# Imogolite Nanotubes: A Flexible Nanoplatform with Multipurpose Applications
## 4 Applications
Hence, INTs could be used as tunable building blocks for filtration applications. Since the successful reinforcement of nylon by clay minerals in 1993 [125], new clay/polymer nanocomposites have emerged during the last two decades [126; 127]. In the case of imogolite nanotubes, effective reinforcement was also extensively explored by using colloidal properties of INTs either for un-modified nanotubes or after functionalization of the outer wall to facilitate their dispersion in the desired polymer matrix (Table 3). Figure 6: Molecular confinement in imogolite nanotubes. (**a**) Selective molecular sieving towards CH\({}_{4}\). Adapted from [75]; (**b,c**) Snapshot of fully hydrated INTs and 2D radial density profiles for oxygen (blue) from water molecules. Adapted with permissions from [117; 118]. Poly(vinyl alcohol) (PVA) is by far the most used polymeric material since PVA/INT can be processed easily from aqueous solutions. In most cases, the mechanical and optical properties of the resulting composite are enhanced, whether in film [84; 128; 129; 130; 131], fiber [132] or membrane form [133; 134; 135] (Figure 7a). Beyond PVA, INT has been incorporated in a wide variety of polymer matrix (Table 3) either for nanotube-reinforced nanocomposites [90; 96; 100; 138; 143; 149; 150] with anisotropic structural properties (Figure 7b) [93; 94; 97; 136; 137; 144; 148; 151; 152], fabrication of hierarchical porous structures [139; 140; 141; 142] or the encapsulation/sustained-release of biomolecules for medical applications (Figure 7c) [153; 154; 155; 156; 157; 158; 159]. Figure 7: Applications of imogolite for designing new (**a**–**c**) hybrid nanocomposites, (**d**) filtration membranes or (**e**) liquid crystals. (**a**) Transparency of in situ synthesized (top) or blend (bottom) PVA/  
INT films. Adapted with permissions from [128]; (**b**) Reversible structural transition of INT hydrogels upon stretching. Adapted with permissions from [148]; (**c**) Encapsulation of lacase in INT hydrogel and its activity. Adapted with permissions from [153]. (**d**) PVA/INT membrane for molecular separation. Adapted with permission from [133]. (**e**) Alignment of a liquid-crystal columnar phase of INTs under electric field. Adapted with permissions from [60]. Beyond their use as nanofillers, INTs present a great potential for water osmosis desalination [161]. The control of nanotube diameter and interface combined with the possibility to capture molecules in a selective manner confer to these tubular structures a wide range of potential applications, from filtration to depollution of water. By combining these properties with the ability of INTs to be dispersed in a polymer matrix, the fabrication of (ultra)thin film nanocomposite (TFN) membranes has started to receive more intention recently (Figure 7d) [133; 135]. Several studies reported that the addition of INTs in TFN substantially increased the water throughput due to preferential water flow through imogolite nanotubes [133; 134; 145; 146; 147]. Thanks to the permanent polarization of INTs with negative charges on the inner surface, significant ion exclusion can be achieved [134; 145; 147] due to the combination of steric hindrance and electrostatic repulsion as shown previously for CNTs [162]. We have shown that INT can be regarded as inorganic counterparts of carbon nanotubes in terms of morphology and aspect ratios [51; 55]. However, a major difference between INT and CNT is that the former is readily water-soluble thanks to their electrical surface charge, in contrast with the latter. As expected for highly elongated colloidal nanorods, an isotropic to nematic liquid-crystal (LC) phase transition can occur in suspensions of INTs [163], which is the result of a competition between orientational entropy and the packing entropy governed by excluded-volume interactions [164]. Surprisingly, very dilute suspensions of INTs also form a columnar liquid-crystal (Figure 7e) [60]. Despite the high dilution, the nanotubes are perfectly organized on a hexagonal lattice with large spacing.