The solid-gas interface

The property possessed by charcoal of absorbing unpleasant smelling gases from the atmosphere has been put to extensive use from the earliest times. 

Whilst charcoal possesses this property to a marked extent similar phenomena are to be noted at all solid surfaces. A gas brought into contact with a solid surface will be adsorbed into the surface to- an extent which is dependent on several factors, the nature of the gas and solid, the partial pressure of the gas and the characteristic structure of the exposed surface. This phenomenon of adsorption is, however, frequently complicated by solution of the gas in the solid to form solid solutions or compounds with the solid. The term sorption has been proposed by McBain to include the two phenomena of absorption and adsorption. Our attention will be limited to the characteristics of adsorption. 

The fact that the surface atoms in the plane of a crystal lattice surface possess the power of cohesion can also be demonstrated readily by such examples where clean pieces of platinum can be welded together, the joining of optically smooth glass surfaces and 

The surface of a crystal is thus chemically unsaturated and the existence of a definite surface energy is due to the fact that the surface is unsaturated. Since the surface of a crystal consists of a regular lattice of orderly distributed atoms or ions the surface adhesional forces may be regarded as being distributed over the centres of these atoms (Haber, J.G.S.I. xxxiii. 50 Z it.f. Elektro-chem. XX. 521, 1914) although in all probability these points represent maxima in a continuous field. 

The problem of the variation in the surface energies of various crystal facets can be attacked from several points of view. Bravais first noted that those planes of a crystal which were most densely packed and were also separated most distantly from the neighbouring parallel plane were those which appeared most frequently in crystals he noted also that a closely packed surface was usually associated with a wide interplanar distance and vice versa. Later Willard Gibbs indicated that the most stable planes on a growing crystal were those possessing the least interfacial surface energy. 

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One of the most important advances in electrochemistry in the last decade was tlie application of STM and AFM to structural problems at the electrified solid/liquid interface [108. 109]. Sonnenfield and Hansma [110] were the first to use STM to study a surface innnersed in a liquid, thus extending STM beyond the gas/solid interfaces without a significant loss in resolution. In situ local-probe investigations at solid/liquid interfaces can be perfomied under electrochemical conditions if both phases are electronic and ionic conducting and this... 

The above discussion relates to diffusion-controlled transport of material to and from a carrier gas. There will be some circumstances where the transfer of material is determined by a chemical reaction rate at the solid/gas interface. If this process determines the flux of matter between the phases, the rate of transport across the gas/solid interface can be represented by using a rate constant, h, so that... 

In all of these systems, the rate of generation at the gas-solid interface is so rapid that only a small fraction is canied away from the particle surface by convective heat uansfer. The major source of heat loss from the particles is radiation loss to tire suiTounding atmosphere, and the loss per particle may be estimated using unity for both the view factor and the emissivity as an upper limit from tlris source. The practical observation is that the solids in all of these methods of roasting reach temperatures of about 1200-1800 K. 

To describe hypergolic heating, Anderson and Brown (A10) proposed a theoretical model based upon spontaneous exothermic heterogeneous reactions between the reactive oxidizer and a condensed phase at the gas-solid interface. In these studies, the least complex case was considered, i.e., the one in which the solid phase is instantaneously exposed to a stagnant (nonflowing) gaseous oxidizer environment. This situation can be achieved experimentally provided the sample to be tested is suddenly injected into the desired environment in a manner designed to minimize gas flow. 

The equations describing the possible nonreactive and/or reactive forms of energy transfer at the gas-solid interface ... 

Volume 19 Volume 20 Volume 21 Volume 22 Simple Processes at the Gas—Solid Interface Complex Catalytic Processes Reactions of Solids with Gases Reactions in the Solid State Additional Section... 

Shrouds are generally placed around grid holes to reduce the velocity at the gas-solids interface and reduce particle attrition. Shrouds simply consist of short pipes centered over the smaller grid holes which have been selected in size and number to operate at a hole velocity defined by Eq. (9). 

A great deal of success was attendant on the early application of PM-IRRAS to the gas/solid interface. Golden et ai (1981) reported the development of instrumentation, using conventional dispersive optics, able to record detailed infrared reflection-absorption spectra from molecules adsorbed on single-crystal Pt without any interference from the gas-phase species. 

Alkali 10ns in aqueous solution are probably the most typical and most widely studied representatives of non-specific adsorption. The electrochemical term of non-specific adsorption is used to denote the survival of at least the primary hydration shell when an ion is interacting with a solid electrode. As pointed out previously, the generation of such hydrated ions at the gas-solid interface would be of great value because it would provide an opportunity to simulate the charging of the interfacial capacitor at the outer Helmholtz plane or perhaps even in the diffuse layer. 

In principle, it should be possible to disentangle that part of 353 needed to increase the gas - solid interface by a unit area in the following manner. Let the bar of Fig. 6... 

Whether the scratch does or does not grow, depends on its sharpness (i.e the radii of curvature of the gas - solid interface at the front of the crack) and on the initial depth 2c of the scratch. A relation between c and 7 was derived by Griffith8see also Petch82). It is... 

In the second case (limit of fast kinetics at the gas-solid interface), the film becomes entirely bulk transport limited, corresponding to the limit of Hebb— Wagner polarization. Since electronic conduction is fast, this situation yields a Warburg impedance for finite length diffusion ... 

Adsorption. Adsorption refers to the adsorption of gas on the gas-solid interface only. It may be van der Waal s adsorption or chemisorption, the latter including activated adsorption. 

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