Ice Clathrate Research Articles

Local Order Parameters Classifying Water Networks of Ice and Cyclopropane Clathrate Hydrates

Clathrate hydrates are crystalline solids with guest molecules in a hydrogen-bonding network of frozen water. Cyclopropane hydrate has a four-phase coexistence point of guest gas, ice, and two hydrates, suggesting ice–hydrate and hydrate–hydrate structural transitions, but little is known about the details. Microscopic approaches such as molecular simulations are expected as a means of elucidation, but these require reaction coordinates that are sensitive to structural changes (e.g., order parameters). Here, 1220 local order parameters (LOPs) and their combinations were used to classify cyclopropane hydrate and ice structures in the vicinity of the four-phase coexistence point. A total of 21 599 490 LOP combinations were automatically and systematically evaluated via supervised machine learning. The optimal (set of) LOP(s) distinguished between structure I and structure II cyclopropane hydrates and ice Ih with high accuracy and robustness.

Computational Analysis of Vibrational Spectra of Hydrogen Bonds in sII and sH Gas Hydrates.

The amount of energy in natural gas hydrates is thought to be equivalent to twice that of all other fossil fuels combined. However, economic and safe energy recovery has remained a challenge till now. To develop a novel method of breaking the hydrogen bonds (HBs) surrounding the trapped gas molecules, we investigated the vibrational spectra of the HBs of gas hydrates with structure types II and H. Two models of 576-atom propane-methane sII hydrate and 294-atom neohexane-methane sH hydrate were built. A first-principles density functional theory (DFT) method was employed using the CASTEP package. The simulated spectra were in good agreement with the experimental data. Compared with the partial phonon density of states of guest molecules, we confirmed that the experimental infrared absorption peak in the terahertz region mainly arose from HB vibrations. By removing the components of guest molecules, we found that the theory of two kinds of hydrogen bond vibrational modes applies. The use of a terahertz laser to enable resonance absorption of HBs (at about 6 THz, to be tested) may therefore lead to the rapid melting of clathrate ice and release of guest molecules.

Overview of the recent studies on high‐pressure impurity ices

AbstractIn the present short review, we briefly overview the history of ice polymorphism under high‐pressure and the recent progress on impurity ices (clathrate ice, salty ice, doped ice, empty ice, etc.), including our contributions due form high‐pressure Raman investigations to these research fields. In particular, the unique roles of impurities in ice, which are (1) acceleration of proton ordering at lower temperatures, (2) offering a clue to form new ice phases, and (3) phase transitions other than the original equilibrium stability region of pure H2O, ice are introduced. Then, as prospects, we address promising impacts on a new area of ice physics with the active use of impurities.

A novel stirred microcalorimetric cell for DSC measurements applied to the study of ice slurries and clathrate hydrates

A novel prototype of a microcalorimetric cell with in-situ stirring has been developed to perform DSC measurements under atmospheric or pressure conditions. After a brief technical description of the apparatus, preliminary tests are presented which analyzed the influence of the stirrer rotation on the heat-flow signal. Experiments were then performed with complex fluids such as ice slurries and clathrate hydrates formed with cyclopentane and with carbon dioxide. They took place in stirred and non-stirred conditions and the results obtained were then compared. It was proven that the rotation of the microstirrer in the measuring cell does not disrupt the heat-flow signal during the analysis. As regards the practical applications tested, the in-situ stirrer efficiently reduces crystallization metastability, increases the water-to-hydrate conversion, and reduces the amount of time needed for analysis. The dissociation enthalpy of cyclopentane (CP) hydrates was measured at atmospheric pressure; it is effectively very difficult to analyze this system with non-stirred calorimetry techniques because the two liquid phases are immiscible. The experimental results, in good agreement with other data found in the literature, showed complete water-to-CP hydrate conversion within a short period of time using a simple protocol. Experiments were also performed under pressure to demonstrate that CO2 hydrate phase equilibrium data could be obtained rapidly and easily. It is therefore our opinion that the potential of this novel technology has been thoroughly demonstrated.

