Molecular investigation of SO2 gas absorption by ionic liquids: Effects of anion type

Absorption of the SO2 gas by five ionic liquids (ILs) containing the 1-ethyle-3-methyl imidazole cation and X- (PF, BETI, NOT, CI- and Br-) anions was studied using molecular dynamic simulations. These 1Ls belong to two different groups, plastic crystals and liquid crystals. The influence of gas on the structure, behavior and physicochemical properties of the ILs was investigated through radial distribution functions (RDFs), mean-square displacement (MSD), coordination numbers, diffusion coefficients, transport numbers, ionic conductivity, density and molar specific heat capacity at constant volume. Shielding effect of ionic interactions by SO2, through weakening the anion-anion interaction, breaks down the structural ordering of the ILs. More microscopic understanding was provided for structure of IL/S02 mixtures by introducing two physical parameters, charge per unit of area of each anion (8) and available space around anion (pm.). The results show %that for a given anion, the higher values for two parameters lead to more ability of the IL to absorb the SO2 gas. We also found an inverse relationship between the size of the ionic aggregates around each ion and its diffusion coefficient. The diffusion coefficient of cation in the pure ILs and IL/S02 gas mixtures was greater than that of the anions and much less than that of the SO2 molecules. In addition, in comparison with purelLs, the presence of SO2 leads to an increase in the diffusion coefficients, conductivity, density and heat capacity of the ionic species of the IL/S02 gas mixtures. 2014 Elsevier B.V. All rights reserved.