Differential formation of allophane and imogolite: experimental and molecular orbital study

Allophane and imogolite are naturally occurring aluminum silicate soil constituents with nano-ball and nano-tube morphology. Wall of the both materials is composed of Al(OH)(3) sheet with orthosilicic acid attached to it. Synthesis of allophane and imogolite can be controlled by addition of alkali and alkaline-earth metal ions. The main reaction product without or with small amounts addition of the metal ions is imogolite, while allophane forms when the metal ions were much added. The effect of metal ions on facilitating allophane formation and inhibition of imogolite formation were greater in the following order of Na, K < Ca, Mg. These metal ions affect the degree of dissociation of Si-OH group of orthosilicic acid, which may causes differential formation of allophane and imogolite. Structure optimization of the proto-imogolite model, precursor of allophane and imogolite, showed that when the Si-OH was undissociated, the shape of proto-imogolite model was transformed to asymmetrical in molecular configuration. This caused curling of the proto-imogolite model, which lead to formation of imogolite tube. On the other hand, when the Si-OH was dissociated, the shape of the proto-imogolite model was transformed to symmetrical configuration. This model curved to make a hollow sphere with placing the orthosilicic acid inside the sphere (allophane). Both of the experimental and molecular orbital calculation results proved that the dissociation of the Si-OH has an important role during the differential formation of allophane and imogolite.