Structural and Dynamical Properties of H₂O and D₂O under Confinement

Area of Science:

• Physical Chemistry
• Materials Science
• Nanotechnology

Background:

• Water (H₂O) and heavy water (deuterium dioxide, D₂O) are crucial substances with distinct properties.
• Previous research primarily focused on bulk differences between H₂O and D₂O.
• Understanding nanoscale behavior is vital for applications in imaging and reactors.

Purpose of the Study:

• To investigate the structural and dynamical properties of H₂O and D₂O.
• To compare their behavior in bulk and under nanoscale confinement within a (14,0) carbon nanotube.
• To elucidate the effects of isotopic substitution and confinement on water's fundamental characteristics.

Main Methods:

• Path integral molecular dynamics simulations were employed.
• Structural properties (bond length, bond angle, hydrogen bonding) were analyzed.
• Dynamical properties (libration frequency, bond stretching, radial breathing mode) were examined.

Main Results:

• In bulk, D₂O showed smaller bond lengths/angles, higher structure, a 4% greater dipole moment, and stronger hydrogen bonding than H₂O.
• Under nanoscale confinement, both H₂O and D₂O exhibited reduced bond lengths/angles and weakened hydrogen bonding.
• Confinement led to lower libration frequencies, higher OH(OD) stretching frequencies, and altered radial breathing modes, with D₂O showing a smaller mode.

Conclusions:

• Isotopic substitution significantly impacts water's bulk properties, with D₂O being more structured and having stronger hydrogen bonds.
• Nanoscale confinement alters water's dynamics and structure, weakening hydrogen bonds and shifting vibrational frequencies.
• The study provides critical insights into heavy water behavior at the nanoscale, relevant for advanced material and technological applications.