Microscopic Study of Proton Kinetic Energy Anomaly for Nanoconfined Water

Abstract

The reported anomalies of the proton mean kinetic energy, Ke­(H), in nanoconfined water, as measured by deep inelastic neutron scattering (DINS), constitute a longstanding problem related to proton dynamics in hydrogen-bonded systems. A considerable number of theoretical attempts to explain these anomalies have failed. The mean vibrational density of states (VDOS) of protons in water nanoconfined inside single wall carbon nanotubes (SWCNTs) is calculated as a function of temperature and SWCNT diameter, D CNT, by classical molecular dynamics (MD) simulation using the TIP4P-2005f water model. The calculated VDOS are utilized for deducing the mean kinetic energy of the water protons, Ke­(H), by treating each phonon state as a harmonic oscillator. The calculation depicts a strong confinement effect as reflected in the drop of the value of Ke­(H) at 5 K for D CNT < ∼12 Å, while absent for larger diameters. The results also reveal very significant blue and red shifts of the stretching and bending modes, respectively, compared to those in bulk ice, in agreement with experiment.

Characterization of Water Structure and Phase Behavior within Metal–Organic Nanotubes

“…The presence of the central OW3 water has been previously identified by computational efforts evaluating water confined within single-walled carbon nanotubes but has not been experimentally characterized in these systems. Several groups have used molecular dynamic simulations to evaluate carbon nanotubes with subtle differences in diameter. − Moid et al have evaluated changes in the confined water structure for single-walled carbon nanotubes with a range of diameters at 1 atm of pressure . They observed a hexagonal ice channel without a central water site for a diameter of 1.22 nm at temperatures up to 250 K. However, when the diameter of the nanotube was changed to 1.36 nm then an eight-membered water ring with an additional water located in the middle of the channel can be observed for this system .…”

Interplay between Local Structure and Nuclear Dynamics in Tungstic Acid: A Neutron Scattering Study

“…According to these and other investigations, water in confined spaces forms otherwise inaccessible liquid and glassy phases that exist over a wide range of temperatures, where protons in the hydrogen bonds (H-bonds) are coherently delocalized and may exhibit nuclear quantum tunnelling. 7−10,14−16 These highly unusual states of water lead to the observation of anomalous effects such as a deficit of nuclear kinetic energy 10,18 and unusual transport properties. 10 The joint effort on both experimental and theoretical fronts led to a paradigm change in modern investigation NCS and augmented with increasingly more sophisticated ab initio modeling protocols.…”

“…Water, especially in the confined state, has been the driving force of the research on nuclear quantum effects (NQEs) in condensed matter systems. , It is largely owing to the investigation of NQEs in water and their relation to water properties − that the experimental techniques such as neutron Compton scattering (NCS), also termed as deep inelastic neutron scattering (DINS), ,,− have risen to prominence. According to these and other investigations, water in confined spaces forms otherwise inaccessible liquid and glassy phases that exist over a wide range of temperatures, where protons in the hydrogen bonds (H-bonds) are coherently delocalized and may exhibit nuclear quantum tunnelling. − ,− These highly unusual states of water lead to the observation of anomalous effects such as a deficit of nuclear kinetic energy , and unusual transport properties …”

Anisotropy of the Proton Kinetic Energy as a Tool for Capturing Structural Transition in Water Confined in a Graphene Nanoslit Pore

“…In recent years, the partial vibrational density of states (pVDOS) approach was utilized for studying proton dynamics in various H containing systems, ,− and it is applied here for studying proton dynamics in nanoconfined water under 2D topology.…”