Heterogeneity in the microstructure and dynamics of tetraalkylammonium hydroxide ionic liquids: insight from classical molecular dynamics simulations and Voronoi tessellation analysis.

Microscopic structural and dynamic heterogeneities were investigated for three ionic liquids (ILs), namely tetraethylammonium hydroxide (TEAH), tetrapropylammonium hydroxide (TPAH), and tetrabutylammonium hydroxide (TBAH), by employing classical molecular dynamics (MD) simulations. Structural heterogeneity was explored through radial distribution functions, domain analysis from Voronoi tessellation, spatial distribution functions, combined distribution functions, and structure factors. Radial distribution functions reveal that TEAH has a slightly different structure than TPAH and TBAH. The domain analysis shows that the three ILs have structural heterogeneity. The cations form continuous domains and the anions form discreet domains. The anions are trapped in the voids formed by the cationic domains. When the cation has sufficiently longer alkyl chains, the polar domains become discontinuous. The total structure factor reveals that a pre-peak is absent in TEAH due to a short alkyl chain. The cation head-anion partial structure factor further confirms this phenomenon. Polar and non-polar domains can be better distinguished in the ILs with longer alkyl chains. This distinctive structural behaviour leads to dynamic heterogeneity, which plays an important role in the applications related to ion transportation. To shed light on dynamic heterogeneity, we have calculated the mean square displacements, vibrational density of states, van Hove correlation, and non-Gaussian parameter. The diffusion of ions is lowest in TPAH among all the ILs. The dynamics are highly sluggish in these ILs. The peaks in the low frequency vibrational density of states undergo redshift with an increase in the alkyl chain length. The van Hove correlation functions and non-Gaussian parameter suggest the presence of dynamic heterogeneity in all three ILs. Dynamic heterogeneity is discussed through van Hove correlation functions and the non-Gaussian (NG) parameter was calculated to account for the deviation from Gaussian behaviour. The NG parameter suggests that TEAH has different dynamics than TPAH and TBAH.