Trends in Physisorption of Ionic Liquids on Boron-Nitride Sheets

The adsorption of ionic liquids (ILs) on the hexagonal boron-nitride (h-BN) surface was studied at the M06-2X/cc-pVDZ level of theory. Three types of ionic liquids based on 1-butyl-3-methylimidazolium [Bmim](+), 1-butylpyridinium [Bpy](+), and butyltrimethylammonium [Btma](+) cations, paired with tetrafluoroborate [BF4](-), hexafluorophosphate [PF6](-), and bis(trifluoromethylsilfonyl)imide [Tf2N](-) anions were chosen as the adsorbates to better understand the trends in adsorption behavior of ILs on the h-BN surface. We have identified the various stable configurations of the h-BN-ionic liquid (h-BN center dot center dot center dot IL) complexes based on their binding energies and investigated the effect of charge transfer behavior and noncovalent interactions on the adsorption of ILs. ChelpG analysis indicated that, upon adsorption of ionic liquids on the h-BN surface, the overall charge on the cation, anion, and h-BN surface changes and the transfer (CT) between ILs and h-BN surface occurs. The order for the magnitude of charge transfer between different ILs and the h-BN surface is as follows: [Bmim][Tf2N] (-0.059e) > [Btma][PF6] (0.036e) > [Bpy][Tf2N] (0.028e) > [Btma][Tf2N] (0.021e) > [Bmim][PF6] (0.009e) > [Bpy][BF4] (0.007e) > [Bpy][PF6] (-0.006e) > [Btma][BF4] (-0.003e) > [Bmim][BF4] (-0.001e), respectively. Orbital energy and density of states (DOSs) calculations also show that the HOMOLUMO energy gap of ILs decreases upon adsorption on the h-BN surface. The order of the HOMOLUMO gap energy changes of ILs upon adsorption on the h-BN surface is as follows: [Btma][PF6] (3.25 eV) > [Btma][BF4] (2.84 eV) > [Bpy][PF6] (2.41 eV) > [Bpy][BF4] (2.29 eV) > [Bmim][BF4] (1.76 eV) > [Bmim][PF6] (1.54 eV) > [Btma][Tf2N] (1.26 eV) > [Bmim][Tf2N] (1.19 eV) > [Bpy][Tf2N] (0.86 eV), respectively. The binding energies based on QTAIM analysis indicate that the [BF4](-), [PF6](-), and [Tf2N](-) anions in the ILs have a stronger interaction with the h-BN surface than [Bmim](+), [Bpy](+), and [Btma](+) cations. The role of cooperative pi center dot center dot center dot pi, CH center dot center dot center dot pi, and X center dot center dot center dot pi (X = N, O, F atoms from anions) interactions on the adsorption of ILs on the h-BN surface was elucidated by analyzing the noncovalent interactions between ILs and the h-BN surface. Energy decomposition analysis (EDA) carried out for the h-BN center dot center dot center dot IL complexes indicates that the contribution of the Delta E-disp component in each complex is also more than electrostatic (Delta E-elect) and orbital (Delta E-orb) components (Delta E-disp > Delta(Eelect) > Delta E-orb), with the exception of the h-BN[Btma][BF4] complex whose Delta E-disp and Delta E-elect components are almost equal. For the complexes with the same cations, dispersion interaction increases by increasing size of anion from [BF4](-) to [PF6](-) and [Tf2N](-). This is confirmed by more favorable enthalpy of adsorption for ILs on the h-BN surface. The thermochemical analysis also indicates that the free energy of adsorption (Delta G(ads)) of ILs on the h-BN surface is negative, and thus, the adsorption occurs spontaneously. Our first-principles study offers fundamental insights into the nature of the physisorption and solvation behavior of ionic liquids on h-BN.