(323d) Self-Diffusion of Alcohols and Water/alcohol Mixtures In Single-Walled Aluminosilicate Nanotubes

Understanding transport phenomena of fluids through nanotubes is of great interest in order to enable potential application of nanotubes as separation devices, encapsulation media for molecule storage and delivery, and sensors, among other applications. Single-walled metal oxide nanotubes are unique and attractive candidates for the above applications as they present a well-defined solid-state structure, precisely tunable length and diameter, as well as a hydrophilic and functionalizable interior for tuning transport and adsorption selectivity. Recent studies on water transport in single-walled aluminosilicate nanotubes have shown that the transport mechanism is governed by fast Fickian diffusion with diffusivities approaching bulk water diffusivity. Here, we study the transport properties of relatively less polar molecules (i.e. methanol and ethanol) as well as water/methanol and water/ethanol mixtures to investigate the influence of liquid-liquid and liquid-surface interactions as well as the effects of competitive transport of different molecules through the nanotubes.

We first employ molecular dynamics (MD) simulations to calculate the axial pure-component self-diffusivities of water, methanol, and ethanol molecules through the aluminosilicate nanotubes at different loadings (ranging from near infinite dilution to near-saturation). The diffusivities for all the three species decreased with increase in loading and were comparable to bulk liquid diffusivities at low molecular loadings, supporting the possibility of attaining high molecular fluxes through the nanotubes. We also study the diffusion of water/methanol and water/ethanol mixtures as a function of mixture composition and examine the possibility of selective transport of molecular species in the present nanotubes. Furthermore, we investigate the dependence of diffusivity on the nanotube diameter and discuss the possibility of tuning the transport properties of the nanotubes for application as nanofluidic components/devices.