Clay hydration water

The arguments presented above lead to the conclusion that the adsorption of nonionic compounds such as halogenated hydrocarbons results primarily from "hydrophobic bonding" or, perhaps more appropriately, the hydrophobic interaction (7). The thermodynamic driving force for hydrophobic interactions is the increase in entropy resulting from the removal, or decrease, in the amount of hydration water surrounding an organic solute in water. Studies have shown that the adsorption of aliphatic amines onto clays (8)... 

Clay minerals behave like Bronsted acids, donating protons, or as Lewis acids (Sect. 6.3), accepting electron pairs. Catalytic reactions on clay surfaces involve surface Bronsted and Lewis acidity and the hydrolysis of organic molecules, which is affected by the type of clay and the clay-saturating cation involved in the reaction. Dissociation of water molecules coordinated to surface, clay-bound cations contributes to the formation active protons, which is expressed as a Bronsted acidity. This process is affected by the clay hydration status, the polarizing power of the surface bond, and structural cations on mineral colloids (Mortland 1970, 1986). On the other hand, ions such as A1 and Fe, which are exposed at the edge of mineral clay coUoids, induce the formation of Lewis acidity (McBride 1994). 

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. 

Surface catalysis affects the kinetics of the process as well. Saltzman et al. (1974) note that in the case of Ca -kaolinite, parathion decomposition proceeds in two stages with different first-order rates (Fig. 16.14). In the first stage, parathion molecules specifically adsorbed on the saturating cation are quickly hydrolyzed by contact with the dissociated hydration water molecules. In the second stage, parathion molecules that might have been initially bound to the clay surface by different mechanisms are very slowly hydrolyzed, as they reach active sites with a proper orientation. 

The swelling of clays in water results from the extra hydration of the interlamellar cations (Fig. 77). This is the best known example of the important phenomenon of intercalation, which is simply the insertion of guest species into an accommodating host, usually, but not exclusively, a layered solid. The degree of swelling, however, is governed by the nature of the interlamellar cation and the sorption isotherm often exhibits steps, as so often occurs with clathrates. 

Magnesium-vermiculite also forms monolayer hydrates with basal-plane spacings of 1.163 and 1.153 nm [23]. These hydrates are distinguished by the configuration of the Mg2+ solvation complexes (outer-sphere surface complexes) in them. The hydrate with the larger basal-plane spacing contains Mg2+ in the centers of flattened tetrahedra formed by water molecules the other clay hydrate contains Mg2+ at the apex of a pyramid whose base comprises three water molecules. 

The clay-water interactions and the quantity of water present in the interlayer space strongly depend on the nature of exchangeable cations. The water in the clay may be acting as hydrate water or additional absorbed water. 

Figure 2 The amount of CO2 consumption (Q) during the hydrate accumulation for model sediments (sand with 7% of montmorillonite clay) with water content (Wjy) 1-17%, 2-10%...

Since water molecules can adsorb on the clay surface, water can be considered as a modifier. To investigate the hydration process of vermiculite, a calorimetric study was performed with the main results as shovm in this section. The vermicuhte sample used was the same as described in the previous section. 

Whatever the extent of [Ain] in the ACH solution, the polycation may be altered during the intercalation step. Hydrolysis of the polymeric cation by hydration water of the clay interlayer as described by Vaughan (3) changes the [AI13] charge. Polymerization of the polycation [AI13] has also been mentioned by Bott et al. (17) and Fripiat (18). 

The solvation of cation and electrical double layer structure near clay surface was studied by neutron diffraction methods (156-158). The intaplay between molecular simulations and neutron diffraction techniques also has been also applied to this clay mineral-water-cation interface system. Park and Sposito (112) simulated the total radial distribution function (TRDF) of interlayer water from Na-, Li-, and K-montmorillonite hydrates as a physical quantity from molecular simulations. They obtained TRDF values from Monte Carlo simulations and directly compared with previously obtained H/ D isotopic difference neutron diffraction results (9,10). 

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