An ultralow-density porous ice with the largest internal cavity identified in the water phase diagram

The recent back-to-back findings of low-density porous ice XVI and XVII have rekindled the century-old field of the solid-state physics and chemistry of water. Experimentally, both ice XVI and XVII crystals can be produced by extracting guest atoms or molecules enclosed in the cavities of preformed ice clathrate hydrates. Herein, we examine more than 200 hypothetical low-density porous ices whose structures were generated according to a database of zeolite structures. Hitherto unreported porous EMT ice, named according to zeolite nomenclature, is identified to have an extremely low density of 0.5 g/cm3 and the largest internal cavity (7.88 Å in average radius). The EMT ice can be viewed as dumbbell-shaped motifs in a hexagonal close-packed structure. Our first-principles computations and molecular dynamics simulations confirm that the EMT ice is stable under negative pressures and exhibits higher thermal stability than other ultralow-density ices. If all cavities are fully occupied by hydrogen molecules, the EMT ice hydrate can easily outperform the record hydrogen storage capacity of 5.3 wt % achieved with sII hydrogen hydrate. Most importantly, in the reconstructed temperature-pressure (T-P) phase diagram of water, the EMT ice is located at deeply negative pressure regions below ice XVI and at higher temperature regions next to FAU. Last, the phonon spectra of empty-sII, FAU, EMT, and other zeolite-like ice structures are computed by using the dispersion corrected vdW-DF2 functional. Compared with those of ice XI (0.93 g/cm3), both the bending and stretching vibrational modes of the EMT ice are blue-shifted due to their weaker hydrogen bonds.

Investigations of the Hydrogen Bonds and Vibrational Spectra of Clathrate Ice XVI

Natural gas hydrates are ice-like crystalline materials formed from natural gas and clathrate ice under high pressure and low temperature. Ice XVI, the first S-II type clathrate ice produced in the lab, was simulated by first-principles density functional theory with the CASTEP code. A 34-molecule supercell was built to mimic the hydrogen-disordered structure. The vibrational spectra were calculated as a reference for inelastic neutron scattering (INS), infrared (IR) absorption, and Raman scattering experiments. Two kinds of H-bond vibration modes corresponding to two different bond strengths were found in our previous studies. In this paper, the statistics of distribution calculated by integrating these two kinds of modes was found to match the phonon density of states (PDOS) very well. We confirmed that the two basic types of H-bonds also appeared in clathrate ice XVI. The typical normal modes were analyzed to illustrate the dynamic process of lattice vibrations.

Temperature and polarization dependent Raman spectra of liquid H2O and D2O

AbstractPolarized Raman spectra of liquid H2O and D2O have been measured over the entire 100 to 4,000 cm−1 vibrational frequency range and 280 to 360 K temperature range. The measured spectra are corrected for wavelength‐dependent detector quantum efficiency and are provided in numerical form to facilitate comparisons with theoretical predictions. Self‐modeling curve resolution is used to decompose the measured spectra into a linear combination of two spectral components whose relative intensities change with temperature but whose shapes are approximately temperature independent. The component whose intensity increases with decreasing temperature contains prominent features in both the high‐frequency OH stretch band region and low‐frequency OH⋯O hydrogen bond stretch band region that resemble those observed in single crystal ice Ih and clathrate hydrates.

Clathrate ice sL: a new crystalline phase of ice with ultralow density predicted by first-principles phase diagram computations.

In contrast to the rich knowledge of water and 17 experimentally confirmed crystalline phases of solid water under positive pressures, water under negative pressure has been poorly explored. In this study, a new crystalline phase of ice with ultralow density (0.6 g cm-3), named "clathrate ice sL", is constructed by nano water cage clusters, and it is predicted to be stable under a lower negative pressure than the experimentally confirmed sII phase by first-principles phase diagram computations, thereby extending the phase diagram of water to negative pressure regions below -5170 bar at 0 K and below -4761 bar at 300 K. In addition, according to our theoretical prediction, the optimal hydrogen storage mass density in the new clathrate ice sL is 7.7 wt% (larger than the 2017 DOE target of 5.5 wt%), which would set a new record of hydrogen storage capacity in clathrate hydrates. The finding of clathrate ice sL not only proposes a new type of crystalline ice under negative pressure but also explores the potential applications of the ultralow density ice phases while extending the water phase diagram and enriching the knowledge of people about water.

Benchmarking the performance of density-functional-based approaches on intermolecular interactions of Helium–water complexes

SynopsisThe He-water systems remain relatively unexplored. As only under extreme conditions, noble gases can be made to react to produce compounds, we investigated the characteristics of the interaction among He atom and water ice channels or clathrate cavities from first-principles calculations. The ability of DFT-based methods to describe such weak interactions might lead to further insights on noble-gas inclusion compounds.

