Montmorillonite silanol

Surface protonation/deprotonation reactions at the edge of the silanol and aluminol sites (>SOH) of montmorillonite, which can be exemplified by the following reactions ... 

Besides the cation exchange in the interlayer space, cations and anions can also undergo sorption on the edge charges of montmorillonite. The edge charges are formed by the protonation and deprotonation of silanol and aluminol sites, and thus they depend on the pH. 

The formation of edge charges of minerals have been discussed in Chapter 1, Section 1.3.21. It has been shown that aluminosilicates (including montmorillonite) have two types of surface (aluminol and silanol) sites, and their protolytic processes have been expressed by Chapter 1, Equations 1.54-1.56. For simplicity, the reaction equations are repeated here. For aluminol sites,... 

FIGURE 2.3 Potentiometric titration curve of copper-montmorillonite in 0.1 mol dm-3 NaC104 solution, m = 50 mg, V = 20 cm3 (upper left). Vs are the experimental points, line is the plotted curve by the surface complexation model. The concentration of surface sites—lower left interlayer cations upper right silanol sites lower right aluminol sites (Nagy and Konya 2004). 

The Concentration of Edge Sites and Intrinsic Stability Constants of Protonation and Deprotonation of Silanol and Aluminol Sites of Montmorillonite Samples Calculated by the Surface Complexation Model... 

For KSF montmorillonite, the number of silanol and aluminol sites was found to be less by an order of magnitude. It is in accordance with the ratios of specific surface areas (10 m2/g for KSF montmorillonite, and 93.5 m2/g for montmorillonite [Istenmezeje]). This is an interesting observation because KSF montmorillonite is an acid-treated substance. Thus, it seems that acidic treatment causes the decrease of the layer charges (the CEC decreases montmorillonite content of Ca-, Cu-, and Zn-montmorillonites is 91%, and that of KSF montmorillonite is 53%). The acidic treatment, however, does not change the nature of silanol and aluminol sites, the stability constants of the edge charge reactions remains the same, and the number of edge sites is proportional to the specific surface area. 

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. 

FIGURE 2.17 The quantity of sorbed valine in the interlayer space and on the silanol sites of copper-montmorillonite, and the concentration of valine species in the solution m = 100 mg, V=20 cm3, c0= le-3 mol dm-3, T = 20°C. The right y-axis shows the concentration of HVal, all other species are labeled on the left y-axis. 

Figure 14.6 Formation of covalent bonds between 1,6-diisocyanatohexane and the silanol and hydroxyl groups in an organomodified montmorillonite as explained in Ref [70].

The probe molecule pyridine has often been used more recently in the study of clay surface acidity by IR methods. Figure 7 shows data of pyridine chemisorption on a synthetic mica-montmorillonite catalyst as an example (62,63) of the types of bands of interest. The spectra are interpreted as showing chemisorption of pyridine at both protic and aprotic sites. The clay was first heated for 15 h at 650°C under vacuum and then cooled and spectrum A taken. Note that there are no bands in the 1400-1700 cm region, the residual hydroxyl near 3450 cm S and edge silanol hydroxyl at 3747 cm . Pyridine vapor was then chemisorbed and spectrum B taken. The bands at 1456 cm and 1547 cm are assigned to Lewis and Bronsted sites, respectively. Since the 3747 cm edge silanol band decreases, it is assumed that these protons are involved in the mechanism. Lewis sites predominated under these particular conditions. 

Unlike the derivatives of montmorillonite and vermiculite, the organic derivatives of the polysilicates, even the air-dried samples, retain high amounts of water in the interlayer space. The air-dried organic derivatives of magadiite contain 5-8 moles of free water per MSiOi, i.e., not bound in the form of silanol groups. The silanol and SiO groups projecting into the interlayer space may provide preferential adsorption sites for water. This water essentially contributes to the stability of the interlamellar structures [147]. 

Most of the early research work has mainly been focused on the nanocomposites of sihcone rubber filled with different clay modified by different intercalating agents [41, 125, 139, 141, 143, 171-173]. Based on the XRD studies in Figure 7.4, Burnside and Giannelis [141] concluded that the delaminated morphology is developed in silanol-terminated PDMS nanocomposites filled with 5 wt% of dimethyl ditallow ammonium bromide-modified Na -montmorillonite. On the contrary, no intercalation or delamination takes place in the benzyldimethyloctadecylammonium salt-modified bentonite and silanol-terminated PDMS nanocomposite. Hexadecyltrimethylammoium ions-modified lithium fluorohectorite (Cj FH) has also been used to prepare PDMS (average M. Wt 400,1750,4200 Da) nanocomposites [139]. 

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