Dispersion of kaolinite and talc in aqueous solution: nano-morphology and nano-bubble entrapment

Nano-morphological structure of well-crystallised kaolinite, micronised talc and micronised talc ground in a ring mill has been compared using field emission scanning electron microscopy (SEM) and atomic force microscopy (AFM). The kaolinite and ground talc dispersed in water immediately after shaking whereas the majority of the micronised talc remains at the air–water interface. AFM micrographs revealed more three-dimensional details of growth steps and attached colloid particles on basal planes of the crystallites than high resolution SEM micrographs. Two-dimensional ordered surface structure on the molecular level was measured by AFM. There are major differences in nano-morphology and surface structure between these three samples. The compact, blocky kaolinite crystals show large, relatively flat (<1.5 nm) basal planes with distinctive step growth on their surfaces. The irregular, thin, curved platelets of the micronised talc have much less complex surface structure with flat, smooth basal surfaces. Micronised talc platelets have much larger aspect ratios than either kaolinite or the ground talc. For micronised talc, the total edge contribution to the specific surface area is 13% whereas ground talc and kaolinite edges contribute 21% and 30–50%, respectively. The low edge contribution to total surface area in micronised talc contributes to its poor dispersion. Highly porous re-entrants and folds are frequently displayed at the edges and between sheets of the micronised talc. These voids, observed in both SEM and AFM micrographs, are filled by air nano-bubbles causing poor dispersion and high floatability when the talc is mixed with water. After grinding, the micronised talc particles are smaller, have reduced aspect ratios and much reduced porosity at edges and between sheets. Hence, the morphological differences in aspect ratio and air entrapment in the porous structure of talc are possible causes for the poor dispersability of micronised talc in comparison to kaolinite and ground talc samples.