Xe Hydrate Research Articles

Ordering and Clathrate Hydrate Formation in Co‐deposits of Xenon and Water at Low Temperatures

AbstractLocal ordering in co‐deposits of water and xenon atoms produced at low temperatures can be followed uniquely by 129Xe NMR spectroscopy. In water‐rich samples deposited at 10 K and observed at 77 K, xenon NMR results show that there is a wide distribution of arrangements of water molecules around xenon atoms. This starts to order into the definite coordination for the structure I, large and small cages, when samples are annealed at ∼140 K, although the process is not complete until a temperature of 180 K is reached, as shown by powder Xray diffraction. There is evidence that Xe⋅20 H2O clusters are prominent in the early stages of crystallization. In xenon‐rich deposits at 77 K there is evidence of xenon atoms trapped in Xe⋅20 H2O clusters, which are similar to the small hydration shells or cages observed in hydrate structures, but not in the larger water clusters consisting of 24 or 28 water molecules. These observations are in agreement with results obtained on the formation of Xe hydrate on the surface of ice surfaces by using hyperpolarized Xe NMR spectroscopy. The results indicate that for the various different modes of hydrate formation, both from Xe reacting with amorphous water and with crystalline ice surfaces, versions of the small cage are important structures in the early stages of crystallization.

Phase transition from structure-H to structure-I in the methylcyclohexane + xenon hydrate system

The methylcyclohexane+Xe hydrate system has been investigated in a temperature range from 274 to 291 K and pressure up to 1 MPa. The structure-H (s-H) type hydrate changes to the structure-I (s-I) type of Xe hydrate at 281.5 K. The overall enthalpy of s-H hydrate formation has been evaluated using the Clapeyron equation. The transition enthalpy change from s-H to s-I is estimated to be about 70 kJ/mol for unit amount of s-H hydrate. Neither cis-1,3-dimethylcyclohexane nor cis-1,4-dimethylcyclohexane generates the s-H hydrate in the presence of Xe.

Phase behavior of xenon hydrate system

The thermodynamical stability boundaries for the Xe hydrate system were investigated in a pressure range up to 70 MPa and a temperature range from 290 to 320 K. In the critical temperature series of the guest molecules, the Xe hydrate is identified to be the border hydrate system having no quadruple point of (hydrate+two liquids+gas). The three-phase coexisting curve of (hydrate+liquid Xe+gas) terminates at the critical end point (289.76 K, 5.86 MPa) of (hydrate+liquid=gas) very close to the critical point of pure Xe fluid. The three-phase coexisting curve of (hydrate+water+gas) exhibits the characteristic ‘S-shape’ as if there would exist the quadruple point. The enthalpy change of Xe hydration was also evaluated from the Clausius–Clapeyron equation; the average value of 65 kJ/mol is almost constant in the whole temperature range along with the three-phase coexisting curve.

A novel approach to the stability of clathrate hydrate

The thermodynamic stability of clathrate hydrates I and II encaging Xe and Ar has been investigated by examining temperature dependence of the dissociation pressure on the basis of the generalized van der Waals and Platteeuw theory developed by Tanaka and Kiyohara, where the coupling of the host lattice vibration with the guest is taken into account in calculating the free energy of formation of hydrates. The predicted dissociation pressures of Xe and Ar hydrates agree reasonably well with experiments in higher temperature range. An agreement for Ar clathrate hydrate at low temperature is poor. This is circumvented by the use of a quantum mechanical partition function for a harmonic oscillator in evaluating the free energy difference between ice and empty hydrate.

The Stability of Clathrate Hydrates: Temperature Dependence of Dissociation Pressure in Xe and Ar Hydrate

Abstract The thermodynamic stability of clathrate hydrates I and II encaging xenon or argon has been investigated by examining the temperature dependence of the dissociation pressure. The evaluation of the stability is made based on the generalized van der Waals and Platteeuw theory developed by Tanaka and Kiyohara [J. Chem. Phys. 98, 4098 (1993)]. In the new treatment, the free energy of formation of hydrates in equilibrium with ice is calculated by taking the coupling of the host lattice vibrations with guests into consideration. The predicted dissociation pressures of Xe and Ar hydrates agree well with experiments in higher temperature range. A poor agreement between experiment and calculation for Ar clathrate hydrate at low temperature is improved by the use of a quantum mechanical partition function for a harmonic oscillator in evaluating the free energy difference between ice and empty hydrate.