Charge determining ions

In case of dolomite and magnesite dissolution a dependence on a fractional order of [H+] has been observed, indicating that adsorption of charge determining ions is involved. 

With this in mind, the impossibility of forming a double layer by electric forces only is obvious. Any ion that may attach to a particle will create a potential that keeps out all other ions of the same sign. Accumulation of a number of identical charges on a surface can take place only if the adsorbing ions experience a non-electric affinity for the surface so that they can move against the adverse potential. The extent to which this occurs depends on the balance between the attractive non-electrostatlc and the repulsive electric forces. In summary the reason for the formation of relaxed double layers is the nonelectric afflnlty of charge-determining ions for a surface the extent to which the double layer develops is determined by the non-electrostatlc electrostatic interaction balance. 

Interfaces AG (non-el) steins from the non-electrostatlc binding of charge-determining Ions. For the simple case where there is only one charge-determining species, for which the non-electrostatic allinity for the surface is independent of coverage (l.e. for a homogeneous surface) it was derived in sec. 1.5.7 that... 

The last step (which also may precede the previous one) is to determine the area A. This also is not always obvious. One choice is the BET-area (chapter 1) but a method based on adsorption from solution (chapter 2) may be more appropriate. In a number of cases, however, it was found that charge-determining ions "see" an area that differs from that obtained from adsorption of uncharged compounds. Silver iodide is a notorious example, to which we shall return in sec. 3.10a. In fact, any time that an example of a° is given, the area used to obtain the data set should be indicated. 

Relaxed interfaces cannot be polarized unless special precautions are taken. Capacitances can of course be obtained as derived quantities by differentiating the surface charge with respect to the surface potentieil if changes In the latter are known, which is possible if the Nemst equation applies. We now discuss direct capacitance measurements on reversible interfaces. To start with, the response of such an interface to an applied field has to be considered. The basic problem is that not only are double layers built up, but also charge transfer across the interfaces takes place and diffusion of charge-determining ions to or from the surface starts to play a role. With regard to these physical processes only the sum-effect is measured, and this sum has to be divided into its parts to obtain the capacitance. Distinctions can be made because the three constituents mentioned react in a different way to the frequency of the external field. 

Ion transfer resistance, caused by the slowness of the transport of charge-determining ions across the (solid-liquid) Interface. This leads to a finite exchange current density and an Ion transfer resistance 6 given by... 

Measuring surface charges from depletion of charge-determining ions ffnds its counterpart in measuring the expulsion of co-ions from the Increase in concentration in solution. From this Increase, called the Donnan ejfect, information on the double layer can be obtained, but it also offers an original method of determining specific areas, i.e. it is at the same time a technique to measure a double layer property and a double layer application. 

Table 3.4. Direction of the shift of point of zero charge and isoelectric point caused by specific adsorption. "Down" and "up" refer to change in the negative logarithm of the concentration of the charge-determining ions.

More relevant for our purposes are models for oxides in which, in some way, the affinity of charge-determining ions is related to the surface electric fleld For example, in the theoiy of Hiemstra et al. proton affinities are computed in terms of bond distances, coordinations and charge distribution in the solid surface, i.e. it is essentially an energy Interpretation. For several oxides there are arguments that this is an acceptable approximation. This model also showed that not all potentially available sites are titrated in the usual pH range. 

The inner part of the DL consists of the adsorption layer and specifically adsorbed counterions. The state of charge-determining ions and specifically adsorbed counterions depend on potentials which are determined by their localisation. 

There is not only a similarity but also a fundamental difference between the approach used to control coagulation in the sol-gel process and that used for metal nanocrystal systems. As previously discussed, in metal oxide particles the surface charge is controlled by the protonation or deprotonation of their hydrous oxide surfaces (M-OH). Thus, the charge-determining ions are H+ and OH. The ease with which protonation or deprotonation occurs will depend on the metal atoms and can be controlled by the pH. 

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