Vapor-liquid equilibria, scaling, and crossover in aqueous solutions of sodium chloride near the critical line

We have developed a crossover scaling model for describing the thermodynamic properties of near-critical binary fluids. We have validated this model for the vapor-liquid equilibrium properties of mixtures of methane and ethane. We then apply the model to a most intriguing binary fluid, namely aqueous solutions of sodium chloride, so as to describe densities and concentrations at vapor-liquid equilibrium and isochoric specific heat capacities in the one-phase region. The description shows crossover from asymptotic Ising-like critical behavior to classical (mean-field) behavior. As the salt concentration increases, the region of Ising behavior decreases, although pure classical behavior is never attained. We suggest that processes of association/dissociation may be responsible for the failure of the model to describe compositions of the vapor phase of the solutions at very low concentrations.