Protons In Ice Research Articles

Tuning the thermodynamic ordering of strongly correlated protons in ice by angstrom-scale interface modification

The static and dynamic behaviour of strongly correlated many-body protons in nanoscale hydrogen-bond networks plays crucial roles in a wide range of physicochemical, biological and geological phenomena in nature. However, because of the difficulty of probing and manipulating the proton configuration in nanomaterials, controlling the cooperative behaviour of many-body protons remains challenging. By combining proton-order sensitive nonlinear optical spectroscopy and well-defined interface modification at molecular/atomic scale, we demonstrate the possibility of extensively tuning the emergent physical properties of strongly correlated protons beyond the thermodynamic constraints of bulk hydrogen bonds. Focusing on heteroepitaxially grown crystalline ice films as a model of a strongly correlated and frustrated proton system, we show that the emergence and disappearance of a high-Tc proton order on the nano- to mesoscale is readily switched by angstrom-scale interface engineering. These results pave a way to designing and controlling emergent properties of correlated proton systems.

Nuclear Quantum Effects on the Electronic Structure of Water and Ice.

The electronic properties and optical response of ice and water are intricately shaped by their molecular structure, including the quantum mechanical nature of the hydrogen atoms. Despite numerous previous studies, a comprehensive understanding of the nuclear quantum effects (NQEs) on the electronic structure of water and ice at finite temperatures remains elusive. Here, we utilize molecular simulations that harness efficient machine-learning potentials and many-body perturbation theory to assess how NQEs impact the electronic bands of water and hexagonal ice. By comparing path-integral and classical simulations, we find that NQEs lead to a larger renormalization of the fundamental gap of ice, compared to that of water, ultimately yielding similar bandgaps in the two systems, consistent with experimental estimates. Our calculations suggest that the increased quantum mechanical delocalization of protons in ice, relative to water, is a key factor leading to the enhancement of NQEs on the electronic structure of ice.

Proton Transport and Related Chemical Processes of Ice.

Excess protons play a key role in the chemical reactions of ice because of their exceptional mobility, even when the diffusion of atoms and molecules is suppressed in ice at low temperatures. This article reviews the current state of knowledge on the properties of excess protons in ice, with a focus on the involvement of protons in chemical reactions. The mechanism of efficient proton transport in ice, which involves a proton-hopping relay along the hydrogen-bond ice network and the reorientation of water, is discussed and compared with the inefficient transport of hydroxide in ice. Distinctly different properties of protons residing in the ice interior and on the ice surface are emphasized. Recent observations of the spontaneous occurrence of reactions in ice at low temperatures, which include the dissociation of protic acids and the hydrolysis of acidic oxides, are discussed with regard to the kinetic and thermodynamic effects of mobile protons on the promotion of unique chemical processes of ice.

Hydrated excess protons and their local hydrogen bond transport network as measured by translational, librational, and vibrational frequencies.

A clear molecular description of excess hydrated protons and their local hydrogen bond transport network remains elusive. Here, the hydrogen bond network of excess hydrated protons in water bridges was probed by measuring their Raman spectra and comparing them to the spectra of protons in ice and water. The proton vibrational spectrum and the hydrogen bond network translational and librational spectra were recorded. The spectra of the water bridge and water exhibit clear differences, indicating the presence of a structure in water bridges when subjected to an electric field of ∼106 V/m that has not been previously reported. The intermolecular Raman spectrum of the floating water bridge exhibits a hydrogen bond stretching band at 150-250 cm-1, librational bands within the 300-1000 cm-1 spectral range, and a large band at 1500-3000 cm-1, which corresponds to the vibrational signature of excess hydrated protons in the water bridge structure. The excess protons are shown to move predominantly at the air/water interface, and the effect of this distribution is a measurable change in the air/water interfacial tension from ∼80 to ∼32 N/m. Therefore, hydrated protons must have a unique water arrangement that enables them to propagate without sinking into bulk water. This local polarized hydrogen bond network in the interfacial water region is characterized by a translational spectrum similar to that of ice V.

