Contribution of the Induced-Dipole Interaction to Methane Aggregation in Water

Apolar molecules in the gas phase have no dipole moments. However, when placed in an aqueous environment, they acquire a dipole moment induced by the electric fields of the surrounding water molecules. Could these induced dipole moments, not present in the gas phase but present in solution, play an important role in the hydrophobic interaction between two apolar molecules? In particular, for two methane molecules, our results show that the interaction between the induced-dipole moments only very weakly plays a role in the aggregation of a pair of methane molecules in water. The induced-dipole-induced-dipole interaction has a magnitude as large as 1 kcal/mol for certain mutual orientations of the induced dipole moments, which is larger than the magnitude of the free energy of aggregation of the methane solutes in water. However, when averaged over all physically occurring conformations for a fixed intersolute separation, this interaction averages to an insignificant value (magnitude less than 0.01 kcal/mol) except, possibly, for some very short intermolecular separation.