Kaolinite Surface acidity

Figure 3. Dependence of the surface acidity of kaolinite on its moisture content. (Reproduced with permission from Ref. 67. Copyright 1971 Marcel Dekker.)...

The change in surface acidity with water content was also demonstrated by the ability of kaolinite to promote acid-catalyzed polymerization (236). Styrene, p-methylstyrene, and p-methoxymethylstyrene polymerized vigorously on kaolinite that was dried at 110°C. At 0.2% wt water content, p-methylstyrene and p-methoxystyrene polymerized and at 0.6% wt water content, only p-methoxystyrene polymerized. The polymerization results are consistent with lower acidity at higher water contents since the susceptibility of these monomers to acid-catalyzed polymerization is in the order p-methoxystyrene > p-methylstyrene > styrene. 

Figure 4.5. Schematic of the kaolinite surface hydroxyls. In addition to the basal OH groups, the schematic is showing the aluminol groups, the silanol groups, and the Lewis acid sites contributing to water adsorption (from Sposito, 1984a, with permission).

Elfarissi F, Pefferkorn E. Kaolinite/humic acid interaction in the presence of aluminium ion. Colloids Surfaces A (in press). 

El-Amamy and Mill (1984) measured the effect of the surface acidity of mont-morillonite and kaolinite on the hydrolysis rate constants for a number of chemicals containing hydrolyzable functional groups that exhibit acid-catalyzed, base-catalyzed, and neutral hydrolysis. The chemicals that were studied included ethyl acetate, cyclohexene oxide, isopropyl bromide, l-(4-methoxyphenyl)-2,3-epoxypropane, and N-methyl-p-tolyl carbamate (MTC). Aqueous suspensions of... 

Polar organic compounds such as amino acids normally do not polymerize in water because of dipole-dipole interactions. However, polymerization of amino acids to peptides may occur on clay surfaces. For example, Degens and Metheja51 found kaolinite to serve as a catalyst for the polymerization of amino acids to peptides. In natural systems, Cu2+ is not very likely to exist in significant concentrations. However, Fe3+ may be present in the deep-well environment in sufficient amounts to enhance the adsorption of phenol, benzene, and related aromatics. Wastes from resinmanufacturing facilities, food-processing plants, pharmaceutical plants, and other types of chemical plants occasionally contain resin-like materials that may polymerize to form solids at deep-well-injection pressures and temperatures. 

The effects of organic molecules and phosphate on the adsorption of acid phosphatase on various minerals, and kaolinite in particular, have been investigated by Huang et al. [97]. The Langmuir affinity constant for AcP adsorption by kaolinite follows the series tartrate (K — 97.8) > phosphate (K= 48.6) > oxalate (K — 35.6) > acetate (K= 13.4). At low concentration, acetate even promoted the adsorption of acid phosphatase. It was considered that competitive interactions between anionic adsorbates can occur directly through competition for surface sites and indirectly through effects of anion adsorption on the surface charge and protonation. 

In summary, the model proposed on the basis of acid-base characteristics of kaolinite platelets explains the pH-dependent charge primarily to the protonation of the hydroxyl groups at the basal gibbsite and the edge surface. We will later illustrate how this charge characteristics (surface protonation) influences the reactivity (dissolution characteristics) of kaolinite. 

Polyelectrolytes provide excellent stabilisation of colloidal dispersions when attached to particle surfaces as there is both a steric and electrostatic contribution, i.e. the particles are electrosterically stabilised. In addition the origin of the electrostatic interactions is displaced away from the particle surface and the origin of the van der Waals attraction, reinforcing the stability. Kaolinite stabilised by poly(acrylic acid) is a combination that would be typical of a paper-coating clay system. Acrylic acid or methacrylic acid is often copolymerised into the latex particles used in cement sytems giving particles which swell considerably in water. Figure 3.23 illustrates a viscosity curve for a copoly(styrene-... 

FIGURE 1.2 Effect of surface on evaporation of phenylacetic acid at 0% relative humidity and 20°C. a, montmorillonite b, mylar plastic c, glass d, cellulose e, kaolinite f, balsa wood g, stainless steel h, platinum. (Adapted from Regnier and Goodwin, 1977.)... 

Kaolinite is the main constituent in china clay used to make porcelain. The layers are largely held together by van der Waals forces. Bentonite is used in cosmetics, as a filler for soaps, and as a plasticizer, and it is used in drilling-muds as a suspension stabilizer. Bentonite and kaolinite clays are used, after treatment with sulfuric acid to create acidic surface sites, as petroleum cracking catalysts. Asbestos also has a layered structure (Section 12.13). 

Likewise, yet in a more pronounced manner, a kaolinite, another clay with a strongly acidic surface (Ho = -3 to -6) (ref. 2), also favors ipso nitration the yield of p-nitroanisole i rises to 27%, boosted by a factor 20 as compared to homogeneous reaction conditions ... 

Adsorbed n-dodecanol and stearic acid react on the surfaces of illite, kaolinite, and montmorillonite to yield hydrocarbons and the pattern of kinetic behavior exhibits a compensation effect (39,291), Table V, R. The... 

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