Plasma electroreflection on platinum and true charge-transfer coefficients

The Role of Anion Adsorption in the Effect of Electrode Potential on Surface Plasmon Resonance Response

Surface plasmon resonance, being widely used in bioanalytics and biotechnology, is influenced by the electrical potential of the resonant gold layer. To evaluate the mechanism of this effect, we have studied it in solutions of various inorganic electrolytes. The magnitude of the effect decreases according to the series: KBr>KCl>KF>NaClO₄ . The data were treated by using different models of the interface. A quantitative description was obtained for the model, which takes into account the local dielectric function of gold being affected by the free electron charge, diffuse ionic layer near the gold/water interface, and specific adsorption of halides to the gold surface with partial charge transfer. Taking into account that most biological experiments are performed in chloride-containing solutions, detailed analysis of the model at these conditions was performed. The results indicate that the chloride adsorption is the main mechanism for the influence of potential on the surface plasmon resonance. The dependencies of surface concentration and residual charge of chloride on the applied potential were determined.

Update on current state and problems in the surface tension of condensed matter

The dual concept of surface energy formally allows application of Gibbs thermodynamics to the surface tension of solids and is unlimited using the classical Lippmann equation for solids that is shown to contradict all available in situ experimental data. At present, the generalized Lippmann equation is believed to be the most universal, since the classical Lippmann equation, the Shuttleworth and Gokhshtein equations could be derived from it. Lately it was evaluated in two opposite ways: the first--the experimental verification of the Gokhshtein equation supports correctness of the generalized Lippmann and Shuttleworth equations; the second--the incompatibility of the Shuttleworth equation with Hermann's mathematical structure of thermodynamics makes invalid all its corollaries, including the generalized Lippmann and Gokhshtein equations. Both approaches are shown here to be incorrect, since the Gokhshtein equation cannot be correctly derived from any of the above-mentioned equations. The Frumkin derivation of the first and second Gokhshtein equations follows from one thermodynamic relationship general for the surface tension of both solid and liquid electrodes. The classical Lippmann equation is also derived from this general relationship as a particular case of the second Gokhshtein equations. We have considered the hierarchy of these equations and discussed the straightforward application of the classical Lippmann equation for solids with an account for elasticity of the surface structured layers of liquids. The partial charge transfer during anion adsorption cannot be measured in electrochemical experiments or reliably estimated by quantum-chemical and DFT calculations. However, it is directly involved in the adsorbate charge that is experimentally accessible by in situ contact electric resistance technique. We present the first quantitative evaluation of charge transfer during halides adsorption on silver from aqueous solutions in dependence on the electrode potential.

Co-adsorbtion of Cu and Keggin type polytungstates on polycrystalline Pt: interplay of atomic and molecular UPD

Second harmonic generation (SHG), electrochemical quartz microbalance (EQCM), and cyclic voltammetry are applied to clarify the structure and properties of Cu adlayers formed in the presence of Keggin polytungstate anions. For 0.02-10 mM CuSO4 solutions, no pronounced suppression of underpotential copper deposition (Cu UPD) by 0.1-10 mM H3PW12O40 (PW12) or H4SiW12O40 (SiW12) is observed in electrochemical experiments. Moreover, coadsorption with polyanions results in an increase of charge in the Cu UPD region. EQCM data demonstrate high surface coverage with polytungstate in the overall potential range and their pronounced co-adsorption with Cu2+ cations under open circuit. The unusual potential dependence of EQCM response of polytungstates is discovered and discussed in terms of anion interactions with adsorbed hydrogen. The SHG response of Cu UPD demonstrates a non-linear dependence on Cu surface coverage, which is interpreted in terms of discontinuous submonolayers consisting of 2D Cu islands. The additives of PW12 or SiW12 decrease copper SHG response at low and high CuSO4 concentrations, with minor effect for a mid range of concentrations. In all mixed solutions, the potential dependence of the SHG response remains typical for Cu UPD, not for polytungstates. SHG transients measured under potential step mode demonstrate that the initial non-steady-state SHG behavior of the adlayer is more close to the behavior of polytungstates, but typical copper features appear at longer wavelength. These facts favor the hypothesis of Cu adatom penetration through anionic adlayers and formation of a metal submonolayer at the vacant areas between large quasi-spherical polyanions, with subsequent transformation into a Pt/Cu/polytungstate layered structure.