ENTROPIC CONTRIBUTIONS TO FREE-ENERGIES OF SOLVATION

Two alternative ways to treat the entropy of solution in the molecular thermodynamics of liquid-phase solutions have proved useful under var ious circumstances. The first is the ideal solution theory in which th e entropic effects are the same as for mixing ideal gases. The second is the theory of Flory and Huggins which is most appropriate for nondi lute solutions of chain molecules in small solvents. The two theories are compared for their ability to describe solutions of alkanes,both s traight-chain and branched, in aqueous solution. The alkanes provide a n especially appropriate testing field because electrostatic effects a re minimal, and it is reasonable to assume that the solvation free ene rgy consists almost entirely of entropic contributions and first-hydra tion-shell effects. The tests show that the experimental data are bett er correlated by the ideal solution theory than by adding an explicit volume-dependent contribution from Flory-Huggins theory as suggested b y Sharp, Nicholls, Friedman, and Honig. We disagree with a recent reco mmendation that one should use Flory-Huggins theory to change the defi nition of the experimental free energy of transfer of a solute from th e gas phase into solution and also with the suggestion that there is a general volume contribution to the free energy of solvation that is w ell modeled by the Flory-Kuggins term.