Determination of Alcohol Solvent Orientation and Bonding at Silver Electrodes Using Surface-Enhanced Raman Scattering: Methanol, Ethanol, 1-Propanol, and 1-Pentanol

Interfacial solvent structure at Ag electrodes in various straight-chain alcohol electrolyte systems has been studied using surface-enhanced Raman scattering (SERS). SERS provides detailed information regarding the nature of interfacial bonding and alkyl chain orientation. Methanol appears to be in a unique hydrogen-bonding environment at the interface, as indicated by a 10-cm^-1 11decrease in v(C-O) frequency from its bulk solution value; C₂ and longer chain alcohols at the interface show no shift in v(C-0) frequency from bulk solution. The alkyl chains are largely parallel to the surface (i.e., greater than 45° relative to the surface normal) and exist in a largely all-trans configuration at electrode potentials positive of the PZC (potential of zero charge). These molecules are oriented with the hydroxyl group at the surface and the alkyl chain tilted slightly away from the surface. At more negative potentials near the PZC, the alkyl chain conformations become more disordered, and the alkyl chains “stand up”, with the methyl group directed into bulk solution. For methanol, two distinct orientations exist simultaneously at the interface in the vicinity of the PZC. The growth of a new methylene band for C₂, C₃, and C₅ alcohols at increasingly negative potentials suggests significant interaction of the methylene hydrogens with the surface. In contrast to the C₃ and C₅ alcohols, the C₁ and C₂ alcohols exhibit little bulk spectroscopic behavior, indicating distinct orientations at potentials negative of the PZC. All observations support the idea that the α-carbon is confined near the surface at all potentials for all four alcohols. The driving force for this behavior is proposed to be the formation of C-H…Ag agostic bonds.