Molecular dynamics investigation of electrostatic properties of pyrrolidinium cation based ionic liquids near electrified carbon electrodes

We performed a series of molecular dynamics simulations to investigate the impact of cation tail variation on the electrostatic properties of a set of pyrrolidinium ionic liquids (ILs) enclosed between neutral and charged model carbon electrodes. In particular, we have focused on 1-alkyl-1-methylpyrrolidinium bis(trifiuoromethylsulfonyl) imide ([Pyrr(1,n)][NTf2]) ILs wherein the cationic alkyl tail is varied from butyl to octyl groups for linear mutations, cyclohexylmethyl (ChxMe) group for cyclic mutation, and ethylhexyl (EtHx) group for branched mutation. The properties studied include charge density (rho(c)) and electrostatic potential (U(z)) profiles, and differential capacitance (C-d) as a function of applied electrode charge density. All the ILs under study render negative potential at zero charge (PZC) which signifies the specific and effective adsorption of negatively charged atoms of anions near the neutral electrodes. Owing to different screening capabilities of the constituent ions in the ILs, the electrostatic potential profiles are observed to be different near the negative and positive electrodes. The oscillations in the electrostatic potential profile near the charged electrodes resemble the oscillations observed in the total charge density profile of the respective IL The differential capacitance versus electrode potential curve for the IL having shortest cationic tail, [Pyrr(1,4)][NTf2], is observed to be slightly deviated from bell shape. However, transformation towards proper bell shape with linear increase in the cationic tail length or by introducing cyclic or branched group is observed. From the charge density profiles, we observe two to three electrical double-layers that are persistent up to 1 to 1.5 nm from the respective electrodes. The analysis of cationic head, cationic tail and ionic number density profiles provides a clearer picture of the interfacial layers. We observe that both the anions and cationic tails are able to approach the neutral electrode up to its closest proximity. Near the charged electrodes the counter-ions are found to be closer to the electrode and the co-ions are pushed to the next layer. (C) 2019 Elsevier B.V. All rights reserved.