Mono-molecular surface layer

Mono-molecular surface layers exhibit strong effects in small colloidal particles. In the visible region even sub-monomolecular ad-layer of sulfides, iodides, oxides or borates lead to a significant modification of the plasmon resonance [9] (Figure 9). Ordered arrays of metal-columns deposited on, imprinted in or etched out of a substrate surface allow the excitation of extended plasmons similar to colloid arrays. Sharp tip structures allow high local fields and thus enable efficient energy transfer applied e.g. in surface enhanced Raman scattering (SERS). 

Figure 5-11 shows a simple model of the compact double layer on metal electrodes. The electrode interface adsorbs water molecules to form the first mono-molecular adsorption layer about 0.2 nm thick next, the second adsorption layer is formed consisting of water molecules and hydrated ions these two layers constitute a compact electric double layer about 0.3 to 0.5 nm thick. Since adsorbed water molecules in the compact layer are partially bound with the electrode interface, the permittivity of the compact layer becomes smaller than that of free water molecules in aqueous solution, being in the range from 5 to 6 compared with 80 of bulk water in the relative scale of dielectric constant. In general, water molecules are adsorbed as monomers on the surface of metals on which the affinity for adsorption of water is great (e.g. d-metals) whereas, water molecules are adsorbed as clusters in addition to monomers on the surface of metals on which the affinity for adsorption of water is relatively small (e.g. sp-metals). 

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. 

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. 

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. 

Aleskovsky, Kolstov, Volkova, and co-workers investigated the chemical modification of silica surfaces by inorganic compounds (274-281). The materials obtained are useful in heterogeneous catalysis, electronics, and so forth. The method for molecular lamination was developed (276-278) by interacting volatile and readily hydrolyzing halides of transitional metals with the surface of hydroxylated silica. This method makes it possible to modify the silica surface with mono- and polymolecular layers of V, Cr, Ti, Fe, Al, B, and so forth. 

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. 

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