Some surface phenomena

This section is dedicated to some selected examples of the very broad spectrum of surface phenomena directly influenced by the dynamics of adsorption of surfactants. There are books and monographs which show these and other examples in much more detail (Adamson 1990, Dorfler 1994, Hunter 1992, Ivanov 1988, Krugljakov Exerowa 1990, Lyklema 1991, Schulze 1984). However, to make the reader acquainted with some of the huge variety of applications, we inserted this section into the book. 

The last mentioned method is particularly connected with the wetting behaviour of a solid surface by a solution. If a hydrophobic solid surface is not wetted in such a way that a 

The terms cohesion work and adhesion work can be defined as follows. When the liquid and the solid are brought into contact with each other a new interface is formed (index 1 corresponds to the liquid, index 2 to the solid phase, index 12 to the interface). The total decrease of Free Energy of this process is given by 

In the last equation 7z -7,2 is the so-called wetting tension b. The cohesion work of the liquid is twice the surface tension value, equal to 27,. The difference between adhesion and cohesion work is the spreading coefficient, which now reads 

Thus the spreading coefficient is equal to b-7, the difference between wetting an surface tension of the liquid. Since the spreading of a liquid presumes a contact angle 0 = 0 between the liquid and the solid phase, a positive value of S is needed. Positive S-values are obtained only if the sinface tension of the liquid is less than of the solid. The surface tension of liquids can be determined very easily, while for solid surfaces it is impossible to determine directly. If we assume that the minimal surface tension of an aqueous solution of a conventional surfactant is in the range of 25 - 30 mN/m, then paraffin can be wetted only if its surface tensions is higher. 

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For liquid-phase catalytic or enzymatic reactions, catalysts or enzymes are used as homogeneous solutes in the hquid, or as sohd particles suspended in the hquid phase. In the latter case, (i) the particles per se may be catalysts (ii) the catalysts or enzymes are uniformly distributed within inert particles or (hi) the catalysts or enzymes exist at the surface of pores, inside the particles. In such heterogeneous catalytic or enzymatic systems, a variety of factors that include the mass transfer of reactants and products, heat effects accompanying the reactions, and/or some surface phenomena, may affect the apparent reaction rates. For example, in situation (iii) above, the reactants must move to the catalytic reaction sites within catalyst particles by various mechanisms of diffusion through the pores. In general, the apparent rates of reactions with catalyst or enzymatic particles are lower than the intrinsic reaction rates this is due to the various mass transfer resistances, as is discussed below. 

Under ideal conditions, charge consumed by the double-layer capacitance and adsorbed reactants will follow the same time course as discussed earlier for ordinary chronocoulometry. Since the ratio of electrode area to solution volume is larger for thin-layer experiments, charge thus accounted for may represent a much greater proportion of the total. This fact points to an advantage of restricted diffusion experiments for studying some surface phenomena. 

Floatability is a- surface phenomenon and some solids are more easily floatable (such as sulfur, sulfides of the metals, graphite) than others (such as oxides, silicates, silica). 

The risk of devitrification rises the longer a glass is kept in a softened or melted state, and it is also linked to how dirty the glass is. Devitrification typically begins as a surface phenomenon, using either dirt or some other surface defect as a nucle-ation point. The devitrification process may be assisted by variations in the exte-... 

Since the surface strains are largely compressive in nature, the tendency to the development of cracking in the surface should be small. However, some surface cracking or shattering does occur in materials which are brittle in bulk, particularly if they cleave easily. The final surface may then be composed, in part or in whole, of systems of cleavage facets (15, 33). This phenomenon has been observed in a number of ionic crystals, such as rock salt and fluorspar, but not so far in metals. 

Interferometers, Applications to the Study of Explosion and Propulsion Phenomena. Two monochromatic light beams arriving out-ofphase at some surface will produce a fringe illumination pattern on that surface. This phenomenon is called interference. If the light beams are in phase the illumination intensity is the sum of the individual intensities, but if they are half-a-wavelength out of phase, the illumination intensity decreases and becomes zero if the individual light beams are of equal intensity. In-between there are gradations... 

Some other materials, such as glass and water, allow visible radiation to penetrate to considerable depths before any significant absorption takes place. Radiation through such scmitranspareiu materials obviously cannot be considered to be a surface phenomenon since the entire volume of the material interacts with radiation. On the other hand, both glass and water ace practically opaque to infrared radiation. Therefore, materials can exhibit different behavior at different wavelengths, and the dependence on wavelength is an important consideration in the study of radiative properties such as emissivity, absorptivity, reflectivity, and transmissivity of materials. 

