Surface coordination

Adsorption of Metal Ions and Ligands. The sohd—solution interface is of greatest importance in regulating the concentration of aquatic solutes and pollutants. Suspended inorganic and organic particles and biomass, sediments, soils, and minerals, eg, in aquifers and infiltration systems, act as adsorbents. The reactions occurring at interfaces can be described with the help of surface-chemical theories (surface complex formation) (25). The adsorption of polar substances, eg, metal cations, M, anions. A, and weak acids, HA, on hydrous oxide, clay, or organically coated surfaces may be described in terms of surface-coordination reactions ... 

The surface is described by a height h which is a function of position, specified by surface coordinates (x,y). It advances at a local time-averaged rate set by the current density. Superimposed on the average rate is a spatially uncorrelated, fluctuating deposition rate r. If the particles remain at the point of deposition, the local rate of growth is determined entirely by the flux. 

Surface Coordination of the Iodo Ligand. The chemisorption of iodine at Au, Pt and Ir surfaces has been demonstrated (15-9). Previous studies with single- and polycrystalline Pt (15-7) showed that aqueous iodide undergoes spontaneous oxidation upon chemisorption to form a monolayer of zerovalent iodine ... 

The E°i(ads) values obtained here indicate that, upon surface coordination, the redox potential of the iodine/iodide couple is shifted in the negative direction by about 0.90 V on Au, 0.76 V on Pt, and 0.72 V on Ir. These chemisorption-induced redox potential shifts can be employed to estimate the ratio of the formation constants for surface coordination of iodine and iodide ... 

The fact that Tdhpz on Au is identical to that on Pt at pH 7 is evidence that ri1-N surface-coordinated DHPz is also formed on Au at this same pH. Since (i) the DHPz isotherm on Au at pH 0 is not stepwise unlike those exhibited by compounds attached in multiple orientational states, and (ii) it has already been shown above that hydroquinone, the homoaromatic analogue of DHPz, is not chemisorbed on Au, it can be argued that i -N surface-coordination of DHPz occurs on Au at pH 0 even at coverages below the saturation value. It can be inferred further that the driving forces for protonation and Au-surface-coordination of the N heteroatom are equally competitive in molar acid. 

The unfettered N-coordination of DHPz to the Pt surface even in molar acid clearly indicates that the surface-coordination strength of DHPz is larger on Pt than on Au. This is consistent with the fact that, although analogous DHPz-Pt coordination complexes exist, none has been reported for Au. 

Some emphasis is given in the first two chapters to show that complex formation equilibria permit to predict quantitatively the extent of adsorption of H+, OH , of metal ions and ligands as a function of pH, solution variables and of surface characteristics. Although the surface chemistry of hydrous oxides is somewhat similar to that of reversible electrodes the charge development and sorption mechanism for oxides and other mineral surfaces are different. Charge development on hydrous oxides often results from coordinative interactions at the oxide surface. The surface coordinative model describes quantitatively how surface charge develops, and permits to incorporate the central features of the Electric Double Layer theory, above all the Gouy-Chapman diffuse double layer model. 

The extent of surface coordination and its pH dependence can again be explained by considering the affinity of the surface sites for metal ion or ligand and the pH dependence of the activity of surface sites and ligands. The tendency to form surface complexes may be compared with the tendency to form corresponding (inner-sphere) solute complexes (Fig. 2.7), e.g.,... 

Some colloid chemists often place these specifically bound cations and anions in the Stern layer (see Chapter 3.2). From a coordination chemistry point of view it does not appear very meaningful to assign a surface-coordinating ion to a layer different than H or OH in a =MeOH group. 

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). 

In the first sequence the dissolution reaction is initiated by the surface coordination with H+, OH, and ligands which polarize, weaken, and tend to break the metal-oxygen bonds in the lattice of the surface. Since reaction (5.7) is rate limiting and using a steady state approach the rate law on the dissolution reaction will show a dependence on the concentration (activity) of the particular surface species, Cj [mol nr2] ... 

Heterogeneous nucleation of CaF2 on Ce02. It occurs only in pH range where Ca2+ and F are specifically bound to the CeOz surface. This surface coordination, accompanied by partial dehydration of the ions, appears to be a prerequisite for the nucleation. IP = Ion Product. (Data H. Hohl)... 

Schematic representation of the various reaction modes for the dissolution of Fe(III)(hydr)oxides a) by protons b) by bidentate complex formers that form surface chelates. The resulting solute Fe(III) complexes may subsequently become reduced, e.g., by HS c) by reductants (ligands with oxygen donor atoms) such as ascorbate that can form surface complexes and transfer electrons inner-spheri-cally d) catalytic dissolution of Fe(III)(hydr)oxides by Fe(II) in the presence of a complex former e) light-induced dissolution of Fe(III)(hydr)oxides in the presence of an electron donor such as oxalate. In all of the above examples, surface coordination controls the dissolution process. (Adapted from Sulzberger et al., 1989, and from Hering and Stumm, 1990.)...

Sulzberger, B., D. Suter, C. Siffert, S. Banwart, and W. Stumm (1989), "Dissolution of Fe(III)(hydr)-oxides in Natural Waters Laboratory Assessment on the Kinetics Controlled by Surface Coordination", Marine Chemistry 28, 127-144. 

Sulzberger, B. (1990), "Photoredox Reactions at Hydrous Metal Oxide Surfaces a Surface Coordination Chemistry Approach", in W. Stumm, Ed., Aquatic Chemical Kinetics, Wiley-lnterscience, New York, pp. 401-429. 

Stumm, W., F. Furrer, and B. Kunz (1983), "The Role of Surface Coordination in Precipitation (Heterogeneous Nucleation) and Dissolution of Mineral Phases", Croat. Chem. Acta 56, 593-611. 

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... 

However, an alternative is to consider the TLM analogue of an inner-sphere surface coordination complex by placing the metal ion in the o-plane (19), e.g.,... 

Figure 2 gives a schematic illustration of the TLM with examples of ion-pair and surface coordination complexes. 

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