AN ESTIMATE OF SHAPE-RELATED CONTRIBUTIONS TO HYDROPHOBIC GIBBS ENERGIES

It is shown that Ben-Naim's quantities for the solvation of hydrocarbons in their own liquids may be calculated from Pitzer's extension of the law of corresponding states. This calculation involves the dissection of solvation quantities into two terms, which may be interpreted as corresponding to (1) the value of the solvation quantity for a hypothetical fluid of simple spherical molecules, with the same critical temperature and pressure as the actual hydrocarbon under consideration, and (2) the contributions that arise as a result of the deviation of the intermolecular potential function from that of simple spherical molecules (''shape'' contributions). The values obtained for the latter contributions support that shape effects may be significant with regard to hydrophobic Gibbs energies; for instance, the value for the strength of the hyprophobic effect derived from liquid alkane dissolution would increase from about 120 to about 170 J/mol Angstrom(2) upon correcting for the calculated shape-entropic contributions (for comparison, we note that correcting for the whole solvation entropy of the liquid hydrocarbon would raise the value to about 200 J/mol Angstrom(2)).