Raman Signatures and Thermal Expansivity of Acetylene Clathrate Hydrate.

The vibrational signatures for the υ2 C≡C and υ1 symmetric C-H stretches of acetylene in cubic structure I clathrate, synthesized under ambient pressure, are reported for the first time. The most diagnostic features are at 1966 for υ2 and 3353 cm-1 for υ1, respectively, and are assigned to acetylene trapped in the large 51262 cages. In addition, the υ2 mode for acetylene occupying the small 512 cages is observed at 1972.5 cm-1, a red shift of 1.5 cm-1 from its gas phase frequency. Unit cell parameters and thermal expansion coefficients are determined via powder X-ray diffraction between 195 and 225 K and are found to be in good correlation with previous single crystal data at 143 K. The calculated density for acetylene clathrate is also reported, with values ranging from 0.985 g/cm3 at 195 K to 0.976 g/cm3 at 225 K. These results are relevant for spectral detection of acetylene-containing compounds on planetary bodies, as well as providing additional insights on the thermal behavior and physical properties of acetylene clathrate.

Formation of a new benzene-ethane co-crystalline structure under cryogenic conditions.

We report the first experimental finding of a solid molecular complex between benzene and ethane, two small apolar hydrocarbons, at atmospheric pressure and cryogenic temperatures. Considerable amounts of ethane are found to be incorporated inside the benzene lattice upon the addition of liquid ethane onto solid benzene at 90-150 K, resulting in formation of a distinctive co-crystalline structure that can be detected via micro-Raman spectroscopy. Two new features characteristic of these co-crystals are observed in the Raman spectra at 2873 and 1455 cm(-1), which are red-shifted by 12 cm(-1) from the υ1 (a1g) and υ11 (eg) stretching modes of liquid ethane, respectively. Analysis of benzene and ethane vibrational bands combined with quantum mechanical modeling of isolated molecular dimers reveal an interaction between the aromatic ring of benzene and the hydrogen atoms of ethane in a C-H···π fashion. The most favored configuration for the benzene-ethane dimer is the monodentate-contact structure, with a calculated interaction energy of 9.33 kJ/mol and an equilibrium bonding distance of 2.66 Å. These parameters are comparable to those for a T-shaped co-crystalline complex between benzene and acetylene that has been previously reported in the literature. These results are relevant for understanding the hydrocarbon cycle of Titan, where benzene and similar organics may act as potential hydrocarbon reservoirs due to this incorporation mechanism.

Experimental Characterization of the Pyridine:Acetylene Co-crystal and Implications for Titan's Surface.

Titan, Saturn's largest moon, has a plethora of organic compounds in the atmosphere and on the surface that interact with each other. Cryominerals such as co-crystals may influence the geologic processes and chemical composition of Titan's surface, which in turn informs our understanding of how Titan may have evolved, how the surface is continuing to change, and the extent of Titan's habitability. Previous works have shown that a pyridine:acetylene (1:1) co-crystal forms under specific temperatures and experimental conditions; however, this has not yet been demonstrated under Titan-relevant conditions. Our work here demonstrates that the pyridine:acetylene co-crystal is stable from 90 K, Titan's average surface temperature, up to 180 K under an atmosphere of N2. In particular, the co-crystal forms via liquid-solid interactions within minutes upon mixing of the constituents at 150 K, as evidenced by distinct, new Raman bands and band shifts. X-ray diffraction (XRD) results indicate moderate anisotropic thermal expansion (about 0.5-1.1%) along the three principal axes between 90-150 K. Additionally, the co-crystal is detectable after being exposed to liquid ethane, implying stability in a residual ethane "wetting" scenario on Titan. These results suggest that the pyridine:acetylene co-crystal could form in specific geologic contexts on Titan that allow for warm environments in which liquid pyridine could persist, and as such, this cryomineral may preserve the evidence of impact, cryovolcanism, or subsurface transport in surface materials.

Low-Temperature Specific Heat Capacity of Water-Ammonia Mixtures Down to the Eutectic.

