The relation between nanotube diameter, length and surface area and pore volume of multi-walled spiral halloysite nanotubes: A theoretical study

Abstract Halloysite (Hal) is a natural clay mineral with nanotube structure. Due to their outstanding specific surface area, loading capacity and selective adsorption ability, Hal have been widely used in the fields of catalysis, adsorption, drug loading and releasing. However, the loading ability of Hal from different halloysite deposits still varies in a large range. The relation between the microstructure properties of Hal and the intrinsic parameters such as length (L), inner/outer diameter (Dinner/Douter) of halloysite nanotubes (HNTs) is still obscure. In this study, we constructed the atomic structures of several multi-walled spiral HNTs with diameters (10–40 nm) in the range of the real size of Hal, and calculated their specific surface area (S) and pore volume (V) to compare with experimental results. By comparing the difference between the calculated values and the theoretical values (based on simple mathematics), the relation between the S/V of HNTs and its Dinner/Douter and L is established using modified equations. The equations were applied to deduce the theoretical values of the specific surface area and pore volumes of HNTs in a wide range of inner/outer diameter (10–200 nm) and length (100–5000 nm). Notably, while the experimental surface area are highly consistent with the theoretical values, the experiment pore volume are mostly lower than the theoretical values, which indicates that the lumen space of HNTs from different deposits are blocked in different degrees by the associated minerals and impurities. It is expected that, in addition to traditional physicochemical purifications, the removal of lumen impurities can help to revive the lumen loading ability of HNTs upto their theoretical limits.