Resolve the melting temperature of water confined in narrow carbon nanotubes

Determine the melting temperatures of water confined within single-walled carbon nanotubes of sub-nanometer diameters at ambient pressure across different diameters, reconciling the large discrepancies among experimental techniques (X-ray diffraction, Raman spectroscopy, photoluminescence) and empirical force-field simulations that report values ranging from approximately 180 K to 450 K.

Background

Understanding the melting behavior of water confined inside carbon nanotubes is critical for both fundamental science and applications such as desalination, water filtration, and nanofluidic devices. However, reported melting temperatures vary dramatically across studies.

Experimental measurements using X-ray diffraction and photoluminescence have reported melting temperatures from about 180 K to 300 K depending on the nanotube diameter, while Raman spectroscopy has suggested values as high as ~450 K for similar diameters. Classical molecular dynamics simulations with empirical force fields also disagree substantially, further contributing to the uncertainty.

The authors note that this disparity has persisted despite numerous investigations, and that until now no computational paper with first-principles accuracy had been available to address it. Their work aims to reduce this uncertainty using a machine-learning potential trained on density functional theory data.