Ice Clathrate Research Articles

Glass Transitions in Pressure-Collapsed Ice Clathrates and Implications for Cold Water

Ice is known to collapse to amorphous ice upon pressurization at low temperatures and shows the unusual feature of multiple distinct solid amorphous water states, which have inspired models of liquid water’s structure and unusual properties. Here, we use heat capacity Cp measurements to show that similarly collapsed ice clathrates display identical glass behavior as amorphous ice but that crystallization above the glass transition temperature Tg of ∼140 K at 1 GPa is inhibited. This effect of the homogeneously distributed “guest molecules” in water reveals a relatively strong reversible Cp increase above Tg but no further transition before crystallization at ∼190 K. This is consistent with a glass–liquid transition of water at Tg, which suggests a new path to study an ultraviscous liquid water network and evaluate water models.

Ice–Clathrate Hydrate–Gas Phase Equilibria for Argon + Water and Carbon Dioxide + Water Systems

In this communication, we report ice–clathrate hydrate–gas equilibrium data (dissociation data) for the argon + water and carbon dioxide + water systems in the temperature ranges of (264.3–271.2) and (260.2–270.7) K, respectively. The equilibrium data were generated using an isothermal pressure-search method. The new equilibrium data for carbon dioxide clathrate hydrates are compared with the equilibrium data reported in the literature to demonstrate the reliability of the experimental technique used in this work. The equilibrium data generated for argon clathrate hydrates are compared with a few sets of experimental data reported in the literature. It is found that such data for the latter clathrate hydrate are indeed scarce in the temperature range studied in our work. A comparison is finally made on the equilibrium conditions of argon, air, oxygen, nitrogen, and carbon monoxide clathrate hydrates.

Water molecular reorientation in ice and tetrahydrofuran clathrate hydrate from lineshape analysis of 17O spin-echo NMR spectra

Lineshape analysis of 17O spin-echo NMR spectra has been used to study water molecular reorientations in ice-Ih and THF Structure II clathrate hydrate. The kinetics was determined by the changes of the lineshapes of the 17O central transitions at different temperatures. A model involving 12 orientations and 4-step jumps of water molecular orientations was proposed. Semi-classical exchange formalism was employed to simulate the lineshape of the central transitions of the 17O nuclei. Lineshape analysis gave the quadrupolar coupling constant CQ = 6.43 MHz and the asymmetry parameter η = 0.935, for both ice-Ih and THF gas hydrate. The theoretical lineshape simulations resulted in activation energies of water molecular reorientations Ea = 55.2 ± 2.1 kJ mol–1 and Ea = 30.5 ± 0.8 kJ mol–1 for ice-Ih and THF hydrate, respectively. The range of dynamic rates in THF clathrate hydrate is such that before melting, a pseudo-isotropic lineshape is observed that retains a second-order quadrupolar shift. The water reorientation process is discussed in light of recent results on Bjerrum defect injection obtained from molecular dynamics simulation and structural studies.

A sublimation technique for high-precision measurements of δ<sup>13</sup>CO<sub>2</sub> and mixing ratios of CO<sub>2</sub> and N<sub>2</sub>O from air trapped in ice cores

Abstract. In order to provide high precision stable carbon isotope ratios (δ13CO2 or δ13C of CO2) from small bubbly, partially and fully clathrated ice core samples we developed a new method based on sublimation coupled to gas chromatography-isotope ratio mass spectrometry (GC-IRMS). In a first step the trapped air is quantitatively released from ~30 g of ice and CO2 together with N2O are separated from the bulk air components and stored in a miniature glass tube. In an off-line step, the extracted sample is introduced into a helium carrier flow using a minimised tube cracker device. Prior to measurement, N2O and organic sample contaminants are gas chromatographically separated from CO2. Pulses of a CO2/N2O mixture are admitted to the tube cracker and follow the path of the sample through the system. This allows an identical treatment and comparison of sample and standard peaks. The ability of the method to reproduce δ13C from bubble and clathrate ice is verified on different ice cores. We achieve reproducibilities for bubble ice between 0.05 ‰ and 0.07 ‰ and for clathrate ice between 0.05 ‰ and 0.09 ‰ (dependent on the ice core used). A comparison of our data with measurements on bubble ice from the same ice core but using a mechanical extraction device shows no significant systematic offset. In addition to δ13C, the CO2 and N2O mixing ratios can be volumetrically derived with a precision of 2 ppmv and 8 ppbv, respectively.

