Water vapor diffusion into a nanostructured iron oxyhydroxide

Water diffusion through 0.4 nm × 0.4 nm wide tunnels of synthesized akaganéite (β-FeOOH) nanoparticles was studied by a coupled experimental-molecular modeling approach. A sorption isotherm model obtained from quartz crystal microbalance measurements suggests that the akaganéite bulk can accommodate a maximum of 22.4 mg of water/g (44% bulk site occupancy) when exposed to atmospheres of up to 16 Torr water vapor. Fourier transform infrared spectroscopy also showed that water molecules interact with (hydr)oxo groups on both the akaganéite bulk and surface. Diffusion reactions through the akaganéite bulk were confirmed through important changes in the hydrogen-bonding environment of bulk hydroxyl groups. Molecular dynamics simulations showed that water molecules are localized in cavities that are bound by eight hydroxyl groups, forming short-lived (<0.5 ps) hydrogen bonds with one another. Diffusion coefficients of water are three orders of magnitude lower than they are in liquid water (D = 0.0-11.1 × 10(-12) m(2)·s(-1)), whereas large integral rotational correlation times are 4 to 15 times higher (τr = 8.4-31.8 ps). Moreover, both of these properties are strongly loading-dependent. The simulations of the interface between the water vapor phase and the (010) surface plane of the akaganéite, where tunnel openings are exposed, revealed sluggish rates of incorporation between interfacial water species and their tunnel counterparts. The presence of defects in the synthesized particles are suspected to contribute to different diffusion rates in the laboratory when compared to those observed in pristine crystalline materials, as studied by molecular modeling.