Untangling the physics of water transport in boron nitride nanotubes.

S. Mistry,R. Pillai,M. K. Borg,D. Mattia

doi:10.1039/d1nr04794a

Abstract

Carbon nanotubes (CNTs) have long been heralded as the material of choice for next-generation membranes. Some studies have suggested that boron nitride nanotubes (BNNTs) may offer higher transport of pure water than CNTs, while others conclude otherwise. In this work, we use a combination of simulations and experimental data to uncover the causes of this discrepancy and investigate the flow resistance through BNNT membranes in detail. By dividing the resistance of the nanotube membranes into their contributing components, we study the effects of pore end configuration, membrane length, and BNNT atom partial charges. Most molecular simulation studies of BNNT membranes use short membranes connected to high and low pressure reservoirs. Here we find that flow resistances in these short membranes are dominated by the resistance at the pore ends, which can obscure the understanding of water transport performance through the nanotubes and comparison between different nanotube materials. In contrast, it is the flow resistance inside the nanotubes that dominates microscale-thick laboratory membranes, and end resistances tend to be negligible. Judged by the nanotube flow resistance alone, we therefore find that CNTs are likely to consistently outperform BNNTs. Furthermore, we find a large role played by the choice of partial charges on the BN atoms in the flow resistance measurements in our molecular simulations. This paper highlights a way forward for comparing molecular simulations and experimental results.

Highlights

The scarcity of fresh drinking water is currently one of the world’s leading causes of malnutrition and other ills.[1]
The cause of the resistance in the non-hydrogenated cases are indicative of trapped water molecules reducing the transport of other flowing water molecules, which is more dominant at the smaller nanotube diameters considered
While there is a small difference at some of the nanotube diameters we have studied where boron nitride nanotubes (BNNTs) have lower end resistance, this difference is much less than the noise, and k1 can be roughly considered to be equal for BNNT-H and Carbon nanotubes (CNTs)

Summary

Introduction

The scarcity of fresh drinking water is currently one of the world’s leading causes of malnutrition and other ills.[1] With (BNNTs) were heralded as the materials of choice for fabricating desalination membranes, potentially offering significantly higher permeance than commercial membranes.[7,8,9] Despite early indications showing that BNNTs might outperform CNTs,[10,11] research into BNNTs lagged behind, primarily due to difficulties in synthesizing laboratory scale membranes.[12]. We synthesized BNNT membranes and showed that they offer advantages of similar selectivity as CNTs but for larger nanotube diameters, which leads to a higher net water permeation.[13] Siria et al.[14] find advantages of using. BNNTs for energy conversion due to the presence of surface charges inside the nanotubes. Other studies, such as Ritos et al.,[18] Wei et al.,[19] Sam et al.[20] and Secchi et al.[21] indicate BNNTs conduct lower water flux when compared to CNTs, especially at D >

Methods

Results

Conclusion