Charge inner-sphere complex

The surface behavior of Na is similar to that of Cs, except that inner sphere complexes are not observed. Although Na has the same charge as Cs, it has a smaller ionic radius and thus a larger hydration energy. Conseguently, Na retains its shell of hydration waters. For illite (Figure 6), outer sphere complexes resonate between -7.7 and -1.1 ppm and NaCl... 

Figure 11.1. Schematic views of various ways in which an organic chemical, i, may sorb to natural inorganic solids (a) adsorption from air to surfaces with limited water presence, (b) partitioning from aqueous solutions to the layer of vicinal water adjacent to surfaces that serves as an absorbent liquid, (c) adsorption from aqueous solution to specific surface sites due to electron donor-acceptor interactions, (d) adsorption of charged molecules from aqueous solution to complementarily charged surfaces due to electrostatic attractions, and (e) chemisorption due to surface bonding or inner sphere complex formation.

In coordination chemistry two types of complex can occur between metals and complexant ligands. Outer-sphere complexes are relatively weak electrostatic associations between a hydrated metal ion and a complexant ligand, and in which both of the charged species retain a hydration shell. In contrast, inner-sphere complexes are stronger interactions in which a covalent bond is formed between a metal ion and a ligand. 

Although each SCM shares certain common features the formulation of the adsorption planes is different for each SCM. In the DDLM the relationship between surface charge, diffuse-layer potential, d, is calculated via the Gouy-Chapman equation (Table 5.1), while in the CCM a linear relationship between surface potential, s, is assumed by assigning a constant value for the inner-layer capacitance, kBoth models assume that the adsorbed species form inner-sphere complexes with surface hydroxyls. The TLM in its original... 

The protonated aluminol sites are the most effective fluoride sorption sites and are usually responsible for the rapid kinetics due to coulombic attraction between the positively charged sites and the negatively charged fluoride species. The reaction with non-protonated sites involves ligand exchange, leads also to the formation of inner-sphere complexes, releases hydroxyl ions, is slow and characterized by a higher activation energy. 

Similar to solution complexation, surface complexation can be distinguished between inner-spherical complexes (e.g. phosphate, fluoride, copper), where the ion is directly bound to the surface, and outer-spherical (e g. sodium, chloride) complexes where the ion is covered by a hydration sleeve with the attraction working only electrostatically. The inner-sphere complex is much stronger and not dependent on electrostatic attraction, i.e. a cation can also be sorbed on a positively charged surface (Drever 1997). 

The adsorption reaction that occurs between metallic ions and the charged surfaces of clay-organics may involve formation of either relatively weak outer-sphere complexes, or strong inner-sphere complexes. 

Equation 4.26, as noted above, has two terms—a chemical complexation term, [-SM(m 1)[H+]/[-SHj[Mm+], and an electrical potential term, exp[(m-l)A /// T]. The electrical potential term is directly dependent on pH (see Eq. 4.27b). Therefore, by regulating the magnitude of the physical attraction between a metal and a surface, the electrical potential indirectly affects the magnitude of surface complexes that could form. Recall that before a chemical complex or inner-sphere complex between two reactants is formed, they must physically collide successfully at a given frequency. When a metal-ion (e.g., contaminant) and a surface are oppositely charged, physical collisions maximize, and, thus, inner-sphere complex formation maximizes. When a metal-ion (e.g., contaminant) and a surface are similarly charged, physical collisions minimize, and inner-sphere complex formation also minimizes. 

Whenever it is feasible, it is useful to study encounter equilibria experimentally, as in the case of Co(III) complexes in water and non-aqueous solvents [7]. There are some weaknesses in equation (7.3) as evidenced by (i) the formation of anionic species by overcompensation of the original positive charge of the metal complex, (ii) lack of evidence of outer-sphere complexation in some cases like Co(en)j+ with Fe(CN) - and (iii) the fact that outer-sphere complexes are preferred over inner-sphere complexes in several systems. 

The solvent molecules form an oriented parallel, producing an electric potential that is added to the surface potential. This layer of solvent molecules can be protruded by the specifically adsorbed ions, or inner-sphere complexed ions. In this model, the solvent molecules together with the specifically adsorbed, inner-sphere complexed ions form the inner Helmholtz layer. Some authors divide the inner Helmholtz layer into two additional layers. For example, Grahame (1950) and Conway et al. (1951) assume that the relative permittivity of water varies along the double layer. In addition, the Stern variable surface charge-variable surface potential model (Bowden et al. 1977, 1980 Barrow et al. 1980, 1981) states that hydrogen and hydroxide ions, specifically adsorbed and inner-sphere... 

The edge charges can also bond ions with opposite charges. This process, however, is not directed clearly by electrostatic forces chemical properties play an important role. The ions are sorbed with no hydrate shell, that is, inner-sphere complexation occurs. These reactions and the surface complexation models for their quantitative treatment are shown in general in Chapter 1, Table 1.7. 

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