Compressional, temporal, and compositional behavior of H2-O2 compound formed by high pressure x-ray irradiation

X-ray irradiation was found to convert H(2)O at pressures above 2 GPa into a novel molecular H(2)-O(2) compound. We used optical Raman spectroscopy to explore the behavior of x-ray irradiated H(2)O samples as a function of pressure, time, and composition. The compound was found to be stable over a period of two years, as long as high pressure conditions (>2 GPa) were maintained. The Raman shifts for the H(2) and O(2) vibrons behaved differently from pure H(2) and O(2) as pressure was increased on the compound up to 70 GPa, indicating that it remains a distinct, molecular compound. Based on spectra taken from different locations in a single sample, it appears that multiple forms of the H(2)-O(2) compound exist. The structure and composition of the starting material plays an important role in compound formation, as we found that hydrogen-filled ice clathrate C(2) (H(2))H(2)O did not undergo the same dissociation as observed in ice VII upon x-ray irradiation until pressure was increased to above 10 GPa.

CO 2 and O 2/N 2 variations in and just below the bubble–clathrate transformation zone of Antarctic ice cores

CO 2 measurements on the EPICA (European Project for Ice Coring in Antarctica) DML ice core in depth levels just below the bubble ice–clathrate ice transformation zone (1230–2240 m depth) were performed. In the youngest part (1230–1600 m), they reveal variations of up to 25 ppmv around the mean atmospheric concentration within centimetres, corresponding to a snow deposition interval of a few years. Similar results are found at corresponding depth regions of the Dome C and the Talos Dome ice cores. Since we can exclude all hitherto known processes altering the concentration of CO 2 in ice cores, we present a hypothesis about spatial fractionation of air components related to episodically increasing clathrate formation followed by diffusion processes from bubbles to clathrates. This hypothesis is supported by optical line-scan observations and by O 2/N 2 measurements at the same depth where strong CO 2 variations are detected. Below the clathrate formation zone, this small-scale fractionation process is slowly smoothed out, most likely by diffusion, regaining the initial mean atmospheric concentration. Although this process compromises the representativeness of a single CO 2 measurement on small ice samples in the clathrate formation zone of an ice core, it does not affect the mean atmospheric CO 2 concentration if CO 2 values are averaged over a sufficiently long depth scale (> 10 cm in case of the EPICA DML ice core).

Guest-free monolayer clathrate and its coexistence with two-dimensional high-density ice

Three-dimensional (3D) gas clathrates are ice-like but distinguished from bulk ices by containing polyhedral nano-cages to accommodate small gas molecules. Without space filling by gas molecules, standalone 3D clathrates have not been observed to form in the laboratory, and they appear to be unstable except at negative pressure. Thus far, experimental evidence for guest-free clathrates has only been found in germanium and silicon, although guest-free hydrate clathrates have been found, in recent simulations, able to grow from cold stretched water, if first nucleated. Herein, we report simulation evidence of spontaneous formation of monolayer clathrate ice, with or without gas molecules, within hydrophobic nano-slit at low temperatures. The guest-free monolayer clathrate ice is a low-density ice (LDI) whose geometric pattern is identical to Archimedean 4.8(2)-truncated square tiling, i.e. a mosaic of tetragons and octagons. At large positive pressure, a second phase of 2D monolayer ice, i.e. the puckered square high-density ice (HDI) can form. The triple point of the LDI/liquid/HDI three-phase coexistence resembles that of the ice-I(h)/water/ice-III three-phase coexistence. More interestingly, when the LDI is under a strong compression at 200 K, it transforms into the HDI via a liquid intermediate state, the first direct evidence of Ostwald's rule of stages at 2D. The tensile limit of the 2D LDI and water are close to that of bulk ice-I(h) and laboratory water.

