Clays and layered silicates

Pol Tner Nanocomposites are novel plastic compounds with a filler having dimensions between 1 and 100 nm. They have attracted much attention in the past because nanocomposites exhibit markedly improved properties like stiffness, thermal flammability, improved barrier properties and others compared to the unfilled matrices [3], Among all potential fillers, those based on easily available clay and layered silicates have been more widely investigated for some time now. 

Two-dimensional structures Clays and layered silicates Kaolinite Al2Si205 (OH)4 Hectorite Naj(Mg3 j Lij )Si40io(OH)2-mH20 Montmorillonite Nuj (Al2 j MGj )Si40io(OH)2-mH20 Niobates and tantalates... 

Clays and layered silicates Montmorillonite, hectorite, saponite, fluoromica, fluorohectorite, vermiculite, kaolinite, magadite... 

The third type of nanocomposite is characterized by only one dimension in the nanometer range. Here the fillers is in the form of sheets of one to a few nanometer thick to hundreds to thousands nanometers long. Clays and layered silicates belong to this family and the composites are known as polymer layered silicates nanocomposites (PLSNs). 

Among all the potential nanocomposite precursors, clay and layered silicates have been most widely investigated, probably because starting clay materials are easily available and because their intercalation chemistry has been studied for a long time [5, 6]. 

In Fig. 9.3, the effect of inorganic fillers with different aspect ratios on the membrane permeability to methanol and proton conductivity is compared [34]. For the same filler concentration, the layered silicate was more effective in reducing methanol permeability. Clays and layered silicates such as laponite have been used by different groups for the development of fuel cell membranes. Examples are Nafion/ mordenite [35] and Nafion/montmoriUonite [36], Nafion/sulfonatedmontmorillonite [37], SPEEK/montmorillonite [38,39], SSEBS/montmoriUonite [40]. 

A number of clays are layered silicate-like materials. Most clays contain finely divided quartz, micas, and feldspars. Iron oxide-rich clays are employed to make pottery and terracotta articles. Clays containing iron oxide and sand are used to make bricks and tiles. Clays rich in calcium and magnesium carbonate are known as marls and are used in the cement industry (Section 12.2). 

Phyllosilicates (clay minerals) - layered silicates such as Al4Si4O10(OH)8 (kaolinite) and (Ca,Na)o.7(Al,Mg,Fe)4(Si,Al)802o(OH)4.nH20 (montmorillonite, also called smectite). 

Clays are layer silicates (phyllosilicates) of particle size less than about 4 pm, produced by the weathering of aluminosilicate rocks. Clay minerals fall roughly into two structural classes the kaolinite type, based on paired sheets of tetrahedral (SiC>44-) and octahedral [A10n(0H) g " or... 

Montmorillonite clays are layered silicates montmorillonite K-10 is a specially manufactured acidic catalyst (Montmorillonite K10, [1318-93-0] A. Comelis, P. Laszlo, M. W. Zettler in eEROS Encyclopedia of Reagents for Organic Synthesis, L. A. Paquette, Ed., John Wiley and Sons, Inc., online reference available at http //www.intersciene.wiley.com)... 

Goethites Condensation of arginine and glucose at 37 °C C and N fractions yields FTIR spectra of solid residue and supernatant humic substances Clay-Size Layer Silicates Gonzalez and Laird (2004)... 

Clay size layer silicates also have the ability to catalyze the polycondensation of phenolic compounds and amino acids. Wang et al. (1985) examined the catalytic effect of Ca-illite on the formation of N-containing humic polymers in systems containing various phenolic compounds and amino acids. The yields and N contents... 

Farmer, V. C., Krishnamurti, G. S. R., and Huang, P. M., 1991, Synthetic allophane and layer-silicate formation in Si02-Al20j-Fe0-Fe203-Mg0-H20 systems at 23°C and 89°C in a calcareous environment Clays and Clay Minerals, v. 39, p. 561-570. 

As with metal cations, anion chemisorption occurs on soil minerals that possess surface hydroxyl groups. The most important minerals in this regard are noncrystalline aluminosilicates (allophanes) oxides and hydroxides of Fe, Al, and Mn and layer silicate clays (edge sites only). It is the H2O or valence-unsatisfied OH ligands bound to surface metal ions (usually Fe, Al, or Mn) that are the sites of chemisorption. In general terms, the surface reaction can be written... 

The capacity of this mechanism to buffer depends on the quantity of exchangeable base cations in the soil. In comparing soils at the same pH, the soil with the highest organic matter and layer silicate clay content should possess the greatest base cation buffer capacity. The CEC of the soil is directly proportional to base cation buffering, assuming that CEC is measured as the sum of base cations. 

Vanadium is found substituted into the structures of layer silicate clays as the (V ) ion, a form that must have very low bioavailability. Its tendency to substitute into oxides and layer silicates as (and perhaps may account for the correlation between clay content and total V content of soils. 

Layer-silicates Recent studies have also demonstrated the potential microbial influence on clay mineral (layer silicates) formation at hydrothermal vents. Bacterial cells covered (or completely replaced) with a Fe-rich silicate mineral (putative nontronite), in some cases oriented in extracellular polymers (as revealed by TEM analysis), were found in deep-sea sediments of Iheya Basin, Okinawa Trough (Ueshima Tazaki, 2001), and in soft sediments, and on mineral surfaces in low-temperature (2-50°C) waters near vents at Southern Explorer Ridge in the northeast Pacific (Fortin etal., 1998 Fig. 8.6). The Fe-silicate is believed to form as a result of the binding and concentration of soluble Si and Fe species to reactive sites (e.g. carboxyl, phosphoryl) on EPS (Ueshima Tazaki, 2001). Formation of Fe-silicate may also involve complex binding mechanisms, whereas metal ions such as Fe possibly bridge reactive sites within cell walls to silicate anions to initiate silicate nucleation (Fortin etal., 1998). Alt (1988) also reported the presence of nontronite associated with Mn- and Fe-oxide-rich deposits from seamounts on the EPR. The presence of bacteria-like filaments within one nontronite sample was taken to indicate that bacterial activity may have been associated with nontronite formation. Although the formation of clay minerals at deep-sea hydrothermal vents has not received much attention, it seems probable that based on these studies, biomineralisation of clay minerals is ubiquitous in these environments. 

Layer silicates, sheet-like phyllosilicates such as the familiar micas, are in primary rocks and in soils. The soil minerals are often called clay minerals. Since other components can also be in the clay fraction, layer silicates is a mom accurate term. A typical layer silicate is a combination of a layer of Al-, Mg-, or Fe(II)-0 octahedra plus one or two layers of Si-0 tetrahedra. The tetrahedral and octahedral sheets bond together by sharing oxygens at the corners of the tetrahedra and octahedra. Layer silicate minerals are differentiated by (1) the number and sequence of tetrahedral and octahedral sheets, (2) the layer charge per unit cell, (3) the type of interlayer bond... 

Surfactants may not only stabilize system against coagulation, but may have an opposite effect, i.e. cause destabilization in cases when the surfactant adsorption proceeds against the polarity equalization rule (Chapter III,2), e.g., during chemisorption of surfactants from aqueous medium on a hydrophilic surface. For example, small additives of cationic surfactants cause coagulation of aqueous dispersions of clays and other silicates due to hydrophobization at T< rmax. Further increase in surfactant concentration results in the formation of a second (hydrophilizing) adsorption layer and leads to an increased... 

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