PNNL

Abstract

Constrained molecular dynamics techniques were used to investigate the mechanism of the transfer of water and ammonia molecules from the gas phase to the liquid phase of water. The computed potentials of mean force or transfer free energies were nearly constant when the solute was more than several angstrom from the Gibbs dividing surface and decreased with no substantial minimum free energy as the solute molecules crossed the interface from the vapor to liquid phase. The computed free energy of solvation for water, estimated from the potential of mean force, was in excellent agreement with the experimental measurement while the corresponding computed solvation free energy for the ammonia molecule somewhat over estimated the experimental value. Possible reasons for the discrepancy are discussed.