Clay properties surface charge density

Clay minerals or phyllosilicates are lamellar natural and synthetic materials with high surface area, cation exchange and swelling properties, exfoliation ability, variable surface charge density and hydrophobic/hydrophilic character [85], They are good host structures for intercalation or adsorption of organic molecules and macromolecules, particularly proteins. On the basis of the natural adsorption of proteins by clay minerals and various clay complexes that occurs in soils, many authors have investigated the use of clay and clay-derived materials as matrices for the immobilization of enzymes, either for environmental chemistry purpose or in the chemical and material industries. 

Some of the functional groups (e.g., OH) on the clay surface exhibit electrical charges. The magnitude of the electrical charge, as well as its character, are controlled by the properties of the surfaces to which the functional groups are bound and by the composition of the surrounding hquid. Sposito (1984) classified the surface charge density of soils as follows ... 

The type of clay present in a soil influences triazine sorption (Brown and White, 1969). Furthermore, variations in surface properties among different samples of the same clay type greatly influence sorption. For instance, sorption of atrazine on 13 clay samples, of which smectite was the dominant mineral, ranged from 0% to 100% of added atrazine (Figure 21.7), and was inversely correlated to the surface charge density of the smectites (Laird et al., 1992). Such data illustrate the complexity of sorption processes and the reason why simple predictive models relying on % OC, % clay, or surface area normalizations may fail to predict accurately the sorption of triazine by a particular soil. 

The chemical interpretation of o-in measured by the Schofield method depends sensitively on the type and concentration of probe electrolyte used. If these properties are chosen so that the cation in the reacting electrolyte neutralizes precisely the exposed functional group charge associated with isomorphic substitutions and dissociated hydroxyls and so that the anion neutralizes only the exposed protonated functional groups, then q+ and q. will have optimal magnitude for the chosen pH value and CTjn will be truly an intrinsic surface charge density. On the other hand, if the cation in the probe electrolyte is not able to displace all of the native adsorbed cations in, e.g., inner-sphere surface complexes, or if the anion cannot displace all of the native anions bound to protonated functional groups, or if either ion does not form only neutral surface complexes in the soil clay, then Ojn will differ from its optimal value. 

The second example concerns the surface heterogeneity of clay minerals. Important problems, such as limited yield of oil recovery arising during oil exploitation, involve interaction of pore filling fluids with the minerals that form the reservoir walls. The clay minerals, due to their relatively high specific surface area and electrical charge density, are the most active for the retention of oil. Illites and kaolinites are the clay minerals that are most frequently found and their wettability properties are believed to be in relation to the heavy oil ends retention process. 

Given the size of clay particles (10-1,000 nm), they are found in solution as colloidal dispersions or gels. At low water content, they can be obtained as dry powders, and can form solid porous materials upon compaction. In all these regimes, their properties crucially depend on the charge density and on the nature of the counterions. Most counterions are mono- or divalent, usually alkaline (most commonly sodium Na" or potassium or alkaline earth cations (most commonly calcium Ca " ). They are not incorporated in the clay layers. Rather, they are located near the surface, either between different layers, in the so-called interlayer porosity, or on the external surfaces of clay stacks (typically 10 layers). Such stacks are called particles, and their assembly to form a porous material then leaves voids called interparticle porosity, with sizes between a few nanometers to tens of nanometers, which are usually saturated by an electrolyte solution. 

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