Adsorption liquid metals

Metal/molten salt interfaces have been studied mainly by electrocapillary833-838 and differential capacitance839-841 methods. Sometimes the estance method has been used.842 Electrocapillary and impedance measurements in molten salts are complicated by nonideal polarizability of metals, as well as wetting of the glass capillary by liquid metals. The capacitance data for liquid and solid electrodes in contact with molten salt show a well-defined minimum in C,E curves and usually have a symmetrical parabolic form.8 10,839-841 Sometimes inflections or steps associated with adsorption processes arise, whose nature, however, is unclear.8,10 A minimum in the C,E curve lies at potentials close to the electrocapillary maximum, but some difference is observed, which is associated with errors in comparing reference electrode (usually Pb/2.5% PbCl2 + LiCl + KC1)840 potential values used in different studies.8,10 It should be noted that any comparison of experimental data in aqueous electrolytes and in molten salts is somewhat questionable. 

At present it is impossible to formulate an exact theory of the structure of the electrical double layer, even in the simple case where no specific adsorption occurs. This is partly because of the lack of experimental data (e.g. on the permittivity in electric fields of up to 109 V m"1) and partly because even the largest computers are incapable of carrying out such a task. The analysis of a system where an electrically charged metal in which the positions of the ions in the lattice are known (the situation is more complicated with liquid metals) is in contact with an electrolyte solution should include the effect of the electrical field on the permittivity of the solvent, its structure and electrolyte ion concentrations in the vicinity of the interface, and, at the same time, the effect of varying ion concentrations on the structure and the permittivity of the solvent. Because of the unsolved difficulties in the solution of this problem, simplifying models must be employed the electrical double layer is divided into three regions that interact only electrostatically, i.e. the electrode itself, the compact layer and the diffuse layer. 

With the addition of a pseudopotential interaction between electrons and metal ions, the density-functional approach has been used82 to calculate the effect of the solvent of the electrolyte phase on the potential difference across the surface of a liquid metal. The solvent is modeled as a repulsive barrier or as a region of dielectric constant greater than unity or both. Assuming no specific adsorption, the metal is supposed to be in contact with a monolayer of water, modeled as a region of 3-A thickness (diameter of a water molecule) in which the dielectric constant is 6 (high-frequency value, appropriate for nonorientable dipoles). Beyond this monolayer, the dielectric constant is assumed to take on the bulk liquid value of 78, although the calculations showed that the dielectric constant outside of the monolayer had only a small effect on the electronic profile. 

The potential of zero charge (pzc) is a characteristic potential for a given interface, and hence is of obvious interest. In the absence of specific adsorption, it can be measured as the potential at which the Gouy-Chapman capacity obtains its minimum this value must be independent of the electrolyte concentration, otherwise there is specific adsorption. For liquid metals the pzc coincides with the maximum of the surface tension (see Section 3.5). 

Valuable information can be obtained from thermal desorption spectra (TDS) spectra, despite the fact that electrochemists are somewhat cautious about the relevance of ultrahigh vacuum data to the solution situation, and the solid/liquid interface in particular. Their objections arise from the fact that properties of the double layer depend on the interaction of the electrode with ions in the solution. Experiments in which the electrode, after having been in contact with the solution, is evacuated and further investigated under high vacuum conditions, can hardly reflect the real situation at the metal/solution interface. However, the TDS spectra can provide valuable information about the energy of water adsorption on metals and its dependence on the surface structure. At low temperatures of 100 to 200 K, frozen molecules of water are fixed at the metal. This case is quite different from the adsorption at the electrode/solution interface, which usually involves a dynamic equilibrium with molecules in the bulk. 

One of the important electrochemical interfaces is that between water and liquid mercury. The potential energy functions for modeling liquid metals are, in general, more complex than those suitable for modeling sohds or simple molecular liquids, because the electronic structure of the metal plays an important role in the determination of its structure." However, based on the X-ray structure of liquid mercury, which shows a similarity with the solid a-mercury structure, Heinzinger and co-workers presented a water/Hg potential that is similar in form to the water/Pt potential described earlier. This potential was based on quantum mechanical calculations of the adsorption of a water molecule on a cluster of mercury atoms. ... 

