Chemical potential prediction in realistic fluid models with scaled particle theory

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A procedure frequently proposed in the literature for calculating chemical potentials relies on the Kirkwood charging process. Numerical problems associated with coupling large repulsive forces can be avoided by estimating the contribution to the chemical potential due to these forces with scaled particle theory. The contribution due to soft repulsive forces and attractive forces can be calculated with the Kirkwood charging process using distribution functions for a test particle obtained from integral equation theories. We have used the accurate HMSA theory of Zerah and Hansen to provide distribution functions in mixtures of Lennard-Jones molecules, and we have used the PYP theory of Lee to scale the distribution functions over the charging process. The theory provides accurate estimates of chemical potentials over a range of densities from gas-like to liquid-like when the reduced temperature kT/ɛ is greater than 2. Accurate results for excess free energy changes of mixing are also obtained at these conditions. At lower temperatures accurate results are obtained for low to moderate reduced densities (ρσ 3⩽0.5).