Specific adsorption, role

It was concluded from this and related works that suppression of the photodissolution of n-CdX anodes in aqueous systems by ions results primarily from specific adsorption of X at the electrode surface and concomitant shielding of the lattice ions from the solvent molecules, rather than from rapid annihilation of photogenerated holes. The prominent role of adsorbed species could be illustrated, by invoking thermodynamics, in the dramatic shift in CdX dissolution potentials for electrolytes containing sulfide ions. The standard potentials of the relevant reactions for CdS and CdSe, as well as of the sulfide oxidation, are compared as follows (vs. SCE) [68] ... 

We have also discussed two applications of the extended ab initio atomistic thermodynamics approach. The first example is the potential-induced lifting of Au(lOO) surface reconstmction, where we have focused on the electronic effects arising from the potential-dependent surface excess charge. We have found that these are already sufficient to cause lifting of the Au(lOO) surface reconstruction, but contributions from specific electrolyte ion adsorption might also play a role. With the second example, the electro-oxidation of a platinum electrode, we have discussed a system where specific adsorption on the surface changes the surface structure and composition as the electrode potential is varied. 

A detailed evaluation shows that the shift of the energies of the surface states with potential is surprisingly large, and approaches 1 eV/V for state B. A completely satisfactory explanation has not yet been given, but specific adsorption of the anion is likely to play a role. 

Hohl, H., L. Sigg, and W. Stumm (1980), "Characterization of Surface Chemical Properties of Oxides in Natural Waters The Role of Specific Adsorption Determining the Specific Charge," in M. C. Kavanaugh and J. O. Leckie, Eds., Particulates in Water, Advances in Chemistry Series, ACS 189, 1-31. 

This effect is explained by the role of the specific adsorption of anions. It is assumed that the specific adsorption of C104 ions is negligible [45, 46], and no chemical changes occur in the course of the electrochemical study of anodic dissolution and corrosion of nickel [47]. 

Adsorption is another phenomenon particularly suited to tracer studies. It plays an important role in the glueing and finishing of wood. In a preliminary study of the adsorption of poly(vinyl acetateJ4C) on smooth geometrically simple surfaces, Weatherwax and Tarrow (66) were able to show that swollen cellophane adsorbed 40 times as much as the other surfaces tested. In the course of this work they were able to measure specific adsorptions of as little as 0.2 pg/cm2 with good accuracy. 

In the limit of a - 0, the ideal Langmuir adsorption isotherm is obtained. See - Frumkin isotherm, and for the role of surface heterogeneity - Temkin isotherm. Refs. [i] Horanyi G (2002) Specific adsorption. State of art Present knowledge and understanding. In Bard A], Stratmann M, Gileadi M, Urbakh M (eds) Thermodynamics and electrified interfaces. Encyclopedia of electrochemistry, vol. I. Wiley-VCH Verlag, Weinheim, pp 349-382 [ii] Calvo EJ (1986) Fundamentals. The basics of electrode reactions. In Bamford CH, Compton RG (eds) Comprehensive chemical kinetics, vol. 26. Elsevier, Amsterdam, pp 1-78... 

We have demonstrated that mobile phase amendment with a range of different IPRs represents a diachronic scientific consideration. It can be speculated that any charged species added to a mobile phase, may play the IPR role they interact with the stationary phase, establishing electrostatic potentials according to their specific adsorption isotherms. It follows that charged analyte retention is altered via both electrostatic interactions with the stationary phase and pairing equilibria in bulk eluent. This indicates how broad in scope and versatile IPC is. 

In this chapter we discuss the rates of adsorption, paying special attention to those few cases where information on the rate of specific adsorption (reaction of an adsorbate in the adsorption layer) is available. Furthermore, we elaborate on the chemical processes involved in the dissolution of minerals and concentrate on the dissolution of oxides, silicates, and carbonates, which play an enormous rx)le in the chemical weathering and erosion. We try to demonstrate that in most cases the rate-determining step in the dissolution is a chemical reaction at the surface of the mineral. Thus we have here an excellent example of the relationship between surface stracture and reactivity. Surface chemistry plays an equally important role in the formation of the solid phase (precipitation, nucleation, and crystal growth). Nature s selectivity is reflected in the creation of a crystal and its growth. 

