Water Adsorption in Single- and Double-Walled Inorganic Nanotubes

Imogolite forms a class of inorganic nanotubes whose inner and outer surfaces can be tuned on a molecular level. The combination of this property with their monodisperse diameters makes imogolites ideal to study water confinement in a one-dimensional environment. Here, we have investigated, by means of infrared (IR) spectroscopy, water confined in two types of germanium-based imogolites: (i) the hydroxylated imogolite (Ge-IMO-OH), a double walled structure with an interstitial space of about 2 angstrom and the interior of the tube covered by Ge-OH groups; and (ii) the hybrid, methylated, imogolite (Ge-IMO-CH3), a single-walled structure with Ge-CH3 groups covering the interior of the tube. In both cases, the external surface is covered by Al-2-(mu OH) groups. Small-angle Xray scattering has also been exploited to determine the nanotube geometry of the two confining systems. Using thermogravimetric analysis, we show here that the interstitial space of Ge-IMO-OH fills with water in a significant proportion. Indeed, at 81% relative humidity (RH), 5% of the weight loss is due to water molecules in this interstitial space, whereas 13% of the weight loss is attributed to water molecules filling the other porosities. The IR study, in turn, reveals that water molecules start filling the interstitial space at very low RH values and are weakly H-bonded while exhibiting a low-frequency bending mode. This low bending value at around 1550 cm(-1) is indicative of a large H-O-H angle (as compared to the liquid water H bond angle). Such a large angle and weak hydrogen bonding are probably related to the strong confinement of water molecules between the two inner walls. Water also fills up the interior of the tube of Ge-IMO-OH from low RH values all the way to maximum RH. In this case, however, water molecules aggregate to form liquid water. In Ge-IMO-CH3, the filling of the porosities outside the tubes increases continuously with hydration. Moreover, a small proportion of weakly H-bonded water molecules enter in the hydrophobic cavity. The number of these internal water molecules is larger than what was observed in the silicon imogolite counterpart Si-IMO-CH3 because of the larger diameter of Ge-IMO-CH3 internal cavity.