Structural and Dynamical Properties of H 2 O and D 2 O under Confinement

Water (H O) is of great societal importance, and there has been a significant amount of research on its fundamental properties and related physical phenomena. Deuterium dioxide (D O), known as heavy water, also draws much interest as an important medium for medical imaging, nuclear reactors, etc. Although many experimental studies on the fundamental properties of H O and D O have been conducted, they have been primarily limited to understanding the differences between H O and D O in the bulk state. In this paper, using path integral molecular dynamics simulations, the structural and dynamical properties of H O and D O in bulk and under nanoscale confinement in a (14,0) carbon nanotube are studied. We find that in bulk, structural properties such as bond angle and bond length of D O are slightly smaller than those of H O while D O is slightly more structured than H O. The dipole moment of D O tends to be 4% higher than that of H O, and the hydrogen bonding of D O is also stronger than that of H O. Under nanoscale confinement in a (14,0) carbon nanotube, H O and D O exhibit a smaller bond length and bond angle. The hydrogen bond number decreases, which demonstrates a weakened hydrogen bond interaction. Moreover, confinement results in a lower libration frequency and a higher OH(OD) bond stretching frequency with an almost unchanged HOH(DOD) bending frequency. The D O-filled (14,0) carbon nanotube is found to have a smaller radial breathing mode than the H O-filled (14,0) carbon nanotube.