Water transport through functionalized nanotubes with tunable hydrophobicity

TL;DR

This study uses molecular dynamics simulations to explore how water flows through functionalized carbon nanotubes with tunable hydrophobicity, revealing nonlinear occupancy and flux behaviors based on atom arrangement and fraction.

Contribution

It introduces a detailed analysis of water transport in heterogeneous nanotubes and proposes an effective interaction model to predict flow behavior based on atom composition.

Findings

Water occupancy varies nonlinearly with hydrophilic atom fraction.

A small fraction (~0.4) of hydrophilic atoms induces spontaneous filling.

Water flux shows two regimes: linear increase and plateau with increasing hydrophilic content.

Abstract

Molecular dynamics simulations are used to study the occupancy and flow of water through nanotubes comprised of hydrophobic and hydrophilic atoms, which are arranged on a honeycomb lattice to mimic functionalized carbon nanotubes (CNTs). We consider single-file motion of TIP3P water through narrow channels of (6,6) CNTs with varying fractions (f) of hydrophilic atoms. Various arrangements of hydrophilic atoms are used to create heterogeneous nanotubes with separate hydrophobic/hydrophilic domains along the tube as well as random mixtures of the two types of atoms. The water occupancy inside the nanotube channel is found to vary nonlinearly as a function of f, and a small fraction of hydrophilic atoms (f ~ 0.4) are sufficient to induce spontaneous and continuous filling of the nanotube. Interestingly, the average number of water molecules inside the channel and water flux through the nanotube are less sensitive to the specific arrangement of hydrophilic atoms than to the fraction, f. Two different regimes are observed for the water flux dependence on f - an approximately linear increase in flux as a function of f for f < 0.4, and almost no change in flux for higher f values, similar to the change in water occupancy. We are able to define an effective interaction strength between nanotube atoms and water's oxygen, based on a linear combination of interaction strengths between hydrophobic and hydrophilic nanotube atoms and water, that can quantitatively capture the observed behavior.