Water in Single-Walled Aluminosilicate Nanotubes: Diffusion and Adsorption Properties

Abstract

Single-walled aluminosilicate nanotubes are attractive materials for construction of nanofluidic devices. They have a well-defined structure, a hydrophilic interior with periodic wide and narrow regions, precisely tunable length and diameter, and a functionalizable interior for tuning mass transport and adsorption properties. We report a computational and experimental investigation that highlights the unique adsorption and diffusive water transport properties of these nanotubes. Axial self-diffusivities of water molecules (at loadings ranging from near-infinite dilution to near-saturation) are calculated by molecular dynamics (MD) simulations, whereas adsorption properties are computed with grand canonical Monte Carlo (GCMC) simulations and are also compared to experimental data. The transport diffusivities are evaluated through the Darken approximation. Water transport in these nanotubes at room temperature was observed to occur via Fickian diffusion. The self-diffusivity decreases with an increase in water content, whereas the transport diffusivity exhibited a maximum at intermediate water content. The diffusivities were comparable to the diffusivity of bulk liquid water and hence are considerably higher than in other nanoporous aluminosilicates such as zeolites. The computed adsorption isotherms exhibited inflections at low partial pressures (∼6 mm Hg) with a large fraction of adsorption occurring in the pores of the nanotube displaying remarkable hydrophilicity. As a combined result of the relatively fast Fickian diffusion of water, hydrophilicity of the nanotubes, and short nanotube lengths, the diffusive water flux through an aluminosilicate nanotube film is predicted to be quite high (102−103 mol m–2 s−1), even at very low pressure differentials across the membrane.