Isomorphism between ice and silica

Both ice and silica crystallize into solid-state structures composed of tetrahedral building units that are joined together to form an infinite four-connected net. Mathematical considerations suggest that there is a vast number of such nets and thus potential crystal structures. It is therefore perhaps surprising to discover that, despite the differences in the nature of interatomic interactions in these materials, a fair number of commonly observed ice and silica phases are based on common nets. Here we use computer simulation to investigate the origin of this symmetry between the structures formed for ice and silica and to attempt to understand why it is not complete. We start from a comparison of the dense phases and then move to the relationship between the different open (zeolitic and clathratic) structures formed for both materials. We show that there is a remarkably strong correlation between the energetics of isomorphic silica and water ice structures and that this correlation arises because of the strong link between the total energy of a material and its local geometric features. Finally, we discuss a number of as yet unsynthesized low-energy structures which include a phase of ice based on quartz, a silica based on the structure of ice VI, and an ice clathrate that is isomorphic to the silicate structure nonasil.

Pressure-induced collapse of ice clathrate and hexagonal ice mixtures formed by freezing

We report thermal conductivity kappa measurements of the pressure-induced collapse of two mixtures of ice and tetrahydrofuran (THF) clathrate hydrate formed by freezing aqueous solutions, THF23 H(2)O and THF20 H(2)O, one containing twice as much excess water than the other. On pressurizing, kappa of the solid mixture first decreases at the onset pressure of approximately 0.8 GPa, as occurs for collapse of pure ice, reaches a local minimum at a pressure of approximately 1.0 GPa, and then increases as occurs for the collapse of the pure clathrate THF17 H(2)O. This shows that in the apparently homogeneous mixture, the ice and the clathrate collapse as if the two were in a mechanically mixed state. The manner in which the clathrate aggregate can arrange in the solid indicates that ice occupies the interstitial space in the tightly packed aggregates and H(2)O molecules belonging to the lattice of one form hydrogen bond with that of the other, a feature that is preserved in their collapsed states. On decompression, the original clathrate is partially recovered in the THF20 H(2)O mixture, but the collapsed ice does not transform to the low density amorph. We surmise that on irreversible transformation to the original clathrate, the aggregates expand. Any pressure thus exerted on the small domains of the collapsed ice with a hydrogen bonded interface with the clathrate aggregates could prevent it from transforming to the low density amorph. Measurements of kappa are useful in investigating structural collapse of crystals when dilatometry is unable to do so, as kappa seems to be more sensitive to pressure-induced changes than the volume.

Change in CO2 concentration and O2/N2 ratio in ice cores due to molecular diffusion

Polar ice cores are unique archives for ancient air. However, a loss of air due to molecular diffusion during storage could affect the composition of the remaining air. We formulate a model with a high spatial resolution (1 mm) calculating the loss of N2, O2 and CO2 in pure clathrate ice in order to determine which layers of an ice core are affected by significant changes in the CO2 concentration and the δ(O2/N2) ratio for storage durations up to 38 years. The results agree with experimental δ(O2/N2) measurements at ice core pieces performed after different storage durations. Additionally, the calculations confirm the importance of the storage temperature and show that the CO2 concentration is less affected than that of δ(O2/N2). Furthermore, guidelines for ice core sample preparation are provided in dependence of storage duration and temperature.

