Diameter and chirality of natural and synthetic imogolite

Sixty years after their discovery, the structure of imogolite is still debated. We have carefully characterized natural and synthetic imogolites by Small-Angle X-ray Scattering, Infrared spectroscopy, Nuclear Magnetic Resonance, and cryo-TEM. We show that the natural imogolite has a significantly smaller diameter and is less polydisperse than the synthetic imogolite. Natural imogolite has an average diameter between the (11,0) and (12,0) configurations. However, surprisingly, the configuration closest to this average size and in better agreement with WAXS data is the (11,1) chiral structure. Concerning the synthetic nanotube, its average diameter is between the (13,0) and (14,0) configurations and measurements tend to show that it is most probably a combination of more than two chiralities. Finally, using a theoretical description of the strain energy as a function of the radius, we show that the observed polydispersity could be explained by the role of proto-imogolites in the formation process of imogolite.