Solvation of Hydrogen Sulfide in Liquid Water and at the Water-Vapor Interface Using a Polarizable Force Field

Molecular dynamics (MD) simulations using the Drude polarizable force field are used to study the solution and interfacial properties of hydrogen sulfide (H2S) in water. Pairwise H2O-H2S Lennard-Jones interactions were optimized to the experimental H2S gas solubility at 298 K. These parameters yield hydration free energies and diffusion coefficients for H2S that are in good agreement with the experiment over 273-323 K and 298-368 K, respectively. H2S is sparingly soluble in water, with a Delta G(hydr)degrees of -0.5 kcal mol(-1). The free energy perturbation (FEP) calculations and analysis of the radial distribution functions show that H2S has limited hydrogen bonding and electrostatic interactions with the water solvent and generally behaves like a hydrophobic solute. These features were confirmed by ab initio MD simulations. Umbrella sampling simulations were used to calculate the free energy profile of the transition of H2S across the water-vapor interface, which showed that H2S has a sizable surface excess, with a Delta G(surf) of 1.3 kcal mol(-1). This high surface excess is consistent with our calculations of the surface tension, which decreases to 20 dyn cm(-1) under high densities of H2S (g). The dipole moment of H2S increases from its gas phase value of 0.98 to 1.25 D in bulk water as it moves across the interface. Adsorbed H2S tends to be oriented perpendicular to the interface, with the sulfur atom pointing toward the vapor phase.