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Tetra-n-butylammonium bromide forms the title semi-clathrate hydrate crystal, C16H36N+.Br-.38H2O, under atmospheric pressure. The cation and anion lie at sites with mm symmetry and seven water molecules lie at sites with m symmetry in space group Pmma. Br- anions construct a cage structure with the water molecules. Tetra-n-butylammonium cations are disordered and are located at the centre of four cages, viz. two tetrakaidecahedra and two pentakaidecahedra in ideal cage structures, while all the dodecahedral cages are empty.

Equilibrium conditions of CH 4 , CO 2 , and C 3 H 8 hydrates confined in small pores of porous glass were determined. The dissociation temperature of each hydrate at a given pressure shifted lower than that for bulk hydrate; the largest shift for CH 4 hydrate was -12.3 K ( 0.2 K for 4-nm-diameter pores and the shift decreased to only -0.5 K for 100-nm pores. CH 4 hydrate experiments at temperatures lower than the quadruple point of 270.6 K in 30-nm porous glass showed no shift of the equilibrium line. All temperature shifts were fitted by the Gibbs-Thomson equation; the best fits for CH 4 , CO 2 , and C 3 H 8 hydrates predicted hydrate-water interfacial energies of 1.7(3) × 10 -2 J/m 2 , 1.4(3) × 10 -2 J/m 2 , and 2.5(1) × 10 -2 J/m 2 , respectively. Both type-I hydrates of CH 4 and CO 2 had interfacial energies within 20% of each other but significantly smaller than the type-II hydrate of C 3 H 8 . Ice formation in the same porous glass fit the Gibbs-Thomson relation with an interfacial energy of 2.9(6) × 10 -2 J/m 2 , which is in good agreement with established values. The estimated interfacial tensions between gas hydrates and water were found to be only weakly affected by the kinds of gas. This indicated that the pore effect on the phase equilibrium was mainly due to the water activity change. The wide range of experiments on pore size, temperature, and the kind of gas allowed us to evaluate the validity of previous model predictions for pore effects on gas hydrate stability.

The dissociation conditions of methane hydrates in confined small pores were measured by the gradual temperature increase method. Significant downward shifts of the dissociation temperature were observed in porous glasses, which had small pores ranging from 100 to 500 Å in diameter, compared with that of the bulk hydrate at a given pressure. Systematic measurements revealed that the temperature offset was in inverse proportion to the pore diameter. The Arrhenius plot of the dissociation conditions suggests that the heat of methane-hydrate dissociation tended to be small compared to that of bulk hydrates in pores smaller than 300 Å in diameter. Applying the Gibbs−Thomson effect to the quantitative analysis of the phenomenon indicated that the dissociation condition of methane hydrates in small pores shifted because of changes in the water activity. The apparent interfacial free energy between methane hydrates and water in the confined condition was estimated to be approximately 3.9 × 10-2 J m-2, which is comparable to that between ice and water in the similar condition.

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This paper reports confirmation of structure II hydrate formation in a methane−2-propanol−water system, which was previously suggested by Østergaard et al. (Ind. Eng. Chem. Res. 2002, 41, 2064−2068) based on a comparison of the phase-equilibrium data with corresponding statistical-thermodynamics predictions. A hydrate crystal sample was prepared with a 16.4 mass % aqueous solution of 2-propanol pressurized with methane and then subjected to a powder X-ray diffraction analysis. The X-ray diffraction pattern thus obtained from the sample indicated that the crystallographic structure of the hydrate was structure II. The pressure−temperature data for aqueous liquid−hydrate−methane-rich vapor three-phase equilibrium in a temperature range from T = 273 to 283 K are also reported.

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