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The three-phase coexistence curve of methane hydrate + saturated water + saturated fluid CH 4 was investigated in the temperature range from 305 to 321 K and pressure range from 98 to 500 MPa. The equilibrium curve increases monotonically on a T-p diagram at these experimental conditions. The Raman spectra of the C-H symmetric vibration mode in the methane hydrate split into two peaks, while a single peak is detected in the fluid CH 4 and water phases. The split of the Raman peak indicates that two kinds of hydrate cages are occupied by the CH 4 molecules. The peak intensity ratio of two types of CH 4 molecules is almost independent of pressure in the range up to 500 MPa; that is, the cage occupancy ratio is constant. The Raman spectrum for the intermolecular vibration mode (O-O stretching) of the water molecules changes linearly with pressure from 207 to 228 cm -1 , and the Raman shifts of the C-H vibration mode in the S-cage and in the water phase vary linearly with pressure from 2915 to 2919 cm -1 and from 2910 to 2916 cm -1 , respectively. On the other hand, the Raman shift of C-H vibration in the M-cage is nearly constant at the lower frequency. That is, the hydrate cage shrinkage is caused by a pressure increase; however, the M-cage contains adequate vacant volume for the CH 4 molecule.

The three-phase coexistence curve of CO 2 hydrate + saturated water + saturated liquid CO 2 was investigated in the temperature range from 289 to 294 K and pressure up to 500 MPa. The temperature maximum point on the equilibrium curve was confirmed at 294 K and 328 MPa. The sign of (dp/dT) of the three-phase coexistence curve changes from positive to negative at this boundary point. The equilibrium curve continues up to the four-phase coexistence point, which lies at a slightly higher pressure than that of melting curve of CO 2 . The laser Raman microprobe spectroscopic analysis reveals that the stretching and bending vibration energies of CO 2 entrapped into the hydrate cage are lower than that of pure CO 2 at the same temperature and pressure. The Raman spectrum for the intermolecular vibration mode (O-O stretching) of water molecules was detected around 200 cm -1 in the hydrate phase. The pressure dependence of the Raman shift reveals that the hydrate lattice constructed of water molecules is shrunk by a pressure increase. The Raman spectroscopy suggests that the hydrate-cage-like structure is provided previously in the aqueous solution in equilibrium.

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We report the experimentally determined phase diagram for an aqueous solution of isotactic-rich poly(N-isopropylacrylamide) (PNiPAM) composed of the sol-gel transition curve and the cloud-point curve. The meso diad content of isotactic-rich PNiPAM is 64%, and it is soluble in water at low temperatures, but undergoes a sol-to-gel transition with increasing temperature in the investigated concentration range of 1.8 wt. %-6.0 wt. %. With a further increase in temperature, the system becomes turbid. The gel formation and clouding behavior are thermally reversible. This is the first observation of thermoreversible gelation under the cloud-point temperature for an aqueous solution of PNiPAM. On the basis of the determined phase diagram, we carried out light scattering experiments to characterize the sol-gel transition behavior as a function of temperature.