity measurements showed an enhancement as expected for Ising criticality and a crossover to regular behavior. The critical enhancement was well described by the mode-coupling crossover function of Bhattacharjee. The closed form resulted from the limiting case (?) matched our experimental data well in the near-critical-point region, while the form deduced from the limiting case (?)â†'∞did not fit our measurements.