An off-lattice theory of solvation: extension of the Flory χ parameter into continuum space

The off-lattice theory of solvation presented here is a generalization of the Flory–Huggins lattice theory of solvation using the fused hard-sphere system as a reference. The contact free energy, a generalization of the Flory χ, is formulated by a perturbation approach. The perturbation term contains the bulk-compression contribution in addition to the contact free energy, and the bulk-compression is shown to compensate with the hard-sphere pressure of the reference system. The contact free energy is shown to be a local thermodynamical quantity and is related to the solvent accessible surface area.