Colloidal Stability of Imogolite Nanotube Dispersions: A Phase Diagram Study

Abstract

In this article, we revisit the colloidal stability of clay imogolite nanotubes by studying the effect of electrostatic interactions on geo-inspired synthetic nanotubes in aqueous dispersions. The nanotubes in question are double-walled aluminogermanate imogolite nanotubes (Ge-DWINTs) with a well-defined diameter (4.3 nm) and with an aspect ratio around 4. Surface charge properties are assessed by electrophoretic measurements, revealing that the outer surfaces of Ge-DWINT are positively charged up to high pH values. A series of Ge-DWINT dispersions have been prepared by osmotic stress to control both the ionic strength of the dispersion and the volume fraction in nanotubes. Optical observations coupled to small and wide-angle X-ray scattering (SAXS/WAXS) experiments allow us to unravel different nanotube organizations. At low ionic strength (IS < 10–2 mol L–1), Ge-DWINTs are fully dispersed in water while they form an arrested gel phase above a given concentration threshold, which shifts toward higher volume fraction with increasing ionic strength. The swelling law, derived from the evolution of the mean intertube distance as a function of the nanotube concentration, evidences a transition from isotropic swelling at low volume fractions to one-dimensional swelling at higher volume fractions. These results show that the colloidal stability of Ge-DWINT is driven by repulsive interactions for ionic strengths lower than 10–2 mol L–1. By contrast, higher salt concentrations lead to attractive interactions that destabilize the colloid suspension, inducing nanotube coagulation into larger structures that settle over time or form opaque gels. Detailed simulations of the WAXS diagram reveal that aggregates are mainly formed by an isotropic distribution of small bundles (less than four nanotubes) in which the nanotubes organized themselves in parallel orientation. Altogether, these measurements allow us to give the first overview of the phase diagram of colloidal dispersions based on geo-inspired imogolite-like nanotubes.

Termination Effects in Aluminosilicate and Aluminogermanate Imogolite Nanotubes: A Density Functional Theory Study

“…Previous computational studies [31,33,34] of periodically repeated AlSi and AlGe NTs have shown a clear correlation between the presence of a permanent polarization at the NT wall and the occurrence of a separation in real-space for the VB and CB edges of the NTs. For pristine hydroxylated NTs, this separation leads to the CB and VB edges being localized outside and inside the NT, respectively.…”

“…Both sides of the imogolite NTs' cavities can be easily functionalized [25][26][27][28]. The synthetic flexibility of imogolite NTs leads to widely tunable properties, with exciting reports as regards the performance of imogolite NT for chemical separation [17,[20][21][22][23][24]26,29], co-photo-catalyst [30], hybrid nanocomposites [31] and colloidal, liquid-crystal applications [32][33][34][35]. In addition to these examples, the potential range of applicability for AlSi AlGe NTs has been recently expanded via synthesis of Fe-doped imogolite NTs, capable of absorbing visible (and UV) light.…”

Influence of the Al/Ge Ratio on the Structure and Self-Organization of Anisometric Imogolite Nanotubes

“…After stirring, a drop was deposited on a carbon-coated copper TEM grid and was left to air dry. High dilution prevents the formation of entangled networks of nanotubes on the grid [36]. The morphological parameters of each sample were performed on more than 200 particles, except for the synthesis at the lowest R' ratio, for which only 50 particles could be measured, using Fiji software [43].…”

“…We report in Figure 8 that the average values obtained for the different samples by using Equations (1) and (2). such as [36,60]:…”

“…It was also suggested that, under basic conditions, hollow nanospheres are formed instead of nanotubes when the precursor ratio R' = [Al]/[Ge] is lowered below 2 [33], which is the ratio of Al and Ge atoms within a non-defective Ge-based INT with formula Ge(Al)2O7H4. Finally, while Si-based INTs are singlewalled (SW) nanotubes, germanium-based nanotubes are obtained as single-walled (SW) or doublewalled (DW) nanotubes depending on the initial concentration of aluminum C[Al] (Figure 1b) [34,35], the latter having an internal diameter of 1.5 nm for an external diameter of 4.3 nm and a limited average length of ~ 20 nm [36]. Some promising attempts have been reported by changing conventional heating (reflux or autoclave) with microwave irradiation, showing an increase in the length of the tubes [37,38].…”