Dielectric relaxation of water inside a single-walled carbon nanotube

We report a molecular dynamics study of anisotropic dynamics and dielectric properties of water confined inside a single-walled carbon nanotube (SWNT) at room temperature. The model includes dynamics of an SWNT described by a realistic potential function. A comparison with simulations assuming a rigid nanotube demonstrates that the popular assumption severely overestimates the dielectric constant for small diameter SWNTs. Simulations of water inside flexible SWNTs with various diameters reveal strong directional dependence of the dynamic and dielectric properties due to the confinement effect. The obtained dielectric permittivity spectra (DPS) identify two different dipolar relaxation frequencies corresponding to the axial and the cross-sectional directions, which are significantly smaller and larger than the single relaxation frequency of bulk water, respectively. The frequency variation increases as the SWNT diameter decreases. The results suggest that DPS can be used as a fingerprint of water inside SWNTs to monitor the water intrusion into SWNTs.