Inner-sphere adsorption

It is also termed chemisorption (especially for gases), inner sphere adsorption and, in the case of ligands, ligand exchange. The binding constants, Kf and for the surface complexes show the same stability trend as do the constants for the equivalent complexation reactions in solution. 

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. 

Bidentate-mononuclear complex A Stem inner-sphere adsorption complex that consists of two atoms bridging between an atom of the adsorbed species and an adsorbent metal atom. Commonly, two oxygens may bridge between an iron atom in the adsorbent and adsorbed arsenate (see Chapter 2). 

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. 

Figure 9.10. Schematic of zeta potential reversal curve A is a normal curve with curve B is reversed owing to an inner-sphere adsorption (adsorption within the slipping plane) of multivalent ions producing 2 (adapted from Taylor and Ashroft, 1972).

Observations indicate that Ca-rich feldspars are more susceptible to organo-ligand promoted dissolution than are the Na-or K-feldspars in experiments, soils and lithified sediment (32.70.73). although the proton-promoted dissolution rates of albite, anorthite and K-feldspar are approximately equal (22). The rate of feldspar dissolution promoted by organo-ligands is proposed to increase when inner-sphere adsorption of organo-ligands on aluminosilicates weakens critical crystal lattice bonds at the site of adsorption (77). 

In order to fully recondle the coordination chemistry and colloid chemistry approaches to inner sphere adsorption, one would like to compare the ligand properties of surface groups of various supports with those of molecules in homogeneous solution. Some elements of such a comparison have already been outlined in ni.C.l, but a systematic parallel study would necessitate a predse characterization of the adsorbed complex at the molecular level. There are indeed examples of this kind of characterization, but they constitute the exceptions rather than the rule. 

Figure 5 A comparison of inner sphere adsorption sites of [Cu(H20)6] on Ti02 (A) and...

Cu(H20)6p adsorption on Ti02 (anatase) has been studied by Ludwig et al. who showed that the support acts as a bidentate ligand The nature of the inner sphere adsorption sites is illustrated in Figure 5A, and can be formulated as Ti-OH-Ti-OH(2), i.e. both a bridging OH (between two TiW)... 

Chromate. Increasing chromate adsorption shifted the PZC of amorphous iron oxide to increasingly lower pH value indicating an inner-sphere adsorption mechanism (54). FTIR analyses of freeze-dried samples containing adsorbed chromate mixed with KBr showed shifts in absorption bands from those... 

To explore this question, the adsorption of glycine on AlOOH was recently investigated at the interface with water [78, 97]. This study confirmed the occurrence of an inner sphere Al-O-C-0 bond that was predicted from calculations at the interface with vacuum, and from numerous experimental studies of carboxylic acid adsorption on alumina polymorphs (see Ref. [78] and references therein). It was found that inner sphere adsorption (shown in Fig. 5.14, right), with -161.6 and -113.6 kJ moF for the anionic and zwittetionic species, respectively is significantly more stable as compared to outer sphere adsorption (-20.5 kJ mol (Fig. 5.14, left)). Beyond the result itself, it is important to note that complex events can now he handled with ab initio methods because they are able to model aU types of forces in reasonable agreement with experiment. 

FIGURE 5.14 Supercells used for the study of glycine adsorption on AlOOH boehmite at the interface with water in the outer-sphere adsorption mode G ft), and inner sphere adsorption mode (right). With permission ftom Motta et al. [78]. 

In the basic Stern model (BSM), also known as diffnse layer model (DLM), the 1 or P plane of the TLM is suppressed, as shown in Figure 12.7 (Hayes et al. 1991 Mattigod andZachara 1996 Zachara and Westall 1998). Thus, outer-sphere complexation is not considered, and two adjustable parameters are eliminated (both capacitances Q and C2). This simplification is reasonable when there is strong inner-sphere adsorption... 

Catalano J, Zhang Z, Fenter P, Bedzyk M. (2006) Inner-sphere adsorption geometry of se(iv) at the hematite (100) water interfece. Journal of Colloid and Interface Science 297 665-671. 

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