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Imogolite and imogolite-like tubular nanocrystals. Formation mechanism, properties and applications

Abstract : Imogolites are aluminosilicate nanotubes naturally occurring in volcanic soils (Yoshinaga and Aomine, 1962). Getting inspiration from this natural clay, it is possible to prepare synthetic aluminosilicate or aluminogermanate nanotubes of formula (OHhAI20 3SixGe(1.x)(OH) which are perfectly monodisperse in diameter (from 2 to 4 nm depending on composition) and polydisperse in length from several tens of nanometers up to several microns (Thill et al. 2012, Amara et al, 2013). The formation mechanism of these nanotubes has been the subject of recent discoveries especially for the aluminogermanate nanotubes. The existence of Double-walled nanotubes has been discovered and their formation mechanism has been explained. A better understanding of the imogolite precursors (proto-imogolite) has been achieved and the growth kinetic of the nanotubes has been studied in situ and modelled. It has recently been discovered that it is possible to prepare hybrid nanotubes having coexisting hydrophobie and hydrophilic surfaces (Bottero et al. 2011, Bac et al. 2009). These hybrid inside/out janus nanotubes can be prepared in two symmetric configurations. Through the grafting of phosphonic acids bearing an aliphatic carbon chain on the outside aluminol surface, nanotubes containing an hydrophilic cavity, that can be dispersed in apolar solvents, are obtained. Alternatively. by replacing the tetraethoxysilane precursor by methyltriethoxysilane, nanotubes possess a hydrophobie nanocavity covered with Si-CH3 groups instead of Si-OH (Figure 1A,B). These nanotubes are easily dispersed in aqueous solutions and are able to trap poorly soluble organic molecules (Figure 1 C) (Amara et al. 2015). We believe that their very original structure brings new and fascinating properties to these nanoparticles. ln particular, we are currently studying the behaviour of such hybrid inside/out janus nanotubes at oil/water interfaces (Picot et al., 2016). When water and oil are mixed in the presence of the nanotubes, stable emulsions are obtained whose size is controlled by the concentration of particles (Figure 1 E). Addition of oil to water containing hydrophilic/hydrophobic nanotubes or water addition to oil containing hydrophobic/hydrophilic nanotubes without mixing led into the formation of a gel (Figure 1 D). When observed after drying with electronic microscopy, a very original hedgehog-Iike structure is observed (Figure 1 F). Therefore, such an oil/water-triggered gel formation signs for a very specifie and original behavior of these hybrid nanotubes originating from their inside/out janus functionality. ln this conference, a review of the recent discoveries on imogolite formation mechanism will be made in detail. We will also present the synthesis of the hydrid janus nanotubes. These hybrid nanotubes have promising properties. We will illustrate their behavior in contact with an oil/water interface (liquid/liquid extraction, gel formation, pickering emulsion and self-assembled nanofluidic devices).
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Pierre Picot, Florent Malloggi, Thibaud Coradin, Antoine Thill. Imogolite and imogolite-like tubular nanocrystals. Formation mechanism, properties and applications. Mediterranean Clay Meeting 2016 (MCM 2016), Sep 2016, Izmir, Turkey. ⟨cea-02346347⟩

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