Potential-Dependent Vibrational Spectroscopy of Solvent Molecules at the Pt(111) Electrode in a Water/Acetonitrile Mixture Studied by Sum Frequency Generation

Sum frequency generation (SFG) vibrational spectra of D2O and/or acetonitrile (CH3CN) on a Pt(111) single-crystal electrode were obtained as a function of applied potential in a 5 mol % water/acetonitrile mixed solvent with different 0.1 molar MSO3CF3 salts (M = H+, Li+, Na+, K+, and Cs+). The results provide a very specific model for the composition of the inner Helmholtz layer as a function of potential and surface charge. Acetonitrile dominates the inner layer with the CN group directed toward the metal at potentials where the metal has a positive charge. As the surface becomes negatively charged, the acetonitrile orientation flips 180°, with the CH3 group pointing toward the surface. At even more negative surface charge, D2O displaces acetonitrile from the inner layer and is the predominant molecule on the surface. Here water is present as an oriented molecule with the oxygen end pointing toward the metal. The potential (and surface charge) where water is the dominant molecule in the inner Helmholtz layer is determined by the solvation energy of the cation.