Silicate clays surface acidity

Significant replacement of monovalent metal cations on layer silicate clay surfaces by protons can occur if the electrolyte concentration is very low. The long-term result, beyond hydrolytic exchange, is acidic decomposition of the clay structure in part, and release of structural AP or Mg + to solution. These multivalent cations may then readsorb onto exchange sites, influencing the rheological properties of clays in very dilute salts. Some of the anomalous behavior of Na -smectites suspended in... 

The H+ and NH forms of homoionic montmorillonite promote the hydrolysis of chloro-s-triazines to the hydroxy analogs (hydroxy-s-triazines) (73). Apparently, the surface acidity of these clays was extremely high, since no degradation was observed in control experiments conducted at pH 3.5 in homogeneous aqueous solution. Russell et al. (73) suggested that the hydroxy-s-triazine products were stabilized in the protonated form at the silicate surface. The IR spectra of these surface complexes agreed with the spectra obtained in 6N HC1, and it was inferred that the pH at the clay surface was 3 to 4 units lower than that measured in suspension. 

The dissociation of water coordinated to exchangeable cations of clays results in Brtfnsted acidity. At low moisture content, the Brrfnsted sites may produce extreme acidities at the clay surface-As a result, acid-catalyzed reactions, such as hydrolysis, addition, elimination, and hydrogen exchange, are promoted. Base-catalyzed reactions are inhibited and neutral reactions are not influenced. Metal oxides and primary minerals can promote the oxidative polymerization of some substituted phenols to humic acid-like products, probably through OH radicals formed from the reaction between dissolved oxygen and Fe + sites in silicates. In general, clay minerals promote many of the reactions that also occur in homogenous acid or oxidant solutions. However, rates and selectivity may be different and difficult to predict under environmental conditions. This problem merits further study. 

This last form of acidity can build up at low pH on the surfaces of variable-charge minerals (A1 and Fe oxides, allophane, and Al-hydroxy and Fe-hydroxy polymers adsorbed on layer silicate clays). 

In general, high-surface-area minerals such as layer silicate clays and allophane dissolve at significant rates in strongly acid soils ... 

Extreme surface acidity is expected only on clay surfaces that have been dried, as will be discussed later in this chapter. Air-dry layer silicate clays, especially when saturated... 

To date, potentiometric titration is still a main approach to study the surface acid base chemistry of clay minerals. Only some papers deal with the dissolution of a solid matrix resulting in various hydrolyzed aluminum species, silicic acid and their product hydrous aluminosilicates, though their interaction with a clay surface should be considered in the modeling description. The surface complexation model (SCM) was successfully applied in a recent paper [6] to interpret surface acid-base reactions involving the dissolution of illite clays during prolonged titration. Voluminous literature on ion adsorption and surface complexation... 

Fillers reinforce and increase the volume of the sealant. Fillers also reduce formulation costs and affect the technical properties of the sealant itself Common fillers for sealants are calcium carbonate (chalk), aluminum silicate (clay), barium sulfate and sihdc acids. Finely divided fillers, such as talc and Microdol 1, reduce the surface tack while simultaneously stiffening the film. Thixotropic fillers, for example fumed silica or Si02, improve the gunnabihty and reduce the sag of the compositions. Pigments are used to color sealants the white pigment used is usually titanium dioxide. 

Section 4.3 sets out the principles underlying the structure of the silicate mineral family. Natural clay deposits are formed by the chemical weathering of rocks -largely as a result of the attack by slightly acidic surface waters. Rainwater,... 

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