Critical behaviour of ionic solutions in non-polar solvents with a liquid-liquid phase transition

Turbidity measurements showing crossover from mean-field to Ising criticality have been reported by Narayanan and Fitter for the liquid-liquid phase transition in ionic solutions of alkyl-ammonium picrates in higher alcohols. The Ising region was found to increase with the dielectric permittivity D for solvents with 4 < D < 8. It was conjectured that the Ising region becomes too small to be observed for lower values of D, which is in accordance with the finding of mean-held criticality in the system triethylhexylammonium triethylhexylborate (N2226B2226) in diphenyl ether (Ph(2)O), where D approximate to 3.7. In order to check this hypothesis, we investigate solutions of salts in non-protonating solvents with D < 2.5. The systems are tetrabutylammonium naphthyl sulphonate (N4444NS) in toluene and tributylheptylammonium dodecyl sulphate (N4447DS) in cyclohexane. The location of the critical points in the corresponding state diagram is in general agreement with the model system of charged hard spheres in a dielectric continuum, i.e. the restricted primitive model (RPM). However, changes of T-c by minute variations of the salt and of the solvent (toluene, xylene, ethylbenzene) cannot be explained by the RPM. We report measurements of the phase diagram and light-scattering results. The amplitudes of the correlation length are up to an order of magnitude larger than those typically found in non-ionic fluids. For the new systems, but also for the solution of N2226B2226 in Ph(2)O, Ising criticality is found in the region of 1 mK < \T - T-c\ < 10 K.