Hydrated montmorillonite

Figure 9.12 Schematic representation of the structures of muscovite mica, (K2Al4(Si6Ali)02o(OH)4], hydrated montmorillonite, [Al4Sig02o(OH)4].xH20 and chlorite, (MgioAl2(Si6Al2)02o(6H)i6], see text.

Using MC simulations Delville and co-workers have investigated the clay-water interface [83-87], The number of hydration layers (2-3) increases suddenly during the swelling process [85]. For hydrated montmorillonite with interlayer sodium counterions it was determined that the water content of the pore is a function of the interlamellar distance. Water molecules are layered in successive shells, whose number (1-4) depends on the available interlayer space [87]. The MD study of structure of water in kaolinite [88] has indicated two types of adsorbed water molecules according to different orientations with respect to the structure of clay sheets with HH vector parallel or perpendicular to the surface. 

All surface complexes between counterions and the clay mineral surfaces were inner sphere due to our use of a monolayer of water. When present, water molecules tended to position themselves in such a way as to be in equatorial association with the counterions, i.e., they did not interpose themselves between a counterion and a clay surface. The equilibrium do0, spacings between hydrated montmorillonite layers in these simulations were reasonable [see, e.g., Brindley (1980) for experimental values], varying from 1.209 0.005 nm (K) to 1.214 0.004 nm (Rb) to 1.223 0.005 nm (Cs). 

Hydrated montmorillonite has water between the silicate-aluminate-silicate layers. The micas (e.g., muscovite) have potassium ions in comparable positions and also have aluminum substituting for silicon in about 25% of the silicate sites. Changes in the proportions of aluminum and silicon in either of these allow the introduction of other cations and the formation of a large number of minerals. The layered structures of some micas are pronounced, allowing them to be cleaved into sheets used for high-temperature applications in which a transparent window is needed. They also have valuable insulating properties and are used in electrical devices. 

The adsorption of neutral molecules on smectites is driven by various chemical interactions hydrogen bonds, ion-dipole interactions, coordination bonds, acid-base reactions, charge transfer, and van der Waals forces. Several polar molecules, such as alcohols, amines, and acids, form intercalation complexes with montmorillonites. The intercalation can be performed from the vapor, liquid, or even solid state. In intercalation from solution, solvent molecules are generally coadsorbed in the interlayer space. Guest molecules may be intercalated in dried clay minerals or may displace the water molecules of hydrated montmorillonite. 

Early work on hydrated montmorillonites was published by Buswell et al [1937] and by Buswell and Dudenbostel [1941]. (See also a review by Nahin [1955].) Further work on montmorillonites was done by Farmer [1958], Kato [1962], and Fripiat et ah [1960], and on vermiculite by Bradley and Serratosa [I960]. 

In the studies by Skipper et al. the number of water layers (and thus molecules) was fixed on the basis of experimental evidence consequently, the stable states or degrees of swelhng were presumed. Quite differently, Karaborni et al. [44] determined, by means of a combination of GCMC and MD, the number of water molecules directly from a series of simulations in which the distance between montmorillonite planes was varied systematically. They observed that swelling proceeded from the dry state through the formation of one, three, and then five layers of water. This is very different from the usually beheved hydration sequence from one layer to two, then to three layers, and so on, which has been intrinsically assumed by Skipper and coworkers. The authors conclude that the complex swelling behavior accounts for many of the experimental facts. This work demonstrates impressively the power of the grand canonical simulation method. 

The diagenetic effects are related to the alteration of rock mineral, shales in particular. Under certain conditions, montmorillonite clays change to illites, chlorites and kaolinites. The water of hydration that desorbs in the form of free water occupies a larger volume. This volume increase will cause abnormal pressures if the water cannot escape. 

Osmotic swelling is a second type of swelling. Where the concentration of cations between unit layers in a clay mineral is higher than the cation concentration in the surrounding water, water is osmotically drawn between the unit layers and the c-spacing is increased. Osmotic swelling results in larger overall volume increases than surface hydration. However, only certain clays, like sodium montmorillonite, swell in this manner. 

Englezos, P. and S. Hull, Phase Equilibrium Data on Carbon Dioxide Hydrate in the Presence of Electrolytes, Water Soluble Polymers and Montmorillonite , CanJ. Chem. Eng, 72, 887-893 (1994). 

Montmorillonite K10 was also used for aldol the reaction in water.280 Hydrates of aldehydes such as glyoxylic acid can be used directly. Thermal treatment of K10 increased the catalytic activity. The catalytic activity is attributed to the structural features of K10 and its inherent Bronsted acidity. The aldol reactions of more reactive ketene silyl acetals with reactive aldehydes proceed smoothly in water to afford the corresponding aldol products in good yields (Eq. 8.104).281... 

