Computational study of water adsorption on halloysite nanotube in different pH environments

The comprehension of structural and energetic features of halloysite nanotube (HNT) in different chemical environments plays a crucial role in developing new HNT based materials. So far these aspects were investigated by means of laboratory techniques that hardly are able to provide hints at atomistic level of detail. Our investigation aims to obtain such accurate informations through density functional theory calculations on HNT models, in order to figure out the most stable forms of HNT under different pH conditions. It turns out that, at low pH, the preferred protonation sites are located in the inner aluminic surface while in alkaline medium the silicic layer can show delocalized negative charges due to the formation of silanol groups. The adsorption of water molecules (WMs) on the HNT inner and outer surfaces in acidic and alkaline environments was investigated as well, so characterizing the hydrogen bonds which determine the adsorption geometry. Further, calculations allowed to verify how the modification of one layer affects the energetics of water adsorption on the other one.