Oriented adsorption

In highly diluted solutions the surfactants are monodispersed and are enriched by hydrophil-hydrophobe-oriented adsorption at the surface. If a certain concentration which is characteristic for each surfactant is exceeded, the surfactant molecules congregate to micelles. The inside of a micelle consists of hydrophobic groups whereas its surface consists of hydrophilic groups. Micelles are dynamic entities that are in equilibrium with their surrounded concentration. If the solution is diluted and remains under the characteristic concentration, micelles dissociate to single molecules. The concentration at which micelle formation starts is called critical micelle concentration (cmc). Its value is characteristic for each surfactant and depends on several parameters [189-191] ... 

A study518 of the mechanism of oxidation of alcohols by the reagent suggested that a reversible, oriented adsorption of the alcohol onto the surface of the oxidant occurs, with the oxygen atom of the alcohol forming a coordinate bond to a silver ion, followed by a concerted, irreversible, homolytic shift of electrons to generate silver atoms, carbon dioxide, water, and the carbonyl compound. The reactivity of a polyhydroxy compound may not, it appears, be deduced from the relative reactivity of its component functions, as the geometry of the adsorbed state, itself affected by solvent polarity, exerts an important influence on the selectivity observed.519... 

In addition to lowering surface tension, surface-active agents contribute to emulsion stability by oriented adsorption at the interface and by formation of a protective film around the droplets. Apparently, the first molecules of a surfactant introduced into a two-phase system act to form a monolayer additional surfactant molecules tend to associate with each other, forming micelles, which stabilize the system by hydrophilic-lipophilic arrangements. This behavior has been depicted by Stutz et al. ( ) and is shown in Figures 1-5. 

Calculations of the contributions toward the adsorption energy by these forces reveal them to be quite appreciable and, indeed, a strong and oriented adsorption will always be found. These calculations result in somewhat too low values when the dipole moment n is used, because of the short distances between the dipoles and the negatively charged attracting centers. In many cases the actual contribution may be 10% higher than the calculated one (39). 

Figure 10. (a) Oriented adsorption of IgG on a pre-coated surface, leading to a biologically more active layer, (b) Prevention of (a) spreading and (b) tilting of adsorbed IgG by pre-adsorbed molecules. The antigen binding sites are represented by the white dots at the top of the F(ab)... 

Figure 2. Schematic mechanisms of solubilization. Clockwise from 5 o clock ion exchange with the surface solution in interior of micelle oriented adsorption formation of oil-soluble salt...

Site Orientation Adsorption Energy(eV) Charge Transfer... 

Adsorbates such as HQ which are capable of more than one surface orientation can undergo a transition from uniformly oriented adsorption to mixed or disordered orientation [60] as the temperature rises about 45°C, as shown in Fig. 29. 

The mechanism of the electron-transfer reaction is not well understood questions concerning how close an approach of the substrate to the electrode is necessary and the role of orientation, adsorption, polarizability, and solvent molecules in the process, are discussed presently. 

Adsorption of the admixture on the hydrating cement grains could decrease flocculation in at least three ways (D44). The first is an increase in the magnitude of the ( -potential if all the particles carry a surface charge of the same sign and sufficient magnitude, they will repel each other. The second is an increase in solid- liquid affinity if the particles are more strongly attracted to the liquid than to each other, they will tend to disperse. The third is steric hindrance the oriented adsorption of a non-ionic polymer can weaken the attraction between solid particles. 

The value of this parameter was determined on the basis of soap adsorption on AAH precipitates at pH = 6.0-6.4. For soap solutions between 0.1-0.3 g/m , an asymptotic shape of the isotherm of adsorption was found and the value of adsorption capacity was determined as 4.5 eq/kg AI2O3. Assuming that an oriented adsorption takes place and that the area covered by a carboxyl group is 30.5 A, the specific surface area of AAH was calculated ... 

Baizer and coworkers established the most brilliant industrial electroorganic synthesis of the hydrodimerization of acrylonitrile to adiponitrile. They extended this hydrodimerization to a variety of activated olefins and in some cases [41 3] paid attention to the stereochemistry of products. However, their stereochemical data were not enough to discuss the stereochemical course of the reaction. Afterward, an attempt was made to provide a working hypothesis in the hydrodimerization of cinnamates by considering an orientated adsorption of radical anion intermediates on a cathode surface, but this was not persuasive because of a lack of experimental data on the stereochemistry of both the starting olefins and products. Recently Utley and coworkers [44-46] have reported stereochemical data of hydrodimers derived from a variety of cinnamic acid esters with chiral alcohol components. 

Figure 7.11 Surfactant films at the water/oil interface in o/w and w/o emulsions (a) formation of a monomolecular film at the oil/water interface for the stabilisation of o/w emulsions (b) stabilisation of w/o emulsions by the oriented adsorption of divalent soap salts (not to scale).

Natta (194) discovered the phenomenon of stereospecific polymerization responsible for the formation of valuable isotactic polymers of a-olefins in the presence of heterogeneous catalysts and of activators and explained it by oriented adsorption. However, the latter is an obligatory condition of the multiplet theory, according to which the reacting atoms come into contact with the sui face, and the substituents must be oriented in one direction, namely, off the surface (195). According to Natta, the electronic properties also play an important part in stereo-specific polymerization. 

Tanabe, K., and Uyeda, N. ( 1974) Oriented adsorption of amino acids on (001) nickel surface. Bull Inst Chem. Res. KyotoUniv. 52, 616-647, Chem. Abstr. 1998,128, 90728g. 

The adsorption double refraction results from orientated adsorption of solvent molecules on the polymer frame. There are indications that this effect plays a part in the double refraction of certain systems However, a theoretical approach to this type of birefringence does not exist. Notwithstanding these difficulties, Treloar ... 

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