Morphological Change in Structure H Clathrates of Methane and Liquid Hydrocarbon at the Liquid-Liquid Interface

The crystal morphology of a structure H (sH) hydrate in a methane (CH4)-liquid hydrocarbon (methylcyclohexane and neohexane)-water system in the temperature range 273.4-277.3 K was investigated in situ by optical scanning microscopy. Near the phase equilibrium condition, the sH hydrate showed a hexagonal shape with respect to its crystalline space group (P6/mmm). As the pressure increased under constant temperature, the edges of the hexagonal sH hydrate grew, and then the sH hydrate changed from a hexagonal to three-dimensional six-branched shape. We found that the branching growth of the sH hydrate appears at a specific degree of sub cooling, Delta T > 4 K. This indicates that the sH hydrate with a snowflakelike crystal shape can be obtained by controlling Delta T.