Integral equation and Monte Carlo study on hydrophobic effects: Size dependence of apolar solutes on solute-solute interactions and tructures of water

Hydrophobic effects for various solute sizes have been investigated by the RISM equation and Monte Carlo simulation. It is shown that the association of solute molecules is enhanced by the hydrogen bonds among water molecules. The degree of association depends on the solute size. For a large apolar solute, the structure of water is enhanced, resulting in the exothermic hydration and the negative entropy change in hydration. For a solute comparable with the water size, the hydrogen bonds among water molecules are strengthened but the net hydrogen bonds number does not increase. These results are also confirmed by the analysis of the geometric patterns formed by the hydrogen bond network. It is shown that the ordered structure of water in the solution of the large solutes gives rise to the larger enthalpy of the hydration, and thus they are more soluble in water than the smaller solutes.