Silicate surface deprotonation

The weathering of silicates has been investigated extensively in recent decades. It is more difficult to characterize the surface chemistry of crystalline mixed oxides. Furthermore, in many instances the dissolution of a silicate mineral is incipiently incongruent. This initial incongruent dissolution step is often followed by a congruent dissolution controlled surface reaction. The rate dependence of albite and olivine illustrates the typical enhancement of the dissolution rate by surface protonation and surface deprotonation. A zero order dependence on [H+] has often been reported near the pHpzc this is generally interpreted in terms of a hydration reaction of the surface (last term in Eq. 5.16). 

In the more alkaline pH-range surface-bound cations may increase the dissolution rate the surface deprotonation increases Coh (cf. Fig. 5.9c) and thus enhances the dissolution. This effect has been observed, above all, with silicates and glasses. 

Some characteristic properties of bentonites (CEC, sorption properties) are mainly governed by the montmorillonite content and the layer charge of montmorillonite. Other properties, however, depend on the circumstances under which the rock is formed. These are particle size distribution, external specific surface area, and surface acid-base properties. The quantity of the edge sites mainly depends on the specific surface area. The protonation and deprotonation reactions take place on the edge sites of other silicates and aluminosilicates present beside montmorillonite, so their effects manifest via surface reactions. Consequently, the origin of bentonite determines all properties that are related to external surfaces. 

The pHpznpc is around pH 2-3. Both the positive surface charge, due to bound protons, and the negative surface charge, due to deprotonation (equivalent to bound OH ), enhance the dissolution rate. The same kind of pH dependence is observed also with silicates. 

The smectite clays do, however, have some important features which make them particularly attractive as catalyst supports. In addition to their high intrinsic surface area, their laminar structure may confer size and shape selectivity to the resultant catalysts. Another important feature is the negative charge on the silicate layers which may be able to polarise reactant molecules and enhance catalytic activity. Finally the intrinsic acidity of clay minerals provides the catalyst with bifunctionality. This may be useful for example in stabilising intermediate carbocations which would otherwise deprotonate. 

The most widely accepted condensation mechanism involves the attack of a nucleophilic deprotonated silanol on a neutral silicate species, as outlined earlier for the condensation in aqueous silicates [Eq. (5.4)]. This condensation mechanism pertains above the PZC (or lEP) of silica (pH > 2) because the surface silanols are deprotonated (i.e., they are negatively charged) and the mechanism changes with the charge on the silanol. Condensation between larger, more highly condensed species, which contain more acidic silanols, and smaller, less weakly branched species is favored. The condensation rate is maximized near neutral pH where significant concentrations of both protonated and deprotonated silanols exist. A minimum rate is observed near the PZC (or lEP). 

The most widely accepted mechanism for silica condensation reactions involves the attack of a nucleophilic deprotonated silanol Si—0 on a neutral silicate species as proposed by Her [43] to explain condensation in aqueous silicate systems. This pertains to reaction (Eq. (17.3)) above the isoelectric point of silica where surface silanols are deprotonated to a significant extent. Note that not all silanol groups are identical, as it depends on the electron density on the... 

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