SEM evidence of structural re-arrangement from gelling to aggregation in Birdwood kaolinite

Birdwood kaolinite from South Australia displays extremely different colloidal stability and particle interactions in comparison with many other kaolinites. Birdwood kaolinite gels at very low solid loadings in aqueous suspension and creams easily on top of the dispersion upon addition of a cationic surfactant (CTAB) and aeration by shacking. The particles have both high surface area and cation exchange capacity as well as X-ray diffraction patterns suggestive of the presence of low level of interstratifications by smectite-like layers in this kaolinite structure. To find the possible cause of this behaviour, the microstructure of Birdwood kaolinite was studied using a cryo-SEM system. SEM observations of aqueous suspension in this kaolinite show voluminous three-dimensional microstructure. This gel structure is composed of individual kaolonite platelets connected at the edges forming an expanded cellular network where the particle basal faces seem to repeal each other. This structure collapses after introduction of CTAB into aqueous suspension. CTAB-treated kaolonite suspension shows compact aggregates, and after aeration by shacking produce microbubbles of air with kaolonite platelets adhered to the bubble walls. This structural re-arrangement after addition of CTAB can be explained by significant charge reduction on negatively charged basal planes of Birdwood kaolonite which allow particles to form compact aggregates.