Survey of Experimental Evidence on Hydrophobic Interaction

Perhaps the simplest indication that nonpolar solutes such as argon or methane dislike the aqueous environment is found in the values of the standard free energy of transfer of these solutes from a nonaqueous solvent to water. Consider, for example, the standard free energy of transfer of methane from ethanol to water at 10°C (r, P constant) 

What does this quantity mean In Chapter 4, we gave three possible interpretations of this quantity (1) the free energy of transferring one mole of methane from ethanol to water, at T and P constant, and at (water) = (alcohol), i.e., the molar concentrations of methane being the same in the two phases (2) the free energy of transferring one S molecule from a fixed position in ethanol to a fixed position in water (multipled by Avo-gadro s number) (3) for S in equilibrium between the two phases. 

Relation (8.7), together with the information in (8.6), can be interpreted in terms of the extent to which the solute molecules favor one environment over the other. Such an interpretation has been used widely (e.g., Kauzmann, 1959 Wishnia, 1962, 1963 Wishnia and Finder, 1964, 1966 Nozaki and Tanford, 1963, 1965 Wetlaufer et aL, 1964 Barone et aL, 1966). 

We have already stressed the fact that the above process does not provide any direct indication of the extent of adherence between two solutes in an aqueous environment. In Section 8.6, we present an example in which such an inference can indeed be misleading. 

The second experimental quantity that, in principle, is suitable for the study of HI, in the sense of attraction between a pair of particles, is the dimerization constant, or the corresponding standard free energy of dimerization of nonelectrolytes in various solvents. 

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