Mono-molecular layer

To maximise separation efficiency requires low H and high N values. In general terms this requires that the process of repeated partitioning and equilibration of the migrating solute is accomplished rapidly. The mobile and stationary phases must be mutually well-dispersed. This is achieved by packing the column with fine, porous particles providing a large surface area between the phases (0.5-4 m2/g in GC, 200-800 m2/g in LC). Liquid stationary phases are either coated as a very thin film (0.05-1 pm) on the surface of a porous solid support (GC) or chemically bonded to the support surface as a mono-molecular layer (LC). 

If the surface of a metal or carbon electrode is covered with a layer of some functional material, the electrode often shows characteristics that are completely different from those of the bare electrode. Electrodes of this sort are generally called modified electrodes [9] and various types have been developed. Some have a mono-molecular layer that is prepared by chemical bonding (chemical modification). Some have a polymer coat that is prepared either by dipping the bare electrode in a solution of the polymer, by evaporating the solvent (ethanol, acetone, etc.) of the polymer solution placed on the electrode surface, or by electrolytic polymerization of the monomer in solution. The polymers of the polymer-modified electrodes are either conducting polymers, redox polymers, or ion-exchange polymers, and can perform various functions. The applications of modified electrodes are really limit-... 

An L-B film is formed by the dispersion of amphoteric molecules at an air-water surface (Figure 8.20). These molecules have a polar group at one end, something like a carboxy substituent (in this respect they resemble the surfactant molecules which make micelles), and a long non-polar aliphatic chain. The polar group stays in the polar water phase, and the aliphatic chain stays in the non-polar air environment. The L-B film at the water surface is then made by the controlled compression of these molecules by means of a floating barrier. The molecules then line up to form a mono-molecular layer on the water surface. 

Solute adsorption is usually restricted to a mono molecular layer, since the solid-solute interactions, although strong enough to compete successfully with the solid-solvent interactions in the first adsorbed monolayer, do not do so in subsequent monolayers. Multilayer adsorption has, however, been observed in a number of cases, being evident from the shape of the adsorption isotherms and from the impossibly small areas per adsorbed molecule calculated on the basis of monomolecular adsorption. 

If the adsorbed particles are packed to form a single layer on the surface, the coverage can be expressed as NML where ML denotes a mono-molecular layer (monolayer). If NML can be accommodated and N, have... 

Adsorption experiments were conducted on chromium, platinum, cadmium, and zinc the sources and preparation of these metal specimens have been reported previously (16). In preparing adsorbed, mono-molecular layers by adsorption directly from the molten pure acid (5), the clean adsorbing substrate was first heated to a temperature just above the melting point of the acid (see Table I), a few crystals of the acid were sprinkled on the surface, and the resulting pool of molten acid was teased over the whole surface with a previously freshly flamed platinum wire. If spontaneous retraction of the liquid acid did not occur, the specimen was allowed to cool and all of the solidified material adhering on top of the adsorbed monolayer was removed by appropriate solvent treatments as discussed below. 

Type I Correspond to monomolecular adsorption postulated by Langmuir. The volume of the gas adsorbed approaches a limiting value, just enough to complete a mono molecular layer even when the gas pressure is rather low. Further increase in pressure hardly produce any further rise in the amount of adsorption. The examples are furnished by adsorption of nitrogen or hydrogen on charcoal at temperatures close to -180 C. 

Figure 15.2. Types of adsorption isotherms (I) monomolecular layer (II and III) multimolecular layers (IV and V) multimolecular layers and condensation in pores (VI) phase transition of a mono-molecular layer on the surface after Brunauer 1945). (Walas, 1988).

As the concentration of the solute is increased, the specific amount adsorbed levels off to a constant value (Fig. 4-1). At the higher concentrations, the surface of the adsorbent is coated with a mono-molecular layer of the solute, and no further material may be adsorbed. One might say that the adsorbent was saturated. 

