Modeling Electrochemical Interfaces in Ultrahigh Vacuum:  Molecular Roles of Solvation in Double-Layer Phenomena

Some virtues of modeling electrochemical systems by dosing interfacial components onto clean metal surfaces in ultrahigh vacuum (UHV) are discussed, with an emphasis on elucidating the nature of double-layer solvation and how solvent molecules influence the intermolecular interactions. This "non situ" strategy (as distinct from ex situ approaches involving electrode transfer to/and from UHV) allows each interfacial component (solutes, ions, solvent) to be added sequentially and in controlled amounts, enabling the various molecular (and hence intermolecular) ingredients that constitute the double layer to be accessed in incremental fashion. The approach also provides an invaluable means of understanding the differences in structure and bonding between analogous electrochemical interfaces and the constituent metal−UHV systems. Such issues are particularly germane with the recent advent of microscopic-level structural information for in situ electrochemical systems. Described specifically here is the UHV-based vibrational characterization of solvent and chemisorbate modes by employing infrared reflection−absorption spectroscopy (IRAS), together with work-function measurements, as a function of interfacial composition. The former provides a sensitive monitor of intermolecular interactions as well as being applicable (albeit with more restrictions) to in situ systems, whereas the latter yields insight into surface-potential profiles and also links the potential scales of metal−UHV and electrochemical interfaces. Several distinct examples aimed at elucidating double-layer solvation effects on Pt(111), recently scrutinized in our laboratory, are discussed. These include examining the progressive solvation of cations, adsorbed anions, and combinations thereof, by water and methanol. Comparisons with vibrational spectra for the solvation of gas-phase (i.e., isolated) ions enables the substantial influence of the metal surface upon double-layer solvation to be explored in detail. The converse role of double-layer charge upon the inner-layer solvent orientation (as exemplified for acetone and acetonitrile) is also found to be considerable and involves long-range forces. The combined influences of solvent and double-layer charge upon chemisorbate structure and bonding are also considered for the archetypical example of carbon monoxide. Marked electrostatic effects of solvation upon CO structure and bonding are seen even in the absence of net charge. The complex short-range influences of added cationic (K+) charge upon the CO adlayer are quenched upon partial K+ solvation. The longer-range electrostatic effects are progressively modified as the chemisorbate layer as well as the ionic charges become fully solvated, so to reveal a simple "Stark-tuning" frequency−potential behavior identical with that familiar in electrochemistry. Some more general implications and applications of such "UHV double-layer modeling" tactics are also briefly considered.