Prediction of a new ice clathrate with record low density: A potential candidate as ice XIX in guest-free form

Using extensive Monte Carlo packing algorithm and dispersion-corrected density functional theory optimization, we predict a new cubic crystalline phase of ice clathrate, named as s-IV, which is composed of eight large icosihexahedral cavities (12464418), eight intermediate dodecahedral cavities (6646), and sixteen small octahedral cavities (6246) per unit cell. Based on DFT calculations, we find that the s-IV ice clathrate with an extremely low mass density of 0.506g/cm3. In the P-T phase diagram of water described by the TIP4P/2005 water model, the s-IV ice clathrate becomes a more stable ice polymorph in the negative-pressure region, e.g., below –3830bar at 0K, below –4882bar at 115K, and below –7292bar at 200K.

Micro-Tomographic Investigation of Ice and Clathrate Formation and Decomposition under Thermodynamic Monitoring.

Clathrate hydrates are inclusion compounds in which guest molecules are trapped in a host lattice formed by water molecules. They are considered an interesting option for future energy supply and storage technologies. In the current paper, time lapse 3D micro computed tomographic (µCT) imaging with ice and tetrahydrofuran (THF) clathrate hydrate particles is carried out in conjunction with an accurate temperature control and pressure monitoring. µCT imaging reveals similar behavior of the ice and the THF clathrate hydrate at low temperatures while at higher temperatures (3 K below the melting point), significant differences can be observed. Strong indications for micropores are found in the ice as well as the THF clathrate hydrate. They are stable in the ice while unstable in the clathrate hydrate at temperatures slightly below the melting point. Significant transformations in surface and bulk structure can be observed within the full temperature range investigated in both the ice and the THF clathrate hydrate. Additionally, our results point towards an uptake of molecular nitrogen in the THF clathrate hydrate at ambient pressures and temperatures from 230 K to 271 K.

Changes in atmospheric carbonyl sulfide over the last 54,000 years inferred from measurements in Antarctic ice cores

AbstractWe measured carbonyl sulfide (COS) in air extracted from ice core samples from the West Antarctic Ice Sheet (WAIS) Divide, Antarctica, with the deepest sample dated to 54,300 years before present. These are the first ice core COS measurements spanning the Last Glacial Maximum (LGM), the last glacial/interglacial transition, and the early Holocene. The WAIS Divide measurements from the LGM and the last transition are the first COS measurements in air extracted from full clathrate (bubble‐free) ice. This study also includes new COS measurements from Taylor Dome, Antarctica, including some in bubbly glacial ice that are concurrent with the WAIS Divide data from clathrate glacial ice. COS hydrolyzes in ice core air bubbles, and the recovery of an atmospheric record requires correcting for this loss. The data presented here suggest that the in situ hydrolysis of COS is significantly slower in clathrate ice than in bubbly ice. The clathrate ice measurements are corrected for the hydrolysis loss during the time spent as bubbly ice only. The corrected WAIS Divide record indicates that atmospheric COS was 250–300 parts per trillion (ppt) during the LGM and declined by 80–100 ppt during the last glacial/interglacial transition to a minimum of 160–210 ppt at the beginning of the Holocene. This decline was likely caused by an increase in the gross primary productivity of terrestrial plants, with a possible contribution from a reduction in ocean sources. COS levels were above 300 ppt in the late Holocene, indicating that large changes in the COS biogeochemical cycle occurred during the Holocene.

A new phase diagram of water under negative pressure: The rise of the lowest-density clathrate s-III.

Ice and ice clathrate are not only omnipresent across polar regions of Earth or under terrestrial oceans but also ubiquitous in the solar system such as on comets, asteroids, or icy moons of the giant planets. Depending on the surrounding environment (temperature and pressure), ice alone exhibits an exceptionally rich and complicated phase diagram with 17 known crystalline polymorphs. Water molecules also form clathrate compounds with inclusion of guest molecules, such as cubic structure I (s-I), cubic structure II (s-II), hexagonal structure H (s-H), tetragonal structure T (s-T), and tetragonal structure K (s-K). Recently, guest-free clathrate structure II (s-II), also known as ice XVI located in the negative-pressure region of the phase diagram of water, is synthesized in the laboratory and motivates scientists to reexamine other ice clathrates with low density. Using extensive Monte Carlo packing algorithm and dispersion-corrected density functional theory optimization, we predict a crystalline clathrate of cubic structure III (s-III) composed of two large icosihexahedral cavities (8(6)6(8)4(12)) and six small decahedral cavities (8(2)4(8)) per unit cell, which is dynamically stable by itself and can be fully stabilized by encapsulating an appropriate guest molecule in the large cavity. A new phase diagram of water ice with TIP4P/2005 (four-point transferable intermolecular potential/2005) model potential is constructed by considering a variety of candidate phases. The guest-free s-III clathrate with ultralow density overtakes s-II and s-H phases and emerges as the most stable ice polymorph in the pressure region below -5834 bar at 0 K and below -3411 bar at 300 K.

