Charge outer-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... 

Complexation of Pu is discussed in terms of the relative stabilities of different oxidation states and the "effective" ionic charge of Pu0 and Pu02+2. An equation is proposed for calculating stability constants of Pu complexes and its correlation with experimental values demonstrated. The competition between inner v outer sphere complexation as affected by the oxidation state of Pu and the pKa of the ligand is reviewed. Two examples of uses of specific complexing agents for Pu indicate a useful direction for future studies. 

As Fig. 2.4 illustrates, a cation can associate with a surface as an inner sphere, or outer-sphere complex depending on whether a chemical, i.e., a largely covalent bond, between the metal and the electron donating oxygen ions, is formed (as in an inner-sphere type solute complex) or if a cation of opposite charge approaches the surface groups within a critical distance as with solute ion pairs the cation and the base are separated by one (or more) water molecules. Furthermore, ions may be in the diffuse swarm of the double layer. 

The stability constant for the outer sphere complex depends on the charge of the reacting species and the ionic strength of the medium, and can be readily calculated from electrostatic considerations. 

The dashed line in the complex in (4.21) and (4.22) indicates an outer-sphere (o.s.) surface complex, Kos stands for the outer-sphere complex formation constant and kads [M 1 s 1] refers to the intrinsic adsorption rate constant at zero surface charge (Wehrli et al., 1990). Kos can be calculated with the help of a relation from Gouy Chapman theory (Appendix Chapter 3). 

In a more restrictive sense, the term "ion exchange" is used to characterize the replacement of one adsorbed, readily exchangeable ion by another. This circumscription, used in soil science (Sposito, 1989), implies a surface phenomenon involving charged species in outer-sphere complexes or in the diffuse ion swarm. It is not possible to adhere rigorously to this conceptualization because the distinction between inner-sphere and outer-sphere complexation is characterized by a continuous transition, (e.g., H+ binding to humus). 

To give some idea of the value of — it is approximately 14 M for interaction of 2 + and 2— charged reactants at p = 0.1 M and 0.15 M for interaction between a cation and a zero charged species. It is necessary to reaffirm the point that the occurrence of outer-sphere complexes, which can be observed in the studies of the Fe(III) —Br , Ni(II) —CHjPO and the ultrasonics of a number of systems, does not... 

AK is estimated as 0 cm mol" for outer-sphere complex formation involving an uncharged species and =3 cm mol for reactants whose charge product is -2. ... 

If the reactants are oppositely charged, the collision complex in (5.18) takes the form of an outer-sphere complex with discernable stability. For the outer sphere redox reaction between Co(NH3)jL"+ and Fe(CN)g, L being a series of pyridine or carboxylate derivatives, saturation kinetics are observed, with the pseudo first-order rate constant (/ obs)> Fe(II) in excess, being given by... 

A proper D mechanism requires that kx be identical to the rate constant for the exchange of solvent (due account being taken of any statistical correction when more than one solvent molecule is present) and the value of k2 (in reality the term fc2/fc i[S] is used because the constants cannot be separated) should be sensitive to the chemical nature of L rather than its size and charge (factors that control Kos in an interchange mechanism). The most convincing demonstration of a D mechanism would be found in cases where k2/k-1[S] is much larger than any value expected for an outer-sphere complex formation constant, but this is not a necessary requirement for the mechanism. 

Stern layer A fixed layer of ions on the surface of a charged particle in an aqueous solution. The Stern layer consists of inner- and outer-sphere complexes. The Stern and Gouy layers comprise the double layer (compare with Gouy layer). 

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. 

The Poisson-Boltzman (P-B) equation commonly serves as the basis from which electrostatic interactions between suspended clay particles in solution are described ([23], see Sec.II. A. 2). In aqueous environments, both inner and outer-sphere complexes may form, and these complexes along with the intrinsic surface charge density are included in the net particle surface charge density (crp, 4). When clay mineral particles are suspended in water, a diffuse double layer (DDL) of ion charge is structured with an associated volumetric charge density (p ) if av 0. Given that the entire system must remain electrically neutral, ap then must equal — f p dx. In its simplest form, the DDL may be described, with the help of the P-B equation, by the traditional Gouy-Chapman [23-27] model, which describes the inner potential variation as a function of distance from the particle surface [23]. 

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. 

Interaction 1 denotes electrostatic forces between humic substances (negatively charged) and metal ions (positively charged). It is a relatively weak interaction (outer-sphere complex) and the cation can be readily exchanged by other weakly bonding cations,... 

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. 

---