Self-diffusivity, hydrogen bonding and density of different water models in carbon nanotubes

Abstract

In this paper, the density, hydrogen bonding and self-diffusivity of water confined in carbon nanotubes are investigated. Molecular dynamics is used to simulate a large variety of nanotubes with various water models. Our results produce, for the first time, the complete trend of these properties from narrow nanotubes, where water shows particularly anomalous behaviour, to large ones where its characteristics are similar to those of bulk.

Keywords:

• carbon nanotubes
• confined water
• molecular dynamics
• water properties

Acknowledgements

This work has been performed under the UCY-CompSci project, a Marie Curie Transfer of Knowledge (TOK-DEV) grant (contract No. MTKD-CT-2004-014199) funded by the CEC under the 6th Framework Program.

Notes

^1. The consequences of the use of flexible nanotubes under various pressure conditions and different bonding potentials in water/CNT systems is investigated in the works of Longhurst and Quirke 13-15.

^2. In this work, properties calculated by letting water molecules enter spontaneously the nanotube are sometimes called ‘natural’ in order to distinguish from those obtained with a prescribed number of molecules in the nanotube.

^3. The initial time t = 0 begins after the statistical steady state is achieved. The molecules, moreover, are followed until they begin to exit the nanotube (approximately 500 ps in Figure 4).

^4. It must be also considered that in our case the nanotubes length and, consequently, the average residence time of a water molecule in the CNT is shorter than in Striolo et al. [32].

^5. Figure 6 compares quantities with different units since the value of α is variable and therefore only a qualitative comparison can be made. Even though the variation of α is relatively small, that data in Figure 6 show higher noise with respect to the previous figures. This is due to the fact that small errors in α are reflected exponentially in D.