Specific adsorption on well defined materials has been the subject of many reviews [8-13]. Specific adsorption plays a key role in transport of nutrients and contaminants in the natural environment, and many studies with natural, complex, and ill defined materials have been carried out. Specific adsorption of ions by soils and other materials was reviewed by Barrow [14,15]. The components of complex mineral assemblies can differ in specific surface area and in affinity to certain solutes by many orders of magnitude. For example, in soils and rocks, (hydr)oxides of Fe(IH) and Mn(IV) are the main scavengers of metal cations and certain anions, even when their concentration expressed as mass fraction is very low. Traces of Ti02 present as impurities are responsible for the enhanced uptake of U by some natural kaolinites. In general, complex materials whose chemical composition seems very similar can substantially differ in their sorption properties due to different nature and concentration of impurities , which are dispersed in a relatively inert matrix, and which play a crucial role in the sorption process. In this respect the significance of parameters characterizing overall sorption properties of complex materials is limited. On the other hand the assessment of the contributions of particular components of a complex material to the overall sorption properties would be very tedious. 

Also the choice of the electrostatic model for the interpretation of primary surface charging plays a key role in the modeling of specific adsorption. It is generally believed that the specific adsorption occurs at the distance from the surface shorter than the closest approach of the ions of inert electrolyte. In this respect only the electric potential in the inner part of the interfacial region is used in the modeling of specific adsorption. The surface potential can be estimated from Nernst equation, but this approach was seldom used In studies of specific adsorption. Diffuse layer model offers one well defined electrostatic position for specific adsorption, namely the surface potential calculated in this model can be used as the potential experienced by specifically adsorbed ions. The Stern model and TLM offer two different electrostatic positions each, namely, the specific adsorption of ions can be assumed to occur at the surface or in the -plane. 

The specific adsorption of halide anions has been studied on Au and Ag single crystals [14]. On Au(l 11), these ions form incommensurate hexagonal monolayers that compress as the electrode potential is changed in the positive direction [19]. However, on Ag(lOO), Br adsorption occurs at the hollow site formed by four metal atoms in a square pattern. This type of commensurate monolayer has a c(2 X 2) surface structure. These studies demonstrate the role of atomic surface structure in determining the extent of adsorption. Differences between adsorption on Ag(lOO) and Au(lOO) are explained in terms of differences in the strengths of the metal-halide bonds [14]. 

Very interesting studies have been made of the pattern for UPD on single crystal metal substrates [49]. A commensurate pattern is usually observed for the formation of a partial monolayer. The theory of UPD and formation of the initial monolayer is an area of active research. Of course, the phenomenon of UPD is restricted to formation of a monolayer. Once this has formed, the deposition process reverts to one of metal ion M"+ on metal M. By comparing the UPD process with anion-specific adsorption, the role of partial charge transfer in these processes is clarified. 

The importance of a specific adsorption mode is recognized especially in heterogeneous enantioselective catalysis, where intimate interactions of prochiral molecule with a modifier result in a specific enantioselectivity [3]. The role of the modifier is to steer the adsorption of the reagent in such a way that enantioselective hydrogenation is enabled. Heterogeneous enantioselective hydrogenation of a-keto esters over Pt catalysts modified with alkaloids has been studied intensively during recent years [4, 5, 6]. However, a-keto esters have just one reactive center and thus enantioselectivity aspects can be considered only. Diones, which possess... 

Carbon nanotubes, generally in the form of multiwalled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs), may exhibit specific adsorption and electronic properties in comparison with activated carbon, due primarily to their peculiar morphology, the role of defects, the probability of opening or closing of the tubes, and so on [5], which are believed to induce cooperative or synergetic interactions between metal or metal oxide... 

---