Since the adsorption is limited to complete coverage by a mono-molecular layer, the surface may be divided into two parts the fraction d covered by the adsorbed molecules and the fraction 1 — 0, which is bare. Since only those molecules striking the uncovered part of the surface can be adsorbed, the rate of adsorption per unit of total surface will be proportional to 1 — 0 that is,... 

Summary In concluding the treatment of physical properties of catalysts, let us review the purpose for studying properties and structure of porous solids. Heterogeneous reactions with solid catalysts occur on parts of the surface active for chemisorption. The number of these active sites and the rate of reaction is, in general, proportional to the extent of the surface. Hence it is necessary to know the surface area. This is evaluated by low-temperature-adsorption experiments in the pressure range where a mono-molecular layer of gas (usually nitrogen) is physically adsorbed on the catalyst surface. The effectiveness of the interior surface of a particle (and essentially all of the surface is in the interior) depends on the volume and size of the void spaces. The pore volume (and porosity) can be obtained by simple pycnometer-type measurements (see Examples 8-4 and 8-5). The average size (pore radius) can be estimated by Eq. (8-26) from the... 

Solid materials adsorb a mono-molecular layer of adsorbate at a given temperature and pressure. A knowledge of the molecular size (diameter) and mass of the adsorbate adsorbed enables one to calculate the specific surface area. 

Figure 11.102. Schematic representation of the dispersion process the polymer matrix is sheared and transported, whereby the agglomerates will be first distributed and destroyed, followed by dispersion (adsorption of a mono-molecular layer of matrix polymer onto the particle surface), and later phase separation. [Reproduced fiom ref 37 with kind permission of Elsevier.]...

On the currently-accepted theory, some of the cations are held close to the surface of the clay particle as a strongly-adsorbed mono-molecular layer (known as the Stern Layer) the remainder form a more diffuse layer in which the concentration of ions falls off exponentially with distance from the surface. 

Figure 12-4. Determination of the cosine of the contact angle d on the interfacial surface tension 7/v of the liquid used on different substrates in contact with air (20° C). PE, poly(ethylene) PTFE, poly(tetrafluorethylene) PEP, poly(hexafluoropropylene) FLA, perfluorolauric acid (mono-molecular layer on platinum). (After R. C. Bowers and W. A. Zisman.)...

Some other experiments by the Marburg group should be mentioned here . They might be named molecular sheet experiments. Here, mono-molecular layers of two different dyestuffs, one the donor and the other the acceptor, were separated by multiple layers of an inert surface-active material, 50 to several 100 A thick. The transfer efficiency from one side of that sandwich to the other was found to decrease with the inverse 4th power of the distance. For two-dimensional layers this corresponds to a / -distance dependence of the elementary transfer process. It must be mentioned, however, that the experimental Rq values are essentially higher than calculated. This might result from the mutual interaction between the close-lying dyestuff molecules in their individual layers. 

Structurally controlled organic thin films are expected for the application in functional sensors and electronic devices [1-5]. Langmuir-Blodgett (LB) technique is one of the most promising methods for preparing mono-molecular layered structure in atomic scale. It is also inevitable that the physical and electronic properties of such advanced thin film are only defined after the detailed characterization of the structure of those films. 

In these terraces, there are also a lot of defects present. Therefore, there are the various types of surface sites present on the catalyst surfaces, which are illustrated in Fig. 3.68. Those surface sites include the kink, step, point defect and the surface atom such as the bonding part of atoms adsorbed and vacancies etc. All these surface sites are very active, even though the equilibrium concentrations at the melting points are far less than 1% of the mono-molecular layer. They play an important role in the migration of atoms on the surface. The physical and chemical features of these surface sites are active. The rates of redox and chemical reactions... 

Under some circumstances, a material can spread out into a monatomic or mono-molecular layer or film. For example, solutions of stearic or oleic acid (both long-chain fatty acids) in a hydrocarbon solvent can be carefully dripped onto water when the solvent evaporates, the remaining fatty acids can arrange themselves into a monomolecular film on the water s surface. Such films have a definite surface coverage (that is, a definite area) depending on the number of fatty acid molecules present. 

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