Effects of Anion on Liquid Structures of Ionic Liquids at Graphene Electrode Interface Analyzed by Molecular Dynamics Simulations

Molecular dynamics simulations of ionic liquids composed of N-propyl-N-methylpyrrolidinium cation ([pmpyro](+)) with (CF3SO2)(2)N- ([TFSA](-)), (FSO2)(2)N- ([FSA](-)), CF3SO3-, CF3CO2- and PF6- anions sandwiched by two charged graphene sheets show that the liquid structures near graphene sheets depend strongly on anion. Long-range charge ordering structures were observed when the ionic liquids were sandwiched with charged graphene sheets. Although the magnitude of the oscillation of cation and anion densities near charged graphene sheets is enhanced by the increase of the charges on the graphene sheets, the range of the charge-ordering structures from the graphene sheets does not largely change. The ranges of the charge-ordering structures from charged graphene sheets observed in the [pmpyro][TFSA], [pmpyro][FSA], [pmpyro][CF3SO3], [pmpyro][CF3CO2] and [pmpyro][PF6] ionic liquids are approximately 40, 80, 60, 60 and 100 angstrom, respectively.