Lower Critical Solution Temperature in the Metastable Region of an Ionic Solution in a Non-Polar Solvent

Previous studies of the critical behavior of ionic solutions in nonpolar solvents showed mainly upper critical solution temperatures. Now, we report for the solution of tetra-n-butyl ammonium bromide (N₄₄₄₄Br) in toluene a nearly closed miscibility loop, where the lower critical solution temperature (T_c = 297.75 ± 0.05 K, x_c = 0.0270 ± 5 × 10⁻⁴) is located in the region, which is metastable in respect to crystallization of the salt. The new observation is in variance to the former findings according to which the phase separation of ionic solutions in nonpolar solvents is similar to that of the model of charged hard spheres in a dielectric continuum, termed restricted primitive model (RPM), which has an upper critical solution point.