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 paper, 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, structural properties such as bond angle and bond length of D2O are slightly smaller than those of H2O while D2O is slightly more structured than H2O. The dipole moment of D2O tends to be 4% higher than that of H2O, and the hydrogen bonding of D2O is also stronger than that of 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 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 D2O-filled (14,0) carbon nanotube is found to have a smaller radial breathing mode than the H2O-filled (14,0) carbon nanotube.