Phyllosilicates water structure

J. W. Stuck and W. L. Banwart, Advanced Chemical Methods for Soil and Clay Minerals Research. Reidel, Dordrecht, The Netherlands, 1980. This book provides an excellent introduction to the use of NMR and INS techniques for the investigation of adsorbed water structure. Many experimental results for adsorbed water on phyllosilicates are presented. 

Interfacial water structures at selected phyllosilicate minerals, including talc, kaolinite, and sepiolite, are analyzed in terms of water distribution and water dipole moment orientation. The behavior of water molecules and wetting characteristics at these surfaces are explained in terms of the mineral surface structure. Simulation details have been reported in previous pnblications (Du and Miller 2007a, 2007b Miller et al. 2007 Nalaskowski et al. 2007). 

In this chapter the surface chemistry of selected nonsulflde flotation systems, including soluble alkali halide salts, phyllosilicates, quartz, and some naturally hydrophobic minerals, were studied using MD simulation. Issues such as water structure and dynamics, solution chemistry, interfacial water structure, and adsorption states for surfactants and macromolecules were examined. It is clear that MD simulation has been validated as a very useful tool to study the surface chemistry of certain flotation systems. As a complement to experimental studies, MD simulation analysis provides further information and understanding at the atomic level to issues such as water structure, particle dynamics, solution viscosities, mineral surface wetting characteristics, surface charge, and adsorption states. A wide application of MD simulation in the study of mineral surface chemistry is expected to have a significant impact on further advances in flotation technology. 

Study of hydrated kaolinites shows that water molecules adsorbed on a phyllosilicate surface occupy two different structural sites. One type of water, "hole" water, is keyed into the ditrigonal holes of the silicate layer, while the other type of water, "associated" water, is situated between and is hydrogen bonded to the hole water molecules. In contrast, hole water is hydrogen bonded to the silicate layer and is less mobile than associated water. At low temperatures, all water molecules form an ordered structure reminiscent of ice as the temperature increases, the associated water disorders progressively, culminating in a rapid change in heat capacity near 270 K. To the extent that the kao-linite surfaces resemble other silicate surfaces, hydrated kaolinites are useful models for water adsorbed on silicate minerals. 

Abstract Clays are ubiquitous constituents of the Earth s crust that serve as raw materials for traditional ceramics. Mineralogically, clays are phyllosilicates or layered aluminosilicates. Bonding is strong within layers, but weak between layers, allowing clays to break into micrometer-sized particles. When mixed with water, clays develop plasticity and can be shaped easily and reproducibly. When heated, clays undergo a series of reactions that ultimately produce crystalline mullite and a silica-rich amorphous phase. Beyond the structure and properties of clays, the science that developed to understand traditional ceramics continues to serve as the framework for the study of advanced ceramics. 

The term clay refers to fine-grained aluminosilicates that have a platy habit and become plastic when mixed with water [11], Dozens of minerals fall under the classification of clays and a single clay deposit can contain a variety of individual clay minerals along with impurities. Clay minerals are classified as phyllosilicates because of their layered structure [12], The most common clay mineral is kaolinite, although others such as talc, montmorillonite, and vermiculite are also abundant. Each of the... 

Clay occurs naturally and has plasticity when water is added. These clay minerals are phyllosilicates, which have water trapped in the mineral structure by polar attraction [2], The data from these solid models are then used to create a full-sized... 

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. 

Any inference concerning the effects of a possibly altered molecular structure of water near the solid surfaces in soil clays must proceed from an acquaintance with the structure of liquid water in bulk and in aqueous electrolyte solutions. In this section, the current picture of the molecular arrangement in bulk water is reviewed. In Sec. 2.2, the same is done for aqueous solutions of inorganic electrolytes. These summaries are followed by discussions of the structure of water near the surfaces of phyllosilicates and the effect of these surfaces on the solvent properties of the water molecule. 

G. W. Brindley and G. Brown, Crystal Structures of Clay Minerals and Their X ray Identification. Mineralogical Society, London, 1980. Chapter 3 of this standard reference contains an excellent discussion of X-ray diffraction studies of adsorbed water on phyllosilicates. 

J. Texter, K. Klier, and A. C. Zettlemoyer, Water at surfaces, Prog. Surface Membrane Sci. 12 327 (1978). This review gives an account of the available data concerning the properties of water on oxide and organic solid surfaces. The general conclusions drawn are similar to those stated in the present chapter for the structure of adsorbed water on phyllosilicates, except that hydrogen bonding of the water to the surface plays a more prominent role. 

Each tetrahedron in the phyllosilicates (sheet silicates) is connected to three other tetrahedrons through an oxide linkage. The phyllosilicates have essentially the same structure as a double chain inosilicate, except that the chains extend into a two-dimensional, sheet-like network, as shown in Figure 12.21. Several important examples of sheet silicates include micas and clays, such as talc and kaolinite. In the clays, such as talc, water molecules are intercalated between the sheets. The water molecules are held in the host by hydrogen bonding and dipole—dipole forces. There are a large number of sites where water can adhere within the sheets, which explains why these materials are so absorbent. While hydrated, clay can be molded into virtually any shape that one desires. When the molecule is heated in a furnace, the intercalated water can often be removed to leave behind a hard and rigid structure. 

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