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

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Solution Chemistry

Background:

• Apolar molecules lack dipole moments in the gas phase.
• In aqueous environments, apolar molecules can develop induced dipole moments due to water's electric fields.

Purpose of the Study:

• To investigate the role of induced dipole moments in hydrophobic interactions.
• To quantify the contribution of induced-dipole-induced-dipole interactions to methane aggregation in water.

Main Methods:

• Computational modeling of apolar molecules (methane) in an aqueous environment.
• Calculation of induced dipole moments and their interaction energies.
• Analysis of interaction magnitudes across various molecular orientations and separations.

Main Results:

• Induced-dipole-induced-dipole interactions can reach magnitudes of 1 kcal/mol for specific methane pair orientations.
• This interaction energy is comparable to the free energy of methane aggregation in water.
• Averaged over all conformations at fixed separations, the interaction becomes insignificant (<0.01 kcal/mol), except possibly at very short distances.

Conclusions:

• Induced dipole moments play a very weak role in the overall hydrophobic interaction driving methane aggregation in water.
• The significant interaction observed in specific orientations is averaged out by the dynamic nature of molecules in solution.
• Hydrophobic interactions are likely governed by factors other than induced dipole-dipole interactions at typical separations.