Meta-Hybrid Density Functional Theory Study of Adsorption of Imidazolium- and Ammonium-Based Ionic Liquids on Graphene Sheet

In this study, two types of ionic liquids (ILs) based on 1-butyl-3-methylimidazolium [Bmim](+) and butyltrimethylammonium [Btma](+) cations, paired to tetrafluoroborate [BF4]-, hexafluorophosphate [PF6](-), dicyanamide [DCA](-), and bis(trifluoromethylsilfonyl)imide [Tf2N]- anions, were chosen as adsorbates to investigate the influence of cation and anion type on the adsorption of ILs on the graphene surface. The adsorption process on the graphene surface (circumcoronene) was studied using M06-2X/cc-pVDZ level of theory. Empirical dispersion correction (D3) was also added to the M06-2X functional to investigate the effects of dispersion on the binding energy values. The graphene(...)IL configurations, binding energies, and thermochemistry of IL adsorption on the graphene surface were investigated. Orbital energies, charge transfer behavior, the influence of adsorption on the hydrogen bond strength between cation and anion of ILs, and the significance of noncovalent interactions on the adsorption of ILs on the graphene surface were also considered. ChelpG analysis indicated that upon adsorption of ILs on the graphene surface the overall charge on the cation, anion, and graphene surface changes, enabled by the charge transfer that occurs between ILs and graphene surface. Orbital energy and density of states calculations also show that the HOMO-LUMO energy gap of ILs decreases upon adsorption on the graphene surface. Quantum theory of atoms in molecules analysis indicates that the hydrogen-bond strength between cation and anion in ILs decreases upon adsorption on the graphene surface. Plotting the noncovalent interactions between ILs and graphene surface shows the role and significance of cooperative pi(...)pi, C-H-...pi, and X-...pi (X = N, O, F atoms from anions) interactions in the adsorption of ILs on the graphene surface. The thermochemical analysis also indicates that the free energy of adsorption (Delta G(ads)) of ILs on the graphene surface is negative, and thus the adsorption occurs spontaneously.