Results from Specific Adsorption Systems

Results From Specific Adsorption Systems.—A number of specific applications have already been discussed the most enlightening have been concerned with monitoring changes in adsorption distributions when the surface is subjected to some treatment, e.g. heating or annealing. However a number of independent studies have been concerned with a specific system and it is interesting to compare the results. 

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] ... 

Self-assembled monolayers (SAMs) provide a versatile system for control of the interfacial properties of inorganic compounds. SAMs result from the spontaneous adsorption and assembly of molecular compounds into crystalline structures. Commonly used molecules consist of a head group with a specific affinity for a particular material, and a terminal group containing a chemical function that becomes exposed... 

A final area of difficulty is in the application of data analysis to specific models of adsorption isotherms. This difficulty results from the fact that different models for adsorption isotherms generate plots of surface versus dissolved concentration that have characteristic shapes. If a plot of observational data results in a curve with a shape similar to that generated by a model, this result is often taken as proof that the particular model applies. Unfortunately, this assumption has been made for situations where many of the basic requirements of the model are violated in the system under study. The Langmuir adsorption isotherm model has suffered considerable abuse by geochemists in this regard. It should be remembered that "shapes" of adsorption isotherms are far from proof that a specific model applies. 

The specific surface area (ssa) of the solids (m g ) obtained after hydrolysis and drying at various temperatures was measured routinely by multi-point BET method at 77K by N2 adsorption in a Fisons Sorptomatic 1900 system. From these isotherms the BET ssa s were determined and the corresponding pore size distribution was also found. Typical results including the adsorption-desorption isotherms as well as the corresponding pore size distributions are shown in Fig.2... 

Computations may be carried out by optimizing the parameters of the applied equations as well as the specific capacity of adsorbed phase for obtaining the best fit of theoretical excess isotherms to the experimental data. Tables presented below contain the characteristics of the adsorption systems from Table 1 illustratively used for computations and the calculation results obtained for Eq.(7) combined with substitutions (27) or (29) under the assumption that n = const. 

Chemical formulas of crystalline salts whose ions tend to be specifically adsorbed do not reflect complex solution chemistry of these salts. Apparently the adsorption systems are simple, but in fact the solutions are multicomponent systems, and the coexisting species can substantially differ in their affinities to the surface. The speciation in the interfacial region can be completely different from that in bulk solution. Many analytical methods do not distinguish between particular species, and the results representing overall sorption behavior of all species involving the element of interest are obtained. Fortunately, some results obtained by means of spectroscopic methods can be resolved into pieces of information regarding... 

Many adsorbates and/or adsorbents are redox sensitive. The specific adsorption in such systems depends on the redox potential, which is very difficult to measure or control, thus, systematic studies in this direction are rare. On the other hand some practical implications are well known, e.g. the uptake of chromates by soils and sediments in enhanced on addition of Fe(II) salts [27] as an effect of a redox reaction, in which Cr(VI) is reduced to Cr(III). A few examples of redox reactions accompanying sorption processes are reported in the column results". The changes of oxidation state in the sorption process are probably more common than it is apparent from literature reports, but they are often overlooked, namely, analytical methods must be specially tailored to observe these changes. 

In view of the pH effects induced by hydrolysis in solution, the blank curve method (titration of the initial solution without the absorbent, Fig. 3.3) is only applicable, when the titration curves of the initial solution without the adsorbent and of the supernatant are identical, i.e. when the hydrolysis is negligible over the pH range of interest. Otherwise the proton adsorption can be obtained by back titration of the supernatant (Section 3.I.B. 2). In both methods (blank curve, back titration) the results need a correction for the acid or base associated with the original adsorbent, which is obtained from titrations at different ionic strengths under pristine conditions (Fig. 3.3). The description of the experimental procedure in the papers on the proton stoichiometry of specific adsorption is often not complete enough to assess if all necessary precautions have been taken into account, and the discrepancies in the results reported by different authors for similar systems are probably due in part to differences in the experimental procedure and interpretation of results. 

---