UNIFIED DESCRIPTION OF INFINITE-DILUTION THERMODYNAMIC PROPERTIES FOR AQUEOUS SOLUTES

A linear expression, asymptotically correct near a solvent's critical point, is derived relating Henry's constant of a solute to the density of the pure solvent. Since the expression is general (i.e. not restricted to the solvent's saturation curve), it may be differentiated to yield other infinite-dilution thermodynamic properties such as partial molar volumes. This enables these properties for a solute to be predicted from a single fit of the Henry's constant expression to solubility data. Such predictions are compared with published measurements of apparent molar volumes and apparent molar heat capacities for aqueous argon, xenon, and ethylene. The volumes are predicted well over a wide range of conditions including the region near water's critical point, where large effects are observed. Agreement with heat capacity data is only qualitative; we show that this may be in part a consequence of a higher order term in the asymptotic expression that becomes important after two differentiations. The divergent behavior of the infinite-dilution properties near the critical point is determined by divergences in pure-solvent properties; the only solute dependence is in the amplitude of the divergence, which is proportional to the slope of the Henry's constant expression. While the experimental Henry's constant data yield a very large linear region, it now appears that the slope in this region is not the true asymptotic slope. Nevertheless, it appears that predictions of infinite-dilution thermodynamic properties based on Henry's constants are adequate over a wide range of conditions.