INFRARED-SPECTROSCOPY OF MODEL ELECTROCHEMICAL INTERFACES IN ULTRAHIGH-VACUUM - EVIDENCE FOR COUPLED CATION-ANION HYDRATION IN THE PT(111) K+,CL- SYSTEM/

Infrared reflection-absorption spectroscopy (IRAS) measurements along with work-function (Phi) and temperature-programmed desorption (TPD) d ata are reported for deuterated water dosed onto Pt(111) in ultrahigh vacuum (uhv) in the presence of adsorbed chlorine and potassium in ord er to assess the nature and extent of hydration of a ''chemisorbed'' ( or specifically adsorbed) anion in the absence and presence of a catio n ''countercharge'', and vice versa. As studied previously (ref 6a), p otassium constitutes an ideal ''cation'' for this purpose since it ind uces large Phi decreases even for small K coverages (theta(K) less tha n or equal to 0.08) that are modified substantially by hydration; this primary solvation also yields downshifted O-D stretching (nu(OD)) vib rations (ca. 2300-2500 cm(-1)) that are diagnostic of cation-induced w ater reorientation, Adsorption of atomic chlorine (dosed as Cl-2) yiel ds milder yet nonmonotonic Phi changes, suggestive of only partial cha rge transfer to form Cl-delta. The presence of adsorbed Cl attenuates the large (ca. 1.2 eV) Phi decreases produced by water dosing onto cle an Pt(111), yet in largely featureless fashion. An absence of strong C l hydration, suggested by these data and from TPD measurements, is als o indicated by lack of solute-induced nu(OD) bands in the correspondin g infrared spectra, except at high Cl coverages (theta(Cl)) and submon olayer D2O dosages. Markedly different behavior, however, is observed in the presence of both Cl and K adsorbates. Addition of roughly stoic hiometric Cl coverages to adsorbed K in the absence of water yields su bstantial (ca. 1.5 eV) increases followed by only minor changes for hi gher theta(Cl) values, suggesting intimate electrostatic K/Cl interact ions involving K+... e(-)/ Cl-... e(+) dipole polarization. Furthermor e, water addition to K/Cl mixtures where theta(Cl) greater than or equ al to theta(K) triggers only minor Phi changes, in contrast to the sit uation for the absence of Cl. This evidence that the interfacial hydra tion of both ''ionic'' solutes is being modified in their combined pre sence is further indicated by corresponding nu(OD) infrared fingerprin ts that differ substantially from corresponding spectra obtained for e ach solute separately, Thus, the K+ ''hydration shell'' nu(OD) signatu re is largely removed in the presence of Cl, and a new feature (at 262 0 cm(-1)) appears; the latter is most likely associated with solvation of Cl-delta-/K+ pairs. Further evidence for the coupled nature of thi s hydration is also gleaned from the marked cooperative influences of Cl and/or K adsorption upon the water desorption temperatures. The qua litative implications of these findings to the conventional picture of solvation in the electrochemical double layer are also noted.