Nature of the pressure-induced collapse of an ice clathrate by dielectric spectroscopy

Collapse of an ice clathrate of type II structure containing tetrahydrofuran as guest molecules has been studied at different pressures by dielectric spectroscopy. The sample was pressurized to 1.3 GPa at 130 K and the resulting collapsed state was pressure cycled. The dielectric relaxation time increases at a progressively rapid rate during pressurizing and then decreases slowly on depressurizing, but the dielectric relaxation time does not reach the value of the original state. With increase in pressure, the limiting high frequency permittivity due to orientation of H(2)O molecules first increases by about 5% until 0.75 GPa and then decreases slightly until 1 GPa, and finally it increases until approximately 1.2 GPa. The decrease is attributed to the loss of contribution from the reorientational motion of tetrahydrofuran molecules and the increase to densification as the structure mechanically collapses completely in the 1-1.25 GPa range. The relaxation time of the collapsed state is comparable with that of the high-density amorph formed on pressure collapse of ice.

Collapse of an ice clathrate under pressure observed via thermal conductivity measurements

Irreversible transformation of the tetrahydrofuran ice clathrate at $\ensuremath{\sim}130\text{ }\text{K}$ was studied by measuring thermal conductivity $\ensuremath{\kappa}$ with increase in pressure $p$. Initially, $\ensuremath{\kappa}$ increases slowly with $p$ up to $\ensuremath{\sim}0.75\text{ }\text{GPa}$ where it levels off, is roughly constant up to $\ensuremath{\sim}0.95\text{ }\text{GPa}$, then decreases up to $\ensuremath{\sim}1.05\text{ }\text{GPa}$. Pressure collapses the clathrate structure, plausibly beginning with lattice distortion, and $\ensuremath{\kappa}$ increases at $\ensuremath{\sim}1.05\text{ }\text{GPa}$ in a sharp sigmoid-shape manner due to large densification until the transformation is complete at $\ensuremath{\sim}1.25\text{ }\text{GPa}$. This is the opposite of that found for ice whose $\ensuremath{\kappa}$ decreases first slowly with increase in $p$ and then rapidly in an inverted sigmoid-shape manner [O. Andersson and H. Suga, Phys. Rev. B 65, 140201 (2002)]. At 1.08 GPa and 131 K, $\ensuremath{\kappa}$ increases with time $t$ (s) according to $\ensuremath{\sim}\text{exp}(t/2945)$, which is also the opposite of the collapse of ice [G. P. Johari and O. Andersson, Phys. Rev. B 70, 184108 (2004)]. The difference in its behavior is attributed to strong phonon scattering from the tetrahydrofuran guest molecules. $\ensuremath{\kappa}$ of the collapsed clathrate is $\ensuremath{\sim}30%$ less than that for the collapsed ice, which is comparable with the 25% lesser $\ensuremath{\kappa}$ of the tetrahydrofuran-water solution from $\ensuremath{\kappa}$ of water at ambient pressure. On depressurizing at 130 K, $\ensuremath{\kappa}$ decreases progressively more rapidly and $\ensuremath{\kappa}$ of the collapsed state at $\ensuremath{\sim}0.3\text{ }\text{GPa}$ is slightly lower than that of the as-made clathrate, showing that its original structure is not recovered.

Hydrogen Nuclear Spin Relaxation in Hydrogen−Ice Clathrate

H2 in D2O ice clathrate has been studied by hydrogen NMR. In a previous report, the H2 line shape was shown to be due to incompletely averaged intramolecular dipolar interactions. Here the relaxation times T1, T1rho, and T2 are reported. T1 passes through a minimum at 10 K, indicating that the rotational transition rate Gamma between the three sublevels of J = 1 passes through the resonance frequency at this temperature. On the cold side, T1 varies as T(-2.6); on the hot side, the rate T1(-1) varies as T(-2) plus a constant (due to paramagnetic impurities). These indicate a two-phonon process drives the rotational transitions Gamma. The spin-echo T2 is nearly independent of temperature and in reasonable agreement with the Van Vleck intermolecular H2-H2 second moment. T1rho deviates from the expected T1rho = T1 behavior above 85 K, revealing an additional slow-motion source of relaxation. The deviation is driven by the hopping of H2 between large cages. Ortho-para conversion is measured to be much slower in the clathrate than in the bulk solid, reflecting the greater distances between the H2 molecules.

