Comparing two tetraalkylammonium ionic liquids. I. Liquid phase structure

X-ray scattering experiments at room temperature were performed for the ionic liquids n-butyl-trimethylammonium bis(trifluoromethanesulfonyl)imide, [N₁₁₁₄][NTf₂], and methyl-tributylammonium bis(trifluoromethanesulfonyl)imide, [N₁₄₄₄][NTf₂]. The peak in the diffraction data characteristic of charge ordering in [N₁₄₄₄][NTf₂] is shifted to longer distances in comparison to [N₁₁₁₄][NTf₂], but the peak characteristic of short-range correlations is shifted in [N₁₄₄₄][NTf₂] to shorter distances. Molecular dynamics (MD) simulations were performed for these ionic liquids using force fields available from the literature, although with new sets of partial charges for [N₁₁₁₄]⁺ and [N₁₄₄₄]⁺ proposed in this work. The shifting of charge and adjacency peaks to opposite directions in these ionic liquids was found in the static structure factor, S(k), calculated by MD simulations. Despite differences in cation sizes, the MD simulations unravel that anions are allowed as close to [N₁₄₄₄]⁺ as to [N₁₁₁₄]⁺ because anions are located in between the angle formed by the butyl chains. The more asymmetric molecular structure of the [N₁₁₁₄]⁺ cation implies differences in partial structure factors calculated for atoms belonging to polar or non-polar parts of [N₁₁₁₄][NTf₂], whereas polar and non-polar structure factors are essentially the same in [N₁₄₄₄][NTf₂]. Results of this work shed light on controversies in the literature on the liquid structure of tetraalkylammonium based ionic liquids.