Organic complexes of unrefined and milled kaolin: An infrared spectroscopic study.

Aim: to characterise the surface sites present on unrefined and ball milled Cornish kaolin, with a view to ultimately determining the mode of interaction between the mineral and industrially significant organic molecules. Milling and temperature were found to affect the structure of kaolin and types of surface sites present. These changes were monitored primarily by variable temperature diffuse reflectance infrared Fourier transform spectroscopy (VT DRIFTS) and other complementary techniques including XRD and TGA. Curve fitting of the VT DRIFT spectra was used to aid characterisation of the types of water present on the mineral surface. The diagnostic probe molecule pyridine was used to identify the changes in reactive acid sites present as temperature increased both before and after milling, and oleic acid was used as a representative adsorbate to analyse the effects of carboxylic acid treatment.As ball milling time increased, so did the kaolin agglomerate size and the amount of surface sorbed water. The types of water present on the surface of unrefined and ball milled Cornish kaolin have been characterised, and grouped into four main types -strongly hydrogen bonded, moderately strongly hydrogen bonded, weakly hydrogen bonded and very weakly hydrogen bonded. The different water environments were observed using DRIFTS in the bending and stretching regions of the spectrum. Changes in the stretching region were generally less distinct, since the bands were broader than in the bending region. However, changes in both regions were elucidated by curve fitting of the VT DRIFT spectra, and certain bands appeared to have similar thermal behaviour.Freshly milled samples had a greater proportion of strongly hydrogen bonded water compared with the other, more weakly bonded types. Aged samples had less total surface sorbed water, and relatively less strongly hydrogen bonded water compared with the more weakly bonded species.Pyridine displaced the more strongly hydrogen bonded water (an effect similar to ageing). A dehydrated halloysite impurity was found which intercalated pyridine. VT XRD showed that deintercalation occurred at c 100 °C. The hydrogen bonding nature of the halloysite-pyridine interactions became less pronounced as milling increased. Pyridine adsorption to kaolin was via Bronsted sites in the unmilled kaolin. As milling time increased the mineral surface took on Lewis acid character and less hydrogen bonding occurred. Bronsted associations were present in all the milled (and unmilled) samples and became more significant as milling time increased (as more surface water was present). In addition to the intercalation reaction between halloysite and pyridine, this probe molecule is likely to bind to exposed (broken) edge sites on kaolin and/or halloysite, or to sorb between the slightly expanded mineral layers at the edges of the mineral stacks.Oleic acid adsorption onto kaolin at pH 3, was via surface adsorption of monodentate. Acid precipitate was loosely associated with the surface via hydrocarbon chain interactions with the adsorbed salt. At pH 9, total surface adsorption was low. Adsorbed species were monodentate in character. Mono- and dioleate were present as loosely bound surface precipitates. At pH 12 the salt was associated with the surface in the bridged bidentate form. Adsorption was high due to hydrocarbon chain associations with micelles, containing some trapped acid species. The precipitate at pH 12 was strongly held, and there was no significant reduction in intensity after washing.