Montmorillonite surface charge

Of the various inorganic soil constituents, smectites (montmorillonite clays) have the greatest potential for sorption of pesticides on account of their large surface area and abundance in soils. Weak base pesticides, both protonated and neutral species, have been shown to be sorbed as interlayer complexes. Sorption of atrazine on smectites ranges from 0 to 100% of added atrazine, depending on the surface charge density of the smectite (36). 

Taking this one step further, perhaps even an inorganic gene may have been provided by clay mineral sources. Earliest clay samples are of a mineral called montmorillonite that consists of sheets of aluminosilicates in which Fe2+, Fe3+ and Mg2+ are substituted for some of the Al3+, and Al3+ is substituted for Si4+. The oxygen content of the layers does not change and the alternative valencies allow the production of positive and negatively charged layers. Dramatically, Paecht-Horowitz and co-workers showed that the amino acid adenylate could be polymerised with up to 50 units on the montmorillonite surface in aqueous solution. Similar condensation reactions for carbohydrates on hydrotalcite surfaces have... 

Clay colloids provide a good example of the kinds of structures that can be formed upon flocculation. The association of plate-like clay particles is complicated by the fact that minerals such as montmorillonite, illite, and kaolinite can exhibit different surface charges at different particle surfaces. 

Sperry, J. M., and J. J. Peirce. 1999. Ion exchange and surface charge on montmorillonite clay. Water Environ. Res. 71 316-322. 

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. 

The role of sodium ions is predicted to be small according to Schulze-Hardy s rule that is, the effectiveness of counterions in the interfacial phenomena is approximately proportional to the sixth power of the counterion charge. The formation of complexes between Na+-ions and anionic sites on the montmorillonite surface is included here for the sake of completeness, although such complexes are subject to reservations because they are unknown in aqueous solutions, and evidence against their physical reality has been presented (Janssen and Stein 1986). Indeed, as will be seen later, they are not prevalent. 

Madrid, L. and Diaz-Barrientos, E., Description of Titration curves of mixed materials with variable and permanent surface charge by a mathematical model. 1. Theory. 2. Application to mixtures of lepidocrocite and montmorillonite, J. Soil Sci., 39, 215, 1988. 

Colloidal particles are also characterized by a large surface area/mass ratio (called specific surface), and by a strong surface charge that favors adsorption. For example, montmorillonite clays and some organic colloids may have specific surfaces of up to 800 m2/g. 

When the amount of modifier on the montmorillonite surface increases, a regular decrease in differential heats of adsorption is observed, with the simultaneous increase of the specific retention volumes per unit area of external surface and Henry s constants for the hydrocarbons considered (Tables 7, 8). At the same time, an increase in the amount of long-chain cationic surfactants on the kaolinite surface leads to the decrease in both differential heat of adsorption and Henry s constant for benzene (Table 8), not surprisingly, because of a more complete covering of the kaolinite surface hy the presorbed modifying layer and a decrease in the charge of the polar NHj groups of the modifier [41]. 

Aluminosilicates such as montmorillonite, kaolinite, illite, and vermiculite are solids that have structures readily accessible to counter ions. The excess negative charge resulting from isomorphic substitution of Al for Si is primarily distributed over the three adjacent surface O atoms of the layer, where it is electrically balanced by mobile, exchangeable cations- Thermodynamically, ion exchange can be interpreted in terms of the interlayer electrostatic interaction between surface charges and hydrated cations in accordance with the classical Eisenman theory (Eisenman, 1983). A comprehensive description has recently been given by Maes and Cremers (1986). 

Tombacz, E. and Szekeres, M.. Surface charge heterogeneity of kaolinite in aqueous suspension in comparison with montmorillonite, Appl. Clay Sci., 34, 105, 2006. 

X-ray difiFraction studies showed that diquat and paraquat were adsorbed in the interlayer spacings of montmorillonite clay with the planes of the pyridine rings parallel to the silicate sheets see paraquat in Figure 5) (30, 53). Adsorption of one compound over another was related to the charge separation on the ions and the surface charge densities of various adsorbents 29, 30, 55, 57). 

Decomposed DDT adsorbed on clay surfaces has been reported by several investigators. Barlow and Hadaway (268, 275) found that DDT decomposed only under anhydrous conditions. Fowkes et al. (287) found that it decomposed on acid clays but not on neutral clays. Lopez-Gonzalez and Valenzuela-Calahoro (288) found more decomposed DDT on basic clays than on acid clays and that DDT decomposed more on vermiculite than on montmorillonite. Vermiculite decomposed more because it has a higher surface charge density than montmorillonite. 

Xu et al. heterocoagulated cationic PMMA latex particles of an estimated 150-200 nm in diameter with various clays, Montmorillonite (GelWhite GP and Cloisite Na+) and (fluoro)hectorites (Laponite RD, RDS, B, S, JS), having plate dimensions between 25 and 600 nm. No details on the stable colloidal armored structures were reported. Mass coagulation was induced in order to obtain a nanocomposite bulk material, which was further analyzed [23]. Chen et al. [24] added Ti02 and SiOi/TiOi nanoparticles with a positive surface charge at a very low pH of 0-2 to both anionic and cationic latexes based on PMMA. A bulk nanocomposite blend was analyzed. 

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