Ionic adsorption at the solid—solution interphase using three in situ methods

• Published in: Journal of electroanalytical chemistry (Lausanne, Switzerland) Switzerland), 1992-11, Vol.339 (1), p.355-400
• Main Authors: Bockris, J.O'M., Gamboa-Aldeco, Maria, Szklarczyk, M.
• Format: Article
• Language: English
• Subjects: Adsorption; Chemistry; Electrochemistry; Exact sciences and technology; General and physical chemistry; Study of interfaces
• ISSN: 1572-6657; 1873-2569
• DOI: 10.1016/0022-0728(92)80463-E

The adsorption of HSO 4 −, Cl − and I − from aqueous solutions of 1 M HClO 4 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 HSO 4 − adsorption, some measurements were made as a function of temperature at constant potential. The dependence on time corresponded neither to the ln Γ t- t nor Γ t-ln t relations expected from previously deduced theoretical models. Up to ca. τ/4, θ 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. (∂θ /∂ log c) E,T was about the same for HSO 4 − and Cl − but around two times higher for I −. The maximum coverage of HSO 4 − 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 HSO 4 − and Cl −. I − results showed little variation with potential before oxide formation. The dependence of (θ) E,c as a function of temperature for HSO 4 − cannot be expressed as a simple law because of the low number of experiments successfully investigated; nevertheless, some approximate values of Δ G°, Δ H° and Δ S° 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 HSO 4 −, 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 HSO 4 − were Δ G° = −32 kJ mol −1 (25°C), Δ H° = −57 kJ mol −1 and Δ S° = −78 J K −1 mol −1 at θ 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.