Ice Ih revisited: No proton tunneling observed in a quasielastic neutron scattering experiment

A large broadening ($\ensuremath{\sim}0.4\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$) of quasielastic neutron scattering (QENS) signal in ${\mathrm{H}}_{2}\mathrm{O}$ ice ${I}_{h}$ at $T=5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ was observed by Bove et al. [Phys. Rev. Lett. 103, 165901 (2009)] and explained by a model of concerted proton tunneling. This result was rather unexpected, as prior studies never showed significant mobility in water at low temperatures and ambient pressure. There were a few attempts of theoretical understanding of the effect. Recent path-integral simulations as well as quantum lattice-gauge theory supported the possibility of the collective tunneling of protons in ice ${I}_{h}$, however other studies stated that concerted tunneling in ice ${I}_{h}$ should have very low frequency. Here, we report on QENS measurements of ${\mathrm{H}}_{2}\mathrm{O}$ ice ${I}_{h}$ at 1.8 and 5 K by using neutron scattering spectrometers with the energy resolution similar to and four times better than the energy resolution in the original experiment of Bove and co-workers. We did not observe any QENS broadening, and the measured spectra for the ice ${I}_{h}$ and the reference vanadium sample were almost identical. Therefore, we conclude that there is no proton tunneling in ice ${I}_{h}$ at temperatures down to 1.8 K measurable on an energy scale of 3.5 $\ensuremath{\mu}\mathrm{eV}$ and above. The literature data on low-temperature heat capacity of ice ${I}_{h}$ support this conclusion.

Observation of nuclear quantum effects and hydrogen bond symmetrisation in high pressure ice

Hydrogen bond symmetrisations in H-bonded systems triggered by pressure-induced nuclear quantum effects (NQEs) is a long-known concept but experimental evidence in high-pressure ices has remained elusive with conventional methods. Theoretical works predicted quantum-mechanical tunneling of protons within water ices to occur at pressures above 30 GPa, and the H-bond symmetrisation transition to occur above 60 GPa. Here we used 1H-NMR on high-pressure ice up to 97 GPa, and demonstrate that NQEs govern the behavior of the hydrogen bonded protons in ice VII already at significantly lower pressures than previously expected. A pronounced tunneling mode was found to be present up to the highest pressures of 97 GPa, well into the stability field of ice X, where NQEs are not anticipated in a fully symmetrised H-bond network. We found two distinct transitions in the NMR shift data at about 20 GPa and 75 GPa attributed to the step-wise symmetrisation of the H-bond.

Entropy-Driven Spontaneous Reaction in Cryogenic Ice: Dissociation of Fluoroacetic Acids.

Chemical reactions are extremely difficult to occur in ice at low temperature, where atoms and molecules are frozen in position with minimal thermal energy and entropy. Contrary to this general behavior, certain weak acids including fluoroacetic acids dissociate spontaneously and more efficiently in cryogenic ice than in aqueous solution at room temperaure. The enhanced reactivity of weak acids is an unexpected consequence of proton-transfer equilibrium in ice. The configurational entropy of protons in ice shifts the acid dissociation equilibrium forward. This configurational entropy, although a solid-state property, is comparatively large in magnitude with the entropy of vaporization and can effectively drive proton-transfer reactions in ice.

Density-Functional Tight-Binding Molecular Dynamics Simulations of Excess Proton Diffusion in Ice Ih, Ice Ic, Ice III, and Melted Ice VI Phases.

The structural, dynamical, and energetic properties of the excess proton in ice were studied using density-functional tight-binding molecular dynamics simulations. The ice systems investigated herein consisted of low-density hexagonal and cubic crystalline variants (ice Ih and Ic) and high-density structures (ice III and melted ice VI). Analysis of the temperature dependence of radial distribution function and bond order parameters served to characterize the distribution and configuration of hundreds of water molecules in a unit cell. We confirmed that ice Ih and Ic possess higher hexagonal symmetries than ice III and melted ice VI. The estimated Grotthuss shuttling diffusion coefficients in ice were larger than that of liquid water, indicating a slower proton diffusion process in high-density structures than in low-density systems. The energy barriers calculated on the basis of the Arrhenius plot of diffusion coefficients were in reasonable agreement with experimental measurement for ice Ih. Furthermore, the energy barriers for high-density structures were several times larger than those of low-density systems. The simulation results were likely related to the suppression of proton transfer in disordered water configurations, in particular, ice with low hexagonal symmetry.

氷エレクトレットの作製と電荷量

Ferroelectric ice, ice XI, existing at the cryogenic temperature of 1 atm is a very interesting material because of its high electrical stability at low temperature and its low environmental impact. However, the ice XI formation process is very difficult. The dielectric properties of ice are governed by the behavior of the hydrogen ions, protons. Protons in ice can hop along lattice defects at a relatively high temperature, e.g., 253 K. At a cryogenic temperature such as 77 K, proton movement in ice is difficult. To obtain polarized ice, we applied DC voltage to normal ice, ice Ih, at 253 K and cool it down to 77 K with voltage application. In this process, protons in ice hop to the cathode side at 253 K and stop moving at 77 K, therefore polarization fixed there. The polarized ice is called "ice electret." The electrical properties and structure of ice electret have not yet been clarified. We produced ice electret using the aforementioned method and observed its depolarization current. We changed the application time and magnitude of voltage in during the formation process and measured the charged stored in the ice electrets produced. The results show that the stored charge in ice electret increases with application time and amplitude of voltage.

