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Gas clathrate hydrates or gas hydrates are made of H-bonded water molecules forming cages, within which gaseous (guest) molecules are encapsulated. The formed clathrate structures, which may be metastable, depend on the nature and on the partitioning of the guest molecules in the water cage. This work focuses on the structural and vibrational properties of nitrogen hydrate in its two clathrate forms (namely, SI and SII) in the thermodynamic ranges 50–200 bar and 150–270 K, together with a comprehensive analysis of the transformation from SI to SII of this gas hydrate. The thermal expansion of both structures has been measured at 1 bar, and the melting of the nitrogen hydrate has been measured at ca. 210 K at 1 bar. Moreover, the SI structure is metastable in the studied pressure region: from time-dependent neutron powder diffraction analysis, it is shown that the SI structure transforms over time to the SII structure with a rate of (1.37 ± 0.17) × 105s–1at 100 K and at 1 bar. The transformation is also characterized by an induction time (i.e., the lifetime of the pure SI structure) of 0.49 day. We have also investigated the guest partitioning of the nitrogen hydrate using high-resolution Raman scattering. Vibrational bands of nitrogen molecules encapsulated in large cages are measured at lower wavenumbers than the one associated with encapsulation in small cages (by 1.1 cm–1in SI and 0.8 cm–1in SII). In the case of the thermodynamically stable SII phase, the dependence of the guest partitioning has been characterized as a function of the pressure–temperature conditions. Variation of the relative cage filling is demonstrated. While the small cages remain singly occupied according to previous neutron diffraction analysis, this variation is attributed to large cages of the nitrogen hydrate that easily catch or release nitrogen guest molecules. This study thus provides new opportunities for preparing nitrogen gas hydrates with a “targeted” structure and relative cage filling not only by varying the pressure and temperature but also by playing with the structural metastability.

In addition to their various applications in the environment, energy and technology fields, gas clathrate hydrates, called gas hydrates (inclusion compounds composed of a H-bonded water network forming cages, inside of which gaseous guest molecules are trapped), are hypothetically involved in the formation of planetesimals, comets, and other planets (e.g., the Saturn’s moon or the Jupiter-family comets). In these issues, CO and N2molecular species often occur. In this work, we have elucidated the molecular selectivity and the guest partitioning of the CO and N2molecules within mixed CO–N2gas hydrate (the two guest species are encapsulated within the cages) as a function of the gas mixture composition at 270 K and 200 bar. By means of micro-Raman spectroscopy, it is shown that CO molecules are preferentially encapsulated within the hydrate cages, with a better selective efficiency for gas mixture below 35% CO in N2gas. In addition, the present Raman investigation gives clear indications about the formation of both SI and SII clathrate structures, depending on the gas mixture composition: below 35% CO, the mixed hydrate is mainly of type SII, whereas above this limit, the formed mixed hydrate is mainly of type SI. Moreover, no preferential CO/N2fractioning in the various types of hydrate cages is observed. In addition to the interest for gas separation technology (e.g., flue gas), such experimental results corroborate, for the first time, theoretical and modeling prediction, showing that the formation of clathrate hydrate could explain the potential N2depletion observed in Jupiter-family comets (e.g., 67 P/Churyumov-Gerasimenko).

Confocal Raman microspectrometry is used to probe, for the first time, the transport of guest molecules along the one-dimensional tunnels in a crystalline urea inclusion compound under conditions of guest exchange in which new guest molecules are introduced simultaneously at bothends of the urea tunnel structure. We focus on the system comprising 1,8-dibromooctane as the original type of guest and pentadecane as the new type of guest, and results are presented for experiments in which the guest exchange process is probed both ex situ and in situ. The Raman data, recorded as a function of position along the tunnel direction and, in the case of the in situ experiments, as a function of time, demonstrate that pentadecane guest molecules enter the tunnels at both ends of the crystal and that transport of guest molecules occurs in bothdirections along the crystal. Mechanistic aspects of this bidirectional transport process are discussed, particularly in relation to the corresponding process for unidirectional transport of guest molecules in urea inclusion compounds reported previously.