Kaolinite water structure

The effect of the kaolinite surface on the structure of water beyond monolayer coverage has not been ascertained conclusively. Multilayers of water are known to form on the clay mineral at relative humidities greater than 20 per cent, and about 10 molecular layers are thought to exist at relative humidities near 98 per cent. Thus an absolute upper limit for the dimension of the region of adsorbed water in a kaolinite suspension should be around 3 nm. Data on the thermodynamic properties (heat of immersion, partial specific entropy, isosteric heat of ackorption) of Li-saturated kaolinite-water systems, which do not show hysteresis, indicate consistently that differences between bulk liquid water and water on the clay are detectable up to about three molecular layers of coverage. Perhaps the lower limit for the dimension of the adsorbed water region is then about 1 nm. 

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). 

The consequence of these partial charges is that one surface of kaolinite is compatible with and attractive to the other surface. This results in increased stability of kaolinite and the formation of relatively stable structures. Some kaolinite particles can be larger than the 0.002 mm upper limit for clay Both surfaces also attract and hold water through these partial charges. The absorptive activity of kaolinite is associated with its surface electrons and partially positive hydrogens, and thus the two faces of kaolinite can attract anions, cations, water, and electrophilic and nucleophilic organic compounds. 

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. 

Water on Hallovsite. Central to the controversy is the observation that clay crystals present a planar array of oxygens (and hydroxyls in the case of kaolinite) which have hexagonal (or nearly) symmetry with a periodicity similar to that found in the crystal structure of ice. Because of this geometric similarity, it has frequently been assumed that water adsorbed on a clay surface will preferentially adopt an ice-like configuration. When looked at in detail, it is difficult to find unequivocal evidence to support this. 

Kaolin Minerals. The 1 1 structures include a group of aluminosilicate minerals which are termed collectively the kaolin minerals specifically these are kaolinite, dickite, nacrite, and halloysite. The basic 1 1 layer for all of these minerals has the composition AlgSigOj-fOHJj, there is a small amount of substitution of iron for aluminum, ana fluoride for hydroxyl ion. All, except halloysite, are normally anhydrous and do not expand (as do the smectites) upon exposure to water and most organic molecules. As a result, they generally have a rather small surface area, on the order of 10 nr... 

Subsequent work showed that a modification of the synthesis procedure produced a 10A hydrate which> if dried carefully, would maintain the interlayer water in the absence of excess water (27). This material is optimal for adsorbed water studies for a number of reasons the parent clay is a well-crystallized kaolinite with a negligible layer charge, there are few if any interlayer cations, there is no interference from pore water since the amount is minimal, and the interlayer water molecules lie between uniform layers of known structure. Thus, the hydrate provides a useful model for studying the effects of a silicate surface on interlayer water. 

Our approach has been to study a very simple clay-water system in which the majority of the water present is adsorbed on the clay surfaces. By appropriate chemical treatment, the clay mineral kao-linite will expand and incorporate water molecules between the layers, yielding an effective surface area of approximately 1000 m2 g . Synthetic kaolinite hydrates have several advantages compared to the expanding clays, the smectites and vermiculites they have very few impurity ions in their structure, few, if any, interlayer cations, the structure of the surfaces is reasonably well known, and the majority of the water present is directly adsorbed on the kaolinite surfaces. 

To the extent that the surfaces of the kaolinite layers resemble the surfaces of other silicate minerals, the structure of the adsorbed hole and associated water can serve as a useful model. To determine the applicability of our model to a specific mineral, it will be necessary to know in some detail the structure of the external surfaces of that mineral. 

Schofield RK, Samson HR (1954) Flocculation of kaolinite due to the attraction of opposite charged crystal faces. Discuss Faraday Soc 18 135-145 Schofield RK, Samson HR (1953) The defiocculation of kaolinite suspensions and the accompanying change-over from positive to negative chloride adsorption. Clay Miner BuU 2 45-51 Schulten HR (2001) Models of humic structures association of humic acids and organic matter in soils and water. In Qapp CE et al. Humic substances and chemical contaminants. Soil Science Society of America, Madison, Wl, pp 73-88... 

The nature of the interfacial structure and dynamics between inorganic solids and liquids is of particular interest because of the influence it exerts on the stabilisation properties of industrially important mineral based systems. One of the most common minerals to have been exploited by the paper and ceramics industry is the clay structure of kaolinite. The behaviour of water-kaolinite systems is important since interlayer water acts as a solvent for intercalated species. Henceforth, an understanding of the factors at the atomic level that control the orientation, translation and rotation of water molecules at the mineral surface has implications for processes such as the preparation of pigment dispersions used in paper coatings. 

The marked difference in the relaxation times for the kaolinite and silica may be attributed to the nature of the surface. Intuitively, the hydrogen bonding which influences the increased structure at the kaolinite surface would be expected to give shorter values for the relaxation time. However this is not observed in the simulations. Instead, shorter values are seen for the silica surface which is a result of water molecules becoming trapped in the cage-like amorphous silica surface. This reflects experimental results where precipitated silica surfaces are microporous and water inclusion in the surface is common. 

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... 

Clay Activation. The clay is heated to about 700 °C to destabilize the kaolinite structure by removing hydroxyl ions as water. This can be either a batch process with the clay in crucibles in a directly fired kiln, or a continuous process in a tunnel kiln, rotary kiln, or other furnace. 

Clays are still another example of sheet structures. Kaolinite has the composition [Al2Si205(0H)4]. It consists of a Si2OT2 sheet bonded to an Al2(OH)6 sheet with two thirds of the (OH)- ions on one side of the Al2(OH)6 sheet replaced by unsatisfied oxygen ions on the Si20 2 sheet. This creates a one-sided molecular structure that attracts water, which is responsible for the ability of wet clay to be shaped easily. The clay becomes rigid when it is dried. 

Yet further, it should be emphasized that the listed site types may not act either solely, as reactions typically proceed through multiple steps, nor independently, as sites may interact. Site interaction is strongly indicated in the case of kaolinites, where spectroscopic properties and/or populations of certain catalytic entities, i.e. structural iron, and O -centers have been shown to be simultaneously modified in the presence of synthetically introduced interlayer water (141). 

Clays are aluminosilicates with a two-dimensional or layered structure including the common sheet 2 1 alumino- and magnesium- silicates (montmorillonite, hectorite, micas, vermiculites) (figure 7.4) and 1 1 minerals (kaolinites, chlorites). These materials swell in water and polar solvents, up to the point where there remains no mutual interaction between the clay sheets. After dehydration below 393 K, the clay can be restored in its original state, however dehydration at higher temperatures causes irreversible collapse of the structure in the sense that the clay platelets are electrostatically bonded by dehydrated cations and exhibit no adsorption. 

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