Specific surface area phyllosilicates

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... 

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... 

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. 

Among the techniques used to characterize silica-supported Ni phases, FTIR spectroscopy is shown to be well adapted to identify ill-crystallized phases generated during the preparation by the competitive cationic exchange method. FTIR spectroscopy permits to discriminate a phyllosilicate of talc-like or serpentine-like structure from a hydroxide-like phase. Samples submitted to hydrothermal treatments have also been characterized by other techniques such as EXAFS and DRS spectroscopies. The pH and the specific surface area strongly influence the nature of the deposited phase, since they control the solubility and the rate of dissolution of silica. The results are discussed in terms of the respective amounts of soluble Si(OH>4 monomers and NP+ complexes at the interface. The relevant parameter as the Ni/Si ratio at the solid-liquid interface is assumed to control the routes to Ni-Si (Ni-Ni) copolyinerization (polymerization) reactions leading to supported Ni phyllosilicates (Ni hydroxide). 

The objective of this work is to investigate the role of parameters such as the pH of the impregnation solution and the specific surface area on the nature of the suppmted phase. Since, for the characterization of a bulk phyllosilicate, the sensitivity of most of the techniques of characterization, depends on its degree of crystallinity [13,14], the identification of supported phases was made by comparison with reference bulk compounds of various degrees of crystallinity. Spectroscopic techniques such as UV-vis diffuse reflectance (DRS), FTIR and... 

The experimental preparation conditions may influence the nature of the supported phases. Medium pH values around 8.5 are already well known to produce larger amounts of supported phyllosilicates [9,24], We have investigated the role of the pH and of the specific surface area of the support, by using EXAFS and FOR spectroscopies only. 

These results show that the formation of a talc-like phase requires a high pH (9.8) of the impregnation solution and a support of high specific sutface area, whereas the formation of a nepouite-like phase requires to operate at medium pH (83). In this case a mixture of phyllosilicates is produced. Ni hydroxide is observed only with the low specific surface area support. 

The dissolution of silica is catalysed by OH ions and the rate of dissolution increases with the specific surface area [28,29]. The Ni/Si ratio of soluble species available near the interface controls the nature of the deposited phase (Ni hydroxide and/or phyllosilicate) which grows via polymerization of Ni-O-Ni (Ni-O-Si) monomers. 

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]. 

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. 

---