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

Reddy, Th Dhileep Nagi and Mallik, Bhabani Shankar (2020) Structure and Conformational Response of Pure and Lithium-Doped Ionic Liquids to Pressure Alterations From Molecular Dynamics Simulations. The Journal of Physical Chemistry B. ISSN 1520-6106 (In Press)

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Abstract

The response of ions in ionic liquids(ILs) to elevated external pressure facilitates the induced structural changes that give a chance to understand change in chemical and physical properties due to 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 ionic liquids (ILs) N-alkyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr1A–TFSI, A=3,6,9) and their mixtures with 30 mol% lithium bis(trifluoromethanesulfonyl)imide (Li–TFSI). The microheterogeneity in these systems was studied by 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 compliments well with the main features of the experimental results. In fact, The pre-peak, 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 towards the addition of Li-salt. The mesoscopic structure of these ILs is affected by high pressure. The height of pre-peak is diminished significantly with the application of high pressure. This decrease in the height of the pre-peak 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-ion causes the absence of charge ordering peak, which is occured 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 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 anion slightly dominates the trans configuration. The conformational change of anion from trans to cis occurs when pressure changes from a low to high value.

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