WATER STRUCTURE AND HYDRATION PROPERTIES OF IMOGOLITE NANOTUBES

Imogolite is a nanotubular aluminosilicate present in the clay fraction of volcanic soils. It has high specific surface areas (~500 m²/g) and is one of the few minerals reactive towards both anions and cations under the same soil physico-chemical conditions, properties which make it an important constituent of the soils where it is present. However, precise determinations of imogolite structure and geochemical reactivity have been hindered by its nano-crystalline character. Structural analyses, until now, were restricted to standard X-ray and electron diffraction techniques, the diffraction peaks being used mainly as fingerprints for the identification of the mineral in soils. In this work, we present a detailed structural characterization of the structure of synthetic imogolite using high-energy X-ray diffraction (HEXRD), neutron diffraction with isotopic substitution (NDIS), transmission electron microscopy (TEM), and molecular dynamics (MD) simulation methods. Theoretical and experimental investigations of the structure of water at the imogolite – water interface revealed the presence of highly structured water shells both at the surface and inside the nanotubes. We used these structural inputs to develop a geochemical multi-site complexation (MUSIC) model of the acidity of surface Al₂-OH groups on the external surface of imogolite and compared this to the acidity of similar sites on the equivalent (but planar) surface of gibbsite. This comparison yielded insights into the influence of surface curvature on mineral reactivity. Our MD simulations also probed the energetics of water adsorption and revealed that the external surface of imogolite is more hydrophobic than that of gibbsite. Ongoing work involving the use of inelastic neutron scattering also will be presented and discussed.