  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 the solvation and
association of apolar solutes in water. While local hydrogen bonding in-
teractions dominate hydrophobic effects involving small solutes, longer ranged
electrostatic and dis- persion 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. Unbalanced long ranged forces acting on interfacial water molecules
are also important in hydrophobic association, illustrated here by analysis of
the association of model methane and buckminsterfullerene solutes.
