Gas solubility and Henry's law near the solvent's critical point

Abstract

It has been experimentally observed, for water and nonaqueous solvents alike, that Henry's constant passes through a maximum and then declines as the temperature is raised from the triple point to the critical point. From classical and nonclassical models, we derive exact relations for the value of Henry's constant and its temperature dependence at the solvent's critical point, showing that the decline of this constant is a universal phenomenon. We demonstrate that the limiting temperature dependence of Henry's constant can be predicted from the thermodynamic properties of the pure solvent and the initial slope of the critical line. The validity of our prediction is tested by comparing it with experimental solubility data for several gases in high-temperature water and benzene. Our predictive model appears valid over a temperature range of at least 15% in temperature below the critical point of the solvent.

Solvation thermodynamics of gas solubility at sub‐ and near‐critical conditions

“…The two partial derivatives of uzm involved in Eqs. 31 and 32 can be straightforwardly written as and (Japas and Levelt Sengers, 1989) where s'(T,p) is the residual entropy per molecule.…”

“…For the case of Henry's constant H2,1(T) and the infinite dilution distribution factor K"(T) along the coexistence curve, Japas and Levelt Sengers (1989) derived the following expressions:…”

“…If valid for orthobaric regions away from the solvent's critical point, these asymptotic expressions become useful linear relationships for correlation purposes because they involve only the solvent's thermodynamic properties and the initial slope of the critical line that depends on the solute-solvent molecular asymmetry (Japas and Levelt Sengers, 1989).…”

“…at the solvent's critical condition, plays an important role in the solvation thermodynamics of dilute mixtures at near-critical conditions by determining the asymptotic behavior of the vapor-liquid distribution factor (Japas and Levelt Sengers, 1989), Henry's constant (Japas and Levelt Sengers, 1989;, the solubility enhancement (Levelt Sengers, 1991b;Chialvo and Cummings, 1994), and the initial departure of the critical line from the critical point of the pure solvent (Chang et al, 1984). If valid for orthobaric regions away from the solvent's critical point, these asymptotic expressions become useful linear relationships for correlation purposes because they involve only the solvent's thermodynamic properties and the initial slope of the critical line that depends on the solute-solvent molecular asymmetry (Japas and Levelt Sengers, 1989).…”

“…This hypothetical process involves an isothermal-isobaric coupling work given by the difference of residual (at constant temperature and either constant pressure or volume) chemical potentials between the infinitely dilute solute and the pure solvent ) (see Appendix A for the multicomponent counterpart), and where H2 and ff are Henry's constant of solute and the fugacity of the pure solvent, respectively, and P = l/kT. Note that this definition of Henry's constant pertains to one-phase system, that is, H,(T,P) (Japas and Levelt Sengers, 1989). Equation 4 relates the change of Gibbs free energy in the solvation process of a solute 2 at infinite dilution with the behavior of ( d P / d~~)~,~, a quantity that characterizes the solvent's local density perturbation (microstructural rearrangement) around the infinitely dilute solute molecule relative to the solvent's local density around any solvent molecule in the ideal solution (pure solvent) .…”