Robust thermodynamic data are essential for the development of geodynamic and geochemical models of ocean worlds. The water-ammonia system is of interest in the study of ocean worlds due to its purported abundance in the outer solar system, geological implications, and potential importance for origins of life. In support of developing new equations of state, we conducted 1 bar specific heat capacity measurements (Cp) using a differential scanning calorimeter (DSC) at low temperatures (184-314 K) and low mass fractions of ammonia (5.2-26.9 wt %) to provide novel data in the parameter space most relevant for planetary studies. This is the first known set of data with sufficient fidelity to investigate the trend of specific heat capacity with respect to temperature. The obtained Cp in the liquid phase domain above the liquidus generally increases with temperature. Deviations of our data from the currently adopted equation of state by Tillner-Roth and Friend[Tillner-Roth R.; Friend D. G.J. Phys. Chem. Ref. Data1998, 27, 63-96]. are generally negative (ranging from +1 to -10%) and larger at lower temperatures. This result suggests that suppression of the critical behavior of supercooled water (rapid increase in specific heat with decreasing temperature) by ammonia starts at a smaller concentration than that set by Tillner-Roth and Friend.[Tillner-Roth R.; Friend D. G.J. Phys. Chem. Ref. Data1998, 27, 63-96]. Cp measurements of the liquid were also obtained in the partial melting domain between the eutectic and liquidus. This novel data set will be useful in future investigations of conditions where such partial melt may exist, such as the ice shell-ocean boundary or the interiors of ocean worlds that may contain relatively large proportions of dissolved ammonia.

Complex organosulfur molecules on comet 67P: Evidence from the ROSINA measurements and insights from laboratory simulations.

The ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instrument aboard the Rosetta mission revolutionized our understanding of cometary material composition. One of Rosetta's key findings is the complexity of the composition of comet 67P/Churyumov-Gerasimenko. Here, we used ROSINA data to analyze dust particles that were volatilized during a dust event in September 2016 and report the detection of large organosulfur species and an increase in the abundances of sulfurous species previously detected in the coma. Our data support the presence of complex sulfur-bearing organics on the surface of the comet. In addition, we conducted laboratory simulations that show that this material may have formed from chemical reactions that were initiated by the irradiation of mixed ices containing H2S. Our findings highlight the importance of sulfur chemistry in cometary and precometary materials and the possibility of characterizing organosulfur materials in other comets and small icy bodies using the James Webb Space Telescope.

Reply to the 'Comment on Cage occupancy of methane clathrate hydrates in the ternary H2O-NH3-CH4 system' by S. Alavi and J. Ripmeester, Chem. Commun., 2022, 58, DOI: 10.1039/D1CC06526B.

Our recent Communication suggested that ammonia in aqueous solution may preferentially destabilize large cages in methane clathrate hydrates. A Comment favored ammonia incorporation instead, but it did not accurately describe our proposed mechanism and relied primarily on studies conducted in different chemical systems and/or which used other preparation methods.

Thermodynamic data and modeling of the water and ammonia-water phase diagrams up to 2.2 GPa for planetary geophysics.

We present new experimental data on the liquidus of ice polymorphs in the H(2)O-NH(3) system under pressure, and use all available data to develop a new thermodynamic model predicting the phase behavior in this system in the ranges (0-2.2 GPa; 175-360 K; 0-33 wt % NH(3)). Liquidus data have been obtained with a cryogenic optical sapphire-anvil cell coupled to a Raman spectrometer. We improve upon pre-existing thermodynamic formulations for the specific volumes and heat capacities of the solid and liquid phase in the pure H(2)O phase diagram to ensure applicability of the model in the low-temperature metastable domain down to 175 K. We compute the phase equilibria in the pure H(2)O system with this new model. Then we develop a pressure-temperature dependent activity model to describe the effect of ammonia on phase transitions. We show that aqueous ammonia solutions behave as regular solutions at low pressures, and as close-to-ideal solutions at pressure above 600 MPa. The computation of phase equilibria in the H(2)O-NH(3) system shows that ice III cannot exist at concentrations above 5-10 wt % NH(3) (depending on pressure), and ice V is not expected to form above 25%-27% NH(3). We eventually address the applications of this new model for thermal and evolution models of icy satellites.

Cage occupancy of methane clathrate hydrates in the ternary H2O-NH3-CH4 system.

