Chain-structure clays

Fig.28. The relation of percent octahedral occupancy to RJ+/(R3+ +R2 +) for layer structure and chain structure clays, = saponite = attapulgite x = sepiolite (nine octahedral positions) o = sepiolite (eight octahedral positions).

Similarly pure beds of kerolite and sepiolite are found in the modern and Pleistocene groundwater wetlands of Amboseli, Kenya (Stoessell and Hay, 1978 Hay and Stoessell, 1984 Hay et al, 1995). The concentration of dissolved silica is also important at higher ratios of Si02 to Mg, chain-structure clay (sepiolite-palygorskite) can precipitate directly from solution, as is also the case at Amargosa. 

Attapulgite and sepiolite are clay minerals with a chain structure. The former has five octahedral positions and the latter either eight or nine. Both have relatively little tetrahedral substitution. The octahedral positions in sepiolite are filled largely with Mg and those in attapulgite with approximately half Mg and half Al. 

If the linking of silicate chains continues in two dimensions, sheets of SiO tetrahedral units result (Table 18.4). Various clays and mica have this sheetUke structure. Clays, which are essential components of soils, are aluminosilicates— some Si + ions are replaced by Al ions plus other cations that take up the additional positive charge. Feldspar, a component of many rocks and a network silicate, is weathered in the following reaction to form clay. 

Figure 2.92. Molecular structures of silicate-based minerals. Shown are (a) chain structures of chrysotile (Mg3Si205(0H)2 - a common member of the asbestos family) and pyroxene (XSiOs, X = Mg, Na, etc.), and (b) sheet/layered structures exhibited by various clays. Reproduced with permission from AUcock, H. R. Introduction to Materials Chemistry, Wiley New York, 2008. Copyright 2008 John WUey and Sons, Inc.

Solar et al. [87] apphed oscillatory measurements to study the durability of nanodispersions in a DGEBA epoxy resin with an organophUic MMT (Nanofil 919) in the presence of vibration or external stress. The dependence of rheological properties on the intercalated/exfoliated structures and the surface characteristics between the polymer chains and clays were then monitored. An increase in both the G and G" moduh was detected as the concentration of nanoclay was increased. A transition from a liquid-hke behavior (G" > G at low temperatures) to a sohd-hke behavior (G > G" at higher temperatures) was also observed, due to the formation of a percolated structure of interconnected tactoids through hydro-phobic interactions (as shown in Figure 4.9). This transition was seen to shift to a lower temperature as the nanoparticle content was increased. 

Due to the TO structure of the clay, the interlayer H-bonding is very strong. This hinders the intercalation of any molecule or chain into the gallery. Hence, the kaolinite clay cannot be used for nanocomposite preparation. 

Intercalated nanocomposites When the polymer chains intercalate into the clay gallery gaps but are unable to break down the layered structure, they are called intercalated nanocomposites (Figure 2.6a). 

Alvarez-Cohen et al. [91] explicitly showed that microbial transformation rates of trichloroethylene (TCE) were proportional to the aqueous TCE concentrations and independent from zeolite-sorbed TCE concentrations. Apparently in contrast to these findings, Crocker et al. [92] reported on the direct bioavailability of naphthalene sorbed to hexadecyltrimethylammonium (HDTMA)-modified smectite clay to Pseudomonas putida 17848, but not to Alcaligenes sp. strain NP-Alk. It should be noted that sorption to the hexadecyl chains of HDTMA resembles more the solubilisation by a surfactant than adsorption to a solid surface. Possibly, hydrophobic surface structures of strain 17848 allowed the close contact with HDTMA, thereby facilitating the uptake of naphthalene by a lipophilic pathway. 

Fig. 2.1 Configurations of the tetrahedral units and chain, double chain, and sheet structures in the silicate and aluminosilicate minerals. (A) Two-dimensional representation of a single silicate tetrahedron. (A ) Two-dimensional representation of an extended silicate chain. (B) Three-dimensional representations of single tetra-hedra in two orientations. The apexes of the tetrahedra point above or below the plane of the paper. (B ) Three-dimensional representations of extended silicate chains showing different orientations of the tetrahedra in two of the many possible configurations. Single chain pyroxenes (C), wollastonite (D), rhodonite (E). Double chains amphiboles (F). Sheets as found in the serpentines, micas, and clays (G).

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