The hydrophobic propensity of water toward amphiprotic solutes: Prediction and molecular origin of the aqueous solubility of aliphatic alcohols

A quantitative expression of the hydrophobic effect for amphiphilic solutes in water is developed in the frame of the nonergodic thermodynamics of mobile order in hydrogen-bonded liquids. In the case of aliphatic alcohols, the new expression leads to reduction of the hydrophobic propensity of water with respect to that exerted towards substances with no hydrogen bonding capacity. The reduction originates from the possible insertion of the alcohol molecules in the weakest hydrogen bond chain of water; hence, strengthening the hydrogen bonding network of water. Combined with the previous solubility model derived from mobile order thermodynamics, the new expression allows correct predictions of the solubility of 86 liquid and solid branched- and straight-chain alcohols in water at 25 degrees C, and provides better understanding of their behavior in aqueous solution. The model is furthermore applied to the estimation of the aqueous solubility of 12 monohydroxysteroids.