CRITICAL-BEHAVIOR OF IONIC-SOLUTIONS IN NONPOLAR-SOLVENTS WITH A LIQUID-LIQUID PHASE-TRANSITION

Turbidity measurements showing crossover from mean-field to Ising crit icality have been reported by Narayanan and Fitter for the liquid-liqu id phase transition in ionic solutions of alkyl-ammonium picrates in h igher alcohols. The Ising region was found to increase with the dielec tric permittivity D for solvents with 4 < D < 8. It was conjectured th at 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 ch eck this hypothesis, we investigate solutions of salts in non-protonat ing solvents with D < 2.5. The systems are tetrabutylammonium naphthyl sulphonate (N4444NS) in toluene and tributylheptylammonium dodecyl su lphate (N4447DS) in cyclohexane. The location of the critical points i n the corresponding state diagram is in general agreement with the mod el system of charged hard spheres in a dielectric continuum, i.e. the restricted primitive model (RPM). However, changes of T-c by minute va riations 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 correlatio n length are up to an order of magnitude larger than those typically f ound in non-ionic fluids. For the new systems, but also for the soluti on of N2226B2226 in Ph(2)O, Ising criticality is found in the region o f 1 mK < \T - T-c\ < 10 K.