Clays surface chemistry

In the current study, we tested the applicability of near infrared spectroscopy coupled with correlation analysis to quantitation of the dependence of montmorillonite spectra on two of the structural features known to be of importance in clay surface chemistry -hydration and exchangeable iron. These clays were... 

Tarapow et al. [83] studied the influence of clay surface chemistry and the processing conditions on the structure and mechanical properties of PP-clay nanocomposites prepared by a melt mixing process. They observed that although the Young s modulus improved with unmodifled clay incorporation, the tensile strength and the impact toughness values decreased. These results were attributed to a poor interfacial adhesion between PP matrix and unmodifled clay. 

The hot-water separation process involves extremely compHcated surface chemistry with interfaces among various combinations of soUds (including both silica sand and alurninosilicate clays), water, bitumen, and air. The control of pH is critical. The preferred range is 8.0—8.5, achievable by use of any of the monovalent bases. Polyvalent cations must be excluded because they tend to flocculate clays and thus raise viscosity of the middlings in the separation cell. 

Simple Models. The surface chemical properties of clay minerals may often be interpreted in terms of the surface chemistry of the structural components, that is, sheets of tetrahedral silica, octahedral aluminum oxide (gibbsite) or magnesium hydroxide (brucite). In the discrete site model, the cation exchange framework, held together by lattice or interlayer attraction forces, exposes fixed charges as anionic sites. 

It has often been argued that clay minerals cannot be used as models for soils in surface chemistry because soil clays are too heterogeneous and impure. The work reported here shows that, for both equilibrium and kinetic studies, standard aluminosilicates are useful models for soils. 

It is reasonable to assume that clay colloids exhibit a similar surface chemistry as clay which is sorbed, bonded, or precipitated in the stationary solid phase. Mineral colloids may be formed when precipitation or dissolution generate particles which are resistant to settling. These particles maybe formed by any number of conditions whereby the solubility of a particular solute is exceeded or a stable solid is disrupted mechanically [21,24]. 

The chemistry of an important group of naturally occurring materials is characterized by surface reactions many clay minerals possess what can be considered surface at its extreme. All clay minerals capable of intracrystalline swelling with separation of the silicate layers are—to overstate it—surface with a silicate layer on each side. Many principles and techniques of surface chemistry were first found with clay minerals. Nevertheless, the clay minerals will not be considered in this article, except for some comparison and analogies with surface compounds. 

Kostka JE, Wu J, Nealson KH, Stucki JW. 1999. The impact of structural Fe(III) reduction by bacteria on the surface chemistry of smectite clay minerals. Geochimica et Cosmochimica Acta 63 3705-3713. 

Chaussidon J, Calvet R (1974) Catalytic reactions on clay surfaces. Third int congr of pesticides chemistry (lUPAC), Helsinki In Coulton E, Albaky NY, Konle E (eds) Environmental quaUty and safety, vol 3. Gerg Thiem. Stuttgart... 

McBride, M.B. (1997) A critique of diffuse double layers applied to colloid and surface chemistry. Clays Clay Min. 45 598-608 McBride, M.B., Goodman, B.A., Russel, J.D., Fraser, A.R., Farmer,V.C. Dickson, D.P.E. (1983) Characterization of iron in alkaline EDTA and NH4OH extracts of podzols. J. Soil Sd. 34 825-840... 

Mineral segregation in industry relies heavily on the selective adsorption of macromolecules onto the surfaces of those minerals that have particular industrial applications. This selectivity is governed mainly by the surface chemistry of the mineral and the type of polymer used as a flocculant. " Effectiveness of flocculation depends upon the charge, concentration and molecular weight of the polymer, and also the pH and salt concentration of the clay suspension. The bonding between the anionic flocculant polyacrylamide (PAM) and clay mineral surfaces has been effectively reviewed recently by Hocking et al and the reader is referred to this should they require an in-depth literature review. For more information on general colloidal chemistry of clay suspensions the reader is referred to the review of Luckham and Rossi." ... 

Zeolite surface chemistry resembles that of smectite clays. In contrast to clays, however, natural zeolites can occur as millimeter- or greater-sized particles and are free of shrink-swell behavior. As a result, zeolites exhibit superior hydraulic characteristics and are suitable for use in filtration systems (Breck 1974) and as permeable barriers to dissolved chemical migration. Internal and external surface areas up to 800 m2 g have been measured. Total cation exchange capacities in natural zeolites vary from 250 to 3000 meq kg 1 (Ming and Mumpton 1989). External cation exchange capacities have been determined for a few natural zeolites and typically range from 10 to 50 percent of the total cation exchange capacity (Bowman et al. 1995). 

In addition to the high density of isomorphic substitutions, clays have several other important features which would tend to maximize the possibility of contributions from energy storage to their surface chemistry. 

Therefore, not only the nature of the surface active sites, as related to the structure, but their availability, as related to the particular texture are discussed as well. The relationship between these two aspects of clay surface reactivity is more heavily emphasized here than is the specific chemistry of reactions occurring on the surface of clays. 

Much effort has been devoted to characterizing the interaction of organic molecules with mineral surfaces. Adsorption of organic amines by clay minerals is one such system which has attracted considerable interest. In addition to concern about the subsurface fate and transport of amine-containing organic pollutants, there is fundamental interest in the ability of organic amines to influence the surface chemistry of layer silicates. Boyd et al. (1.2) have demonstrated recently that the presence of quaternary ammonium cations, such as the hexadedyltrimethyl-... 

Two specific approximations were developed to solve the problems of surface chemistry the periodic approximation, where quantum-chemical methods employ a periodic structure of the calculated system, and the cluster approximation, where the model of solid phase of finite size is created as a cutoff from the system of solid phase (it produces unsaturated dangling bonds at the border of cluster). The cluster approximation has been widely used for studying the interactions of molecules with all types of solids and their surfaces [24]. This approach is powerful in calculations of systems with deviations from the ideal periodic structure like doping and defects. Clay minerals are typical systems having such properties. 

John M. Thomas was born In South Wales and took his Bachelor s degree at the University of College at Swansea and completed his Ph.D. at Queen Mary College, of the University of London. For 9 years from 1969 he was Head of the Department of Chemistry at the University College of Wales In Aberystwyth. He then moved to the University of Cambridge as Head of the Department of Physical Chemistry. He Is a Fellow of the Royal Society and of the Indian Academy. He was Baker Lecturer at Cornell University In 1963. He conducts research In solid-state and surface chemistry, dealing with materials such as carbons, organic molecular crystals, clays, and zeolites, and Is Interested In catalysis and In the chemical consequences of crystalline Imperfections. 

Zeolite membranes are amenable by surface modification with a variety of chemical functional groups using simple silane chemistry, which may provide alternative surface chemistry pathways for enzyme immobilization. In this context, Shukla et al. [238] have recently used a chemically modified zeolite-clay composite membrane for the immobilization of porcine lipase using glutaraldehyde to provide a chemical linkage between the enzyme and the membrane. The effects of pH, temperature, and solvent on the performance of such biphasic zeohte-membrane reactors have been evaluated in the hydrolysis of olive oil to fatty acids. 

Giudici-Orticoni MT, Leroy G, Nitschke W, Bmschi M (2000) Characterization of a new dihemic c(4)-type cytochrome isolated from Thiobacillus ferrooxidans. Biochem 39 7205-7211 Gustafsson JP (2001) The surface chemistry of imogolite. Clays Clay Minerals 49 73-80 Guthrie GD, Bish DL, Reynolds RC (1995) Modeling the X-ray diffraction pattern of opal-CT. Am Mineral 80 869-872... 

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