Nature of proton transport in a water-filled carbon nanotube and in liquid water

TL;DR

This study uses ab initio path-integral molecular dynamics to show that quantum nuclear effects enable barrierless proton transfer in both bulk water and water-filled carbon nanotubes, with confinement facilitating transfer through molecular prealignment.

Contribution

It reveals that quantum nuclear effects are crucial for barrierless proton transfer and that confinement in nanotubes promotes transfer via molecular prealignment, a novel insight into proton transport mechanisms.

Findings

Quantum nuclear effects enable barrierless PT in water and nanotubes.

Prealignment of water molecules facilitates PT in nanotubes.

Proton under confinement is a fluxional defect, not a fixed hydration state.

Abstract

Proton transport (PT) in bulk liquid water and within a thin water-filled carbon nanotube has been examined with ab initio pathintegral molecular dynamics (PIMD). Barrierless proton transfer is observed in each case when quantum nuclear effects (QNEs) are accounted for. The key difference between the two systems is that in the nanotube facile PT is facilitated by a favorable prealignment of water molecules, whereas in bulk liquid water solvent reorganization is required prior to PT. Configurations where the quantum excess proton is delocalized over several adjacent water molecules along with continuous interconversion between different hydration states reveals that, as in liquid water, the hydrated proton under confinement is best described as a fluxional defect, rather than any individual idealized hydration state such as Zundel, Eigen, or the so-called linear H7O3+ complex along the water chain. These findings highlight the importance of QNEs in intermediate strength hydrogen bonds (HBs) and explain why H+ diffusion through nanochannels is impeded much less than other cations.