A computational and experimental investigation of halloysite silicic surface modifications after alkaline treatment

Density functional theory calculations were performed in order to investigate the formation of silanol groups on the outer surface of halloysite nanotubes, as possible products of the reactions with water in alkaline environments. The results, discussed in terms of energy release and structural properties of the modified surface, suggest that the formation of various kinds of silanol group constellations, and even the extraction of orthosilicic acid, are highly exothermic reaction in the presence of hydroxide ions. Thermogravimetric analysis, FT-IR, XRD and X-ray fluorescence spectrometry performed on halloysite treated at high pH conditions indicate that the adsorption of water on the outer surface occurs with higher percentage after the treatment, confirming the increased number of silanols groups.