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 di scussed, with an emphasis on elucidating the nature of double-layer so lvation and how solvent molecules influence the intermolecular interac tions. 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 co ntrolled amounts, enabling the various molecular (and hence intermolec ular) ingredients that constitute the double layer to be accessed in i ncremental fashion. The approach also provides an invaluable means of understanding the differences in structure and bonding between analogo us electrochemical interfaces and the constituent metal-UHV systems. S uch issues are particularly germane with the recent advent of microsco pic-level structural information for in situ electrochemical systems. Described specifically here is the UHV-based vibrational characterizat ion of solvent and chemisorbate modes by employing infrared reflection -absorption spectroscopy (IRAS), together with work-function measureme nts, as a function of interfacial composition. The former provides a s ensitive monitor of intermolecular interactions as well as being appli cable (albeit with more restrictions) to in situ systems, whereas the latter yields insight into surface-potential profiles and also links t he potential scales of metal-UHV and electrochemical interfaces. Sever al distinct examples aimed at elucidating double-layer solvation effec ts 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., isolate d) ions enables the substantial influence of the metal surface upon do uble-layer solvation to be explored in detail. The converse role of do uble-layer charge upon the inner-layer solvent orientation (as exempli fied for acetone and acetonitrile) is also found to be considerable an d involves long-range forces. The combined influences of solvent and d ouble-layer charge upon chemisorbate structure and bonding are also co nsidered for the archetypical example of carbon monoxide. Marked elect rostatic effects of solvation upon CO structure and bonding are seen e ven in the absence of net charge, The complex short-range influences o f added cationic (K+) charge upon the CO adlayer are quenched upon par tial K+ solvation. The longer-range electrostatic effects are progress ively modified as the chemisorbate layer as well as the ionic charges become fully solvated, so to reveal a simple ''Stark-tuning'' frequenc y-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.