Allophanes dispersibility

Escudey, M. and Galindo, G., Effect of iron oxide coatings on electrophoretic mobility and dispersion of allophane, J. Colloid Intetf. Sci., 93, 78, 1983. 

These products can be broadly defined as coatings that contain the urethane or urea groups. To a lesser extent, groups such as allophanate or biuret can be present. They are available one or two component systems, able to cure at room or higher temperature. These coatings can be based on linear PU dispersions, or crosslinkable dispersions and they can also be produced by solvent free processes. 

Coarse clays (0.2 to 2 nm) of allophanic soils dispersed after less severe chemical treatments seem to contain similar constituents together with undispersed allophane. It does not appear possible at this stage to decide whether the latter material should be regarded as undispersed aggregates or as gel-like particles or both. This matter is further discussed later. 

When clay fractions are removed from soils containing allophane using methods of clay dispersion, which are satisfactory for most soils, surface area measurements of the residual coarse fractions show that much clay-size material has not been effectively dispersed. Various degrees of dispersion of soil allophane are achieved by dispersion in aqueous alkaline media... 

In general, allophane in soils may be appreciably dispersed in aqueous media of low salt content provided the pH is above 10 or below 4. After dispersion, maximum flocculation again occurs in the pH range 5.5 to 6, which corresponds to the isoelectric pH range of allophane (Fieldes [1958], Fieldes and Schofield [I960]). 

Allophane appears more difficult to disperse in the presence of gibbsite, and the addition of small amounts of gibbsite to apparently stable suspensions of allophane causes gradual flocculation (Birrell and Fieldes [1952]). 

Without treatment for iron removal the alkaline dispersion yields generally much larger particles than acid dispersion, indicating possible attack by acid or dispersal of a different fraction. After treatment for iron removal, particles from both kinds of dispersion show evidence of attack and dissolution, the total extent of the attack being apparently greater in the case of acid dispersion. Effects of different treatments are also illustrated in Figures 12 c and d, which show a fibrous form of allophane extracted by acid dispersion of an Egmont soil from volcanic ash and a coarse particulate amorphous material extracted from the same soil by alkali dispersion. 

Figure 12. Electron micrographs of some allophanic clays separated from New Zealand soils, (a) Clay dispersed in sodium carbonate at pH 9 from Kaharoa soil (b) clay dispersed in sodium carbonate at pH 9 from Egmont Ash soil (c) fibrous clay dispersed with acid from Egmont Ash soil (d) coarse glassy particles in clay dispersed with alkali from Egmont Ash soil.

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