The atomic structure of imogolite nanotubes: A 50 years old issue reinvestigated by X-ray scattering experiments and molecular dynamics simulations

Imogolites are clay nanotubes consisting of a curved di-octahedral gibbsite-like layer [${\left(\mathrm{OH}\right)}_3{\mathrm{AlO}}_3$] and of isolated $\left(\mathrm{OH}\right){\mathrm{SiO}}_3$ tetrahedra connected by three mutual oxygen atoms to the octahedra, with N silicon tetrahedra along a circumference. There is a consensus that synthetic imogolites have a larger N value than natural ones, which may depend on the method of synthesis. However, N values reported in the literature over the last fifty years are given as uncertain or are not consistent. In this work, we reinvestigate the structure of synthetic imogolite nanotubes for which a value of N = 12 was reported. We perform X-ray scattering experiments on nanotubes in suspension and on the same sample as a dry powder after removing water. The analysis of diffractograms is based on Molecular Dynamics simulations allowing us to obtain imogolite nanotubes with different N values. We find that the sample is made of a mixture of nanotubes with N = 13 and N = 14. Our study points towards strong deformation of the nanotubes, assembled in bundles, in their dry state, which has to be taken into account in the analysis of measured diffractograms.