FREE-ENERGIES OF SOLUTION OF RARE-GASES AND ALKANES IN WATER AND NON-AQUEOUS SOLVENTS - QUANTITATIVE ASSESSMENT OF THE HYDROPHOBIC EFFECT

Standard free energies of solution of the rare gases and alkanes in water and in over 16 nonaqueous solvents are tabulated. It is shown that it is difficult to reach unambiguous conclusions about the existence of the hydrophobic effect from plots of ΔGs° against solute radius or solute volume, partly because of inherent uncertainties in the values of the solute radius. It is found that ΔGs° values for both rare gases and alkanes in all nonaqueous solvents are well correlated through linear equations: ΔGs°(solvent) = mΔGs°(benzene) + c and ΔGs°(solvent) = IR + d, where R is a solute parameter related to solute radius. When applied to ΔGs° values for solution in water, these equations show conclusively that the solution of alkanes (but not rare gases) in water is quite anomalous. The -CH2- increment for partition of n-alkanes between hexane and water is 0.92 kcal mol-1in favor of hexane, and can be separated into a favorable gas → hexane contribution of 0.74 kcal mol-1and an unfavorable gas → water contribution of 0.18 kcal mol-1. The latter is further dissected into a true (unfavorable) hydrophobic contribution of 0.54 kcal mol-1and a favorable normal solvent effect of 0.36 kcal mol-1. Methods for the estimation and prediction of ΔGs° values in nonaqueous solvents are discussed. © 1979, American Chemical Society. All rights reserved.