Clay materials show a different behavior. They are either cation-poor or cation-rich sheet silicates. They can swell by taking up varying amounts of water between the sheets. If the intercalated cations are hydrated as in montmorillonite, they act as cation exchangers. Montmorillonite, especially when it has intercalated Ca2+ ions, has thixotropic properties and is used to seal up drill holes. The effect is due to the charge distribution on... 

A theoretical model for the adsorption of metals on to clay particles (<0.5 pm) of sodium montmorillonite, has been proposed, and experimental data on the adsorption of nickel and zinc have been discussed in terms of fitting the model and comparison with the Gouy-Chapman theory [10]. In clays, two processes occur. The first is a pH-independent process involving cation exchange in the interlayers and electrostatic interactions. The second is a pH-dependent process involving the formation of surface complexes. The data generally fitted the clay model and were seen as an extension to the Gouy-Chapman model from the surface reactivity to the interior of the hydrated clay particle. 

The inductive effect of the carbon chain in the clay phase amounts to (only) 5 to 7 % of the effect in the gas phase. Ammonium cations in the interlamellar region of clay minerals are therefore less hydrated than in equilibrium solution. The free energy of alkylammonium exchange increases with charge density from Laponite (42) < Red Hill montmorillonite (40) < Camp Berteau montmorillonite (41) in line with the smaller interlamellar hydration status of the adsorbed cation at higher charge density. 

Selectivity decreased through the weathering sequence mica > i 11 ite = vermicul ite > montmorillonite. Selectivity for K over Ca has been ascribed to the low hydration number and polarizability of K (19), to wedge sites at the weathered edge of clay... 

However, when protonated TEMPAMINE adsorbs by cation exchange on fully hydrated layer silicate clays (10, 11), the spectrum becomes less symmetrical as shown in Figure 5. The beidellite and montmorillonite spectra have line shapes typical for nitroxide molecules with rotational frequencies on the order of 10 Hz (17). 

The amount of adsorbed chemical is controlled by both properties of the chemical and of the clay material. The clay saturating cation is a major factor affecting the adsorption of the organophosphorus pesticide. The adsorption isotherm of parathion from an aqueous solution onto montmorillonite saturated with various cations (Fig. 8.32), shows that the sorption sequence (Al > Na > Ca ) is not in agreement with any of the ionic series based on ionic properties. This shows that, in parathion-montmoriUonite interactions in aqueous suspension, such factors as clay dispersion, steric effects, and hydration shells are dominant in the sorption process. In general, organophosphorus adsorption on clays is described by the Freundhch equation, and the values for parathion sorption are 3 for Ca +-kaoUnite, 125 for Ca -montmorillonite, and 145 for Ca -attapulgite. 

Rearrangement reactions catalyzed by the clay surface were observed for par-athion (an organophosphate pesticide) when it was adsorbed on montmorillonite or kaolinite in the absence of a liquid phase. The rate of rearrangement reactions increased with the polarization of the hydration water of the exchangeable cation (Mingelgrin and Saltzman 1977). Table 14.1 summarizes a series of reactions catalyzed by clay surfaces, as reported in the literature. 

Montmorillonite, one of the most commonly encountered smectites, is similar to pyrophyllite (2 1) but has some interlayer cations and extra water. In pyrophyllite the layers are neutral because Si " in the tetrahedral sheet is not replaced by Al. In the smectites there is substitution of Al for Si " in the tetrahedral sheets, and occasionally Al appears in octahedral locations as well (for the names assigned to the end members, see Brindley and Brown, 1980, pp. 169-170.) The charge imbalances of the substitutions are compensated by interlayer cations, usually Na or Ca. These cations are easily exchangeable. The hydration level of the smectites is also variable. These minerals are very responsive to changes in water content as well as to the salt contents of the water. Other liquids that might be associated with the minerals and temperature can also effect changes in the chemical and crystal structure. 

Figure 7 shows the representative bright field HRTEM images of nanocomposites of NR and unmodified montmorillonite (NR/NA) prepared by different processing and curing techniques. It is apparent that the methodology followed to prepare the nanocomposites by latex blending facilitates the formation of exfoliated clay structure, even with unmodified nanoclays. It has been reported in the literature that hydration of montmorillonite clay leads to extensive delamination and breakdown of silicate layers [94, 95]. It has also been shown that NA disperses fully into the individual layers in its dilute aqueous dispersion (clay concentration <10%)... 

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