Does a pair of methane molecules aggregate in water?

Area of Science:

• Computational chemistry
• Physical chemistry
• Molecular modeling

Background:

• Understanding methane-water interactions is crucial for various chemical and geological processes.
• Accurate modeling of these interactions requires precise force fields and simulation techniques.

Purpose of the Study:

• To investigate the potential of mean force (PMF) between two methane molecules in aqueous solutions using ab initio molecular dynamics.
• To compare simulation results with experimental data and other computational methods.

Main Methods:

• Ab initio molecular dynamics (MD) simulations were employed.
• Polarizable force fields were used for methane-CH4, water-H2O, and methane-water interactions.
• Potential of mean force (PMF) between methane pairs was calculated.

Main Results:

• The calculated PMF showed qualitative agreement with empirical force field simulations but differed from DFT-BOMD and MC simulations.
• The ab initio MD results for the PMF global minimum depth better matched experimental estimates.
• Water molecules were found to influence methane-methane interactions, causing both attraction and repulsion depending on configuration.

Conclusions:

• Ab initio MD simulations provide a more accurate description of methane-methane interactions in water compared to DFT-BOMD, likely due to better handling of dispersion forces.
• Water's influence on methane pair interactions is complex, with average configurations favoring separation by a water molecule.
• The structure of water around methane solutes is similar to bulk water, without enhanced tetrahedral ordering.