Dried hybrid imogolite nanotubes as solids with a changeable surface area: an insight into textural properties based on the correlation between nitrogen gas adsorption, immersion calorimetry into water, and small angle X-ray scattering

Abstract

Powdered samples of three imogolite materials with an inner surface covered with Si–CH₃ groups (hybrid IMO-CH₃) were prepared by ambient drying at 323 K, freeze drying, and spray drying. Reliable estimates of the intra-tube (S_A), inter-tube (S_B), and inter-bundle (S_C) specific surface areas of these samples were inferred from a model-guided correlation between the results of measurements by small angle X-ray scattering (SAXS), nitrogen gas adsorption, and immersion calorimetry into water. Since the SAXS studies indicated no significant deformations of IMO-CH₃ nanotubes upon drying, further studies by gaseous N₂ adsorption at 77 K indicated the intra-tube and inter-bundle specific surface areas as being only accessible to this adsorbate. The outer curved surfaces of IMO nanotubes, including the inter-tube surface areas, were evaluated based on the calorimetric measurements of the enthalpy of immersion into water, using the enthalpy of immersion per unit surface area for a modelled curved surface of gibbsite as the conversion factor. Given the uncertainty in the determination of surface areas, the IMO-CH₃ samples were found to possess the limiting values of specific surface areas ranging between 1150 and 1480 m² g⁻¹. In contrast to the two other materials, ambient-dried IMO-CH₃ was characterized by the highest value of S_C parameter due to the much smaller bundles formed by the constituent imogolite nanotubes. The accessibility of these surface areas, together with the hydrophobic and hydrophilic surface domains, was demonstrated to depend on the nature of the surrounding medium and the size of the adsorbing species.