Effects of anions and alkyl chain length of imidazolium-based ionic liquids at the Au (111) surface on interfacial structure: a first-principles study

The interfacial structure and adsorption mechanism of imidazolium-based ionic liquids (ILs) on Au (111) surface were investigated via first-principles calculation. Electron density analysis and Bader charge analysis were used to explore the electronic structure of Au (111)-ILs interface. Computations show that the alkyl chain length and anions play a significant role in designing Au (111)-ILs interfacial structure. On the one hand, the stability of interface and adsorption energy tend to be enhanced as the alkyl chain length increases. It attributes to the methylene group of alkyl chain which could easily anchor on the gold interface. On the other hand, the difference in anions makes the adsorption behavior quite different. The adsorption energy follows the order: [Cnmim] [Br] > [C(n)mim][Cl] > [C(n)mim][TFSA] > [C(n)mim][OAc] > [C(n)mim][PF6] > [Cnmim][BF4]. The nonfluorinated ILs (containing Br, Cl, and O atoms of anions) always have a drastic charge transfer among gold-ILs interface. However, the larger van der Waals (vdWs) volumes of the fluorinated anions have a more diffused electron density which lead to the relatively weak interaction. To sum up, a detailed and systematic investigation of the variation of anions and alkyl chain length of ILs which will affect the interfacial structure is fully studied. The above study could be helpful to understand electrode-electrolyte microscopic interface and design of functional materials for energy storage.