Outer-sphere complexes, surface coordination

For the silica gel (Figure 3A), the solution was removed slightly less effectively, and more Cs was left (ca. 0.0020 atoms/A2). The spectral behavior is quite similar to that of boehmite, except that there is a peak due to surface Cs coordinated by only water molecules and not in contact with the surface oxygens (so-called outer sphere complexes)at 30% RH. Complete dynamical averaging among sites at frequencies greater than ca. 10 kHz occurs at 70% RH and greater. 

A prototypical example of a molecular probe used extensively to study the mineral adsorbent-solution interface is the ESR spin-probe, Cu2+ (Sposito, 1993), whose spectroscopic properties are sensitive to changes in coordination environment. Since water does not interfere significantly with Cu11 ESR spectra, they may be recorded in situ for colloidal suspensions. Detailed, molecular-level information about coordination and orientation of both inner- and outer-sphere Cu2+ surface complexes has resulted from ESR studies of both phyllosilicates and metal oxyhydroxides. In addition, ESR techniques have been combined with closely related spectroscopic methods, like electron-spin-echo envelope modulation (ESEEM) and electron-nuclear double resonance (ENDOR), to provide complementary information about transition metal ion behaviour at mineral surfaces (Sposito, 1993). The level of sophistication and sensitivity of these kinds of surface speciation studies is increasing continually, such that the heterogeneous colloidal particles in soils can be investigated ever more accurately. 

Inner-sphere complexes are relatively stable in comparison to outer-sphere complexes under equivalent solution conditions (i.e. pH, ionic strength), and in a competitive situation will tend to displace less stable adsorbates. This is a fundamental property of coordination reactions, and explains the observed trends in metal uptake preference observed in lichen studies (Puckett et al., 1973). Metal sorption results previously attributed to ion exchange reactions are more precisely described as resulting from competitive surface complexation reactions involving multiple cation types. Strictly speaking, each metal adsorption reaction can be described using a discrete mass law relation, such as... 

There are two widely accepted mechanisms for adsorption of solutes by a solid surface. Outer-sphere surface complexation, or non-specific adsorption, involves the electrostatic attraction between a charged surface and an oppositely charged ion in solution (Fig. 3). The adsorbed ion resides at a certain distance from the mineral surface. Inner-sphere complexation, also termed specific adsorption, involves the formation of a coordinative complex with the mineral surface (Kingston et al., 1972 Fig. 3). Inner-sphere complex bonds are more difficult to break than outer-sphere complex bonds and result in stronger adsorption of ions. 

It is important to distinguish between outer-sphere and inner-sphere complexes. In inner-sphere complexes the surface hydroxyl groups act as a-donor ligands, which increase the electron density of the coordinated metal ion. Cu(II) bound in an inner-sphere complex is a different chemical entity from Cu(II) bound in an outer-sphere complex or present in the diffuse part of the double layer. The inner-spheric Cu(II) has different chemical properties for example, it has a different redox potential with respect to Cu(I), and its equatorial water is expected to exchange faster than that in Cu(II) bound in an outer-sphere complex. As we shall see, the reactivity of a surface is affected, above all, by inner-sphere complexes. 

I nations 82 and 83 distinguish between inner-sphere and outer-sphere complexes of a surface bound Pb(II). Actual bond lengths, conformations, and site energies for coordination complexes on a real surface may vary from one site to n not her. The postulated reactions do, however, provide a means of estimating how adsorption density responds in a semiquantitative way to changing medium conditions. See, for example, Schindler and Stumm (1987) for a comprehensive discussion of acid-base and complexation equilibria at oxide-water interfaces. 

An intermediate situation between the non-specific electrostatic adsorption and the highly specific irmer sphere complex formation is the formation of an outer sphere complex where the original ligands remain coordinated to the metal ion, but form weak specific bonds with the surface, for instance hydrogen bonds. 

Some ions or molecules are attracted to the surface by non-specific electrostatic attractions and are able to penetrate the Stern layer and bind chemically on surface sites. Most often, these ions are complexing anions, easily hydrolyzable cations or neutral molecules forming true coordination complexes with surface groups. Depending on the strength of the interaction between the adsorbed species and the surface, inner sphere or outer sphere complexes ate formed. These species are called physisorbed or chemisorbed respectively. This phenomenon is known as specific adsorption, or surface complexatioii, and a chemical term appears in the free enthalpy of adsorption. Specific adsorption is further discussed in Chapter 9. 

It is important to distinguish between outer-sphere and inner-sphere complexes. In inner-sphere complexes the surface hydroxyl groups act as o-donor ligands which increase the electron density of the coordinated metal ion. Cu(II) bound inner-... 

Similarly, surface protonation tends to increase the dissolution rate, because it leads to highly polarized interatomic bonds in the immediate proximity of the surface central ions and thus facilitates the detachment of a cationic surface group into the solution. On the other hand, a surface coordinated metal ion, e.g., Cu2+ or Al3+, may block a surface group and thus retard dissolution. An outer-sphere surface complex has little effect on the dissolution rate. Changes in the oxidation state of surface central ions have a pronounced effect on the dissolution rate (see Chapter 9). 

Surface complexation models attempt to represent on a molecular level realistic surface complexes e.g., models attempt to distinguish between inner- or outer-sphere surface complexes, i.e., those that lose portions of or retain their primary hydration sheath, respectively, in forming surface complexes. The type of bonding is also used to characterize different types of surface complexes e.g., a distinction between coordinative (sharing of electrons) or ionic bonding is often made. While surface coordination complexes are always inner-sphere, ion-pair complexes can be either inner- or outer-sphere. Representing model analogues to surface complexes has two parts stoichiometry and closeness of approach of metal ion to... 

Further use of relaxation data, now studying the water and the ligand protons34 36, leads to an estimate of the outer sphere hydration of the lanthanides. We know there are no water molecules in the first coordination sphere of course. These outer sphere relaxation data for the different cations are proportional to susceptibilities and electron relaxation times and become very useful in the study of the inner sphere hydration of other complexes M(dipic) (H20)x and M(dipic)2(H20)y, see below. Note that there is no evidence of further association of the Ln(III) tris-dipicolinate complexes with small cations such as sodium ions. Later we shall show that these anions can bind to biological cationic surfaces and act as shift or relaxation probes. 

It involves an outer-sphere mechanism (see Section 3.1.4) because the four carboxylate groups in the monovalent [Co(III)EDTA] complex are coordinated to the metal center, whereas in the divalent [Co(II)EDTA]2- complex, one carboxylate group is free to coordinate with the surface and thus it adsorbs through an entirely different path. 

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