Layered aluminosilicates

Halloysite is an economically viable material that can be mined from deposits as a raw mineral. The size ofhalloysite tubules varies within 0.5-10 microns in length and 15-200 nm in inner diameter, depending on the deposit. There are between 15 and 20 aluminosilicate layers rolled in the multilayer tubule walls with a layer spacing of... 

Kaolinite is easily cleaved perpendicular to the c direction, since the interactions between the aluminosilicate layers are much weaker than the intralayer interactions. Therefore as a part of this work the structure and surface energy, of the resulting 001 surface are considered. The surface energy may be evaluated from the energy of a single layer surface block of clay in a vacuum, U, and the energy of a portion of the bulk clay, U, containing the same number of atoms as the surface block. 

The molecular size and the cross-sectional area of coumarine 1 were calculated to be 3.2x10.4x7.5 (A) and 78 A, respectively. From the data of observed d-spacings and the calculated molecular size, three possibilities for the conformation of coumarine molecules could be proposed. In the dl-type [shown in Fig. 5(a)], since the thickness of one aluminosilicate layer was about 9.6 A, the full clearance space was estimated to be about 3.6 A. This value was almost equal to the thickness of the planar coumarine molecule. Therefore, it was considered that coumarine molecules were "flat" on the silicate surfaces and covered each exchangeable cation site without any overlap. In the dh-type [shown in Fig. 5(b,c)], the measured d-spacing was 18.5 A, so that the interlamellar spacing was evaluated to be about 8.9 A, in which the coumarine... 

Many clay minerals have aluminosilicate layer structures. For example, in kaolinite, Al2(0H)4[Si205] (Fig. 7.5), the Al3+ are all in octahedral locations. Clay minerals of the smectite or swelling type, such as montmo-rillonite, can absorb large amounts of water between the aluminosilicate... 

Figures 2.21 and 2.22 refer to the adsorption of low molecular weight aliphatic alcohols from alcohol + benzene mixtures on montmorillonite. This adsorbent Is a so-called swelling clay mineral, meaning that it consists of packages of thin (aluminosilicate) layers that, under certain conditions, swell to give ultimately a dispersion of the individual sheets. Upon this swelling the specific surface area increases dramatically, it can readily reach several hundreds of m g" On adsorption from solution the swelling is determined by the extent to which one or both of the component(s) penetrate(s) between these sheets. In other words, we are dealing here with a non-inert adsorbent. The gas adsorption equivalent has been illustrated in fig. 1.30.

The stabilizing effect of the aluminosilicate layer of DAY-Saim and DAY-T can be explained by the elimination of the terminal silanol groups and the blocking of the energy-rich Si-O-Si bonds at the crystal surface, where the water molecules attack the framework. In this case, the question of stability of high-silica faujasites is transferred to the question of stability of the aluminosilicate structures and, accordingly, transferred from the alkaline to the acid mechanism of decomposition which is rate-controlling. 

An aluminosilicate layer at the crystal surface generated subsequently by an alumination of DAY-S zeolite or directly by the steaming of NaY in order to get DAY-T zeolite stabilizes the high-silica faujasites. The decomposition follows the acid hydrolysis of usual aluminosilicates. Consequently, in this case the kinetic model described in ref [5] is valid at least for the surface layer. 

A nanomaterial can be loosely defined to be any material containing heterogeneity at the nanoscale in one or more dimensions. In the broadest sense, then, the following are nanomaterials phase-separated glasses or crystals with domains in the nanoregime, zeolites and mesoporous materials with pores of nanometer dimensions, clays with nanometer sized alternations of aluminosilicate layers and interlayer hydrated cations, and nanoscale leach layers at the mineral-water interface. 

These data suggest very negative entropies of hydration for H2O molecules surrounding cations sandwiched between aluminosilicate layers. If this structuring of water is a general phenomenon, it may have significant implications for diffusion and reactivity at layer and nanoparticle surfaces. 

