Alkyl Methyl Imidazolium-Based Ionic Liquids at the Au(111) Surface: Anions and Alkyl Chain Cations Induced Interfacial Effects

The structure, stability, and adsorption mechanism on the Au(111) surface of hydrophilic/hydrophobic ionic liquids (ILs) with various lengths of the alkyl chain group in the cation were investigated using first-principles approaches including electrostatic potential (ESP), electron density analysis, and dispersion-corrected density functional theory (DFT-D3) methods. A suitable selection of ILs is considered. Indeed, we treat ILs of general formula [CₙMIm]⁺[X]⁻, where X = Cl–, PF₆–, and TFSA–, MIm = methylimidazolium and where we varied the alkyl chain (Cₙ) length for n = 0, 2, 4, 6, 8, and 10. We found that the adsorption energies (Eₐdₛ’s) of fluorinated ILs are lower (by ∼30%) than those of nonfluorinated ILs. Computations show that the nature of the anion within the ILs and the alkyl chain length play a very important role to alter the interfacial interactions between ILs and gold surface. Indeed, the alkyl groups of the cation affect the cation–anion interaction strength within ILs because of specific modes of adsorption of alkyl chains on the Au(111) surface. Also, structural-induced adsorption effects are observed during the variation of alkyl chain and anions. These are due to the modification of the local properties of the gold surface upon IL adsorption, where we identify, in some specific cases, an enhancement of the charge transfers between the ILs and the Au(111) surface through van der Waals and dispersive interactions.