Phyllosilicates surface area

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. 

Among the minerals found in the earth s crust, those belonging to the phyllosilicate family, namely the clays, are especially interesting from the point of view of their surface activity. Being finely divided, their specific surface area is large. Their... 

Sepiolite/ Palygorskite Phyllosilicates Marine sediments, arid soils, high Si and Mg levels R Moderately high CEC.15 surface area, and sorptive properties... 

Smectites Phyllosilicates Mica and/or vermiculite alteration C High CEC high surface area high shrink-swell capacity... 

However, on high surface area silica, the same procedure gives rise to the formation of nickel phyllosilicates. 

Smectites, by virtue of their large surface area, are particularly sensitive to the exchangeable cation type. In nature, the most common cations are Ca and Na. Calcium ions reduce bound water layer thickness and provide some deformation resistance by cross-linking clay platelet surfaces. Sodium smectite, on the other hand, has the highest affinity for water of any common phyllosilicate, and is therefore used in water-based muds (WBM) to build viscosity and to suspend fine-grained materials used to increase the mud weight. The most geochemically sensitive shales are almost invariably smectitic with saline pore fluids. 

Due to the number of intermolecular forces and entropic effects that can be implied in the interaction of phosphohydrolases with soil constituents, studies on homogeneous and simplified systems are best suited for understanding the basic phenomena. Mont-morillonite is a clay mineral with well-defined surface properties. It is a 2 1 phyllosilicate, in which each clay platelet consists of one layer of octahedral alumina between two layers of tetrahedral silica. It has a large specific surface area (800 mVg) mainly represented by basal surfaces whose electrical charge originates from isomorphic... 

Typical results of specific surface area determinations on phyllosilicates by nitrogen gas/water vapor or nitrogen gas/CPB adsorption are listed in Table 1.7. For Mg-vermiculite and Na-montmorillonite, the measured adsorption specific surface area is close to that calculated from the unit cell dimensions and structural formula. For illitic mica, the area is about 14 per cent of the ideal crystallographic value, indicating that this mineral forms particles containing about seven phyllosilicate layers that cannot be penetrated by water vapor or CPB. 

Table 1.7. Specific surface areas of phyllosilicates determined by nitrogen, water vapor, or iV-cetyl pyridinium bromide adsorption...

Table 1.8 lists specific surface area values for illitic micas as determined by nitrogen gas adsorption and by negative chloride adsorption.The specific surface areas calculated from N2 gas adsorption with the help of Eq. 1.7 show no particular trend with type of exchangeable cation. The mean value of 5, 11.2 0.5 x 10 m kg", suggests that the mineral forms particles containing seven phyllosilicate layers, as indicated previously. The external surfaces of these particles are expected to repel anions, and therefore the specific surface area determined by negative chloride adsorption should also be around lO m kg" . As shown in Table 1.8, however, the values of 5k, obtained with Eq. 1.18, are always less than 5 and decrease sharply with increasing radius of the... 

Figure 6.7. Total potential energy per unit area for the interaction of two charged parallel phyllosilicate surfaces, with 10 mol

More recent x-ray absorption studies of Co sorption complexes on quartz [77] suggest that for quartz at relatively low density coverage, first-sorbed Co species provide energetically favorable nucleation sites for the subsequent formation of multinuclear hydroxide-like surface complexes. This is also seen for Ni " " adsorbed onto silica [78] where the first species adsorbed are seen as nickel phyllosilicates and not hydroxides. Subsequent layers are either more nickel phyllosilicate or nickel hydroxide depending on the activity of the sdica. If the silica is more active, for example due to smaller particles or greater surface area, then more metal-sUicate species are formed either at the... 

The most widely used layered silicates for polymer nanocon sites are the smectite clays such as montmorillonite. They offer a high aspect ratio and a high surface area. Sodium montmorillonite is 2 1 phyllosilicate, constructed of repeating triple-layers with two silica tetrahedral sheets fused to an edge-shared octahedral sheet of alumina. The physical dimensions for these silicate sheets are approximately one hundred to several hundred nanometers in lateral... 

In an excellent series of papers Burattin, Che and Louis [5-8] have proposed the molecular details of DP with urea studying the important system of nickel on silica. By variation of the silica specific surface area, nickel concentration and DP time, the authors concluded that turbostratic nickel hydroxide is the main phase deposited when short reaction times and low silica surface area are applied. Longer reaction times and higher silica surface area led to 1 1 nickel phyllosilicate of increasing crystallinity. The overall reaction mechanism is depicted in Figure 6.5 and is now discussed in some detail. Following the papers mentioned, the authors describe the key steps of the mechanism as follows. 

Almost the same Ni phases are obtained whether silica is porous or not. One difference is that for short DP times, the Ni(II) phase on nonporous silica of low surface area is already a mixture of 1 1 nickel phyllosilicate and nickel hydroxide (5). In addition, as observed by TEM, the Ni(II) phase is better crystallized and the surface of contact between the support and the Ni(II) phase is larger with nonporous silica (Figure 14.6b) than with porous ones (Figure 14.6a), because of the more regular shape of the silica particles. 

It can be noted that the brucitic layer of Ni(II) bonded to silica acts as nuclei for the growth of supported 1 1 nickel phyllosilicate or supported nickel hydroxide. The heterocondensation reaction is faster than the olation one, but it is limited by the concentration and diffusion in solution of silicic acid arising from silica dissolution, which itself depends on the silica surface area, i.e., on the extent of support-solution interface. 

This mechanism enables us (i) to explain why nickel hydroxide or nickel phyllosilicate are obtained, depending on the silica surface area and on the DP time, and (ii) to interpret the changes in the pH curves, the nature of the Ni(II) phase as a function of several parameters of preparation the concentrations of urea and nickel nitrate, and the silica loading (5). 

Nanoclay is the term generally used when referring to a clay mineral with a phyllosilicate or sheet structure with dimensions of the order of 1 nm thick and surfaces of perhaps 50-150 nm. The mineral base can be natural or synthetic and is hydrophilic. The clay surfaces can be modified with specific chemistries to render them organophilic and therefore compatible with organic polymers. Surface areas of nanoclays are very large, about 750 m /g. When small quantities are added to a host polymer, the resulting product is called a nanocomposite. 

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