Simulation of Liquid and Supercritical Hydrogen Sulfide and of Alkali Ions in the Pure and Aqueous Liquid.

A polarizable model for hydrogen sulfide (H2S) is optimized based on the experimental properties of the monomer and of the bulk liquid. The model is characterized by rigid SH bonds but flexible HSH angle and the polarizability is based on the Drude oscillator model. Bonded parameters and atomic charges are based on the experimental properties of the gaseous monomer. Atomic Lennard-Jones (LJ) parameters are adjusted based on the density of H2S around the critical point (in the temperature range 363-393 K and pressure range 8.023-10.013 MPa). The model gives binding energies for H2S dimers, trimers, and tetramers in good agreement with ab initio MP2(full)/6-311++G(d,p) results. It shows a liquid structure in very good agreement with neutron diffraction data. The model also gives density, self-diffusion coefficient, heat of vaporization, and dielectric constant of liquid hydrogen sulfide at the normal boiling point in good agreement with experimental data. In addition, the model is transferable to high temperature and pressure conditions, as evidenced from simulations up to 542.2 K and 40 MPa. The model is used in combination with the SWM4-NDP water model, with LJ parameters between the S and O atoms adjusted to reproduce the experimental hydration free energy of H2S. Simulations suggest that, in its first solvation shell, a single H2O molecule is solvated by 10 H2S molecules while a single H2S molecule is solvated by 20.5 H2O molecules. Pair-specific LJ parameters between alkali ions (Li(+), Na(+), K(+), Rb(+), Cs(+)) and the S atom are adjusted to reproduce ab initio binding energies of the ion-H2S pairs at the CCSD(T) level. Simulations based on these parameters show that alkali ions have higher coordination numbers and lower solvation free energies in liquid H2S than in liquid water or liquid ammonia. The model is also used to investigate the preferential solvation of the ions in aqueous solutions with a 10% H2S mole fraction. Results show that the ions are preferentially solvated by water in their first solvation shell but have no significant selectivity to either ligands in their second shells.