Bentonite edge charge

The parameters obtained by others for SWy-2, BSAB, and MX-80 cannot be compared to the previously discussed data because the silanol and aluminol sites as well as the deprotonation processes (Equations 2.4 and 2.5) were treated together. Calcium bentonite (Istenmezeje) shows similar intrinsic stability constant for SWy-1 bentonite, but the number of edge sites is different. Note, however, that the specific external surface areas are also very different 21.4 m2/g for SWy-1, and 93.5 m2/g for Istenmezeje montmorillonite (Table 2.1). The ratio of the specific surface area (Istenmezeje/SWy-1) is 4.4, and the ratio of the total number of edge sites (silanol + aluminol) is 5.3, which are in fairly good agreement if the surface charge density is the same. 

Some characteristic properties of bentonites (CEC, sorption properties) are mainly governed by the montmorillonite content and the layer charge of montmorillonite. Other properties, however, depend on the circumstances under which the rock is formed. These are particle size distribution, external specific surface area, and surface acid-base properties. The quantity of the edge sites mainly depends on the specific surface area. The protonation and deprotonation reactions take place on the edge sites of other silicates and aluminosilicates present beside montmorillonite, so their effects manifest via surface reactions. Consequently, the origin of bentonite determines all properties that are related to external surfaces. 

The second mechanism, first proposed by van Olphen (8,11-14), assumed structure formation in bentonite gel to be due to edge-to-flat surface asssociation of the plate-like particles as a result of electrostatic attraction between the oppositely charged double layers at the surface. This so-called "house of cards structure" is likely to occur provided the pH of the suspension is below the isoelectric point of the edges, which are then positive and become attracted to the negatively charged faces. 

The relationship between lEPs and mechanical properties of dispersions of clay minerals is more complex than for oxides. Specific results for bentonites can be found in [394] (Tables 3.1317, 3.1318, and 3.1321). Eor kaolin, tlie maximum in the yield stress was at pH 5.3, while the potential was negative at pH > 4. This lack of coincidence of the IEP and the maximum in the yield stress is due to different charges of faces and edges [1019]. 

Platelets are held together by cations. They impart a positive charge to the edge of the particles. These interlayer cations play a key role in the physicochemical properties of bentonite and in the stability of aqueous dispersions. Normally calcium is predominant and the clay swells to a moderate extent when dispersed in water. When Ca ions are replaced by Na, e.g., by reacting with Na2CC>3, the bentonite is said to be activated. This activation makes the clay much more swellable. 

Sodium silicates are also used to provide the fluid with a yield stress large enough to hold the particles at high water content. The mechanism is completely different from that of bentonite platelets that, having opposite charges on the faces and on the edges, gel the fluid by forming card house structures. Here, sodium silicate reacts with lime or calcium chloride to form a calcium silicate gel. It is this gel that provides the yield stress required to hold the particles. 

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