Probing the Effect of Side Alkyl Chain Length on the Structural and Dynamical Micro-heterogeneities in Dicationic Ionic Liquids.

Abstract

The molecular dynamics simulations and Voronoi tessellation analysis of two dicationic ionic liquids (DILs) including [C5(mim)2][NTf2]2 and [C5(mim)2C4][NTf2]2 have been carried out to investigate the effects of side alkyl chain length on the structural and dynamical micro-heterogeneity of these DILs. Radial distribution functions (RDFs), spatial distribution functions (SDFs), and also neighborhood analysis of ions have been calculated to determine the arrangement of the nearest neighboring ions. To better understand the hydrogen-bonding network, microstructures, inter- and intramolecular orientations of ions in the studied DILs, different kinds of combined distribution functions (CDFs) were computed and analyzed. Also, qualitative and quantitative analyses of the structural heterogeneity were explored through total/partial structure factors, heterogeneity order parameters (HOPs), and domain analysis from Voronoi tessellation. The results showed that the side alkyl chains in DILs have significant effects on their micro-organizations in such a way that [C5(mim)2C4][NTf2]2 with longer side chains has more microstructural heterogeneity than [C5(mim)2][NTf2]2 where the linkage alkyl chain is the same in both of them. Furthermore, to shed light on the dynamical heterogeneity, ion pair, ion cage, and hydrogen-bond stabilities and also the reorientation dynamics of ions have been investigated. Results demonstrated that local dynamics differences originate from local structural heterogeneity.