Near-infrared spectra of H2O under high pressure and high temperature: Implications for a transition from proton tunneling to hopping states

The nature of protons in ice VII up to 368°C and 16GPa was investigated with synchrotron near-infrared spectroscopy. The absorption band of the first OH stretching overtone mode divided into doublet peaks above 5GPa at room temperature, suggesting that proton tunneling occurs at the overtone level. As the temperature increased, the doublet peaks gradually reduced to a singlet. This result implies that thermally activated protons hop between the two potential minima along the oxygen–oxygen axis. A pressure–temperature diagram for the proton state was constructed from the changing band shape of the overtone mode.

Spherical momentum distribution of the protons in hexagonal ice from modeling of inelastic neutron scattering data

The spherical momentum distribution of the protons in ice is extracted from a high resolution deep inelastic neutron scattering experiment. Following a recent path integral Car-Parrinello molecular dynamics study, data were successfully interpreted in terms of an anisotropic Gaussian model, with a statistical accuracy comparable to that of the model independent scheme used previously, but providing more detailed information on the three dimensional potential energy surface experienced by the proton. A recently proposed theoretical concept is also employed to directly calculate the mean force from the experimental neutron Compton profile, and to evaluate the accuracy required to unambiguously resolve and extract the effective proton potential from the experimental data.

Isotope Effect of Proton/Deuteron Diffusion Constant in Ice

AbstractThe isotope effect of the proton diffusion constant in ice Ih was studied by using a time‐resolved emission technique. For that purpose we used three photoactive molecules that are sensitive to the excess mobile proton in ice. We found that the diffusion constant isotope effect was smaller by less than a factor of two in the temperature range of 240–270 K. Previously, we reported that the proton diffusion constant in ice is very large, in fact, 10 times greater than that of water at 295 K.

Indication of a Very Large Proton Diffusion in Ice Ih. 2. Fluorescence Quenching of Flavin Mononucleotide by Protons

A time-resolved emission technique was employed to study the effect of excess protons on the fluorescence quenching process of flavin mononucleotide (FMN) in methanol-doped ice samples. We found that an excess of protons in ice has a very large effect on the fluorescence quenching whereas in liquid water the proton fluorescence quenching is rather small. We analyzed the experimental data using the Smoluchowski diffusion-assisted binary collision model. Under certain assumptions and approximations, the calculated proton diffusion constant in ice in the range of 245−265 K is about 10 times that of water at 295 K.

Indication of a Very Large Proton Diffusion in Ice Ih

A time-resolved emission technique was employed to study the photoprotolytic cycle of the 2-naphthol-6,8-disulfonate (2N68DS) photoacid in the presence of a low concentration of a strong HCl acid. We found that an excess of protons in ice has a very large and profound effect on the photoprotolytic cycle. The excess proton reacts with the deprotonated form of the photoacid. An analysis of the experimental data reveals that the proton diffusion constant in ice in the temperature range of 240−270 K is much larger than in the liquid state. Under certain assumptions and approximations, the calculated proton diffusion constant in ice is 10 times larger than in water at 295 K, i.e., DH+ice = 1.2 × 10−3 cm2/s.

Internal motions of a quasiparticle governing its ultrafast nonlinear response

A charged particle modifies the structure of the surrounding medium: examples include a proton in ice, an ion in a DNA molecule, an electron at an interface, or an electron in an organic or inorganic crystal. In turn, the medium acts back on the particle. In a polar or ionic solid, a free electron distorts the crystal lattice, displacing the atoms from their equilibrium positions. The electron, when considered together with its surrounding lattice distortion, is a single quasiparticle, known as the Fröhlich polaron. The basic properties of polarons and their drift motion in a weak electric field are well known. However, their nonlinear high-field properties--relevant for transport on nanometre length and ultrashort timescales--are not understood. Here we show that a high electric field in the terahertz range drives the polaron in a GaAs crystal into a highly nonlinear regime where, in addition to the drift motion, the electron is impulsively moved away from the centre of the surrounding lattice distortion. In this way, coherent lattice vibrations (phonons) and concomitant drift velocity oscillations are induced that persist for several hundred femtoseconds. They modulate the optical response at infrared frequencies between absorption and stimulated emission. Such quantum coherent processes directly affect high-frequency transport in nanostructures and may be exploited in novel terahertz-driven optical modulators and switches.

