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

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.