Surface electrical states

This technique can be applied to samples prepared for study by scanning electron microscopy (SEM). When subject to impact by electrons, atoms emit characteristic X-ray line spectra, which are almost completely independent of the physical or chemical state of the specimen (Reed, 1973). To analyse samples, they are prepared as required for SEM, that is they are mounted on an appropriate holder, sputter coated to provide an electrically conductive surface, generally using gold, and then examined under high vacuum. The electron beam is focussed to impinge upon a selected spot on the surface of the specimen and the resulting X-ray spectrum is analysed. 

As we have seen, the electric state of a surface depends on the spatial distribution of free (electronic or ionic) charges in its neighborhood. The distribution is usually idealized as an electric double layer one layer is envisaged as a fixed charge or surface charge attached to the particle or solid surface while the other is distributed more or less diffusively in the liquid in contact (Gouy-Chapman diffuse model, Fig. 3.2). A balance between electrostatic and thermal forces is attained. 

In all physical and chemical processes, and in particular those of relevance to geochemistry, that involve the oxide/aqueous solution interface, it is important to understand the general, non-specific characteristics of that interface before focussing on those specific processes or mechanisms of interest. Due to the structure of mineral surfaces, the mineral oxide/aqueous solution interface will invariably acquire a net charge or electrostatic potential relative to the bulk solution. The electrical state of the interface will depend in part on the chemical reactions that can take place on the mineral surface, and in part on the electrolytic composition of the aqueous environment. 

There are two ways to control the electrical state determination at constant charge, oM, or at constant cell potential. From a thermodynamic point of view, isotherms with respect to relative surface excesses may be determined at constant charge or at any well-defined constant potential. However, the interpretation and physical meaning of the results may be significantly more difficult in the case when constant cell potential (-> cell voltage) is used. 

The AGf, AGAa AGAs. and AGss values, and, correspondingly, In fi and a values depend on the electric state of the surface, i.e., on the electrode potential or charge. This isotherm was deduced by Frumkin [i] (and named after him soon) as a general case of the -> Langmuir isotherm, which corresponds to a = 0. A statistical derivation of the Frumkin isotherm is available [ii] various model considerations and relations to other types of isotherms are discussed in [iii]. Another typical form of the Frumkin isotherm is... 

Inner electric (or internal electric) potential, — within the phase a is related to the -> electric field strength E in the interior of the phase by -Acj> = E. It is the sum of the - outer (or external) electric potential iff and the - surface potential ya, and it characterizes the electrical state of any phase a [i—iv]. When the free electrostatic charge in the phase a turns to zero, fa = 0 and inner potential is not measurable. See also - Lange. 

Finally, using response theory it is also possible to determine some excited states properties like the dipole moment, from response of the excited state energy to an applied constant electric field, or forces [222,230,231]. It is then possible to perform Molecular Dynamics simulations on electronic excited states surfaces, to describe the dynamics of photo-chemical reactions for example [210,232]. 

The quantity Wm, represents the total reversible work obtainable in the given change this may include other forms of work, e.g., electrical or surface work, in addition to work of expansion. The latter is equal to PAV ( 3g), and so Wrav. — PAV represents the reversible work, exclusive of work of expansion, that can be obtained from a given change in state. This quantity is sometimes referred to as the net work, and is represented by so that by equation (25.10),... 

The experimentally controlled variables which are explicit in equation (10.9.12) are the activity of the adsorbate and the temperature T. The experimentally observed quantity is the surface excess Fa. The dependence of the adsorption on the electrical state of the interface is expressed through the local effective field E perpendicular to the interface and the average dipole moments of the adsorbate (/7a) and water molecules p ) in the same direction. The contribution of the last term is much larger under most circumstance for adsorption at the electrode solution interface than at the solution air interface. As a result, further treatment of the two problems is quite different. 

Ii is now known that /-electrons and their orbitals, hybrid and other wise, are responsible for the bonding within the metal and at the surface. The type of bond in the bulk leads to properties such as crystal structure and dimensions, melting temperature, mechanical strength, magnetic state, and electrical conductivity. Surface bonds determine adsorption and surface mechanisms. The ability of a molecule to bond with the surface depends upon two factors < 1) geometric or ensemble, and (2) electronic or ligand. 

Calculations give values of the contact angle of water on quartz of )/ 4° which is close to experimental data. For this calculation based on Eq. (10D.2), the third component, i.e. the repulsive structural force, was introduced into the equation for the disjoining pressure. Thus, the phenomenon of the incomplete wetting of silicate surfaces is connected to the influence of the electrical state of the film-gas interface. A small contact angle is caused by structural repulsive forces. A still higher effect can be provided by recharging the film-gas surface, which is corroborated experimentally (Zorin et al. 1979). 

Two of the most relevant parameters to characteiize the electrical state of the carbon surface in solution are both the isoelectric point (lEP) and the point of zero of charge (PZC). The lEP is defined as the pH where the charge at the slipping plane pH of the Stem Layer is zero. Generally it is obtained when an electric field, a pressure gradient, etc. is used to move the charged particles of colloids in solution microelectrophoresis, electroosmosis methods, etc. 

The Freundlich and Langmuir theories, which use distribution coefficients Kd to set the ratios of sorbed to dissolved ions, are applied widely in groundwater studies (Domenico and Schwartz, 1990) and used with considerable success to describe sorption of uncharged organic molecules (Adamson, 1976). The models, however, do not account for the electrical state of the surface, which varies sharply with pH, ionic strength, and solution composition. Freundlich... 

Surface complexation models, on the other hand, account explicitly for the electrical state of the sorbing surface (e.g., Adamson, 1976 Stumm, 1992). This class of models includes the constant capacitance, double layer, and triple layer theories (e.g., Westall and Hohl, 1980 Sverjensky, 1993). Of these, double layer theory (also known as diffuse layer theory) is most fully developed in the literature and probably the most useful in geochemical modeling (e.g., Dzombak and Morel, 1987). 

The electrical state of any phase a can be characterized by its internal potential, which is a sum of the external (or outer) potential induced by free electrostatic charges of the phase and the surface potential x [6, 23-26] ... 

Steady state surface photovoltage (SPV) spectroscopy is useful for determining the nature of the junction between the nanocrystalline film and substrate. SPV studies of Ti02 films on Sn02 confirm that an electric field exists at the interface, driving electrons into the substrate. This is consistent with the values of the work functions of the two materials in vacuum (4.85 eV for Sn02 F, 5.15 eV for... 

The early work on passive iron by Priestley, Bergman, Wenzel, and Keir (see Vol. Ill, index, p. 846), had been forgotten until the phenomenon was rediscovered by N. W. Fischer, G. Wetzlar and G. T. Fechner. The latter published a complete German translation of Keir s paper. Experiments of 1825 (before Wetzlar s) by J. F. W. Herschel were first published in 1833 attributed the passivity to a certain permanent electric state of the surface of the metal. Passivity was once more rediscovered by Schonbein. The passivity of copper and bismuth in nitric acid was discovered by T. Andrews." ... 

---