It appears that the incorporation of metal adatoms into adsorbate structures stabilizes the reaction intermediates, and therefore, can be expected to be a general phenomenon on catalytic metal surfaces, at least for metal particles large enough to be considered as metallic. The dynamic processes of incorporation, release, and mass transport of metal adatoms may occur on the time scale of surface reactions and affect the reactive behavior of the intermediates, that is to say, the reaction kinetics. Indeed, STM studies have shown that the kinetic oscillation in some surface reactions can be partially attributed to the spatial organization of reactive species on the surfaces and the structural change in such complex surfaces on the time scale of reaction [69]. The structural complexity of the active surfaces and the origin of unusual surface reaction kinetics are of interest, and may be connected. Recently, such a relationship was established in the autocatalytic decomposition of formate and acetate on the Ni(llO) surface [21]. 

Although catalysis is a surface phenomenon, the spectroscopic investigation of the surface properties of tin-antimony oxides do not appear to have been pursued with an excessive degree of vigor. This may in part reflect the comparatively recent development of spectroscopic techniques with these specific powers, but it might also reflect the reluctance that some potentially... 

Surface-enhanced Raman scattering is a phenomenon where the cross section for Raman scattering for molecules adsorbed on some surfaces is enhanced by many orders of magnitude compared to the scattering cross section for the same molecule in the bulk—in solution, for example. 

The requirements for an antiozonant are believed to be more stringent. Since ozone attack is essentially a surface phenomenon, it is assumed (44-46) that an antioxidant can only be effective in the surface of the rubber. However, it has been found (47) that extraction makes very little difference to the antiozonant activity of MADA-B in rubber (see Table 16). The mechanism of antiozonant action may therefore require some revision. 

Although radiation can travel in a vacuum, it originates from matter. All forms of matter emit radiation through the mechanisms of electronic transitions and lattice vibrations. In most solids and liquids, radiation emitted from the interior is strongly absorbed by adjoining molecules. Therefore, radiation from these materials can be treated as a surface phenomenon. Radiation in gases and some semitransparent sohds and liquids, however, must be treated as a volumetric phenomenon. 

When radiation falls on a solid body, a definite fraction p may be reflected and the remaining fraction 1 — p enters the solid to be either transmitted or absorbed. Most solids (other than glasses, certain plastics, quartz, and some minerals) absorb radiation of all wavelengths so readily that, except in thin sheets, the transmissivity T is zero, and all nonreflected radiation is completely absorbed in a thin surface layer of the solid. The absorption of radiation by an opaque solid is therefore a surface phenomenon, not a volume phenomenon, and the interior of the solid is not of interest in the absorption of radiation. The heat generated by the absorption can flow into or through the mass of an opaque solid only by conduction. 

Vink has produced evidence in total conflict with previous experience which is suggested to show that the photo-oxidation of polypropylene is a bulk reaction rather than a surface phenomenon. ESCA studies, however, have shown that the photo-oxidation of polypropylene is clearly a surface phenomenon.In a recent study by Kollmann and Wood the photo-oxidation of polypropylene was found to be dependent upon the intensity of the light source. Thus, for unstabilized polymer the rate was proportional to I , whereas for stabilized polymer the rate was proportional to 1O 8-0.9 Thej-e appears to be some conflict in the literature as to whether chemical changes during the photo-oxidation of polyolefins correlate with the changes in mechanical properties.This has always been a difficult... 

Alfin polymerization of butadiene is unique in being a surface phenomenon. It is dependent upon specific components which must be present in proper proportions for the best reactions. Polymerization appears to be limited strictly to particular areas on the aggregate. These areas can be dispersed or poisoned by the presence of some compounds. Polymerization takes place by a radical mechanism, confined, however, to the surface where the monomer is adsorbed in a position ideal for polymerization. 

Experimental results with low concentration feeds or under conditions where M is close to 1.0 are in good agreement with the theoretical predictions. However, when the wall concentration becomes high, the solvent flux often cannot be controlled by adjusting the pressure difference. Thus, Eq. 117-401 no longer holds Some other phenomenon must be controlling the solvent flux. Careful examination of the membrane surface after these experiments shows a gel-like layer covering the membrane surface. This gel layer alters the flux-pressure drop relationship and controls the solvent flow rate. 

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