The liquid-liquid phase transition in ionic solutions: Coexistence curves of tetra-n-butylammonium pricrate in alkyl alcohols

We report coexistence curves of the liquid-liquid phase transition in ionic solutions. The phase diagrams of tetra-n-butylammonium pricrate solutions in a series of alkanols (2-propanol, 1-decanol, 1-dodecanol, 1-tridecanol a nd 1-tetradecanol) are determined either by measuring the refractive index in the two phase region in one sample of near-critical composition as a fun ction of the temperature, or by direct observation of the composition depen dent phase separation temperatures. With the exception of the 2-propanol sy stem, the critical points are in accordance with the predictions by the res tricted primitive model. The coexistence curves are analyzed in terms of di fferent composition variables, of which the volume fraction seems to be the most appropriate one. For the volume fraction, deviations from asymptotic Ising behavior are observed which are equally well described by a critical exponent slightly different from the Ising value or by Wegner corrections. Although the deviations are quite small, they show a systematic increase wi th decreasing dielectric constant of the solvent, thus suggesting an approa ch to the mean-field case. The significance of this finding is, however, we akened by the fact that the corrections to scaling are also affected by the choice of the composition variable. For all investigated systems, the diam eter of the coexistence curve shows a pronounced nonanalytic temperature de pendence. (C) 1999 American Institute of Physics. [S0021-9606(99)51205-1].