Simple Anionic Adsorption from Solution

Simple Anionic Adsorption from Solution. - Here the relevant equation would be (11), and again the equilibrium would be pH dependent but in this case shifts to the left at a high pH. It is also dependent upon the ionic affinity which increases with anionic polarizability and ionic charge, e.g., I Br Cr F- and SOf Q . 

---

Complex Ionic Adsorption from Solution. - Much of the metal adsorption that normally is of importance in catalyst preparative chemistry concerns the interaction with complex ions. Obviously, for example, all metal anionic adsorption must of necessity involve the metal as a complex ion. This tendency to form complex ions can cause substantial deviations from what one might normally expect from considerations of simple ion-exchange equilibria. This is true in the case of ferric chloride, for example reactions (12)-(15). 

Figure 6.1 A simple electrostatic adsorption mechanism illustrating the protonation-deprotonation chemistry of surface hydroxyl groups on oxide surfaces (which are neutral at the PZC) and the corresponding uptake of anionic or cationic complexes. Proton transfer to or from the surface can significantly affect the solution pH.

The influence of the cations and anions has been discussed separately with the solution properties and reactions in the main focus. It has, however, been known over 100 years that anions play a crucial role for the stabilization and coagulation of colloids. More recently, the contribution of anions on the stabilization of particles, biocolloids, and bubbles has received renewed attention. - In these papers, it has been pointed out that there exists a collaborative interaction between cations and anions upon adsorption of one of the complexes from solution. At high concentrations this effect renders the simple indifferent ions specific and selective to each other. It is also seen as a dependency on the acid-base pair chosen for the regulation of the pH. This effect certainly needs to be added as an extension to (correction of) the DLVO theory. However, as shown in this paper, it is just as probable that the anion and cation collaborate during the adsorption and formation of gels and precipitates at the surface. The presence of such mixed phases has been confirmed experimentally, e.g., during the formation of hydroxoapatite in silica gel layers. ... 

Note that, only when the solute mass balance equation is substituted, does the adsorption equation for the weak acid anion differ from that of the simple monovalent anion. 

Values of the PZC at the Hg solution interface are shown as a function of electrolyte concentration in fig. 10.6. In the case of NaF, the PZC with respect to a constant reference electrode is independent of electrolyte concentration. However, in the cases of the other halides, the PZC moves to more negative potentials as the electrolyte concentration increases. The latter observation is considered to be evidence that the anion in the electrolyte is specifically adsorbed at the interface. Specific adsorption occurs when the local ionic concentration is greater than one would anticipate on the basis of simple electrostatic arguments. Anions such as Cl , Br , and 1 can form covalent bonds with mercury so that their interfacial concentration is higher than the bulk concentration at the PZC. Furthermore, the extent of specific adsorption increases with the atomic number of the halide ion, as can be seen from the increase in the negative potential shift. A more complete description of specific adsorption will be given later in this chapter. 

Figure 2 A very simple model of electrostatic adsorption on a negatively charged oxide surface with formation of a "double layer" (surface + diffuse layer). Small dosed drcles are cations, larger open drcles are anions, oq" surface charge density x distance from the surface into the solution k thickness of double layer < ) electric potential c ix) and c (x) local concentrations in cations and anions, respectively. The shaded area represents the excess of cations over anions in the diffuse layer, and therefore the amount of cations that are electrostatically adsorbed.

According to Fig. 7.3.1 the isotherm slopes are approximately equal in formamide and methanol whereas the slope for the aqueous system is considerably larger. The mutual repulsion of adsorbed anions is therefore evidently stronger in methanol and formamide than in water. The interaction parameter is also found to depend strongly on the anion for a given solvent. For example in the formamide system the second virial coefficient (which is directly related to the interaction parameter) for adsorption of 1 ions is 310 A /ion compared with 2000 A /ion for Cl ions. Thus the simple adsorption model of point charges undergoing lateral coulombic repulsion represents a considerable oversimplification in non-aqueous solutions as in aqueous solutions. Studies of adsorption of halide anions from mixed electrolyte solutions in formamide and methanoF reveal complex behaviour which cannot be explained in terms of a simple model. 

---