Structure and Conformational Response of Pure and Lithium-Doped Ionic Liquids to Pressure Alterations from Molecular Dynamics Simulations

The response of ions in ionic liquids (ILs) to elevated external pressure facilitates the induced structural changes that give a chance to understand the change in chemical and physical properties because of perturbation. Employing classical molecular dynamics simulations, we report various structural properties of IL mixtures with Li-salt under varying pressure. Here, we aim to explore the effect of pressure on three ILs N-alkyl-N-methyl-pyrrolidinium bis­(trifluoromethanesulfonyl)­imide (Pyr_1A-TFSI, A = 3, 6, 9) and their mixtures with 30 mol % lithium bis­(trifluoromethanesulfonyl)­imide (Li-TFSI). The microheterogeneity in these systems was explored by investigating the total and partial structure factors. Intramolecular distribution functions were calculated to study the conformational changes. The distribution of clusters of alkyl chains provides information about aggregation behavior. The calculated total structure factors compliment well with the main features of the experimental results. The prepeak, charge-ordering peak, and main peaks from simulations are in good agreement with the experimental results. The three pure ILs are structurally similar, which show both polarity and charge-ordering peaks. The addition of Li-salt makes the charge-ordering peak disappear. We find that three ILs respond quite differently to the addition of Li-salt. The mesoscopic structure of these ILs is affected by high pressure. The height of the prepeak is diminished significantly with the application of high pressure. This decrease in the height of the prepeak is due to the reduction of alkyl chain aggregation. This motivated us to calculate the aggregation of the alkyl chain quantitatively. The number of alkyl chains present in a given cluster fades with the rise in pressure. The addition of Li-salt decreases the tendency of alkyl chain aggregation. The presence of lithium-ions causes the absence of the charge-ordering peak, which occurs at around 8.0 nm^–1 in pure ILs. From the partial structure factors, the charge-ordering is observed to be present in Li-containing mixtures. The conformation change in the ionic entity is also observed; the distance between the nitrogen atom and terminal carbon of the alkyl chain in cation decreases with increasing the pressure. When the Li-ion is present in the mixture, the cis configuration of anions slightly dominates the trans configuration. The conformational change of anions from trans to cis occurs when pressure changes from a low to a high value.