Surface activities of clays

The activity of clay minerals, proven in the reactivity of terrestrial (15-16), and postulated in Martian (j ) soils, is disproportionate to their quantity, relative to other minerals. This is the result of several factors small particle size, high specific surface area, Bronsted and Lewis acidity, redox and other potentially catalytically active sites common to clay minerals, and a limited capacity for size exclusion (which is influenced by the number and valence of exchangeable cations ( )). 

Kodama and co-workers [58] have reported TG-DSC curves for the analysis of the interaction between vulcanisation accelerators (tetramethylthiuram disulphide, dibenzothiazolyl disulphide, diphenylguanidine and N-cyclohexyl-2-benzothiazolyl-sulphenamide) and fillers (carbon black, white carbon, hard clay and CaC03). The initial melting point (MP) of the accelerators was largely influenced by the fillers. The higher the surface activity of the filler is, the lower and wider the melting range becomes. 

The second example concerns the surface heterogeneity of clay minerals. Important problems, such as limited yield of oil recovery arising during oil exploitation, involve interaction of pore filling fluids with the minerals that form the reservoir walls. The clay minerals, due to their relatively high specific surface area and electrical charge density, are the most active for the retention of oil. Illites and kaolinites are the clay minerals that are most frequently found and their wettability properties are believed to be in relation to the heavy oil ends retention process. 

It is apparent that at low moisture content (<10% for the Na-saturated clay mineral and <5% for the Ca-, or Mg-saturated clay mineral), where water is not available for hydrolysis, hydrolysis does not occur. This low moisture content corresponds with the saturation of the cation s first hydration shell. As the moisture content is increased to the upper limit of bound water (50% moisture content), a significant enhancement of the hydrolysis of the epoxide is observed. When the moisture content exceeds the upper limit of bound water (>50%), the rate constant for the hydrolysis of the epoxide was reduced by a factor of 4. It was concluded that water in excess of sorbed water diminishes the catalytic activity of clay surfaces by reducing the concentration gradient across the double layer, effectively raising the surface pH closer to that of the bulk water. In similar studies with MTC, the addition of water to oven-dried Na-montmorillonite and Na-kaolinite retarded the hydrolysis rate of the carbamate. This observation is consistent with the fact that MTC exhibits only neutral base-catalyzed hydrolysis. 

The properties of a material that determine its suitability as a filler and the properties it imparts to rubber are its grain-size, surface area of particles and surface activity of particles (i.e. the ability of the particle s surface to bond with the rubber matrix). These properties of clay minerals, especially kaolinite, and their appropriateness as fillers are explained below. 

Acid activation has long been known as a means for increasing (dramatically) the catalytic cracking activity of clays (e.g., montmorUlonite) (Rupert et al., 1987). Upon acid treatment, the surface area of the clay is also increased. Typically, sulfuric acid is used in the treatment. The acid attacks and dissolves the octahedral layer that is sandwiched between two tetrahedral silica layers in the clay. The attack takes place uniformly on the edges of the octahedral layer, and eventually removes this layer. Thus, by optimal treatment (i.e., at a proper combination of acid concentration, temperature, and time) one can achieve a high surface area. 

Gas turbine fuels can contain natural surfactants if the cmde fraction is high in organic acids, eg, naphthenic (cycloparaffinic) acids of 200—400 mol wt. These acids readily form salts that are water-soluble and surface-active. Older treating processes for sulfur removal can leave sulfonate residues which are even more powerful surfactants. Refineries have installed processes for surfactant removal. Clay beds to adsorb these trace materials are widely used, and salt towers to reduce water levels also remove water-soluble surfactants. In the field, clay filters designed as cartridges mounted in vertical vessels are also used extensively to remove surfactants picked up in fuel pipelines, in contaminated tankers, or in barges. 

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