Entropy/enthalpy compensation: hydrophobic effect, micelles and protein complexes

Molecular interpretations are here presented of the hydrophobic effect, which is the cause of the low solubility of apolar substances in water. The solubilization process of substances such as the noble gases consists in the formation of a cavity in the solvent with expulsion of nw water molecules. The process is associated to an entropy/enthalpy (S/H) compensation linearly dependent upon the temperature. The observed enthalpy ΔHapp, either determined calorimetrically or by van’t Hoff equation, shows ΔCp,app ≠ 0 and positive. We set ΔCp,app = nwCp,w where Cp,w is the isobaric heat capacity of water. The number nw (nw > 0) of relaxed water molecules is proportional to the size of the solute molecule and hence of the cavity. The term nwCp,wT is actually an entropy term, which compensates for part of the reaction enthalpy (ΔH0 0) of water molecules. The presence of polar groups and/or charges in the solute molecule, on the other hand, exerts on the water molecules the same action as that produced by micellisation (nw