Water-Structure-Specific Entropic Dominance in the Filling of Boron Nitride Nanotubes

Abstract

The filling of nanometer and sub-nanometer channels/ tubes with water governs applications ranging from desalination and filtration to nanoscale energy conversion. Here, we report the most nonintuitive entropy-dominated filling of mildly hydrophilic boron nitride nanotubes (BNNTs) with diameters ranging from 0.85 to 1.69 nm. For all the BNNT sizes, water inside the BNNT is more stable than water in the bulk. The factor dictating the favorable nature of the entropy depends on the specific-BNNT-diameter-dictated structure of the water molecules. For example, for the 0.85 nm BNNT, the rotational entropic component dominates due to the presence of a significant fraction of the dangling water OH bonds that do not participate in hydrogen bonding. The fraction of such dangling OH bonds (the average HBs per molecule) decreases (increases) with an increase in the BNNT diameter, leading to a progressively reduced contribution of the rotational entropy. For the 1 nm BNNT, the translational entropic component dominates due to the enhanced in-plane motion of the water molecules caused by the single-file nature of water molecules spanning a significant radial expanse inside the BNNT. For larger BNNTs, translational entropy decreases with an increase in the BNNT diameter, although it remains the dominant factor governing the entropy-driven filling of BNNTs. This favorable translational entropy for larger BNNTs can be associated with (1) the presence of chain-like regions formed by water in 1.13 nm BNNT and (2) the presence of a high specific water volume and reduced number of HBs per molecule (as compared with bulk water) in 1.27, 1.41, 1.55, and 1.69 nm BNNTs.

Strength, number, and kinetics of hydrogen bonds for water confined inside boron nitride nanotubes

“…In the (8, 8) BNNTs, as the nanotube diameter increases, the ordered single-file structure disappears, and we start to see high-density clusters of water molecules joined by low-density chain-like regions. 30 This water structure results in the appearance of the second hydration PMF well situated close to the HB basin (see Fig. 2).…”

“…In these extreme confinements, i.e. , in (6, 6) and (7, 7) BNNTs, where water takes a single-file structure 30 with N HB close to 2 (see Table 1), a given water molecule is forced to form HBs with the same two neighboring water molecules for the entire duration of the simulations. As a result, if a pair of water molecules is found to form an HB through one of the four hydrogen atoms at t = 0, the probability of finding the same HB at t = ∞ would be approximately 0.25 (one of four possible HBs).…”

“…In addition, we also carried out bulk water simulations to highlight the effect of the BNNT-based confinement on water–water hydrogen bonding relative to water in the absence of any confinement. The detailed water structure and the energetics dictating these structures have been discussed in our previous paper; 30 we avoid repeating such discussions here for the sake of brevity.…”

“… 29 In our recent study, we established that such disruption of hydrogen bond (HB) formation, associated with the presence of dangling –OH bonds, leads to the spontaneous filling of boron nitride nanotubes (BNNTs) of sub-nanometric diameters. 30 …”

“…In our previous study, we have delineated in great detail the manner in which BNNTs of different diameters lead to entropy-driven water filling of BNNTs, with the factors dictating the favorable entropy changes and the corresponding water structure being strongly dictated by the BNNT diameters. 30 In this paper, through all-atom molecular dynamics (MD) simulations, we study two critical issues: (1) the effect of confinement on the water–water interactions that govern the hydrogen bonding and (2) the effect of confinement on the kinetics of the water–water HBs. In order to understand the effect of confinement on the interactions governing the water–water hydrogen bonding, we probe the PMFs (potential of mean forces) dictating the water–water HBs.…”