AN ESTIMATE OF SHAPE-RELATED CONTRIBUTIONS TO HYDROPHOBIC GIBBS ENERGIES

It is shown that Ben-Naim's quantities for the solvation of hydrocarbo ns in their own liquids may be calculated from Pitzer's extension of t he law of corresponding states. This calculation involves the dissecti on of solvation quantities into two terms, which may be interpreted as corresponding to (1) the value of the solvation quantity for a hypoth etical fluid of simple spherical molecules, with the same critical tem perature and pressure as the actual hydrocarbon under consideration, a nd (2) the contributions that arise as a result of the deviation of th e intermolecular potential function from that of simple spherical mole cules (''shape'' contributions). The values obtained for the latter co ntributions support that shape effects may be significant with regard to hydrophobic Gibbs energies; for instance, the value for the strengt h of the hyprophobic effect derived from liquid alkane dissolution wou ld increase from about 120 to about 170 J/mol Angstrom(2) upon correct ing for the calculated shape-entropic contributions (for comparison, w e note that correcting for the whole solvation entropy of the liquid h ydrocarbon would raise the value to about 200 J/mol Angstrom(2)).