Basal surfaces, negatively charged

Although structural charges do not reside at the surface, they produce an electric field that emanates in all the directions affecting the surface potential.20 This potential, which is negative with respect to that of the solution bulk, drives the particular cation adsorption properties of basal surfaces. As mentioned previously,2 Figure 4.6... 

Micas. Micas or muscovites are 2 1 minerals with strong layer charges due to the iso-morphous substitution of AP for Si" in the tetrahedral sheets. The excess of net unbalanced negative charges, ideally 1 charge per formula unit, is compensated by K" ions strongly bonded between adjacent basal planes. Consequently, micas have a relatively low CEC and a low surface area (Table 7-4)... 

The absolute value of the zeta potential decreases until a plateau is reached at a certain polyanion concentration. A contrary effect is obtained in the case of adding a polycation. A partial stabilization of the kaolin particles can be realized due to an adsorption of anionic charged macromolecules at the edges of the kaolin platelets. By adding a polycation to the kaolin dispersion, an adsorption at the negative basal surface becomes possible, and the iep of the particles is reached very quickly at low PEI concentrations. A further addition of PEI leads to an increase of the zeta potential while flocculation was observed. This is because the adsorption of cationic polymer can cause a face-to-face association that can generate polymer-kaolin complexes. 

The configuration of water molecules between the basal planes in vermiculite depends sensitively on the nature of the cation complexed on these surfaces to balance the negative charge produced by isomorphic substitution of for in the tetrahedral sheet. Because of this substitution, the siloxane ditrigonal cavities exhibit a relatively ItKulized... 

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