Environmentally significant organic molecules adsorbed onto mineral samples from non-aqueous media: Diffuse reflectance (DRIFTS) infrared spectroscopy

DRIFTS, was used to study small organic molecules, with and without functional groups, adsorbed to the surface of γ-alumina and kaolinite clay from a non-polar solvent. Adsorbed water is present on the surface of both minerals and this influences the adsorption of organic molecules from, the non-polar solvent, onto the mineral surface, particularly the interaction of polar functional groups with the surface. The carboxylic acid functional group (on myristic or salicylic acid) is not dissociated in non-polar solvent, thus the presence of carboxylate bonded to the minerals indicates interaction of this polar organic functional group with the adsorbed water layer on the mineral surface. The results imply the displacement of the H from the carboxyl group being accommodated, via the water layer, by a negatively charged surface site (rather than being taken up by the hexane solvent). Carboxyl groups were also evident on some surfaces, particularly samples treated with the more highly concentrated solutions and not rinsed with fresh solvent. The washing step removed all, or a significant proportion, of the carbonyl species from the surface, supporting this group being present as a physic-sorbed layer formed by dry down. The non-polar octacosane was found after washing with fresh solvent (hexane), on the surface of kaolinite but not γ-alumina. This implies interaction with uncharged siloxane surface but not charged alumina type surfaces. This will be CHECK USING DEUTERATED OCTACOSANE It was possible to use DRIFTS spectra from a set of prepared samples to identifying the molecular density for monolayer coverage (resistant to washing) of myristic acid on γ-alumina to be in the region of 2.0 molecules per nm. This was over double that observed using salicylic acid adsorbed from aqueous solution, with the same supply of γ-alumina. The technique was able to identify myristic acid down to coverage of 0.02 molecules per nm. This was the same as the lower sensitivity observed using salicylic acid adsorbed from aqueous solution, with the same supply of γ-alumina.