The solid phase of soils can consist of many Al-bearing materials, including organic matter oxides and hydroxides of Al noncrystalline aluminosilicates layer silicate clays and various primary minerals. Any model of Al solubility that is based on the dissolution reaction of only one of these materials is likely to be too simple. Nevertheless, some of these models will now be considered in turn. 

Kaolinite can be neoformed from solutions of A1 and silica only at acid pH in alkaline solutions, the tetrahedrally coordinated A1 ion in Al(OH)4 is unable to form the gibbsitelike sheet that is the precursor of the 1 1 aluminosilicate layer. Even in acid solutions, however, kaolinite crystallization is very slow at room temperature, probably because Al(OH)3 is insoluble except at very low pH. Natural organic chelating agents, such as oxalate, increase the solubility of A1 at acidic pH, thereby increasing the rate of kaolinite neoformation. 

The proof of reversibility in primary mineral weathering would be instances where primary mineral structures have formed under earth-surface conditions. There are reports that secondary quartz can slowly precipitate at room temperature from solutions supersaturated with monosilicic acid. More typically, however, precipitated silica in soils is structurally disordered, in the form of chalcedony or opal. In fact, as long as alumina is present, silica does not precipitate as a separate phase, reacting instead to form aluminosilicates (layer silicates, imogolite, or allophane). 

Soils with less than 4% potassium saturation of the CEC are termed potassium frxers . Potassium is fixed especially in the clay mineral vermiculite, where it is entrapped in openings of the surface layers of oxygen atoms in adjacent aluminosilicate layers. In smectites and hydrous micas saturated with either Na-, Ca- or Mg-ions, the potassium ions displace these hydrated cations from the peripheral interlayer space, which leads to a collapse of this space and a decrease in CEC. 

Smectite clays consist of 1 nm thick aluminosilicate layers separated by sodium and calcium counterions. As little as 1 to 5 wt% of these layered clays can significantly improve the mechanical properties of nylon, polyolefins, and other polymers (78). Delaminating the clay structure by replacement of the sodium or calcium ions with a polymer-compatible surfactant, a quaternary ammonium ion surfactant, for example, is essential to generate a large polymer-clay interfacial area, as shown in Figure 11.21 (79). Ammonium ion head groups of the dispersant bind to the surface of the clay by Coulombic forces, and the hydrophobic alkyl tails bind to the polymer by van der Waals forces. 

A formamide intercalation adduct of dickite, Al2Si20s(0H)4, is the first clay-mineral intercalate that has been shown to possess an ordered structiure of interlamellar molecules. Thus, the formamide molecules lie over vacant octahedral sites in the aluminosilicate layers. 

A new composite material was introduced in 1987 with the discovery of a nylon-6/clay hybrid (NCH) [201]. The hybrid was prepared by the in situ thermal polymerization of s-caprolactam with 8% or less montmorillonite, the clay material containing 1-nm thick exfoliated aluminosilicate layers. It exhibited a truly nanometer-sized composite of nylon-6 and layered aluminosilicate. Figure 2.14 depicts conceptually the NCH synthesis and its fine structure. The NCH exhibited high modulus, high strength, and good gas-barrier properties. The unique and superior properties led to the commercialization of NCH. It has also created a new class of nanocomposites and worldwide interest. 

Anisotropic crystals. Some important materials have crystal structures that when cleave can result in the producrion of both positively and negatively charged surfaces (49). This is the case of the aluminosilicate layers in clays. 

Fig. 8. Molecular representation of sodium montmorillonite, showing two aluminosilicate layers with the Na+ cations in the interlayer gap or gallery. The octahedral (Oh) alumina layer is shown as blue aluminum atoms surrounded by red oxygen atoms. The tetrahedral (Td) silicate layers are shown as yellow silicon atoms surrounded by red oxygen atoms. Hydrogen atoms are white and sodium (Na+) cations are shown in green.

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