On the role of the definition of potential models in Gibbs ensemble phase equilibria simulations of the H2S-pentane mixture

Effect of partial charge parametrization on the fluid phase behavior of hydrogen sulfide

The effect of partial charge parametrization on the fluid phase behavior of hydrogen sulfide is investigated with grand canonical histogram reweighting Monte Carlo simulations. Four potential models, based on a Lennard-Jones + point charge functional form, are developed. It is shown that Lennard-Jones parameters can be tuned such that partial charges for the sulfur atom in the range -0.40 < q(s) < -0.252 will lead to an accurate reproduction of experimental vapor-liquid equilibria. Each of the parameter sets developed in this work are used to predict the pressure composition behavior H2S-n-pentane at 377.6 K. While the mixture calculation provides a means of reducing the number of candidate parameter sets, multiple parameter sets were found to yield an excellent reproduction of both the pure component and mixture phase behavior.

Transferable Potentials for Phase Equilibria. 8. United-Atom Description for Thiols, Sulfides, Disulfides, and Thiophene

An extension of the transferable potentials for phase equilibria united-atom (TraPPE-UA) force field to thiol, sulfide, and disulfide functionalities and thiophene is presented. In the TraPPE-UA force field, nonbonded interactions are governed by a Lennard-Jones plus fixed point charge functional form. Partial charges are determined through a CHELPG analysis of electrostatic potential energy surfaces derived from ab initio calculations at the HF/6-31g+(d,p) level. The Lennard-Jones well depth and size parameters for four new interaction sites, S (thiols), S(sulfides), S(disulfides), and S(thiophene), were determined by fitting simulation data to pure-component vapor-equilibrium data for methanethiol, dimethyl sulfide, dimethyl disulfide, and thiophene, respectively. Configurational-bias Monte Carlo simulations in the grand canonical ensemble combined with histogram-reweighting methods were used to calculate the vapor-liquid coexistence curves for methanethiol, ethanethiol, 2-methyl-1-propanethiol, 2-methyl-2-propanethiol, 2-butanethiol, pentanethiol, octanethiol, dimethyl sulfide, diethyl sulfide, ethylmethyl sulfide, dimethyl disulfide, diethyl disulfide, and thiophene. Excellent agreement with experiment is achieved, with unsigned errors of less than 1% for saturated liquid densities and less than 3% for critical temperatures. The normal boiling points were predicted to within 1% of experiment in most cases, although for certain molecules (pentanethiol) deviations as large as 5% were found. Additional calculations were performed to determine the pressure-composition behavior of ethanethiol+n-butane at 373.15 K and the temperature-composition behavior of 1-propanethiol+n-hexane at 1.01 bar. In each case, a good reproduction of experimental vapor-liquid equilibrium separation factors is achieved; both of the coexistence curves are somewhat shifted because of overprediction of the pure-component vapor pressures.