Effect of Size and Temperature on Water Dynamics inside Carbon Nano-Tubes Studied by Molecular Dynamics Simulation.

Amit Srivastava,Dirar Homouz,Jamal Hassan

doi:10.3390/molecules26206175

Abstract

Water transport inside carbon nano-tubes (CNTs) has attracted considerable attention due to its nano-fluidic properties, its importance in nonporous systems, and the wide range of applications in membrane desalination and biological medicine. Recent studies show an enhancement of water diffusion inside nano-channels depending on the size of the nano-confinement. However, the underlying mechanism of this enhancement is not well understood yet. In this study, we performed Molecular Dynamics (MD) simulations to study water flow inside CNT systems. The length of CNTs considered in this study is 20 nm, but their diameters vary from 1 to 10 nm. The simulations are conducted at temperatures ranging from 260 K to 320 K. We observe that water molecules are arranged into coaxial water tubular sheets. The number of these tubular sheets depends on the CNT size. Further analysis reveals that the diffusion of water molecules along the CNT axis deviates from the Arrhenius temperature dependence. The non-Arrhenius relationship results from a fragile liquid-like water component persisting at low temperatures with fragility higher than that of the bulk water.

Highlights

Water transport through nano-channels is a fundamental process in many emerging applications, such as drug delivery [1,2], intracellular solute transport [3], cancer therapy [4], and water treatment technologies [5]
Bernardina et al [19] used infrared spectroscopy to study water dynamics in single-wall carbon nano-tube (SWCNT) with different diameters, and they observed that water molecules confined in carbon nano-tubes (CNTs) remain loosely bound, even for filled tubes
For CNTs with larger diameters, more water molecules fill the space inside the CNT and organize in the coaxial tubular sheets

Summary

Introduction

Water transport through nano-channels is a fundamental process in many emerging applications, such as drug delivery [1,2], intracellular solute transport [3], cancer therapy [4], and water treatment technologies [5]. Water exhibits different physical properties when confined in nano-scale geometries compared to that of bulk [6,7,8,9,10]. Recent advances in both theoretical and experimental techniques have resulted in noticeable progress in investigating nano-confined water systems. Many of the recent studies have focused on the structure and dynamics of water molecules inside CNTs and their temperature and size dependence. Bernardina et al [19] used infrared spectroscopy to study water dynamics in single-wall carbon nano-tube (SWCNT) with different diameters, and they observed that water molecules confined in CNT remain loosely bound, even for filled tubes

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