(520c) Dissecting Hydrophobic Hydration and Association

We use appropriately defined short ranged reference models of liquid water to clarify the different roles local hydrogen bonding, van der Waals attractions, and long ranged electrostatic interactions play in determining the structure and thermodynamics of the solvation and association of apolar solutes in water. While local hydrogen bonding interactions dominate hydrophobic effects involving small solutes, longer ranged electrostatic and dispersion interactions are found to be increasingly important in the description of interfacial structure around large solutes. The hydrogen bond network sets the solute length scale at which a crossover in solvation behavior between these small and large length scale regimes is observed. This network is also found to be crucial for the existence of the phenomena of entropy convergence. Unbalanced long ranged forces acting on interfacial water molecules are also important in hydrophobic association, as will be illustrated here through analysis of the association of model methane and buckminsterfullerene solutes.