Structural and dynamical properties of H₂O and D₂O under confinement

Water (H₂O) is of great societal importance and there has been a significant amount of research on its fundamental properties and related physical phenomena. Deuterium dioxide (D₂O), 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 H₂O and D₂O have been conducted, they have been primarily limited to understanding the differences between H₂O and D₂O in the bulk state. In this report, using path integral molecular dynamics simulations, the structural and dynamical properties of H₂O and D₂O in bulk and under nanoscale confinement in a (14,0) carbon nanotube are studied. We find that in bulk, the dipole moment of D₂O tends to be 4% higher than that of H₂O and the hydrogen bonding of D₂O is also stronger than H₂O. Under nanoscale confinement in a (14,0) carbon nanotube, H₂O and D₂O exhibit a smaller bond length and bond angle. The hydrogen bond number decreases, which demonstrates weakened hydrogen bond interaction. Moreover, confinement results in a lower libration frequency, and higher OH(OD) bond stretching frequency with an almost unchanged HOH(DOD) bending frequency. The D₂O-filled (14,0) carbon nanotube is found to have a smaller radial breathing mode than the H₂O-filled (14,0) carbon nanotube.