Novel Phase-Transition Behavior in an Aqueous Electrolyte Solution

We have investigated the near-critical behavior of the susceptibility of a ternary liquid mixture of 3-methylpyridine, water, and sodium bromide as a function of the salt concentration. The susceptibility was determined from light-scattering measurements performed at a scattering angle of 90% in the one-phase region near the locus of lower consolute points. A sharp crossover from asymptotic Ising behavior to mean-field behavior has been observed at concentrations rang- ing from 8 to 16.5 mass0 NaBr. The range of asymptotic Ising behavior shrinks with increasing salt concentration and vanishes at a NaBr concentration of about 17 mass0, where complete mean-field-like behavior of the suscep- tibility is observed. A simultaneous pronounced increase in the background scattering at concentrations above 15 mass0, as well as a dip in the critical locus at 17 mass0 NaBr, suggests that this phenomenon can be iInterpreted as mean-field tricritical behavior associated with the formation of a microhetero- geneous phase due to clustering of the molecules and ions. An analogy with tri- critical behavior observed in polymer solutions as well as the possibility of a charge-density-wave phase is also discussed. In addition, we, have observed a third soap-like phase on the liquid liquid iInterface in several binary and ternary liquid mixtures.