Adsorption of methanesulfonate anion on Au(111) and its interaction with water molecules at the electrical double layer

The adsorption of methanesulfonate anion ( ), from methanesulfonic acid (MSA), on a Au(111) electrode surface and its interaction with interfacial water molecules have been investigated by a combination of electrochemical and spectroscopic techniques, including cyclic voltammetry, laser-induced temperature jump (LITJ) method, and Attenuated Total Reflection-Surface Enhanced Infrared Reflection Absorption Spectroscopy (ATR-SEIRAS). Results from cyclic voltammetry indicate that the adsorption strength of methanesulfonate on Au(111) is intermediate, falling between that of sulphate and perchlorate anions. A marked voltammetric spike located at potentials above the completion of the anion adsorbed adlayer indicate the existence of a disorder/order phase transition. The magnitude and position of this spike suggest the existence of strong lateral interactions between adsorbed methanesulfonate anions, likely due to van der Waals forces. Voltammetric measurements in diluted solutions identified the potential of zero charge (pzc) at approximately 0.57 V vs. SHE. Furthermore, the LITJ experiments determined the potential of maximum entropy (pme) to be around 0.44–0.49 V vs. SHE, slightly negative to the pzc. This is in accordance with earlier results for sulphate adsorption and suggests a preferential orientation of water molecules with their oxygen atoms towards the Au surface in the absence of an electric field. The study is complemented with ATR-SEIRAS studies using quasi (111) Au films. Those studies confirmed the potential-dependent adsorption of methanesulfonate anion and revealed significant changes in the structure and orientation of interfacial water molecules, due to the influence of the anion on the hydrogen-bonding network at the electrode surface.