Anomalous Behavior of Proton Zero Point Motion in Water Confined in Carbon Nanotubes

The momentum distribution of the protons in ice Ih, ice VI, high density amorphous ice, and water in carbon nanotubes has been measured using deep inelastic neutron scattering. We find that at 5 K the kinetic energy of the protons is 35 meV less than that in ice Ih at the same temperature, and the high momentum tail of the distribution, characteristic of the molecular covalent bond, is not present. We observe a phase transition between 230 and 268 K to a phase that does resemble ice Ih. Although there is yet no model for water that explains the low temperature momentum distribution, our data reveal that the protons in the hydrogen bonds are coherently delocalized and that the low temperature phase is a qualitatively new phase of ice.

The conductivity properties of protons in ice and mechanism of magnetization of liquid water

From a study of electrical conductivity of protons in the hydrogen-bonded chains in ice we confirm that the magnetization of liquid water is caused by proton transfer in closed hydrogen-bonded chains occurring as a first order phase transition, through which the ice becomes liquid water. We first study the conductive properties of proton transfer along molecular chains in ice crystals in our model. Ice is a typical hydrogen-bonded molecular system, in which the interaction of localized fluctuation of hydrogen ions (H+) with deformation of a structure of hydroxyl group (OH) results in soliton motion of the protons along the molecular chains via ionic and bonded defects. We explain further the quantum conductive properties of proton transfer and determine its mobility and conductivity under constant electric-field using a new theory of proton transfer, which agree with experimental values. From features of first order phase-transition for ice, and some experimental data of pure and magnetized water we confirm further that there are not only free water molecules, but also many linear and closed hydrogen-bonded chains consisting of many polarized water-molecules in the liquid water. Thus a ring proton-current, which resembles to a '' molecular current '' or a '' small magnet '' in solids, can occur in the closed hydrogen-bond chains under action of an externally applied magnetic field. Then the water molecules in the closed chains can be orderly arrayed due to the magnetic interaction among these ring proton currents and the externally applied magnetic field. This is just the magnetized effect of the water. In such a case the optical and electronic properties of the water, including the dielectric constant, magnetoconductivity, refraction index, Raman and Infrared absorption spectra, are changed. We determine experimentally the properties of the magnetized water which agree with the theoretical results of our model. However, the magnetized effect of water is, in general, very small, and vanishes at temperatures above 100 degrees C.

Percolation Model for Proton Immobility in Ice

A simple Ising model and a statistical theory of gel formation in a polymer matrix were combined to explain the effect of temperature on the mobility of protons in ice. Cowin et al observed that protons in ice bulk were immobile at temperatures below 190 K, and suddenly become mobile at temperatures above 190 K (Nature 398, 405 (1999)). We proposed here that the transport of protons is controlled by a percolation mechanism. The proton transport is facilitated by the occurrence of easily reoriented water molecules in the ice bulk. The fraction of this molecule depends on temperature and was calculated using a simple Ising model. The formation of network of these molecules which provides pathways for proton transport was calculated using a statistical theory of gel formation in a polymeric system. Our model succeeded to reproduce the variation of potential difference between the ice film surfaces with respect to temperature as observed by Cowin et al based on a soft landing experiment.

How 'spin ice' freezes.

The large degeneracy of states resulting from the geometrical frustration of competing interactions is an essential ingredient of important problems in fields as diverse as magnetism, protein folding and neural networks. As first explained by Pauling, geometrical frustration of proton positions is also responsible for the unusual low-temperature thermodynamics of ice and its measured 'ground state' entropy. Recent work has shown that the geometrical frustration of ice is mimicked by Dy2Ti2O7, a site-ordered magnetic material in which the spins reside on a lattice of corner-sharing tetrahedra where they form an unusual magnetic ground state known as 'spin ice'. Here we identify a cooperative spin-freezing transition leading to the spin-ice ground state in Dy2Ti2O7. This transition is associated with a very narrow range of relaxation times, and represents a new form of spin-freezing. The dynamics are analogous to those associated with the freezing of protons in ice, and they provide a means through which to study glass-like behaviour and the consequences of frustration in the limit of low disorder.

Incoherent inelastic neutron scattering measurements on dome-fuji antarctic ice

We measured the incoherent inelastic neutron scattering (IINS) in ice (depth, 201–2201 m) recovered from Dome-Fuji (DF), Antarctica. We found that the peak intensity of librational vibration in the IINS spectra of DF ice changes with the ice temperature (Ti) in the ice sheet: the intensity at 97 meV decreases with increases in depth from 201 to 2201 m. From the IINS spectra, we calculated the Ti dependence of the order parameter (the ratio of ordered protons in ice), which is consistent with the theory of the order-disorder phase-transition from proton arrangements in ordinary ice to proton-ordered arrangements at Ti = 237 K. The results indicate that Antarctic ice in which Ti ≤ 237 K contains proton-ordered arrangements.