Colloidal phase behavior of high aspect ratio clay nanotubes in symmetric and asymmetric electrolytes

Hypothesis: Imogolite nanotubes (INTs) are anisometric particles with nanometric diameters, as carbon nanotubes. But unlike the latter, the specific structure of imogolite nanotubes gives appreciable properties, such as monodispersity in diameter. Aluminogermanate double-walled INTs (Ge-DWINTs) are obtained with variable aspect ratios by controlling the synthesis conditions. It thus appears as an interesting model system to investigate how aspect ratio and ionic valence influence the colloidal behavior of highly anisometric 1D objects. Experiments: The nanotubes were synthesized by a 5 or 20 days hydrothermal treatment to modify the aspect ratio while the electrostatic interactions was investigated by comparing the colloidal stability in symmetric and asymmetric electrolytes. The phase behavior and their related microstructure were determined by optical observations and small-angle X-ray scattering measurements, coupled with interparticle distance modelling. Findings: We evidenced that colloidal suspensions of Ge-DWINTs prepared in NaCl are guided by repulsive double layer forces, undergoing different liquid crystal phase transitions before stiffen into a glass-like state. We found that the microstructure can be rationalized by taking into account the anisometric nature of the particles. By contrast, dispersions prepared with asymmetric electrolytes are governed by strong attractive forces and thus form space-filling gels containing large nanotubes aggregates.