Water in Carbon Nanotubes Is Not the Same Old Stuff

TL;DR

This study reveals that water confined in carbon nanotubes exhibits a unique phase with distinct proton momentum distribution and a phase transition linked to hydrogen bond reordering, representing a new form of ice.

Contribution

It demonstrates that water in nanotubes forms a novel 2-D ice phase with unique quantum properties, differing from known ice phases.

Findings

Protons in nanotube water have lower kinetic energy than in ice Ih.

A phase transition occurs between 230K and 268K involving hydrogen bond reordering.

The 2-D ice layer in nanotubes is a new phase with coherent proton delocalization.

Abstract

The momentum distribution of the protons in ice Ih, ice VI, high density amorphous ice and water in carbon nanotubes at low temperatures has been measured using deep inelastic neutron scattering. We find that the momentum distribution for the water in the nanotubes is qualitatively unlike that in any other phase of water or ice. The kinetic energy of the protons is 35mev less than that in ice Ih at the same temperature, and the high momentum tail of the distribution, characteristic of the molecular covalent bond and the stretch mode of the proton in the hydrogen bonds, is not present. We observe a phase transition between 230K and 268K in the nanotube data. The high momentum tail is present in the higher temperature measurement, which resembles that of ice Ih at the same temperature. Molecular dynamics simulations show the phase transition to be associated with the reordering of the hydrogen bonds of the 2-D ice layer, coating the interior of the nanotube at low temperatures, into a 3-D structure at 268K. We conclude that the protons in the hydrogen bonds in the 2-D ice layer are coherently delocalized, and that the 2-D ice layer is a qualitatively new phase of ice.