Phyllosilicates, structure

It is useful as a point of departure, to briefly describe the basic crystal lattice common to phyllosilicates. The elementary character is the SiO tetrahedral linkage of an essentially two-dimensional, hexagonally symmetric, network. One side of this "sheet network is coordinated with other cation-oxygen complexes joined by an important component of covalent bonding while the other is coordinated by essentially ionic bonding or van der Waals type bonds. The key to phyllosilicate structures is the oxygen network which determines the shape and extent of the structure. 

Clay minerals are hydrous layer silicates of colloidal dimensions, with most if not all of the individual platy particles in the colloidal range of c. 1 nm-1 pm (van Olphen, 1976 Van Damme et al., 1985). The term phyllosilicate (phyllo = leaf like) is applied to the broad group of hydrous silicates with layer structures. The essentia] components of the phyllosilicate structure are two-dimensional tetrahedra and octahedra of oxygen atoms (or ions). The coordinating atoms (or cations) in the centre of the tetrahedra are for the most part Si, but Al3 or Fe3+ may also be present. The coordinating cations in the octahedra are usually Al3, Mg2+, Fe3 or Fe2. Some clay structures (e.g. hectorite) can be synthesized in a reproducible and relatively homogeneous form. 

DE Inosilicates with 2-periodic multiple chains 9.DG. Inosilicates with 3-periodic single and multiple chains 9.DH. Inosilicates with 4-periodic single chains 9.DJ. Inosilicates with 4-periodic double and triple chains 9.DK. Inosilicates with 5-periodic single chains 9.DL. Inosilicates with 5-periodic double chains 9.DM. Inosilicates with 6-periodic single chains 9.DN. Inosilicates with 6-periodic double chains 9.DO. Inosilicates with 7-, 8-, 10-, 12- and 14-periodic chains. 9.DP. Transitional ino-phyllosilicate structures 9.t)Q. Unclassified inosilicates 9.E Phyllosilicates... 

Figure 1.3. The three layer types for phyllosilicate structures in soil clays. All shown here are dioctahedral, with hydroxyl groups shown as shaded circles.

Fujii and coworkers reported the synthesis and detailed structural analyses of alkylammonium/magnesium phyllosilicate hybrids [88], which were prepared by hydrothermal reaction from a mixture ofoctadecyldimethyl(3-trimethoxysilylpropyl)-ammonium chloride, silica sol, and magnesium hydroxide Mg(OH)2. The structure of the hybrid compound was studied by XRD, TEM, electron diffraction, high-resolution solid-state NMR, TG-DTA/MS, and elemental analysis. The resulting analytical information confirmed the unit structure, which consists of a 2 1... 

Fig. 2.17 Three structure candidates (A, B, and C) of alkyl-ammonium/magnesium phyllosilicate hybrids. Reprinted with permission from [88], K. Fuji etal., Chem. Mater., 2003, 75,1189.

Fig. 8.2 PXRD pattern of ethlyenediamine-functionalized magnesium phyllosilicate showing reflections indexed according to the 2 1 trioctahedral phyl losi I icate structure of talc.

Fig. 8.3 SEM images of hexadecyl-functionalized magnesium phyllosilicate showing (A) intact spheroids (scale bar = 20pm) and (B) fractured spheroid with foam like interior (scale bar = 20pm). (C) TEM image of a wall fragment showing lattice fringes corresponding to a periodic lamellar structure (scale bar = 50 nm).

Layered materials are of special interest for bio-immobilization due to the accessibility of large internal and external surface areas, potential to confine biomolecules within regularly organized interlayer spaces, and processing of colloidal dispersions for the fabrication of protein-clay films for electrochemical catalysis [83-90], These studies indicate that layered materials can serve as efficient support matrices to maintain the native structure and function of the immobilized biomolecules. Current trends in the synthesis of functional biopolymer nano composites based on layered materials (specifically layered double hydroxides) have been discussed in excellent reviews by Ruiz-Hitzky [5] and Duan [6] herein we focus specifically on the fabrication of bio-inorganic lamellar nanocomposites based on the exfoliation and ordered restacking of aminopropyl-functionalized magnesium phyllosilicate (AMP) in the presence of various biomolecules [91]. 

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. 

The clay minerals can now be discussed in terms of their relationship with the phyllosilicates (sheet silicates). It is important to keep clearly in mind here the difference between clay - the material which is dug out of the ground, and which may be a mixture of different clay minerals, together with various nonclay minerals (such as quartz, pyrite, etc), as well as unaltered rock fragments and incorporated organic material (Grim, 1968) - and the clay minerals themselves, which are crystalline compounds of specified stoichiometry and structure. At this stage, we are only considering the structure of the clay minerals. 

Brown, G. (1984). Crystal structure of clay minerals and related phyllosilicates. In Clay Minerals Their Structure, Behaviour and Uses, ed. Fowden, L., Barrer, R.M. and Tinker, P.B., Royal Society, London, pp. 1 20. 

Study of hydrated kaolinites shows that water molecules adsorbed on a phyllosilicate surface occupy two different structural sites. One type of water, "hole" water, is keyed into the ditrigonal holes of the silicate layer, while the other type of water, "associated" water, is situated between and is hydrogen bonded to the hole water molecules. In contrast, hole water is hydrogen bonded to the silicate layer and is less mobile than associated water. At low temperatures, all water molecules form an ordered structure reminiscent of ice as the temperature increases, the associated water disorders progressively, culminating in a rapid change in heat capacity near 270 K. To the extent that the kao-linite surfaces resemble other silicate surfaces, hydrated kaolinites are useful models for water adsorbed on silicate minerals. 

Phyllosilicates are clay-related compounds with a sheet structure such as talc, mica, kaolin, etc. for which the nucleation mechanism of PET is known to be heterogeneous, although still uncertain. 

Figure 7.2 Silicate anion structures (o) orthosilicate, (6) pyrosilicate, (c) three-silicate ring, (d) six-silicate ring, (e) pyroxene, (/) amphibole, and (g) phyllosilicate.

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