Surface charge of colloids

One of the applications of zeta potential measurements is in the determination of surface charges of colloids such as proteins. We look into this briefly in Section 12.10 and derive the equation connecting the surface charge to the zeta potential under some simplifying assumptions. 

Particle-Particle Interaction Colloids, Coagulation, and Filtration 14.3 SURFACE CHARGE OF COLLOIDS... 

Table 14.3. Summary of Components of Surface Charge of Colloidal Particles"...

Also useful are the measurements of zeta potential. Particles in suspensions typically acquire a surface charge by the adsorption of ions on the surface, dissolution of the material, chemical reaction, or preferential adsorption of a specific additive or impurity ions from the solution. Surface charge of colloidal particles can be inferred through the measurement of the zeta potential (the particle... 

The above terminology ( inert vs. specific ) was adopted for studies of the surface charging of colloids. Different experimental methods are used and different quantities are measurable for colloids than for the Hg electrode, but the model of an electrical double layer is analogous. Studies of pH-dependent surface charging of colloids are usually carried out in the presence of an inert electrolyte and an acid or base (used to adjust the pH) with an anion or cation in common with the inert electrolyte. Products of dissolution of the solid are also present in solution at low concentration (we are only interested in sparingly soluble solids), but are ignored in most studies. Sometimes, the concentration of dissolution products is measured, and very occasionally the concentration of dissolution products (which are water-soluble salts) is controlled by addition of these salts to the dispersion. The effect of addition of Al(iii) salt on the potential of alumina was studied in [35]. At the lEP, the solubility of Al species is low thus, the lEP was not very different from that in a 1-1 electrolyte. The solubility problem is discussed in more detail in Section 1.6. 

In various problems, the approximation of constancy of either the surface potential, or the surface charge of the particle is used. For a charged electrolytic mixture the condition qs = const is usually used, whereas in colloidal chemistry problems, where the surface charge of colloids can change, one sets C = const. 

Surface Charge of Colloidal Particles From Equations 5.21 and 5.22, we get dlny... 

For unmodified silica dispersions, the surface charge is the main stabilizing factor. Surface charge of colloidal siUca dispersions can be measured by cationic polyelectrolyte titration [54], The surface charge of two unmodified silica dispersions and the corresponding silane-modified siHca dispersions were made by titration of the said dispersions with a Polybrene solution, 4 g per liter. All silica dispersion had a concentration of 5 g SiO per liter. To compensate for the presence of soluble silicate species in the silica dispersions, the latter were quantitatively determined by using the ammonium heptamolybdate method [55]. 

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