Transitions in pressure collapsed clathrate hydrates.

Type II clathrate hydrates (CHs), or ice clathrates, are inclusion compounds in which a hydrogen-bonded cage-like structure of H2O accommodates molecules of suitable size, known as "guest molecules". CHs have similar local geometrical arrangements of the hydrogen-bonded water network as ice and both are known to collapse to amorphous states on isothermal pressurization at temperatures below about 140 K. Moreover, the collapsed CH states undergo a glass, or glass-like, transition at 140 K on heating at 1 GPa, which is identical to that of collapsed ice, or high density amorphous ice. Here we use thermal conductivity and dielectric measurements to study the transition behavior of two type II CHs with tetrahydrofuran and 1,3 dioxolane, respectively, as guest molecules. After their collapsed states have been heated to well above the glass transition at 1 GPa, we find transitions corresponding to the high to low density amorphous ice transition of ice with only slightly shifted temperature-pressure (T-p) coordinates compared to those of pure water. Thus, collapsed CHs show the same transition behaviors as cold water, which provide the basis for the model that explains the unusual temperature and pressure behaviors of water's properties in terms of two distinct types of liquid water. Collapsed CHs are, however, more stable than collapsed ice and can therefore be studied in a wider T-p interval. The results suggest sluggish homogenizing and phase separation processes, which affect the transition behaviors of collapsed CHs.

Structure of the Ice–Clathrate Interface

Clathrate hydrates are crystals in which water molecules form hydrogen-bonded cages that enclose small nonpolar molecules, such as methane. In the laboratory, clathrates are customarily synthesized from ice and gas guest under conditions for which homogeneous nucleation of hydrates is not possible. It is not known how ice assists in the nucleation of clathrate hydrates or how ice forms on clathrate hydrate in the case of self-preservation. There is no lattice matching between any plane of ice and clathrate hydrates; therefore, an interfacial transition layer has to form to connect the two crystals. Here, we use molecular dynamic simulations to study the structure of ice–clathrate interfaces produced by alignment and equilibration of the crystals, competitive growth of ice and clathrate from a common solution, nucleation of hydrate in the presence of a growing ice front, and nucleation of ice in the presence of clathrate hydrates. We find that the interfacial transition layer between ice and clathrate is always disordered and has a typical width of two to three water layers. Water in the interfacial transition layer has tetrahedral order lower than either ice or clathrate and higher than liquid water under the same thermodynamic conditions. The potential energy of the water in the interfacial transition layer is between those in liquid water and the crystals. Our results suggest that the disordered interfacial transition layer could assist in the heterogeneous nucleation of clathrates from ice and ice from clathrates by providing a lower surface free energy than the ice–liquid and clathrate–liquid interfaces.

Differentiating bubble-free layers from melt layers in ice cores using noble gases

AbstractMelt layers are clear indicators of extreme summer warmth on polar ice caps. The visual identification of refrozen meltwater as clear bubble-free layers cannot be used to study some past warm periods, because, in deeper ice, bubbles are lost to clathrate formation. We present here a reliable method to detect melt events, based on the analysis of Kr/Ar and Xe/Ar ratios in ice cores, and apply it to the detection of melt in clathrate ice from the Eemian at NEEM, Greenland. Additionally, melt layers in ice cores can compromise the integrity of the gas record by dissolving soluble gases, or by altering gas transport in the firn, which affects the gas chronology. We find that the easily visible 1 mm thick bubble-free layers in the WAIS Divide ice core do not contain sufficient melt to alter the gas composition in the core, and do not cause artifacts or discontinuities in the gas chronology. The presence of these layers during winter, and the absence of anomalies in soluble gases, suggests that these layers can be formed by processes other than refreezing of meltwater. Consequently, the absence of bubbles in thin crusts is not in itself proof of a melt event.

Experimental study on the effect of ammonia on the phase behavior of tetrahydrofuran clathrates.

