An Insight into Solubility of H2S in Choline Based Ionic Liquids from Molecular Dynamics Simulation

Abstract

Solvation free energy of six choline based ILs were computed under molecular dynamics (MD) simulation using Bennet Acceptance Ratio (BAR) method. H2S removal from the natural gas can be accomplished by absorption process using a solvent. Degree elimination of gases in liquid can be estimated by excess chemical potential (μex), Henry’s law constant (kH), enthalpy (hex), and entropy (sex). The Optimised Potential Liquids Simulation (OPLS) force fields used for ILs were validated against experimental density in order to get valid results of solvation free energy. The percentage error of the predicted density of six choline based ILs were below than 2.0% which is a good agreement with the previous study. The highest value of kH was 109.23 atm gained by choline thiocyanate [Chl][tcy] and the lowest value of kH was 23.25 atm obtained by choline acetate [Chl][act]. There are many features that influence the solubility of H2S for example length of alkyl chain, free volume effect, molar density, type of anions and hydrogen bonding interaction. In order to understand the behaviour of H2S molecules in ILs system, the radial distribution function (RDF) of H2S-ILs and RDF for specific atom were analysed. Basically, the shorter the alkyl chain, increase in molar density of ILs, reduce the free volume thus provide less space for H2S to occupy and decrease insolubility.