Measurements of the work-function changes, Delta Phi, on Pt(111) for continuously increasing solvent exposures theta(S)(*) and in the presence of various coverages of potassium. theta(K), in ultrahigh vacuum (UHV) at 90 K are reported with the objective of ascertaining how the surface 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 vibrational spectroscopic studies from this laboratory. Since potassium dosage yields interfacial electron transfer to form K+ together with surface 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 Delta Phi-theta(K) traces obtained for these different solvent dosage regions shows that the basic charge-potential features characteristic of the solvated double laver require only ionic solvation, even though complete metal surface solvation modifies significantly the electrostatic behavior. While surface solvation by the different species examined in the absence of charge yield substantially dissimilar Phi values, (i.e., differing "potentials of zero charge"), the charge-potential characteristics are relatively insensitive to the solvent. This finding, comparable to that obtained for in-situ electrochemical interfaces. indicates 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) data for CO-saturated Pt(111) indicates that the CO adlayer plays a role in dielectric screening. Effectively theta(K)-independent Delta Phi responses were obtained with ammonia-solvated Pt (111), however, suggestive of the formation of solvated electrons. Specific comparisons are made between the UHV-based charge-potential behavior with that for in-situ electrochemical interfaces and for ionizable high-nuclearity Pt carbonyl clusters in nonaqueous media. The latter systems, in particular, exhibit closely similar surface charge-potential characteristics to the corresponding UHV-based Pt(111) interfaces.