Clathrate hydrates, ice-like crystalline compounds in which small guest molecules are enclosed inside cages formed by tetrahedrally hydrogen-bonded water molecules, are naturally abundant on Earth and are generally expected to exist on icy celestial bodies. A prototypical example is Saturn's moon Titan, where dissociation of methane clathrates, a major crustal component, could contribute significantly to the replenishment of atmospheric methane. Ammonia is an important clathrate inhibiting agent that may be present (potentially at high concentrations) in Titan's interior. In this study, low-temperature Raman experiments are conducted to examine the dissociation point of tetrahydrofuran clathrates, an ambient-pressure analogue of methane clathrates, over a wide range of ammonia concentrations from 0 to 25 wt %. A phase diagram for the H2O-THF-NH3 system is generated, showing two main results: (i) ammonia lowers the dissociation point of clathrate hydrates to a similar extent compared to the melting of water ice and (ii) THF clathrate exhibits a "liquidus-like" behavior in the presence of ammonia, with a eutectic temperature of about 203.6 K. As temperatures higher than this estimated eutectic are anticipated within Titan's icy crust, these results imply that partial dissociation of clathrates can occur readily and may contribute to outgassing from the interior.

A review of the brittle ice zone in polar ice cores

AbstractMaintaining ice-core quality through the brittle ice zone (BIZ) remains challenging for polar ice-core studies. At depth, increasing ice overburden pressurizes trapped air bubbles, causing fracture of cores upon exposure to atmospheric pressure. Fractured ice cores degrade analyses, reducing resolution and causing contamination. BIZ encounters at 18 sites across the Greenland, West and East Antarctic ice sheets are documented. The BIZ begins at a mean depth of 545 ± 162 m (1 standard deviation), extending to depths where ductile clathrate ice is reached: an average of 1132 ± 178 m depth. Ice ages in this zone vary with snow accumulation rate and ice thickness, beginning as young as 2 ka BP at Dye-3, Greenland, affecting ice >160 ka BP in age at Taylor Dome, Antarctica, and compromising up to 90% of retrieved samples at intermediate-depth sites. Effects of pressure and temperature on the BIZ are explored using modeled firn-column overburden pressure and borehole temperatures, revealing complex associations between firn densification and BIZ depth, and qualitatively supporting expected thinning of the BIZ at low ice temperatures due to shallower clathrate stability. Mitigating techniques for drilling, transport, sampling and analysis of brittle ice cores are also discussed.

The deep water cycle and flood basalt volcanism

Experimental data combined with numerical simulations suggest that fast-subducting oceanic slabs are cold enough to carry a significant amount of H2O into the deep mantle in antigorite; with increasing depth, this mineral undergoes solid–solid transitions to phase A and then to phase E and/or wadsleyite. Ice VII and clathrate hydrates can also be stable under pressure and temperature (PT) conditions of cold slabs and represent other potential phases for water transport into the deep mantle. Cold slabs are expected to stagnate in the mantle transition zone. With time, they are heated to the temperature of the ambient transition zone and release excess water as a H2O-bearing fluid. This may cause voluminous melting of overlying upper mantle rocks. If such a process operates in nature, magmas geochemically similar to island-arc basalts are expected to appear in places relatively remote from active arcs at the time of their emplacement. Dolerites of the southeastern margin of the Siberian flood basalt province, located about 700 km from suggested trench, were probably associated with the fast (cold) subduction of the Mongolia–Okhotsk oceanic slab, and originated by the dehydration of the stagnant slab in the transition zone. We show that the influence of the subduction-related deep water cycle on Siberian flood basalt magmatism gradually diminished with increasing distance from the subduction zone. Thus, the unique size and volume of the Siberian flood basalt province could have originated due to long-term underflow beneath Siberia with or without the existence of a lower mantle plume.

Hypervelocity Impact Effect of Molecules from Enceladus’ Plume and Titan’s Upper Atmosphere on NASA’s Cassini Spectrometer from Reactive Dynamics Simulation

The NASA/ESA Cassini probe of Saturn analyzed the molecular composition of plumes emanating from one of its moons, Enceladus, and the upper atmosphere of another, Titan. However, interpretation of this data is complicated by the hypervelocity (HV) flybys of up to ~18 km/sec that cause substantial molecular fragmentation. To interpret this data we use quantum mechanical based reactive force fields to simulate the HV impact of various molecular species and ice clathrates on oxidized titanium surfaces mimicking those in Cassini's neutral and ion mass spectrometer (INMS). The predicted velocity dependent fragmentation patterns and composition mixing ratios agree with INMS data providing the means for identifying the molecules in the plume. We used our simulations to predict the surface damage from the HV impacts on the INMS interior walls, which we suggest acts as a titanium sublimation pump that could alter the instrument's readings. These results show how the theory can identify chemical events from hypervelocity impacts in space plumes and atmospheres, providing in turn clues to the internal structure of the corresponding sources (e.g., Enceladus). This may be valuable in steering modifications in future missions.