Role of salts on the strength of pairwise hydrophobic interaction

The strengthening of pairwise hydrophobic interaction upon salt addition to water is due to the increased number density of aqueous salt solutions, according to scaled particle theory. Two CH4 molecules occupy the same volume both when they are far apart and when they are in contact, however, the contact configuration is favoured because the solvent-excluded volume decreases upon association and there is a substantial gain of translational entropy of solvent molecules. The same decrease in solvent-excluded volume upon association causes a different entropy gain depending on the number density of the solution: the larger the number density, the larger will be the number of solvent particles that gain translational freedom.