Effect of Ionic Strength on the Bundling of Metal Oxide Imogolite Nanotubes

Abstract

Significant developments have been proposed over the past decade in the synthesis of aluminosilicate and aluminogermanate imogolite-like nanotubes. But, while liquid phase synthesis is well-controlled, it is not the case for the nanotube arrangement in the dry state. In particular, nanotubes are found to self-assemble in bundles of various sizes, which may impact the properties of the final product. Here, we investigate the effect of ionic strength on bundling of aluminogermanate single-walled imogolite nanotubes (Ge-SWINT) in aqueous suspensions and in the resulting powders after solvent evaporation. The nanotube arrangement as a function of salt concentration was studied by X-ray scattering experiments and simulations. In aqueous suspension, nanotubes bundling occurs only at high ionic strength (IS > 8 × 10–2 mol·L–1), while beyond this threshold, the increase of electrostatic repulsions induces a complete stabilization of individual nanotubes. After solvent evaporation, nanotube arrangement is shown to be dictated principally by the initial concentration of salt. Beyond an ionic strength of ∼10–3 mol·L–1 in the starting suspension, all Ge-SWINT samples tend to form large bundles in powder, whose lattice parameters are independent of the initial salt concentrations. These experimental results clearly show that the positive surface charge of imogolite can be used to control nanotubes bundling by anion condensation.