The critical locus of a simple fluid with added salt

A single-component, nonionic fluid that exhibits a standard gas-liquid critical point is considered under the addition of a salt at concentration phi. The critical loci, T-c(phi), p(c)(phi), and p(c)(phi) are studied perturbatively. Since the Debye screening length, xi (D), diverges as phi (-1/2) in the pure-fluid limit, singular behavior of the critical loci is to be expected when phi -->0. Using classical (i.e., van der Waals-type) theory for the solvent+ion mixture combined with Debye-Hackel theory for the ionic interactions, we find a phi (3/2) dependence of the critical loci so that, e.g., (d(2)T(c)/d phi (2)) diverges when phi --> 0. After extending the theory to allow for ion pairing, experimental data for T-c(phi) for solutions of sodium chloride in water are analyzed and found to support a phi (3/2) anomaly of the predicted magnitude and sign. The latter turns out to be a consequence of the density dependence of the dielectric constant of pure water.