Unraveling the structural and dynamic heterogeneities of hydroxyl-functionalized di-cationic ionic liquids (HFDILs): An integrated ab initio and molecular dynamics approach

Abstract

This study comprehensively investigates the effects of hydroxyl groups in side chains and cation symmetry on intra- and inter-molecular structures, dynamics, and thermodynamic properties of two hydroxyl-functionalized di-cationic ionic liquids (HFDILs). Using a combination of molecular dynamics (MD) simulations and quantum chemistry (QC) calculations, we compare the influence of these effects on various properties with corresponding non-hydroxyl DILs reported in previous works. The structure of HFDILs is explored using angle distribution, radial distribution, spatial distribution, and combined distribution functions. Also, the averaged noncovalent interaction (aNCI) analysis is used to characterize the weak noncovalent interactions in fluctuating environment of HFDILs. Results showed that the oxygen atoms of anion have the strongest interaction with the hydrogen atom of hydroxyl group, followed by the hydrogen atom at the top of the imidazolium ring. Domain analysis and Voronoi calculations are performed to investigate the number of polar and non-polar subunits, revealing that symmetric HFDILs with two hydroxyl groups in their side chains have greater structural heterogeneity than asymmetric ones with only one hydroxyl group due to the larger accumulation of non-polar subunits. To investigate dynamical heterogeneity, we calculate the non-Gaussian parameter, van Hove correlation, hydrogen bond, ion pair, ion cage, and reorientation dynamics. Results indicated that symmetric HFDILs have slower dynamics, greater dynamic heterogeneity, and more stability than asymmetric ones. Finally, the structure and strength of ion triplet interactions are examined using quantum mechanical methods, revealing that symmetric HFDILs are more stable than asymmetric ones. This study provides valuable information for understanding DILs and their potential applications in various fields.