Interfacial Structure of Pyrrolidinium Cation Based Ionic Liquids at Charged Carbon Electrodes: The Role of Linear and Nonlinear Alkyl Tails

Carbon electrodes are very significant components in energy storage devices, and it is equally important to understand how electrolytes or similar materials interact with the electrode. In the present molecular dynamics study, we explore the effect of cationic tail alteration by introducing the branched or cyclic alkyl groups or by changing the tail length on the interfacial layering of pyrrolidinium-based ionic liquids (ILs) near carbon electrodes with +/- 16 and +/- 32 mu C cm(-2) charge density. The ILs chosen for this purpose are 1-alkyl-1-methylpyrrolidinium bis (trifluoromethylsulfonyl)imide, P-yrr1,n(+)/NTf2-, with pentyl, heptyl, cyclohexylmethyl (ChxMe), and ethylhexyl (EtHx) cationic tail. To estimate the effects of cation tail modification and increasing electric field, We analyze the number density profiles, orientational order parameters, and dihedral distributions near the electrified interfaces. We observe that, as the applied field is doubled, while the number density of cations near the negatively charged electrode is doubled, the increase in the number density of anions near the positively charged electrode becomes more than double. Analysis of orientational order parameter for vectors in the cationic pyrrolidinium ring and in S-N-S plane of the anion reveals a picture where these ionic species are aligned at an angle indicating that ions are not perfectly parallel or perpendicular to the axis normal to the electrodes with +/- 16 mu C cm(-2) charge density. This picture changes when the charge density is increased to +/- 32 mu C cm(-2), where the alignment of the vectors is perpendicular to the reference axis perpendicular to the electrode. Further, investigation of the distribution of C-S-S-C pseudodihedral angle in the anions shows that the tendency of the anions to procure an orientation in which their oxygen atoms are facing toward the positively charged carbon electrode increases on increasing the applied electric field.