Molecular dynamics of ice-nanotube formation inside carbon n

The first order phase transition of a water cluster confined in a dynamic single-walled carbon nanotube is investigated using a classical molecular dynamics (MD) method. The formation of ice-nanotube is monitored through the structure factor and potential

Molecular dynamics of ice-nanotube formation inside carbon nanotubes

Junichiro Shiomi, Tatsuto Kimura2 and Shigeo Maruyama1,*

1

1

Department of Mechanical Engineering, The University of Tokyo

7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

Department of Mechanical Engineering, Kanagawa University, 3-27-1 Kanagawa-ku, Yokohama-shi, Kanagawa, 221-8686, Japan

2

The first order phase transition of a water cluster confined in a dynamic single-walled carbon nanotube is investigated using a classical molecular dynamics (MD) method. The formation of ice-nanotube is monitored through the structure factor and potential energies. The transition temperature and its diameter dependence obtained by the simulations agree well with those of previously reported experiments. The transition temperature of the ice-nanotube was shown to take a maximum value of around room temperature with the number of the ring members n=5. Potential energy contribution to the phase change is generally dominated by that of the intrinsic water-water interaction, while that of water-carbon interaction plays a significant role on determining the dependence of transition temperature on the nanotube diameter.

*Corresponding author: Tel: +81-3-5841-6421, Tel: +81-3-5800-6983. E-mail address:

I.

INTRODUCTION

Investigation of water confined in low dimension holds much importance as it is a key system in bioscience1 and nanotechnology under aqueous environments. The confinement is expected to alter the phase transition and various transport properties of water from those of bulk water. An ultimate realistic case of such low-dimensional systems is water confined in single-walled carbon nanotubes (SWNTs). An anomalous phase transition of water to ice-nanotube was first predicted by classical molecular dynamics (MD) simulations of water inside a static SWNT under high pressure (axial pressure of 50-500 MPa)2. It was found that the water experiences a first order phase transition to form an ice-nanotube (I-NT), where the number of members of the circumferential ring (n) was determined by the diameter (d) of the surrounding SWNT. Molecular simulations have been also used to explore detail structures of the confined water and their energetic properties3,4. The existence of ice-nanotubes was first confirmed in experiments using the X-ray diffraction analyses5,6. Experiments were performed at around the saturated vapor pressure, where condensation of water inside SWNTs occurred around 315-330 K.

The experiments delivered a striking feature of the phase change that the ordering transition temperature increases, even to room temperature, as the nanotube diameter i.e. n of I-NT decreases. This trend is opposite from that of the bulk water in a glass capillary tube7 and hence indicates a crossover of physics from bulk to atomic scale phenomena on reducing the diameter6.

In the current study, we investigate the diameter dependence of the transition temperature of a saturated water cluster locally confined in an SWNT by monitoring instantaneous molecular structures and potential energy. While the transition temperature has been calculated for various nanotube diameters (or n) by MD simulations2 and Grand canonical Monte Carlo simulations4, the current work is first to provide the direct comparison with the experiment in terms of the dependence of the transitional temperature on the nanotube diameter by simulating the realistic phase-change process without artificial pressure treatments. Unlike the earlier MD calculations of the transition2, the model includes the carbon-carbon interaction dynamics based on a potential function that have been shown to exhibit the phonon density of states of carbon nanotubes with a sufficient accuracy8-10.

The first order phase transition of a water cluster confined in a dynamic single-walled carbon nanotube is investigated using a classical molecular dynamics (MD) method. The formation of ice-nanotube is monitored through the structure factor and potential

certain cut-off function. B*ij represents the effect of the bonding condition of the atoms. As for the potential parameters, we employ the set that was shown to reproduce the force constant better (table 2 in Ref. 14). It has been shown that the formulation of potential function exhibits phonon dispersion with sufficient accuracy8-10. The inclusion of the lattice vibrations of carbon nanotubes enables us to incorporate the realistic heat transport from an

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σ σ SWNT to the water cluster. The thermal boundary qiqje2

OOOO φ12=4εOO conductance between the SWNT and the confined R +∑∑4πεr, (1) Rij 12 12 0ij liquid water was previously computed to be typically

about 5 MW/m2K16.

where R12 represents the distance of oxygen atoms, A typical simulation begins with an initial and σOO and εOO are Lennard-Jones parameters. The condition with liquid water locally confined in an Coulombic interaction is the sum of 9 pairs of point SWNT. Figure 1(a) shows the picture of a water charges. The SPC/E potential is known to predict cluster of 192 molecules adsorbed inside a (9, correct phase change temperature and is widely used 9)-SWNT at room temperature or higher. The system 12

in micro/nano heat transfer. The potential function was subjected to a periodic boundary condition in the between water molecules and carbon atoms were axial direction. The cast SWNT (L=20.2 nm) is represented by Lennard-Jones function of the longer than sum of the cut-off distance of the distance between the oxygen in the water molecule Coulombic …… 此处隐藏：20160字，全部文档内容请下载后查看。喜欢就下载吧 ……