Characterization of nano-domains in ionic liquids with molecular simulations

Nano-domains in ionic liquids (IL) that involve cations with long attached alkyl chains are characterized by analyzing their structural and energetic features. Nitrate anions that are paired with guanidinium-based cations, to which alkyl chains of increasing length from butyl to decyl are attached, are simulated with molecular dynamics simulations. All simulations utilize our previously developed force field for guanidinium-based ILs, which is based on the charge distribution in the actual liquid. Even though a substantial amount of positive charge of +0.2 e is distributed over the long alkyl chains, their aggregation is observed, especially in the IL that involves the longest alkyl chains. These aliphatic nano-domains are stabilized by attractive van der Waals interactions between the alkyl chains that compensate their mutual electrostatic repulsion. Also, within these domains the charged alkyl chains induce a positive electrostatic potential of significant strength. The ring moieties and guanidinium group of the cations, however, are tightly coordinated with the anions and both together form the second type of domain. Our simulations verify the structure of previously reported nano-domains in imidazolium-based ILs by using the more realistic liquid phase charge distribution for the simulations. Furthermore, the first observation of nano-domain formation in a non-imidazolium-based IL demonstrates that the aggregation of alkyl chains is a general phenomenon in ILs that occurs when alkyl chains of sufficient length are involved. An increase of the alkyl chain length fundamentally changes the nano-structure of an IL and thereby its properties, which can be exploited for example to modify the solvation properties of ILs.