Many-body interactions in liquid methanol and its liquid/vapor interface: A molecular dynamics study

Abstract

Many-body interactions in liquid methanol and its liquid/vapor interface are evaluated using classical molecular dynamics techniques. The methanol molecule carries a molecular polarizability to account for induction energies and forces. The computed dipole moment for the methanol molecule changed from 1.7 to 2.8 D, respectively, from the vapor to the liquid phases. This result indicated that there are significant many-body interactions in this complex molecular system. The computed average molecular dipole moment in liquid methanol at room temperature is in good agreement with experimental measurements. The computed average dipole moments of methanol molecules near the interface are close to their gas phase values, while methanol molecules far from the interface have dipole moments corresponding to their bulk values. The structural and thermodynamic properties of the liquid methanol as well as the surface tension of its liquid/vapor interface are in good agreement with the experiments, demonstrating the high quality of our potential model and simulation approaches. A constrained molecular dynamics technique was used to investigate the transport mechanism of a methanol molecule across the methanol liquid/vapor interface. The computed transfer free energy changed gradually as the methanol molecule approached the Gibbs dividing surface, and it crossed the interface with no substantial minimum free energy. The computed solvation free energy of the methanol molecule in liquid methanol estimated from the free energy profile (4.25 kcal/mol) is in good agreement with the corresponding experimental measurement (4.89 kcal/mol).