Accurate Prediction of Enhancement Factors for Water Flow Through Boron Nitride Nanotubes

Abstract

Water in nanoconfined spaces, such as nanotubes, exhibit anomalous yet intriguing behaviour compared to bulk water, a better understanding of which can enable us to realize a sustainable future. Nanotubes are atomically thin sheets (e.g., graphene or hexagonal boron nitride) that have been rolled into tubes. Boron nitride nanotubes (BNNTs) have been explored for a wide variety of applications ranging from water desalination to osmotic power harvesting since their prediction and experimental discovery in 1994 and 1995, respectively. However, even after three decades of research, water flow through BNNTs is not fully understood at a fundamental level. In this thesis, we considered several aspects that were not given enough attention in previous studies of nanoconfined flow through BNNTs. For instance, no simulation work has modelled the changes in the partial charge distribution when a flat sheet is rolled into a tube, up to this point. To address this knowledge gap, we employed electronic density functional theory (DFT) calculations to accurately estimate quantum-mechanically derived partial charges on boron (B) and nitrogen (N) atoms in BNNTs of varying lengths and diameters. We observed a spatially varying charge distribution inside both armchair and zigzag nanotubes of finite length. Performing DFT calculations for longer BNNTs is computationally intractable even using state of the art resources. To solve this issue, we performed DFT calculations for shorter BNNTs and devised a charge assignment scheme to predict partial charges for longer BNNTs, thus overcoming the need to perform expensive DFT calculations. Subsequently, we performed molecular dynamics (MD) simulations to calculate enhancement factors (EFs), that quantify the extent to which the Hagen-Poiseuille equation is disobeyed at the nanoscale, for BNNTs of varying lengths and diameters. To elucidate the effects of electrostatic interactions, we used three different kinds of partial charge distributions on B and N atoms in a BNNT: (i) bulk partial charges from pristine hBN sheets (±0.907e, where e is the magnitude of charge on an electron), (ii) accurate partial charges obtained from DFT calculations, and (iii) the typical partial charge on carbon atoms in carbon nanotubes (0.0e). BNNTs with the bulk and zero partial charges exhibited the lowest and the highest flow enhancements, respectively, whereas those with accurate partial charges had intermediate EFs. We also incorporated atomic vibrations into our study and discovered, surprisingly, that these vibrations lead to a reduction in the water flow through BNNTs. Finally, we also investigated the effect of vacancy defects in a BNNT on water flow and observed that a single boron and diboron vacancy defects do not affect water flow if atomic vibrations are considered. Our results demonstrate the combined role of atomic vibrations, electrostatic interactions, and defects in modulating water flow through BNNTs and unravel partially the reasons for ultra-low flow EFs in BNNTs. Overall, we believe that the insights developed in this thesis can aid in the fabrication of tailor-made nanofluidic devices which can be employed for sustainability applications in the upcoming decades.