Water Transport through Nanotubes with Varying Interaction Strength between Tube Wall and Water.

We present the results from extensive molecular dynamics simulations to study the effect of varying interaction strength, εNT-OW, between the nanotube atoms and water's oxygen atom. We find the existence of a narrow transition region (εNT-OW ≈ 0.05 - 0.075 kcal/mol) in which water occupancy within a nanotube and flux through it increases dramatically with increasing εNT-OW, with the exact location defined by nanotube diameter and length. This transition region narrows with increasing nanotube diameter to nearly a step-change in water transport from no flow to high water flux between εNT-OW= 0.05 kcal/mol to 0.055 kcal/mol for tube diameter 1.6 nm. Interestingly, this transition region (εNT-OW= 0.05 - 0.075 kcal/mol) also coincides with water contact angles close to 90° on an unrolled nanotube surface hinting at a fundamental link between nanotube wetting characteristics and water transport through it. Finally, we find that the observed water flux is proportional to the average water occupancy divided by the average residence time within the nanotube, with a proportionality constant found to be 0.36, independent of the nanotube diameter and length.