Adsorption of Hydrophobic and Hydrophilic Ionic Liquids at the Au(111) Surface

Electrode-electrolyte microscopic interfacial studies are of great interest for the design and development of functional materials for energy storage and catalysis applications. First-principles-based simulation methods are used here to understand the structure, stability, energetics, and microscopic adsorption mechanism of various hydrophilic and hydrophobic ionic liquids (ILs; 1-butyl 3-methylimidazolium [BMIm]⁺[X]^-, where X = Cl, DCA, HCOO, BF₄, PF₆, CH₃SO₃, OTF, and TFSA) interacting with a metallic surface. We have selected the Au(111) surface as a potential electrode model, and our computations show that ILs (anions and cations) adsorb specifically at some selective adsorption sites. Indeed, hydrophilic anions of ILs are strongly adsorbed on the gold surface (via Au-Cl and Au-N bonds at Au(111)), whereas hydrophobic anions are weakly bonded. The [BMIm]⁺ is always found to be stabilized parallel to the metal surface, irrespective of the nature of the anion, through various kinds of noncovalent interactions. Mulliken, Löwdin, and Hirshfeld charge analyses reveal that there is significant charge transfer between ILs and the surface that may enhance the charge transfer mechanism between the surface and electrolytes for electrochemical applications. Our study shows that the electrostatic and van der Waals interactions are in action at these interfaces. Moreover, we show that there are several covalent and noncovalent interactions between ILs and the metal surface. These interactions play an essential role to maintain the electrostatic behaviors at the solid-liquid interface. The present findings can be helpful to predict specific selectivity and subsequent design of materials for energy harvesting and catalysis applications.