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

Water (H 2 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 2 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 2 O and D 2 O have been conducted, they have been primarily limited to understanding the differences between H 2 O and D 2 O in the bulk state. In this paper, using path integral molecular dynamics simulations, the structural and dynamical properties of H 2 O and D 2 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 2 O are slightly smaller than those of H 2 O while D 2 O is slightly more structured than H 2 O. The dipole moment of D 2 O tends to be 4% higher than that of H 2 O, and the hydrogen bonding of D 2 O is also stronger than that of H 2 O. Under nanoscale confinement in a (14,0) carbon nanotube, H 2 O and D 2 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 2 O-filled (14,0) carbon nanotube is found to have a smaller radial breathing mode than the H 2 O-filled (14,0) carbon nanotube.