Behavior of a Solute Near the Critical Point of an Almost Pure Solvent

Abstract

Abstract A lattice‐gas model is used to analyse the behavior of the properties of a solute near the critical point of a pure solvent. The dimensionless activity coefficient γ 2 = z 2 /p 2 exhibits striking anomalies near the critical point of the pure solvent. Singularities in both the isothermal compressibility, K T , and the constant volume heat capacity, C v , of the pure solvent are reflected in the behavior of γ 2 . The limit.γ* 2 , of this activity coefficient as the solute density tends to zero is found be to a particularly simple function of the density and the internal energy density of the pure solvent. Isotherms and the coexistence curve in the γ* 2 ‐P and γ* 2 ‐p 1 planes are discussed. It is found that the partial molar volume of solute v 2 , can become very large (either positive or negative) on the critical locus of the mixture, and diverges at the pure solvent critical point. The limiting value of the partial molar volume of solvent,v 1 , at the pure solvent critical point depends upon the path of approach and can even be negative. Negative values of v 1 and v 2 are found to be associated with critical azeotropy and “incipient azeotropy” in this model. The partial‐molar enthalpies and internal energies also exhibit anomalies. It is suggested that the anomalous behavior of the activity coefficient may be responsible for anomalies in the rates of chemical reactions of solutes near the critical point of a solvent.