Water Confined in Boron Nitride Nanotubes: Role of Entropic Effects

The thermodynamic factors dictating the stability of water confined in sub-nanometer and nanometer-wide channels are of great interest in nanofluidics. This study reports an entropy-dominated filling of mildly hydrophilic boron nitride nanotubes (BNNTs) with diameters ranging from 0.85 to 1.69 nm. The factors governing such entropic favorability depend on the BNNT-diameter-dictated water structure. Due to a pronounced dangling water-OH bond fraction, the rotational entropic component is dominant in the 0.85 nm diameter BNNT. Further, the water rotational entropy steadily decreases with an increase in the tube diameter, following the same trend as the fraction of dangling water-OH bonds. The translational entropic component is most dominant in the 1 nm wide BNNT due to the enhanced in-plane motion of water molecules that form a single-file structure spanning a significant radial expanse. In BNNTs with a diameter larger than 1 nm, while the translational entropy decreases with an increase in tube diameter, it remains the dominant component in stabilizing water inside these BNNTs. Such a favorable translational entropy for water in larger BNNTs can be attributed to the presence of high specific water volume and a lower number of hydrogen bonds per molecule as compared to bulk water.