The electrical double-layer (edl) properties pose a fundamental problem for electrochemistry because the rate and mechanism of electrochemical reactions depend on the structure of the metal-electrolyte interface. The theoretical analysis of edl structures of the solid metal electrodes is more complicated in comparison with that of liquid metal and alloys. One of the reasons is the difference in the properties of the individual faces of the metal and the influence of various defects of the surface [1]. Electrical doublelayer properties of solid polycrystalline cadmium (pc-Cd) electrodes have been studied for several decades. The dependence of these properties on temperature and electrode roughness, and the adsorption of ions and organic molecules on Cd, which were studied in aqueous and organic solvents and described in many works, were reviewed by Trasatti and Lust [2]. 

T1he adsorption of metal ions from aqueous solutions is a phenomenon of immediate interest to workers in many diverse disciplines. The incorporation of metals into geological sediments, removal of metal ions from industrial and civic effluent, interference of trace metal ions in analytical and electroanalytical chemistry, ore flotation, metallurgical leaching processes, and the stability of ceramic slips are all processes which are controlled to a large extent by interaction of metal ions with solid-liquid interfaces. 

Interestingly, protein adsorption is also a field of biological interfacial chemistry which parallels that of synthetic materials at the solid - liquid interface. A number of spectroscopic advances have been made which allow FT-IR to be used in kinetic monitoring of protein adsorption on metals and "biocompatible" polymers. In addition to providing in - situ measurements of total adsorbed protein, FT-IR can also yield information about perturbation of protein secondary structure in adsorbed layers. 

An example of the application of the MPE technique is given in Table 4.6 for AI2O3, which was studied using Sn and Co as the auxiliary liquids. The dihedral angles xp, xp° and 0 in these systems were measured by Nikolopoulos (1985). For both systems, at each temperature, it was found that the difference between xp and xp° is of the order of the experimental error, meaning that metallic vapours on A1203 surfaces was negligible. This result is to be expected for a non-reactive metal /oxide system in... 

Liquid metals wet well metallic substrates (0 interfacial interactions unless the metallic surfaces are oxidized. Adsorption of oxygen on solid substrates can increase contact angles by tens of degrees. Intrinsic wetting seems to be slightly improved in systems with some solubility or which form intermetallics. The main effect of interfacial reactions is the disruption of oxide layers covering the metallic surfaces, allowing the formation of real metal-metal interfaces. 

Figure 6.14. Possible isotherms of liquid metal/oxide interfacial energy in the non-reactive range when enrichment of the interface in oxygen occurs by a) Gibbsian adsorption or b) 2D phase...

Understanding chemical reactivity at liquid interfaces is important because in many systems the interesting and relevant chemistry occurs at the interface between two immiscible liquids, at the liquid/solid interface and at the free liquid (liquid/vapor) interface. Examples are reactions of atmospheric pollutants at the surface of water droplets[6], phase transfer catalysis[7] at the organic liquid/water interface, electrochemical electron and ion transfer reactions at liquidAiquid interfaces[8] and liquid/metal and liquid/semiconductor Interfaces. Interfacial chemical reactions give rise to changes in the concentration of surface species, but so do adsorption and desorption. Thus, understanding the dynamics and thermodynamics of adsorption and desorption is an important subject as well. 

For compound semiconductors, one of the simplest approaches involves the monolayer adsorption of metal salts from liquid solutions, and a subsequent conversion of the salt to the desired semiconductor. This second step can be carried out in solution or in the gas phase. 

Whatever the solid product may be, it seems rather remarkable that the two interfacial tensions vs. the solid should be so closely matched, particularly when the interfacial tensions of the two liquids vs. fresh glass differ by at least 41 dynes, and probably by several hundred dynes. It may well be, however, that the approximation (Equation 7) is not valid for this new solid product, on account of adsorption of metal atoms. 

The first example is the measurement at a mercury (Hg) electrode. After the invention of polarography, an enormous body of work on the adsorption of organic molecules has been made at a mercury electrode surface. However, mercury is a liquid metal and its hanging drop changes its shape in response to the change of the surface tension, thus to the electrode potential. It is a difficult task to measure the reflection change at a mercury drop electrode surface, since exclusion of the perturbation due to the change of the shape of the electrode is critical. 

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