Temperature−Pressure Phase Diagram of a D2O−D2 System at Pressures to 1.8 kbar

Using a volumetric technique, the deuterium solubility, X, in heavy water (L), low-pressure hexagonal ice (I h), and high-pressure cubic clathrate ice (sII) is studied at deuterium pressures up to 1.8 kbar and temperatures from -40 to +5 degrees C. The triple point of the L + I(h) + sII equilibrium is located at P = 1.07(3) kbar and T = -4.5(8) degrees C. The molar ratios D2/D2O of phases at the triple point are X(L) = 0.020(5), X(Ih) = 0.012(5), and X(sII) = 0.207(5).

Inelastic neutron scattering study of hydrogen in d8-THF∕D2O ice clathrate

In situ neutron inelastic scattering experiments on hydrogen adsorbed into a fully deutrated tetrahydrofuran-water ice clathrate show that the adsorbed hydrogen has three rotational excitations (transitions between J=0 and 1 states) at approximately 14 meV in both energy gain and loss. These transitions could be unequivocally assigned since there was residual orthohydrogen at low temperatures (slow conversion to the ground state) resulting in an observable J=1-->0 transition at 5 K (kT=0.48 meV). A doublet in neutron energy loss at approximately 28.5 meV is interpreted as J=1-->2 transitions. In addition to the transitions between rotational states, there are a series of peaks that arise from transitions between center-of-mass translational quantum states of the confined hydrogen molecule. A band at approximately 9 meV can be unequivocally interpreted as a transition between translational states, while broad features at 20, 25, 35, and 50-60 meV are also interpreted to as transitions between translational quantum states. A detailed comparison is made with a recent five-dimensional quantum treatment of hydrogen in the smaller dodecahedral cage in the SII ice-clathrate structure. Although there is broad agreement regarding the features such as the splitting of the J=1 degeneracy, the magnitude of the external potential is overestimated. The numerous transitions between translational states predicted by this model are in poor agreement with the experimental data. Comparisons are also made with three simple exactly solved models, namely, a particle in a box, a particle in a sphere, and a particle on the surface of a sphere. Again, there are too many predicted features by the first two models, but there is reasonable agreement with the particle on a sphere model. This is consistent with published quantum chemistry results for hydrogen in the dodecahedral 5(12) cage, where the center of the cage is found to be energetically unfavorable, resulting in a shell-like confinement for the hydrogen molecule wave function. These results demonstrate that translational quantum effects are very significant and a classical treatment of the hydrogen molecule dynamics is inappropriate under such conditions.

Rotation and Diffusion of H2 in Hydrogen−Ice Clathrate by 1H NMR

A recently reported hydrogen-ice clathrate carries up to four H(2) in each large cage and one H(2) in each small cage. We report pulsed proton NMR line shape measurements on H(2)-D(2)O clathrate formed at 1500 bar and 250 K. The behavior of the two-pulse spin-echo amplitude with respect to the nutation angle of the refocusing pulse shows that intramolecular dipolar broadening, modulated by H(2) molecular reorientations, dominates the line width of the ortho-H(2). Dipolar interaction between H(2) guests and host D atoms explains the echo variation with the relative phases of the pulses. From 12 to 120 K, the line width varies as 1/T, demonstrating that the three sublevels of J = 1 are split by a constant energy, epsilon. The splitting arises from distortion in the otherwise high-symmetry cages from frozen-out D(2)O orientational disorder. Above 120 K, further line-narrowing signals the onset of H(2) diffusion from cage to cage. At the lowest temperature, 1.9 K, the spectrum has Pake powder doublet-like features; the doublet is not fully developed, indicating a broad distribution of order parameters and energies epsilon.

