Microscopic adsorption structures of imidazolium-based ionic liquids, [C1mim]PF6, [C4mim]PF6, [C1mim]Cl, and [C4mim]Cl, on the surfaces of graphene plates, coronene and circumcoronene, were studied in detail by quantum chemical computation at the DFT/B3LYP/6-311g level of theory. The large size graphene (circumcoronene) is suitable to determine the specific adsorption property of the ionic liquids appropriately. The interplay between the anion−cation interaction energies and the ionic liquid/graphene adsorption energies substantiate the nature of the ionic liquid. Adsorption of [C1,4mim]PF6 is featured by adsorption of the anion and the cation each interacting immediately onto the plate surface while the immediate interaction of only the cation is favorable in the adsorption of [C1,4mim]Cl. This can be due to the low cation−anion interaction energy and thus high adsorption energy in [C1,4mim]PF6. The converse is true in the case of [C1,4mim]Cl. The heats of adsorption for [C1,4mim]PF6 ionic liquids on the circumcoronene are almost the same (89.574 and 88.969 kJ·mol−1, respectively) and are about 9 times that of [C1,4mim]Cl (10.122 and 10.548 kJ·mol−1, respectively). While density of states of [C1,4mim]PF6 ionic liquids are higher than that of [C1,4mim]Cl, the band gap energy indicates the relative stability of the former over the latter. Band gap energies of the [C1,4mim]PF6/circumcoronene system decrease more than those of the [C1,4mim]Cl/circumcoronene system with respect to the corresponding ionic liquid. Charge transfer between the anion and the cation is favorable upon the adsorption of [C1,4mim]Cl, while the favorable charge transfer in the adsorption of [C1,4mim]PF6 is between ionic liquid atoms and those of circumcoronene. Anion type has the major contribution to the adsorption, whereas the role of alkyl group is minor. The HOMO energies of [C1,4mim]PF6 are lower than those of [C1,4mim]Cl. Upon adsorption, the increase in HOMO energies of the former is larger than that of the latter. On the other hand, the decrease in the LUMO energies of latter is slightly larger than that of the former. Overall the adsorption of hydrophobic [C1,4mim]PF6 ionic liquids is favorable both structurally and energetically.

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