Montmorillonite water structure

Montmorillonite-Calcite (me) Mixture. Heated mixtures of montmorillonite and calcite yielded the phases given in Table I. Although the montmorillonite structure persisted through 400 °C, it underwent dehydroxy lation between 400 and 500 °C. Grim and Bradley (16) have shown that the general layered structure is able to survive the elimination of the (OH) water with moderate readjustments. This structure produces an X-ray diffraction pattern like that given in Table III. Table III represents data close to those observed in this study. This phase is called dehydroxylated montmorillonite in Table I. This phase disappeared between 700 and 800 °C as a result of the complete destruction of the montmorillonite crystal structure. Calcite decomposed between 500 and 600 °C to form lime that was present through 900 °C. 

H. P6z6rat and J. M6ring, Recherches sur la position des cations 6changeables et de I eau dans les montmorillonites, Compt. Rend. Acad. Sci (Paris) 265 529 (1967). R. K. Hawkins and P. A. Egelstaff, Interfacial water structure in montmorillonite from neutron diffraction experiments, Clays and Clay Minerals 2H PM)). 

Interlayer Molecular Structure and Dynamics in Li-, Na-, and K-Montmorillonite-Water Systems... 

Greathouse JA, Cygan RT (2006) Water structure and aqueous Uranyl(VI) adsorption equilibria onto external surfaces of beidellite, montmorillonite and pyrophillite results from molecular simulations. Environ Sci Technol 40 3865-3871... 

Fewer controlled experiments have been carried out for purely aquatic systems. Montmorillonite complexes with benzylamine at concentrations below 200 pg/L decreased the extent of mineralization in lake-water samples, although a similar effect was not noted with benzoate (Snbba-Rao and Alexander 1982). Even in apparently simple systems, general conclusions cannot therefore be drawn even for two structurally similar aromatic compounds, both of which are readily degradable nnder normal circumstances in the dissolved state. 

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... 

Chatteijee, A., Ebina, T., and Mizukami, F. 2005. Effects of water on the structure and bonding of resorcinol in the interlayer of montmorillonite nanocomposite—a periodic first principle study. J. Phys. Chem. B 109 7306-7313. 

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. 

The Li ions were introduced in two different ways either before or after Zr intercalation. The montmorillonite (Weston L-Eccagun) was first exchanged with NaCl (IN) and washed. Two montmorillonites with reduced charge were prepared following the Brindley and Ertem method (13). Part of the Na+ montmorillonite was first saturated with LiCl (IN) and washed. The Li+ clay thus obtained and Na+ clay suspension were stirred for 24 hours at 25°C and dried on glass plate. The films were then heated at 220°C for 24 h in order to allow Li diffusion in the clay structure. Two different Li concentrations (F=0.4 and F=0.6) were used. The Na Li+ modified montmorillonite were dispersed in water acetone solution (1/1). The ZrOCla, 8H2O solution was added to the Na+Li+ montmorillonite (0.02g.l l Zr/Clay=5.CEC). The suspension was stirred with NaOH solution (0.1 N) up to a OH/Zr ratio of 0.5. The final pH of the suspension was 1.85. After two hours of reaction at 40°C the Zr pillared clay was washed up to constant conductivity of the solution, freeze-dried and calcined at different temperatures up to 700°C (Eni-02 and EIII-03). 

Ion exchange reactions between cations present in groundwater and in the interlayer space of the montmorillonite structure. These are mainly the replacement of 2Na+ by Ca2+ and Mg2+ present in ground-waters. This triggers the dissolution of carbonates according to equilibria like the one exemplified by equation (1). The ion-exchange reactions can be described by ... 

The peculiar layer structure of these clays gives them cation exchange and intercalation properties that can be very useful. Molecules, such as water, and polar organic molecules, such as glycol, can easily intercalate between the layers and cause the clay to swell. Water enters the interlayer region as integral numbers of complete layers. Calcium montmorillonite usually has two layers of water molecules but the sodium form can have one, two, or three water layers this causes the interlayer spacing to increase stepwise from about 960 pm in the dehydrated clay to 1250, 1550, and 1900 pm as each successive layer of water forms. 

Figure 17. Proposed phase relations where K is a mobile component and Al, Fe are immobile components at about 20°C and several atmosphere water pressure for aluminous and ferric-ferrous mica-smectite minerals. Symbols are as follows I illite G = non-expanding glauconite Ox = iron oxide Kaol = kaolinlte Mo montmorillonite smectite N nontronitic smectite MLAL aluminous illite-smectite interlayered minerals Mlpe = iron-rich glauconite mica-smectite interlayered mineral. Dashed lines 1, 2, and 3 indicate the path three different starting materials might take during the process of glauconitization. The process involves increase of potassium content and the attainment of an iron-rich octahedral layer in a mica structure.

There are more complicated structures intermediate between pyrophyllite and talc with variable substitution of A1J and Mg2. Electroneulrality is maintained by hydrated cations between layers. Thus the montmorillonites arc unusual days forming thixotropic aqueous suspensions that arc used as well-drilling muds and in nondrip puints. They are derived from the formulation AU(OH)jSi40 ,-x-H2o with variable amounts of water, Mg3+ (in place of some Al5 ), and compensaUng cations. M"+ (M = Ca in fuller s earth, which is converted to bentonite, M = Na). Vermiculite likewise has variable amounts of water and cations, (t dehydrates to a talc-like structure with much expansion when heated (see page 750). 

In the absence of water, none of the chemical transformations described above occurs noticeably. The low diffusion coefficient of alkyl-ammonium cations between the montmorillonite layers (2) together with the strong acid character of residual water (3, 4) in this situation might provide a favorable situation which, perhaps, does not exist on other silicate surfaces with a more open structure. 

Mineral Surfaces. Organic matter is chemically adsorbed (deriva-tized) at the surfaces of clay minerals, zeolites, and related minerals (105) and is at times protected, concentrated, and degraded by contact with the solid surfaces. For example, porphyrins are protected (106), as are optically active amino acids by montmorillonite (107). This may result in part from the position of the organic matter in lattice spaces, as shown by Stevenson and Cheng (108) for proteinaceous substances keyed into hexagonal holes on interlamellar surfaces of expanding lattice clays, or from the fact that there are ordered structures at solid-water interfaces (109). 

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