Structure and stability of ethane hydrate crystal

The three-phase coexistence curve of the ethane hydrate+saturated water+saturated liquid ethane was investigated in the temperature range from 290 to 324 K and pressure range from 20 to 479 MPa. The equilibrium curve increases monotonically on a T-p diagram at these experimental conditions. The Raman spectra of the C–C symmetric stretching vibration mode of the C2H6 molecule split into two peaks, while a single peak is detected in the liquid ethane and aqueous phase. The split of the Raman peak reveals that the C2H6 molecules are entrapped in both the S- and M-cages of hydrate lattice, although the ethane hydrate has been believed as a typical gas hydrate which consists of filled M-cage and empty S-cage. The C–C vibration peak of ethane in the S-cage is very weak in a low pressure region and gradually increases in the high pressure region, that is, the degree of occupancy is considerably high at high pressures. The C–C vibration energy in the M-cage is almost independent of pressure while those in the aqueous solution and in the S-cage increase with pressure. The O–O vibration energy of the ethane hydrate lattice shows very weak pressure dependence while those of methane and CO2 hydrates are increased clearly with pressure.