L.G.J. Fokkink

Temperature dependence of the electrical double layer on oxides: rutile and hematite

Abstract The surface charge σ0 on aqueous suspensions of hematite and rutile was measured as a function of pH in various KNO3 concentrations and at different temperatures. Two types of congruences were observed: (a) if σ0 is plotted as a function of the pH minus the pzc, curves on these two oxides coincide at the same KNO3 concentrations and (b) the same is observed for σ0 as a function of temperature. Thermodynamic analysis enabled us to separate double layer parameters into oxide-specific and generic contributions. Standard Gibbs energies, enthalpies, and entropies for proton adsorption were obtained.

Specific ion adsorption on oxides: Surface charge adjustment and proton stoichiometry.

In this paper a quantitative analysis of the coadsorption ratio for oxides, r OH , that is the number of OH − ions coadsorbing with each specifically adsorbing metal ion at given pH, is given. This ratio is an important double-layer parameter. It is shown that because of the high accumulation of charge on the surface and the effective compensation of this charge by countercharge in the Stern plane, r OH can be established with satisfactory precision using Gouy-Stern theory only. Computed r OH values depend primarily on the distance between the pH and the pzc and to a lesser extent on the indifferent electrolyte concentration. If the Stern layer capacitance is high, it is not a critical parameter. Agreement with data in the literature and new measurements for Cd 2+ adsorption on rutile and hematite is satisfactory.

Temperature dependence of cadmium adsorption on oxides. I: Experimental observations and model analysis

Abstract The influence of temperature on the adsorption of cadmium on rutile (TiO2) and hematite (α-Fe2O3) is measured at different values of the (initial) surface charge density and salt concentration. The formation of cadmium hydroxo-complexes in solution is simultaneously investigated. Experimental adsorption results are analyzed in terms of the Frumkin-Fowler-Guggenheim model, using a Gouy-Stern double layer picture. Thermodynamic analysis points to a great similarity between the binding of Cd2+ with hydroxyl groups on the surface and with OH− ions in solution. Both processes are entropically driven.

Electrical double layers on thiol-modified polycrystalline gold electrodes

Abstract The solid | liquid interfacial tension, capacitive current and differential capacitance of alkyl-thiol-modified gold electrodes in K 2 SO 4 are measured simultaneously as a function of the electrode potential. The chain length of the thiols is varied between C 10 H 21 SH and C 22 H 45 SH and the ion concentrations range from 10 −4 to 10 −2 M K 2 SO 4 . It is found that these alkyl thiols form extremely densely packed self-assembled layers. The potential-dependent wetting of the thiol-modified electrodes depends strongly on the chain length of the thiol: the shorter the chain, the stronger the influence. From the potential dependence of the contact angle, the Helmholtz energy of the electric double layer is derived. It is found that measured double-layer capacitances are consistent with the model derived from the wetting method: a large and linear potential decay takes place within the thiol layer and a diffuse charge develops at the electrolyte side of the interface. The relative permittivity is independent of the chain length and is about 2. The K 2 SO 4 concentration affects the measured double-layer capacitance in a consistent manner, but it does not influence the wettability significantly. It is concluded that the dependence of the wettability on the electrode potential originates in the formation of an electrical double layer and that potential-induced conformational changes within the thiol layer are insignificant.

Thermodynamics of proton charge formation on oxides. Microcalorimetry

Abstract Calorimetric heats of proton adsorption on rutile and hematite in solutions of KNO3 at the point of zero charge (p.z.c.) agree well with the corresponding enthalpies derived from the shift of p.z.c. with temperature. The enthalphy of charge formation depends on the nature of the oxide, but is practically independent of surface charge and electrolyte concentration. This observation is consistent with a high capacitance nature of the oxide—solution interface. In the theoretical analysis absence of specific adsorption or chemisorption of ions other than protons has been assumed. The electrostatic contribution to the enthalpy of double layer formation is shown to strongly decrease with increasing inner layer capacitance. The relationships between calorimetry and temperature congruence of surface charge—pH curves are discussed in some detail.

Single-electron tunneling up to room temperature

Ultrasmall (5 nm in lateral diameter) double-barrier tunnel junctions have been realized using metal particles (2-5 nm in diameter) sandwiched in between a metallic substrate and the metallic tip of a scanning tunneling microscope (STM). The particles were either grown by e-beam evaporation or prepared using colloid chemistry. Two electrical transport effects, in good agreement with the semi-classical theory of single-electron tunneling, have been found at room temperature: the Coulomb gap and the Coulomb staircase. The interpretation in terms of single-electron tunneling is further supported by liquid helium temperature STM measurements on identical samples.

Specific adsorption of organic cations at the silver iodide-electrolyte interface.

Abstract The adsorption of monovalent tetrabutylammonium cations and bivalent bipyridinium cations diquat and paraquat on AgI has been studied as a function of surface charge density in 0.1 M indifferent (1–1) electrolyte. Adsorption results are analyzed according to the Frumkin—Fowler—Guggenheim isotherm. The model allows analysis of the several chemical and electrostatic free enthalpy contributions both to the lateral and to the adsorbent—adsorbate interactions. Hydrophobic interactions are shown to be the major non-Coulombic driving force for adsorption. The influence of adsorbate valency is consistently accounted for by the model.

Temperature dependence of cadmium adsorption on oxides. 2. Thermodynamics and microcalorimetry of the Cd(NO3)2-rutile system.

Abstract The enthalpy of cadmium adsorption onto rutile (TiO 2 ) was determined microcalorimetrically. Thermodynamic analysis of the temperature dependence of cadmium adsorption provides an independent estimate of this enthalpy. By introducing a model to account for enthalpic contributions due to OH − co-adsorption, we show that results from both methods are consistent. Adsorption of the cadmium ion as such is enthalpically unfavourable. However, because of concomitant OH − /H + adsorption, both the overall adsorption and desorption are exothermic processes.

Application of metallization patterns to glass

Abstract Metallization patterns are applied to the selection plates of the Zeus panel to be able to direct the electrons through the substructure to the proper colour pixel at the phosphor screen. The patterns, which also partially cover the insides of the holes in the selection plates, are applied using a subtractive process with either vacuum-deposited aluminium or wet-chemically applied silver. Different options for the lithographic resist are discussed, i.e. a cataphoretic negative resist and a sprayed positive resist. Satisfactory results have been obtained with both options. Adhesion of the metal to glass has been accomplished by application of a chromium adhesion layer in the case of the vacuum-deposited metal and by mechanical anchoring in the case of the wet-chemically applied metallization.