# Imogolite Nanotubes: A Flexible Nanoplatform with Multipurpose Applications
## 4 Applications  
It is well recognized that imogolite nanotubes remain insulating materials with large band gaps. To tackle this issue, structural modification of the outer wall has also been conducted this time by isomorphic substitution of Al\({}^{3+}\) by Fe\({}^{3+}\)[102; 103; 104; 105; 106; 107]. The effect of Fe incorporation acts as specific coordination centers for organic moieties and leads to higher adsorption efficiency [104; 105], while altering the electronic properties of the nanotubes even for low substitution rates [109]. Other isomorphic substitutions have been explored, on the theoretical background, by replacing both Al and Si atoms by elements of groups III (Ga, In) & IV (C, Ge, Sn) respectively [110], or by substituting the inner tetrahedrons by phosphorous and arsenic derivatives [111]. Figure 5: Illustration of outer surface modification of imogolite nanotubes. (**a**) Dispersion state of unmodified and poly(methylmethacrylate) (PMMA) grafted single-walled aluminosilicate imogolite nanotubes (Si-SWINT) in water/chloroform mixtures. (**b**) Flowchart of the modification and the immobilization of the initiator (BMPOPO\({}_{4}\), see Table 2) used for surface-initiated polymerization of methyl methacrylate (MMA). Adapted with permissions from [90; 92]. The purpose of this section is to provide the reader with a concise but comprehensive overview of the applications that have been explored for this class of nanotubular clay minerals. Because of its unique structure with well-defined porosity and interfaces, INTs were first recognized as a molecular sieving material. A large number of studies have been devoted to gas adsorption of neutral and reactive molecules. Molecules interact mainly with the inner silanol (germanol) sites, which are accessible only after careful pretreatment to remove confined water [112]. The tetrahedral sites present interesting acidic properties, which enhance interactions with adsorbed molecules such as NH\({}_{3}\), benzene, CO, CO\({}_{2}\) and CH\({}_{4}\) (Figure 6a) [43; 64; 72; 75; 113; 114; 115; 116]. Recent results show that modification of the inner surface of INTs results in dramatically enhanced CO\({}_{2}\)/CH\({}_{4}\) and CO\({}_{2}\)/N\({}_{2}\) selectivity [72; 75; 113]. Several works also demonstrate that hybrid INTs, dispersed in a liquid medium, spontaneously confine inside them organic molecules added to the suspension [77; 87]. The peculiar structure of INTs also offers a unique opportunity to understand deeply the structural and dynamical properties of water confined in relation to the nature of the interface and at the nanometer scale. They certainly deserve comparison with, for instance, carbon nanotubes (CNT) which present unique transport properties [11; 12; 119; 120]. Despite the excitement generated by this topic for carbon nanostructures, there is little experimental data available on water confined inside INTs up to now. Water vapor adsorption isotherms reveal strong water uptake at low relative pressure (20 wt% at P/P\({}_{0}\) < 0.2) followed by a pseudo-plateau up to P/P\({}_{0}\) = 0.85 (30 wt%) [64]. Water structuring was recently investigated during the filling of INTs by infrared spectroscopy. Unlike CNTs [121; 122], these experiments suggested that the silanol pending groups played a non-negligible role in the way in which water molecules interact with the inner wall [123]. In contrast, the properties of water confined in INTs has been mainly investigated on the theoretical background. Molecular dynamics simulations revealed the predominant hydrophilicity character of the inner surface of INTs (Figure 6b) [65; 117; 124]. Indeed, radial density profiles suggested a strong structuration of water molecules in specific adsorption sites with complex interactions with adjacent molecules (Figure 6c) [118]. Furthermore, the simulations confirmed the key role of the inner pending function, whose flexibility is critical in the adsorption of hydrogen-bonding molecules [124].