Non-linear dielectric effect in ice clathrates

Non-linear dielectric experiments were carried out in ice clathrates containing propylene oxide, tetrahydrofurane, acetone and 1,4-dioxane. It is shown that small dipolar guest molecules preserve rotational freedom.

Phase transitions and equilibrium hydrogen content of phases in the water–hydrogen system at pressures to 1.8 kbar

Using a volumetric technique, the hydrogen solubility X in liquid water (L), low-pressure hexagonal ice (I h), and high-pressure cubic clathrate ice (sII) is studied at hydrogen pressures up to 1.8 kbar and temperatures from−36 to+20 °C. The triple point of the L+I h+sII equilibrium is located at P=1.07(5) kbar and T=−10(1) °C. The hydrogen concentrations of phases at the triple point are X L =0.17(5), and X sII=2.3(1) wt.% H2. The thermal stability and the process of decomposition of the clathrate phase at ambient pressure are studied by neutron diffraction.

Low-energy excitations of guest molecules in clathrates and the Boson peak

The heat capacity, Cp, of seven ice clathrates, four β-quinol clathrates and one squarate clathrate containing rare gas atoms or molecules as guests in their cage-like structures at T<30 K has been analysed, and compared against that of empty β-quinol-clathrate, two channel-type thiourea clathrates containing polar molecules and four canonical glasses. Ethylene oxide–ice clathrate shows a peak in Cp/T3 at ca. 7.5 K, and argon–β-quinol clathrate at ca. 16 K. Other clathrates show an approach toward a Cp/T3 peak at 5<T<12 K. On the basis of their (i) Cp, (ii) Raman scattering, (iii) far infra-red absorption, and (iv) density of vibrational states data, it is shown that low-energy (2–8 meV) excitations are intrinsic to clathrates. Their features are remarkably similar to those of the Boson peak, which has been interpreted in terms of translational vibrations of 20–100 atoms group or of strongly cohesive nanometer size zones separated by softer zones in the heterogeneous glass structures. Since only translational (rattling) and/or rotational (torsional oscillations or librations) motions of atoms or molecules can occur in clathrate cages and channels, and nanometer size clusters cannot form, the similarity noted above indicates that the microstructural origin of the Boson peak in glasses may need reconsideration. Interaction between the guest and cage molecules explains satisfactorily neither the low-energy (2–8 meV) features of clathrates, nor its low-thermal conductivity relative to ice.

Molecular inertial effects in liquids: Poley absorption, collision-induced absorption, low-frequency Raman spectrum and Boson peaks

The far-infrared absorption spectra of low-viscosity liquids contains a broad peak whose frequency decreases with increase in temperature and height increases. This is known as the Poley absorption. In the theory of itinerant oscillations of molecules in a liquid, the frequency of the Poley absorption peak is inversely proportional to the square root of a molecule's moment of inertia. Dipolar molecules confined to symmetrical cages in the structure of an ice clathrate crystal also show a peak in the far-infrared region. This has also been interpreted in terms of rotational oscillations of the dipolar molecule with its frequency being inversely proportional to the square root of the molecule's moment of inertia. The low-frequency Raman and the far-infrared absorption spectra (or the dielectric loss spectra in the THz frequency range) of a glass and a supercooled liquid also show a peak and several other features depending on the manner of presenting the Raman scattering and infrared absorption data. Data on these features are collected from the literature and compared here. It is shown that despite the differences arising from molecule-specific effects, these three features are remarkably similar, thus indicating that the vibrational effects attributed to the Boson peak and to other features of the low-frequency Raman scattering are likely to admit to the same underlying mechanisms as the Poley absorption. There is also a collision-induced absorption in liquids in the far-infrared region. Its equivalence in the Raman-scattering studies is yet to be recognized.