The incorporation of ammonia inside methane clathrate hydrate is of great interest to the hydrate chemistry community. We investigated the phase behavior of methane clathrate formed from aqueous ammonia solution. Ammonia's presence decreases methane occupancy in the large cages, without definitive Raman spectroscopic evidence for its incorporation inside the structure.

Titan Lakes Simulation System (TiLSS): A cryogenic experimental setup to simulate Titan's liquid hydrocarbon surfaces.

Titan's hydrocarbon lakes play an important role in the chemistry, geomorphology, and climate of the satellite. Our knowledge of their composition relies mainly on thermodynamic modeling and assumptions based on Cassini Radar and VIMS (Visible and Infrared Mapping Spectrometer) data. Several thermodynamic models have been used to calculate the composition of these lakes, and their results on even the major lake components (methane, ethane, propane, and nitrogen) exhibit large discrepancies. Recent Cassini radar observations revealed an echo from the lake's bottom. A low loss factor of attenuation is needed within the lakes to interpret these observations, and it has been suggested that the lakes are dominated by methane. Cassini VIMS data obtained on the North Pole lakes at three-year intervals showed no detectable surface level change, which is consistent with ethane being their primary constituent. This additional discrepancy between thermodynamic models and Cassini data strongly shows the need for experimental measurements under realistic Titan conditions in order to better constrain the thermodynamic models. We designed and built a cryogenic experimental platform allowing the simulation of Titan's lakes. This facility, named Titan Lakes Simulation System (TiLSS), produces liquid hydrocarbons in equilibrium with a gas phase mimicking Titan's atmosphere. Samples of the condensed liquid are injected directly into a gas chromatograph allowing the direct measurement of its chemical components and their abundances. To test the overall operation of the system, a gas mixture of methane and ethane was condensed under 1.5 bar of nitrogen and analyzed. Results from this proof of concept test are in good agreement with experimental studies previously published.

Cometary science. Subsurface properties and early activity of comet 67P/Churyumov-Gerasimenko.

Heat transport and ice sublimation in comets are interrelated processes reflecting properties acquired at the time of formation and during subsequent evolution. The Microwave Instrument on the Rosetta Orbiter (MIRO) acquired maps of the subsurface temperature of comet 67P/Churyumov-Gerasimenko, at 1.6 mm and 0.5 mm wavelengths, and spectra of water vapor. The total H2O production rate varied from 0.3 kg s(-1) in early June 2014 to 1.2 kg s(-1) in late August and showed periodic variations related to nucleus rotation and shape. Water outgassing was localized to the "neck" region of the comet. Subsurface temperatures showed seasonal and diurnal variations, which indicated that the submillimeter radiation originated at depths comparable to the diurnal thermal skin depth. A low thermal inertia (~10 to 50 J K(-1) m(-2) s(-0.5)), consistent with a thermally insulating powdered surface, is inferred.

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.

Thermodynamic model for water and high-pressure ices up to 2.2 GPa and down to the metastable domain.

We propose a thermodynamic model of the properties of liquid water and ices I, III, V, and VI that can be used in the ranges of 0-2200 MPa and 180-360 K. This model is the first to be applicable to all H(2)O phases in these wide ranges, which exceed the stability domain of all phases. Developing empirical or semiempirical expressions for the specific volumes of liquid water or ices has been necessary. The model has been tested on available experimental data sets. The specific volume of liquid water is reproduced with an accuracy better than 1%. The error on the specific volume of ices remains within 2%. The model has also been used to describe the melting curves of high-pressure ice polymorphs and compared with new Simon equations fitting available data. Our calculations suggest a slight revision of the triple point positions in the H(2)O phase diagram. We have ensured the reliability of our model up to 1.5 GPa, and we have shown that it can be used with good confidence up to 2.2 GPa. In order to show the validity of this model in the low-temperature domains, the melting curve of ice Ih in the water-ammonia system has been modeled. This curve is reproduced with good accuracy down to 180 K, at a 1 bar pressure. It shows that this model can be used in further studies for modeling equilibriums involving liquid or solid phases of H(2)O under pressure and for investigating the effect of inhibitors in complex water-rich systems.