Tuning the properties of confined water in standard and hybrid nanotubes: An infrared spectroscopic study

Imogolite is a natural nanotubular aluminum silicate clay mineral found in volcanic soils. Its well-defined, tunable structure makes it a good candidate for studying water confinement in a one-dimensional (1D)structure. Water confinement in self-sustaining imogolite thin films was studied using infrared spectroscopy. Two types of synthetic imogolites were investigated: pristine imogolite (IMO-OH) with a hydrophilic inner surface covered with Si–OH groups and hybrid imogolite (IMO-CH₃) with a hydrophobic inner surface covered with Si–CH₃ groups. Both imogolites have an outer surface that is covered with Al–OH groups. Infrared spectra were recorded in the 20–4, 000 cm^-1 spectral range as a function of relative humidity. Analysis of the O–H stretching band provides information on the H bonding of confined water molecules inside and outside the IMO-OH tubes. The scenario for water filling as a function of relative humidity is determined for both systems. Adsorption begins in the IMO-OH tubes at the lowest relative humidity (< 10%). The inner surface of the tubes is first covered with water molecules; then, the central part of the tubes is filled, leading to very strong H bonds and a structured spectrum. In contrast, the H bonds of water adsorbed on the outer surfaces of these tubes are weaker. A different scenario is observed for water inside IMO-CH₃: Weakly H-bonded water molecules are present, similar to that observed incarbon nanotubes. Water confinement in imogolites is governed by the hydrophilicity of the inner walls. At similar partial pressures, the degree of H bonding depends on the interactions between water and the nanotube wall.