IONIC ADSORPTION AT THE SOLID-SOLUTION INTERPHASE USING 3 INSITU METHODS

The adsorption of HSO4-, Cl- and I- from aqueous solutions of 1 M HClO4 was examined using radiotracer Fourier transform IR spectroscopy and ellipsometry. The dependence of the adsorption was measured as a function of time of equilibrium, concentration at constant potential, and at various potentials at constant concentration. For the HSO4- adsorption, some measurements were made as a function of temperature at constant potential. The dependence on time corresponded neither to the GAMMA(t)-t nor GAMMA(t)-in t relations expected from previously deduced theoretical models. Up to ca. tau/4, theta(t) was found to be proportional to the adsorption time. The dependence of adsorption on concentration at constant potential was carried out over an extended region, i.e. from 10(-9) to 10(-2) M. In the concentration range up to 10(-5) M the behaviour followed the Henry isotherm and at concentrations up to 10(-3) M the results followed a logarithmic law. (partial derivative theta/partial derivative log c)E,T was about the same for HSO4- and Cl but around two times higher for I-. The maximum coverage of HSO4- and Cl- was reached at ca. 0.25 and ca. 1 for I-. The dependence on potential at constant concentration was essentially linear until oxide formation for HSO4- and Cl-. I- results showed little variation with potential before oxide formation. The dependence of (theta)E,c as a function of temperature for HSO4- cannot be expressed as a simple law because of the low number of experiments successfully investigated; nevertheless, some approximate values of DELTAG-degrees, DELTAH-degrees and DELTAS-degrees were derived. An isotherm was derived in such account that the heterogeneity of the surface, ion-ion repulsion through a simple imaging model and ion-ion dispersive forces were considered. In the calculations, electrostriction effects on ion size were computed. Evidence for partial charge transfer was found: 0.2, 0.6 and 0.9 for HSO4-, Cl- and I- respectively. Indications of water displacement from the surface was found to be consistent with one displaced entity represented by a water dimer. The thermodynamic parameters derived for HSO4- were DELTAG-degrees = -32 kJ mol-1 (25-degrees-C), DELTAH-degrees = -57 kJ mol-1 and DELTAS-degrees = -78 J K-1 mol-1 at theta(ss) = 0.22. From these values, the Pt-O bond was estimated to be ca. one-quarter of that expected for the isolated Pt-O bond in the gas phase. The entropy was consistent with immobile adsorbed ions with hindered rotation.