Henry constants in non-ideal fluid mixtures

Abstract

Infinite-dilution chemical potentials (or Henry's constants) of highly non-ideal binary Lennard-Jones mixtures were calculated using Widom's test particle method in the canonical and Kirkwood's charging method in the isothermal-isobaric ensemble. For large solutes at high densities, the results were significantly different from previous values obtained using the umbrella sampling test-particle method in the canonical ensemble. The difference can be attributed to the much more severe system size dependence of the canonical ensemble for large solutes using umbrella sampling methods. Simulations were carried out at a variety of temperatures and densities for infinitely dilute mixtures with C ≡ ε _AB /ε _BB ⩽ 2 and D ≡ (σ _AB /σ _BB )³ ⩽ 3·5 (Here ε and σ are the Lennard-Jones energy and size parameters, A and B refer to the solute and solvent respectively.) It was found that the test particle method is applicable to mixtures at reduced density ρ* ≡ ρσ³ _BB ⩽ 0·5 with C ⩽ 2 and D ⩽ 3·5. For higher densities and/or larger C and D, the Kirkwood method should be used. The Kirkwood method is also preferred in simulating systems near phase boundaries because the structural changes induced by the addition of non-ideal solutes can be represented.

The simulated chemical potentials were compared to the van der Waals I (vdWI) conformal solution theory predictions. For 1 ⩽ C ⩽ 2 and 0 ⩽ D ⩽ 3·5 the agreement was within the numerical uncertainties of the simulation results. However, the simulation values were found to be consistently lower than the vdWI values. For C ⩽ 1·0, the vdWI values were significantly higher than the simulation results, the differences being larger than the statistical uncertainties. These results seemed to indicate that the vdWI theory tends to overestimate the entropic or repulsive contribution to the chemical potential while the energetic or attractive contribution is reasonably well represented.