Permanent negative charge

Montmorillonite is a laminar and expandable clay with wet binding properties and widely available throughout the world. The layers have permanent negative charges due to isomorphic substitutions. The scientific interest of montmorillonite lies in its physical and chemical properties as well as its low price. Consequently, the industrial application of montmorillonite is an attractive process [1]. On the other hand, among numerous reports published so far, crystallization of zeolite Beta draws much attention because of its unique characteristics, in particular, acidity and acid catalysis. It is reasonable to conceive that a catalyst system based on Beta/montmorillonite composite with suitable composition should provide a good catalytic capacity. 

Clay minerals have a permanent negative charge due to isomorphous substitutions or vacancies in their structure. This charge can vary from zero to >200cmol kg" (centimoles/kg) and must be balanced by cations (counter-ions) at or near the mineral surface (Table 5.1), which greatly affect the interfacial properties. Low counter-ion charge, low electrolyte concentration, or high dielectric constant of the solvent lead to an increase in interparticle electrostatic repulsion forces, which in turn stabilize colloidal suspensions. An opposite situation supports interparticle... 

In the study of soil science, most attention has historically been paid to the aluminosilicate clays, which dominate the properties of temperate soils, the first to be scientifically studied. More recently, the importance of the amorphous aluminosilicates has been shown in young soils, in soils derived from volcanic ash and in leached, acidic soil (e.g. podzols or spodosols). The hydrous oxides are especially important components of old, highly weathered soils, such as those found in the tropics (e.g. oxisols). This is an important distinction as the charge on the aluminosilicate clays is predominantly a permanent negative charge, while the amorphous aluminosilicates and hydrous oxides have a variable,... 

CEC). This permanent negative charge is the major characteristic of temperate soils, in which the aluminosilicate clays dominate the reactive fraction. The consequences of this are discussed in relation to ion-exchange processes in Section 5.5. 

Figure 6. Schematic representation of electrostatic sorption and surface complexation involved in Na -Ca2+ exchange at the mineral-water interface. =SOHn represents a surface hydroxyl (variable-charge) site =Sp represents a site of fixed (permanent) negative charge.

The permanent charge of clay minerals is due to lattice imperfections or defects, plus isomorphous substitutions. Sposito (1989) suggests that the permanent negative charge of illites, smectites, and vermiculites in mol sites/kg, ranges from 1.9 to 2.8. 0.7 to 1.7, and 1.6 to 2.5, respectively. 

Kaolinite is a 1 1 (T-O) phyllosilicate. The fundamental unit of its structure is an extended sheet of two constituents a silica-type layer of composition (Si4O10)4- and a gibbsite-type layer of composition (0H)6A14(0H)204 (see schematic representation in Fig. 10). Ideally, kaolinite crystals are not permanently charged. However, due to isomorphic substitution of Si by Al at the siloxane surface, kaolinite platelets carry a small, permanently negative charge (Van Olphen, 1977). Lim et al. (1980) and Talibudeen (1984) postulate that the permanent charge of kaolins is caused by contamination with small amounts of 2 1 phyllosilicates rather than a consequence of isomorphic substitution. 

The permanent negatively charged surface sites at the siloxane layer (XO) are accessible to ion-exchange reactions with cations, such as Na+, K +, Ca2 +, and Al3+ (inset in Fig. 14) (Schindler et al., 1987). 

Figure 14. Surface protonation and ion exchange equilibria at the kaolinite surfaces. The inset represents the protonation and ion-exchange reactions at the permanent negatively charged surface sites of the siloxane layer (0.1 MNaN03, [Al] = 1.6x 10-4 M, [XO], = 1.46x 10 3 M). The excess proton density, rHV, at the surface hydroxyl group is displayed as a function of pH. Surface protonation is interpreted as a successive protonation of two distinct types of OH groups localized at the gibbsite and edge surfaces. The pHZPC of the edge surface is about 7.5.

The absence of tangential ion conduction also results in a weaker liquid phase tangential electric field. As such, the AC electrospray behavior was found to be insensitive to liquid conductivity [12]. This passivity of the liquid phase is compounded by the formation of a thin, highly conducting, permanent negatively charged... 

Most commonly used layered silicate is montmorillonite clay, which is composed of micron-sized particles. The particles are constructed of platelets with thickness of lnm and width of 100-200 nm. Platelets have permanent negative charge and they are held together by charge balancing cations such as Na" or Ca [2-i] ions. The significant disruption of individual silicate layers in polymer matrix with nanoscopic dimensions (exfoHated structure) leads to improvements of the nanocomposite properties. However, in many cases, the isolated silicate layers are not completely dispersed throughout the polymer matrix, instead, the clay particles in polymer matrix maintain the hierarchical architecture, and an interlayer expansion occurs (intercalated structure). 

CEC, as exemplified in Table 8.3, due to permanent negative charge. On the other hand, anion-exchange capacity is generally low and present only at low pH values. The determination of CEC has been widely described in the literature (Sumner and Miller 1996 Zelazny, He, and Vanwormhoudt 1996 and references therein). 

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