INFRARED-SPECTROSCOPY OF MODEL ELECTROCHEMICAL INTERFACES IN ULTRAHIGH-VACUUM - COUPLED INFLUENCE OF DOUBLE-LAYER ANION CATION SOLUTES UPONACETONE SOLVENT CHEMISORPTION ON PT(111)/

The effects of adsorbing chlorine and/or potassium on the infrared spe ctra of acetone dosed onto Pt(111) in ultrahigh vacuum (UHV) are repor ted with the objective of ascertaining the manner and extent to which the oxygen bound (eta(1)) chemisorbed state of acetone is influenced b y 'specific anion' adsorption, along with the cation countercharge in this 'UHV electrochemical model' system. Acetone was chosen as a model solvent in view of its dipolar properties along with the presence of an infrared chromophore, the carbonyl stretching vibration, the freque ncy as well as the intensity of which is sensitive to the dipolar orie ntation and coordination of adsorbed acetone. The presence of co-adsor bed chlorine atoms stabilizes substantially the perpendicular solvent dipole orientation, as deduced from a marked intensity increase in the ca. 1640cm(-1) band due to the eta(1) chemisorbed state of acetone. T his effect, which can be understood most simply in terms of an attract ive electrostatic interaction between the Cldelta- metal image and Cde lta+ = Odelta- oriented dipoles, is opposite to that observed in the p resence of adsorbed K+ alone. In the latter case, even very low fracti onal coverages (around 0.02) of K+ are sufficient to eliminate essenti ally the eta(1) chemisorbed acetone state as deduced from the removal of the ca. 1640 cm(-1) band; this finding indicates that long range (o ver two to three solvent diameters) cation-solvent interactions are op erative [cf. I. Villegas and M.J. Weaver, J. Am. Chem. Sec., 118 (1996 ) 458]. In the presence of co-adsorbed chlorine + potassium mixtures, the eta(1) acetone state is also largely absent even for marked (five- to 10-fold) stoichiometric deficiencies of the cation relative to the 'anion' solute component. This latter result can be rationalized on t he basis of the anticipated greater ionicity of the adsorbed cation (K +) relative to the chemisorbed anion (Cldelta-) species. These finding s are corroborated by means of measurements of the work function chang es induced on Pt(111) by dosing acetone in the presence of varying cov erages of chlorine and/or potassium. The implications to our understan ding of the interplay between interfacial ion-solvent and solvent-surf ace interactions in electrochemical systems are noted on a more genera l basis. (C) 1997 Elsevier Science S.A.