MODELING ELECTROCHEMICAL INTERFACES IN ULTRAHIGH-VACUUM - INFLUENCE OF PROGRESSIVE CATION AND SURFACE SOLVATION UPON CHARGE-POTENTIAL DOUBLE-LAYER BEHAVIOR ON PT(111)

Measurements of the work-function changes, Delta Phi, on Pt(111) for c ontinuously increasing solvent exposures theta(S)() and in the presen ce of various coverages of potassium. theta(K), in ultrahigh vacuum (U HV) at 90 K are reported with the objective of ascertaining how the su rface charge-potential properties of such ''UHV electrochemical model' ' interfaces are altered by progressive solvation. The solvents-water, methanol, acetonitrile, acetone, and ammonia-span a range of dipolar and other solvating properties and have been utilized in related vibra tional spectroscopic studies from this laboratory. Since potassium dos age yields interfacial electron transfer to form K+ together with surf ace electronic charge, the corresponding Delta Phi-theta(K) plots for various solvent dosages extracted from the above data provide surface charge-potential (sigma-phi) curves for systematically varying extents of interfacial solvation. In contrast to the large (1-3 eV) monotonic solvent-induced Phi decreases observed in the absence of ionic charge , the presence of predosed K+ yields initial Phi increases associated with cation solvation, followed by Phi decreases due primarily to the ensuing metal surface solvation. Examination of the corresponding Delt a Phi-theta(K) traces obtained for these different solvent dosage regi ons shows that the basic charge-potential features characteristic of t he solvated double laver require only ionic solvation, even though com plete metal surface solvation modifies significantly the electrostatic behavior. While surface solvation by the different species examined i n the absence of charge yield substantially dissimilar Phi values, (i. e., differing ''potentials of zero charge''), the charge-potential cha racteristics are relatively insensitive to the solvent. This finding, comparable to that obtained for in-situ electrochemical interfaces. in dicates that the effective ''interfacial solvent dielectric constant'' varies by only 2-fold or less. Delta Phi-theta(K) data obtained by K dosing after solvent addition yielded larger -Delta Phi values (i.e.. smaller capacitances), consistent with more complete K+ solvation and/ or larger K+ surface separations. Corresponding Delta Phi-theta(K) dat a for CO-saturated Pt(111) indicates that the CO adlayer plays a role in dielectric screening. Effectively theta(K)-independent Delta Phi re sponses were obtained with ammonia-solvated Pt (111), however, suggest ive of the formation of solvated electrons. Specific comparisons are m ade between the UHV-based charge-potential behavior with that for in-s itu electrochemical interfaces and for ionizable high-nuclearity Pt ca rbonyl clusters in nonaqueous media. The latter systems, in particular , exhibit closely similar surface charge-potential characteristics to the corresponding UHV-based Pt(111) interfaces.