Spheres adsorption onto

The third class of redox species are couples located near the conduction band of WSe2- The only outer-sphere example found, which is suitable for use in aqueous electrolytes, is Ru(NH3)e3+. Its reduction is characterized by an immediate onset upon accumulation in the semiconductor and a tafel slope of 130 mV/decade. The reduction mechanism appears to be direct reduction of the Ru(NH3)e3+ by electrons from the accumulation layer. The only member of the forth class of redox species is triiodide ion. It is characterized by adsorption onto the semiconductor surface as was demonstrated by the first application of chronocoulometry to a semiconductor electrode (another demonstration of the reproducibility and low background currents on... 

In the following section, we describe the case of adsorption of a Sn complex onto a palladium oxide suspension. In an alkaline medium (a basic PdO hydrosol), chlorides in the SnCL complex are substituted in the coordination sphere of tin(IV) by hydroxo anions, which are in excess, yielding the stannate Sn(OH)g complex. The Sn Mossbauer spectroscopy spectrum of a bimetallic sol (frozen in liquid nitrogen) is compared with a true stannic solution. At the same tin concentration, it shows the changes in the Sn environment due to adsorption onto the PdO surface (Fig. 13.27). The isomer shift S is found to be close to zero for the stannate solution and increases when contacted with the PdO suspension, indicating a modification of the coordination sphere of tin. The increase in 5 can be correlated to an increase in the core level electronic density of tin. The quadrupole splitting A, is related to a modification of the symmetry of the close environment of tin, due to adsorption of Sn(OH)g complexes onto the PdO colloidal nanoparticles. 

The link between colloids and surfaces follows naturally from the fact that particulate matter has a high surface area to mass ratio. The surface area of a 1cm diameter sphere (4jtr ) is 3.14 cm, whereas the surface area of the same amount of material but in the form of 0.1 pm diameter spheres (i.e. the size of the particles in latex paint) is 314 000 cm. The enormous difference in surface area is one of the reasons why the properties of the surface become very important for colloidal solutions. One everyday example is that organic dye molecules or pollutants can be effectively removed from water by adsorption onto particulate activated charcoal because of its high surface area. This process is widely used for water purification and in the oral treatment of poison victims. 

Bidentate-binuclear complex A Stem inner-sphere adsorption complex that consists one adsorbed atom bonding to two separate metal oxides. As discussed in Chapter 2, an arsenate may adsorb onto two separate pairs of iron and oxygen atoms. 

Inner-sphere complex A type of Stern layer, where a chemical species bonds directly onto the surface of a solid material (adsorbent) in an aqueous solution. The formation of inner-sphere complexes is called chemisorption. Stern inner-sphere adsorption complexes are further divided into monodentate, bidentate-mononuclear, and bidentate-binuclear types (compare with outer-sphere complex). 

Monodentate A Stern inner-sphere adsorption complex where each adsorbed species attaches onto only one atom on the adsorbent surface. 

Strontium adsorption onto soil minerals is an important retardation mechanism for Sr " ". Chen et al. (1998) investigated the adsorption of Sr " " onto kaolinite, illite, hectorite, and montmorillonite over a range of ionic strengths and from two different electrolyte solutions, NaNO3 and CaCb- In all cases, the EXAFS spectra suggested Sr adsorbed to clay minerals as an outer-sphere mononuclear complex. Sahai et al. (2000) also found that on amorphous silica, goethite, and kaolinite substrates, Sr"+ adsorbed as a hydrated surface complex above pH 8.6. On the other hand, Collins et al. (1998) concluded from EXAFS spectra that Sr " " adsorbed as an inner-sphere complex on goethite. 

Bostick et al. (2002) studied Cs+ adsorption onto vermiculite and montmorillonite with EXAFS and found that Cs+ formed both inner-and outer-sphere complexes on both aluminosihcates. The inner-sphere complexes bound to the siloxane groups in the clay structure. Combes et al. (1992) found that NpOj adsorbed onto goethite as a mononuclear surface complex. Waite et al. (1994) were successful in describing uranyl adsorption to ferrihydrite with the diffuse layer model using the inner-sphere, mononuclear, bidentate surface complex observed with EXAFS. 

FIGURE 9.15 Schematic representation of structural models of Pb(ll) adsorption onto goe-thite surfaces, (a) Inner-sphere surface complex (b) surface polymerization anchored by some inner-sphere bound Pb(II) ions. (Reprinted with permission from Roe et al. 1991, 367—373. Copyright 1991 American Chemical Society.)... 

These examples (by no means exhaustive) illustrate the general aspects of cation adsorption onto oxide minerals inner-sphere complex formation, preferentially edge or corner binding as opposed to basal plane binding, eventually forming small polynuclear complexes or surface polymers but more rarely surface precipitates. All that is true of transition metal and p-block representative cations alkaline metals normally bind only electrostatically, and alkali earth metals most often do the same. 

We finally would like to emphasize that the variational approach of Muthukumar and colleagues [56, 107] provides a good description of the scaling behavior of the critical charge density for polyelectrolyte-sphere interactions, whereas the same approach fails to predict the correct dependence in the case of adsorption onto a cylindrical surface in the limit Ka< l (see Sect. 4.3). 

Fig. 10 Critical surface charge densities obtained by the WKB approach for polyelectrolyte adsorption onto planar, cylindrical, and spherical surfaces. The asymptotic scaling relations for a cylinder (rod) (45) and a sphere (53) are indicated by dotted lines [48]...

Comparing the results obtained by the WKB method with the exact solutions for the planar and spherical surface, we find, within 2% error, quantitative agreement in the planar case. For a sphere, we find the same asymptotic dependence of critical adsorption behavior for a wide range of geometries. The main advantage of the WKB method is a unified approach for the various geometries based on the same level of approximations. It can be applied at the same level of complexity to virtually any shape of the polylectrolyte-surface adsorption potential. Recent advances in polyelectrolyte adsorption under confinement [49,167] and adsorption onto low-dielectric interfaces [50] have been presented. 

Studies of the adsorption onto various substrates such as mica (190-192), silica (193, 195), aqueous magnesium chloride and sulfate solutions (196), porous media (197), and polystyrene spheres (198) and latexes (199) has been done. 

Figure 6.8 Illustration of colloid-templated nanoparticle assemblies. The process involves the layer-by-layer adsorption of charged polymers and oppositely charged nanoparticles onto the surfaces of the colloidal template. The colloidal core particles may then be removed to generate a hollow sphere of nanoparticles, held together by electrostatic interactions with the linear polymer glue ...

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