The Surface Chemistry of Imogolite

Abstract

Abstraet--Imogolite is a tubular aluminosilicate which is common in Andosols and Spodosols. The high pH at point-of-zero charge at the outer parts of the tube and the anomalously high chloride adsorption of imogolite suggested that there may be structural charge associated with this mineral. The structural charge may arise because of changes in bond valence imposed by the incorporation of orthosilicate anions in a gibbsite-type sheet. By using a Basic Stern Model approach, it is shown that the surface charge properties of imogolite are explained if the mean AI-O bond valence of the outer -A12OH groups is higher than the inner -A12OHSiO 3 groups. Hence, a weak positive charge is developed on the outer tube walls whereas a negative charge develops in the tubular pores. The best model fits were obtained where either one or two units of structural charge per unit cell of tube were assumed. The model may also explain why imogolite tubes are normally aggregated in large bundles in close hexagonal packing, because bound counterions may hold the tubes together. However, to arrive at good model descriptions, the deprotonation of -AlzOH groups must occur at a higher pH than that expected when assuming that all surface oxygens form two hydrogen bridges with H20. A more precise structure of imogolite is required to test fully this hypothesis.

Hydroxyl vacancies in single-walled aluminosilicate and aluminogermanate nanotubes

“…The VB edge is in fact localized inside the nanotube cavity, while the CB edge faces the outer side of the nanotube. In line with [9], these findings confirm a strong radial anisotropic electron affinity with an ensuing enhanced Brønsted acidity (basicity) for the outer (inner) tube surfaces as suggested in [22] (see SI for further details). In order to assess the effect of OH vacancies on the global electronic structure of the tube, one hydroxyl fragment was eliminated both on the inner, Al − Si(Ge) − OH invac , and on the outer, Al − Si(Ge) − OH outvac , side.…”

“…On this basis, it is straightforward to evaluate the actual dipole moment across the tube walls from ∆V values calculated in DFT. From the shift in the electrostatic potential (averaged both angularly and along the tube axis, see with [22] (and at odds with [9]) the calculated dipole directions agree with the positive (negative) charge accumulation at the outer (inner) surface. Despite its larger dipole, Al-Ge is characterized by lower inner and outer plateaus of V with respect to Al-Si.…”

Imogolite flocculation under alkaline conditions

“…The variable charge of imogolite is explained as the positive charge appears on the outer surface of the tube under acidic conditions and the negative charge appears on the inner surface of the tube under alkaline conditions (Cradwick et al 1972;Gustafsson 2001). Accordingly, anions will be distributed outside the imogolite under acidic conditions (Fig.…”

Large-scale density functional theory simulation of inorganic nanotubes: a case study on Imogolite nanotubes

“…With the exception of three DFT contributions focused on point defects and Fe-doping in AlSi and AlGe NTs, 50,51,55 the structural and electronic characterisation of defects, dopant and termination effects in Imo-NTs and their role in tuning the NT electronic and spectroscopic properties as well as its chemical reactivity is to date unexplored. In addition, AlSi NTs walls are experimentally known to develop an intrinsic polarisation, 56 with accumulation of negative (positive) charges on the interior (exterior) of the NT cavity (Fig. 2).…”