Perturbed hard-body fluid analysis of the global effects of solvation on conformational thermodynamics

The molecular perturbed hard-body fluid (PHF) model is used to extract repulsive (cavity formation) and attractive (cohesive) solvent excess perturbations to the conformational equilibria of 1,2-dichloroethane (DCE) and trans-1,2-dichlorocyclohexane (T12D) dissolved in diethyl ether. This represents the first combined theoretical/experimental global analysis of the complete set of solvent excess thermodynamic functions (ΔGx, ΔSx, ΔVx, ΔHx, ΔUx, and ΔAx) for any chemical process. Cavity formation is modeled by treating molecules either as hard spheres or anisotropic hard bodies while cohesive interactions are treated using the van der Waals mean field approximation. The results are used to test PHF approximations and elucidate solvent contributions to each thermodynamic function. Cohesive interactions are invariably found to favor the more polar isomer while cavity formation favors the more polar isomer of DCE but the less polar isomer of T12D. This difference is shown to be consistent with independent excluded volume calculations and to explain the stronger pressure dependence of the excess isomerization volume of DCE than T12D.