Interfacial solvent structure in butan-1-ol, butan-2-ol and 2-methylpropan-1-ol at Au and Ag electrodes from surface enhanced Raman scattering and capacitance measurements

Surface enhanced Raman scattering (SERS) and differential capacitance have been used to study interfacial solvent structure in LiBr electrolyte solutions of three isomers of butanol, butan-1-ol, butan-2-ol and 2-methylpropan-1-ol, at Au electrodes. The potential-dependent spectral behaviour in the regions containing the v(C-C), v(C-O) and v(C-H) modes of these isomers at Au electrodes is compared to that observed at Ag electrodes as a function of rational potential. Differential capacitance data are treated using the Hurwitz-Parsons analysis to extract surface coverage of specifically adsorbed Br^- as a function of electrode potential. Potential of zero charge (p.z.c.) values are estimated from these data. Based on a quantitative comparison of spectral data at Ag and Au electrodes as a function of rational potential, the solvent structures at Au electrodes are concluded to resemble those previously proposed for Ag electrodes. The potential-dependent orientations of these solvents appear to be driven by interactions of the O non-bonding electrons and the alkyl chain with the electrode, and hydrogen bonding of the hydroxy group with specifically adsorbed Br^-. At potentials positive of the p.z.c. at both metals, butan-1-ol and butan-2-ol hydrogen bond with specifically adsorbed Br^- with their alkyl portions relatively close to the electrode surface. As more negative potentials are applied, Br^- is repelled from the surface, and the alkyl groups move away from the electrode surface insofar as is possible. Only minor changes in orientation are observed as a function of potential for 2-methylpropan-1-ol at these electrodes due to the symmetry of its alkyl structure. The interaction of the O atom of 2-methylpropan-1-ol with the electrode is similar to that observed for the other butanol isomers. This interaction places the alkyl groups of the molecule at the surface in an orientation which is largely potential-insensitive.