Interferometric observation of salt concentration distribution in liquid phase around THF clathrate hydrate during directional growth

The directional growth experiments of tetrahydrofuran (THF) clathrate hydrate were carried out from the solution. The diffusion processes of solutes (THF and salt) in the solution were observed near the growth interfaces using a Mach-Zehnder interferometer. During the growth of hydrate from off-stoichiometric solution, the transport of guest molecules (THF), caused by the concentration difference between the solid and liquid, was clearly observed near the interface. During the growth from THF-17H₂O solution, the THF concentration was uniform in the solution even near the interface. The composition was found to be stoichiometric for hydrate formation. The effect of salt as the inhibition of hydrate formation was investigated using the stoichiometric solution with the addition of 3 wt% sodium chloride. Analyzing the interference fringes, the salt concentration distribution was obtained around the growing hydrate. The salt concentration increased with time due to the rejection of salt at the growth interface and decreased sharply with the distance from the interface into the bulk concentration. Theses results lead to the conclusion that the freezing temperature of hydrate is considerably reduced by the local increase of salt concentration. Finally, it should be emphasized that the present method is very useful for the research of fundamental process for hydrate formation, i.e. mass transport process in the solution.