Solid-vacuum

The liquid-solid interface, which is the interface that is involved in many chemical and enviromnental applications, is described m section A 1.7.6. This interface is more complex than the solid-vacuum interface, and can only be probed by a limited number of experimental techniques. Thus, obtaining a fiindamental understanding of its properties represents a challenging frontier for surface science. 

There are several approaches to gain the required surface sensitivity with diffraction methods. We review several of these here, emphasizing the case of solid/vacuum interfaces some of these also apply to other interfaces. 

Imaging/mapping Sample requirements Solid, vacuum compatible... 

Sample requirements Any solid vacuum-compatible material... 

Fig. 1 illustrates possible setups that have been used in various studies. Scheme (a) allows the simulation of two equivalent interfaces between aqueous and non-aqueous phases. Scheme (b) simulates two equivalent aqueous/non-aqueous and two equivalent non-aqueous/vacuum interfaces and can be used to avoid the interactions between the aqueous phase and its images. Scheme (c), usually used when the non-aqueous phase is solid, simulates simultaneously an aqueous/non-aqueous, a solid/vacuum and an aqueous/gas interface. In addition, a confining wall at large distances from the aqueous/gas interface may be employed to prevent the loss of molecules from the simulation cell. 

Electrochemical reactions are driven by the potential difference at the solid liquid interface, which is established by the electrochemical double layer composed, in a simple case, of water and two types of counter ions. Thus, provided the electrochemical interface is preserved upon emersion and transfer, one always has to deal with a complex coadsorption experiment. In contrast to the solid/vacuum interface, where for instance metal adsorption can be studied by evaporating a metal onto the surface, electrochemical metal deposition is always a coadsorption of metal ions, counter ions, and probably water dipols, which together cause the potential difference at the surface. This complex situation has to be taken into account when interpreting XPS data of emersed electrode surfaces in terms of chemical shifts or binding energies. 

These measurements have verified that the work function of an electrode, emersed with the double layer intact, depends only on the electrode potential and not on the electrode material or the state of the electrode (oxidized or covered with submonolayer amounts of a metal) [20]. Work function measurements on emersed electrodes do not serve the same purpose as in surface science investigations of the solid vacuum interface. At the electrochemical interface, any change of the work function by adsorption is compensated by a rearrangement of the electrochemical double layer in order to keep the applied potential i.e. overall work function, constant. Work function measurements, however, could well be used as a probe for the quality of the emersion process. Provided the accuracy of the measurement is good enough, a combination of electrochemical and UPS measurements may lead to a determination of the components of equation (4). 

The use of a solid vacuum vessel as the rigid envelope, on the other hand, is better tor repetitive testing where an integral test is to be made. When solid envelopes are used it is also possible to recover the helium once the test has been completed. 

The electrochemical interface has a very large electrical capacity (— 10"s F cm-2) compared with the solid—gas or solid—vacuum interface due to the existence of an ionic space charge localized at a short distance from the electrode. 

If both electron populations are considered, it is possible to find an implicit analytical solution to (17.29), inside (x < 0) and outside (x > 0) the target, by imposing the continuity of the potential and the electric field at the solid-vacuum interface, x = 0. In the region x <0, the solution reads [94]... 

Nutshe-filter/centrifuge Recovery of solids Liquid-solid Vacuum to atmospheric pressure, -10-100°C... 

Optical techniques are not limited to solid-vacuum interfaces like charged particle techniques, so their further development can expand the range of surface structural studies to solid-solid, solid-liquid, solid-gas and liquid-gas interfaces. 

Molecular adsorbates of increasing size should be a fertile area of research. Such research could lead to the study of biological surfaces. The surface structure at solid-gas, solid-liquid, and solid-solid interfaces should be explored and compared with the results for the solid-vacuum interface. Structural properties of thin films, electrodes, and composite materials can be obtained in this way. 

Immemorial wetting (which we simply call immersion and denote by subscript imm ) is a process in which the surface of a solid, initially in contact with vacuum or a gas phase, is brought in contact with a liquid without changing the area of the interface. Here, a solid-gas (or solid-vacuum) interface is replaced by a solid-liquid one of the same area. 

Electrochemistry shares many concepts with surface science, and for the last two decades there has been an exchange of methods and ideas between these two neighboring disciplines. However, the electrosorption valency has no equivalent in surface science, since experiments at the solid/gas or solid/vacuum interface cannot be performed at constant potential. However, for low coverages, and near the potential of zero charge, the electrosorption valency can be related to the dipole moment of the adsorbate, which can be measured both in surface science and, though with greater difficulty, also in electrochemistry. In the following, we point out the relation between these two quantities. 

The surface energies of several materials have been determined by measuring the change of the lattice constant (Table 2). One problem of the technique lies in the preparation of the sample. Only a limited number of substances can be prepared as small spherical particles with a defined radius on a carbon support. Often the particles are not spherical, which limits the applicability of the above equation. The surface stress can only be determined for the solid/vacuum interface, not in gas or liquids. In addition, the interpretation of diffraction effects from small particles becomes increasingly difficult with diminishing particle size (43,44],... 

Figure 13 Plot of electronic charge density as a function of distance across a solid-vacuum interface as calculated from a jellium model...

I) Techniques used to characterize solid-gas and solid-vacuum Interfaces are useful. If not indispensable. In establishing the properties of solid surfaces In contact with liquids ... 

Books. M. W. Roberts, Chemistry of the Metal-Gas Interface , Oxford University Press, Oxford, 1978 F. C. Tompkins, Chemisorption of Gases on Metals , Academic Press, London, 1978 Experimental Methods in Catalysis Research , ed. R. B. Anderson and P. T. Dawson, Academic Press, London, 1976 Chemistry and Physics of Solid Surfaces , ed. R. Vanselow and S. Y. Yong, CRC Press, Cleveland, Ohio, 1977 Advances in Characterisation of Metal and Polymer Surfaces , ed. L. H. Lee, Academic Press, New York, 1976 K. Tamaru, Dynamic Heterogeneous Catalysis , Academic Press, London, 1978 The Solid-Vacuum Interface , ed. A. van Oostrom and M. J. Sparnay, Surface Sci., 1977, 64 Electron Spectroscopy , ed. C. R. Brundle and A. D. Baker, Academic Press, New York, 1977, Vol. 1 Auger Electron Spectroscopy (Bibliography 1925—1975) , compiled by D. T. Hawkins, Plenum, New York, 1977. 

This equation is applicable to any interface, allowing the energies of extension per unit area of the solid-vacuum, solid-vapor, solid-liquid, and liquid-vapor... 

Immersional wetting corresponds to a process where a solid—vacuum or solid-vapor interface is replaced by a soHd—liquid one. When starting from a solid-vacuum interface, the free energy variation during the process is (per unit area)... 

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