Investigation of the intermolecular voids at the dissolution of CO2 in ionic liquids

Abstract

Empty intermolecular volume in pure liquids and solutions is an important factor determining the mobility of molecules and the ability to dissolve other substances. The investigation of computer models gives insight into the role of intermolecular voids in these processes. In this work, we demonstrate an application of the Voronoi-Delaunay method to describe quantitatively the voids in ionic liquids on the example of the dissolution of carbon dioxide. Molecular dynamics models of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Cnmim][NTf2]) ionic liquids (ILs) with different lengths of the alkyl substituent n (from 2 to 8 carbon atoms in the alkyl chain), as well as their mixtures containing CO2 at different concentrations have been studied. The contributions of the anions and the cations to the empty volume of the pure ILs and the mixtures are studied. It was shown that the increase in the empty volume with the growth of the alkyl substituent length of the cation is mainly determined by the methyl groups of the substituent, while the empty volume related to the anions and the imidazole ring of the cations practically does not change. CO2 molecules add additional empty volume to the mixture, instead of filling available voids preliminarily formed in a pure IL. The apparent molar volume of CO2 molecules in the studied ILs exceeds their intrinsic volume, defined as their Voronoi volume. However, the distribution of the interstitial spheres radii remains practically the same in the pure ILs and in the mixtures. Moreover, the radial distribution of large intermolecular voids (comparable to the size of a CO2 molecule) in pure ILs does not correspond to the real distribution of CO2 molecules in the mixtures.