The influence of the permanent dipole and quadrupole moments in water on the thermodynamic characteristics of the hydrophobic effect

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Abstract
A thermodynamic perturbation theory that derives from the work of Gray, Gubbins, and Stell is used to analyze the influence of the permanent dipole and quadrupole moments in water on a variety of thermodynamic functions that characterize hydrophobic solvation and hydrophobic interactions. The model used for water was a generalized Stockmayer potential. Experimental values were used for the dipole moment and for the three components of the quadrupole tensor of water, and the parameters for the Lennard-Jones part of this potential were taken from Finney et al.’s polarizable electropole model for water. This potential function involves reduced dipole and quadrupole moments that are shown to be within the range of validity of Stell’s Padé approximant. Induced moments were not considered. The solute–solute and solute–solvent pair potential functions were taken to be the same as the Lennard-Jones part of the solvent–solvent pair potential function. The effects of three types of anisotropies, namely, dipole forces only, generalized quadrupole forces only, dipole–dipole plus generalized quadrupole–generalized quadrupole plus dipole–generalized quadrupole forces, are considered separately. Values obtained for the Henry’s law constant, the heat of solution, the partial molal volume, and the partial molal heat capacity, at infinite dilution, are compared with ranges of experimental values for these quantities for the rare gases in water. The closeness of our results to those data, for our solvent with all the anisotropies simultaneously turned on, is, with the exception of the partial molal volume, remarkable good. The reason for this one failure is discussed. Most surprising perhaps was the result, again for the solvent with the most complicated anisotropy, that it was possible, over a restricted temperature range, to mimic the drop in the osmotic second virial coefficient that is found in real aqueous systems at ordinary temperatures. A physical explanation for this behavior is given.

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