Chapter 8 - Colloid Clay Science
Section snippets
Particle and Aggregate Structure
Clay mineral particles, in particular those of smectites, are never crystals in the strict sense (Brindley and Brown, 1980, Moore and Reynolds, 1997, Plançon, 2001). Many crystallographers are dismayed at the particles clay scientists often call ‘crystals’. In fact, a smectite ‘crystal’ is more equivalent to an assemblage of silicate layers than to a true crystal (Fig. 8.1). Montmorillonite (Mt) particles seen in the electron microscope never have the regular shape of real crystals but look
Hydrates of 2:1 Clay Minerals
The 2:1 clay minerals form hydrates with one, two, three or four pseudo-layers of water molecules between the silicate layers. The state of hydration changes with the water vapour pressure, with the water content, and, in salt solutions, with the type and concentration of salts, and is dependent on the layer charge and the interlayer cation density. Typical basal spacings are 1.18–1.24 nm (water pseudo-monolayer), 1.45–1.55 nm (water pseudo-bilayer), and 1.9–2.0 nm (four water pseudo-layers). The
Fractionation of Clay Dispersions
Clay minerals with a certain degree of purity can be separated from raw clay samples by sedimentation techniques. The first step consists of removal of iron oxides and organic materials. These materials not only affect the properties of colloidal dispersions but also prevent optimal peptization of clay particles and successful fractionation by sedimentation. To prepare colloidal dispersions, it is important to remove carbonates and silica (see Chapter 7.1). Carbonates can release calcium or
Coagulation by Inorganic Salts
Since the colloidal state of dispersed clay minerals is decisive in many practical applications, the coagulation of kaolinite and Mt dispersions was investigated for many decades (Jenny and Reitemeier, 1935, Kahn, 1958). Unlike other colloidal dispersions, well-dispersed clay minerals (kaolinites, smectites, illites, palygorskite) in the sodium form may be coagulated by very low concentrations of inorganic salts. The critical coagulation concentration, cK, of sodium chloride varies between 3
Flocculation by Bridging and Charge Neutralization
Macromolecules can flocculate colloidal dispersions by two different mechanisms: bridging between the particles and charge neutralization (Fig. 8.17) (Chaplain et al., 1995, Lagaly et al., 1997, Theng, 2012).
Bridging requires that the macromolecules can attach to the surface of two approaching particles and that the bridging part of the macromolecules is compatible with the solvent (the solvent has to be a better-than-theta solvent). In aqueous dispersions, a certain low salt concentration is
Modes of Aggregation
The most well-known mode of aggregation is the house-of-cards model, where the clay mineral particles are held together by edge/face contacts (Fig. 8.24A) (Hofmann, 1961, Hofmann, 1962, Hofmann, 1964). This type of network forms only when the edges are positively charged, or in a slightly alkaline medium above the critical salt concentration. Formation of edge/face contacts below pH ≈ 6 is due to hetero-coagulation between the positive edges and the negative faces of the particles or silicate
Clay Mineral Hybrid Films
The formation and properties of hybrid films of clay minerals bridge clay colloid science and materials science. If appropriate conditions are selected, clay mineral platelets settle to form a sediment where the platelets preferentially adopt a parallel orientation (see Section 8.6.3). Drying produces oriented films that are often used in spectroscopic investigations and X-ray diffraction and are considered for possible new applications of clay minerals (Fitch et al., 1998, Fendler, 2001). Such
Formation of Nanoparticles in Clay Minerals
Clay mineral particles provide confined volumes of nano-sized dimensions for the formation of colloidal particles. The confined space between particles or layers of clay minerals limits the particle growth (Fig. 8.44).
Formation of colloidal metal particles was observed decades ago during the oxidation of octahedral Fe2 + ions in micas by interlayer silver cations. The silver atoms aggregated to Ag0 particles outside the interlayer spaces (Sayin et al., 1979). Giannelis et al. (1988) described
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