House-of-cards structure

A good state of dispersion of the organoclay has been found in the CR matrix. The exfoliated structure can be directly observed from the of the OMMT-filled CR composite (left-hand image in Fig. 30). It is noticed from this micrograph that all silicate layers are exfoliated and distributed very nicely throughout the whole rubber matrix. It is also observed that some of the exfoliated clay platelets form a house of cards structure (right-hand image in Fig. 30). 

Phenomenon (iii) is responsible for so-called house of cards structure with very large irregular mesopores. 

In dilute suspensions clays tend to form gels. The classical model is the house of cards structure of kaolinite in which the face-to-edge association leads to an open 3-D structure (van Olphen, 1965). In the case of smectite-water systems it now seems more likely that the microstructure is mainly controlled by the face-to-face interactions (Van Damme et al., 1985). 

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. 

FIG. 5.20 Schematic of the house-of-cards structure derived from clay platelet edge-face interactions. (Supplied by and used with the permission of Southern Clay Products.)... 

The X-ray diffraction pattern of saponite exhibits a relatively weak and broad (001) reflection compared with montmorillonite, indicating a lack of long-range layer ordering. This characteristic of saponite can be ascribed to the three-dimensional voluminous house-of-cards structure. The house-of-cards structure also contributes to the high catalytic activity in comparison to montmorillo-nites, which do not form the house-of-cards structure. Alkylation activity... 

SH for CPNCs containing a small amount of clay to the presence of house-of-cards structures, resulting in the yield behavior. The flow-induced structures were different in shear and elongation. 

Okamoto, M., Nam, P. H., Maiti, R, Kotaka, T., Hasegawa, N., and Usuki, A., A house of cards structure in polypropylene/clay nanocomposites under elongational flow. Nano Lett., 1, 295-298 (2001b). 

Fig. 3 (a) Structure of hectorite (Laponite XLG [Mgj 34Lio.66Si802o(OH)4]Nao.66)- (b) Exfoliation in aqueous media and formation of house-of-cards structure [39]... 

Because of the house-of-cards -structure, and the limited size and stacking of the platelets of all saponites prepared in this work, the specific surface areas and pore volumes are extremely high, which is favourable for application in organic reactions. Except for Zn-saponites, all surface areas and pore volumes are comparable (Co-saponite) or even higher (Ni- and Mg-saponite) than those reported thus far for pillared clays [10]. Calcination of the synthetic saponites at 450°C for 4 hours do not affect strongly the textural properties the BET surface area and pore volume decrease by 1 to 10 percent. 

It is possible to prepare saponites with specific surface areas between 100 and 750 nfilg and pore volumes of 0.03-0.32 ml/g, all displaying a house-of-cards structure. The thermal stability of the synthetic saponites is high, which enables us to dehydrate the clay minerals efficiently, prior to the catalytic reaction. 

The plasticity of clay suspensions results in part from the house of cards structure formed by the attraction of the oppositely charged basal surfaces and edges (Fig. 4.6). 

The kinetics of the anisotropic growth of flat mica crystals which form a glass-ceramic of house of cards structure was also investigated by Holand et... 

Figure 3-10 House-of-cards structure in machinable fluormica glass-ceramic. Note phase-separated residual borosilicate glass and affinity of siiiceous droplets for mica flakes. Black bar = 1 pm.

Figure 2.22 The house-of-cards structure of a thixotropically solidified kaolinite suspension.

Classical studies have included those of Freundhch (1935), who introduced the term thixotropy ( change by touching ) and Cashen (1963) more recent reports were made by Cheng (2007), li et al. (2008), and Kelessidis (2008). This reversible sol-gel transition is caused by particle interactions induced by van der Waals forces that lead to aggregation under the occlusion of liquid so as to form a house-of-cards structure. The structure of a thixotropically solidified gel of kaolinite is shown schematically in Figure 2.22. 

Although the dy/dt(fj and i7 (r) dependencies of suspensions with structural viscosity/dUatancy (Figure 2.16) and thixotropy/rheopexy (Figure 2.21) behaviors show similar tendencies, there is a fundamental difference grounded in the different temporal characteristics of these phenomena. While structurally viscous systems with Newtonian or Bingham characteristics respond spontaneously to a change in shear stress, thixotropic and rheopectic systems require some time to recover-that is, to reconstruct the house-of-cards structure after collapse. 

Several other thickeners may be used Swellable clays, such as sodium montmor-illonite (commercially known as Bentopharm ). These clay particles consist of very thin plates and they can produce a gel in the continuous phase by a mechanism known as House-of-Card structure. The plates have negative surfaces and positive edges and they produce the House-of-Card structure by edge-to-face association. Fumed silica, such as AerosU 200 (manufactured by DeGussa), can also be used. These finely divided particles produce gels in the continuous phase by association between the particles forming chains and cross-chains. 

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