Formation and Growth of Tetrahydrofuran Hydrate at the Ice/Hexane Interface

The formation and growth of the tetrahydrofuran (THF) clathrate hydrate (CH) are studied at the interface between ice and hexane containing THF by a confocal Raman microscopy. Raman spectroscopic measurements across the ice/hexane interface have clearly detected. the formation of the THF CH under an appropriate condition. The ring breathing mode of THF found around 915 cm(-1) is the most efficient probe for a change in the molecular environment of THF; we can distinguish the THF CH, THF in hexane, and THF in water on the basis of the band shift of this mode. The temperature is a critical parameter governing the formation of the THF CH. The THF CH is formed at -2.0 degrees C, and its layer thickness increases with time; a layer thickness of ca. 30 mu m is reached 160 min after the contact of ice with THF/hexane. This has been supported by the analysis of the Raman spectra in the higher wavenumber range as well. In contrast, an induction time longer than 100 min is necessary for the THF CH formation at -2.5 and -5.0 degrees C, and the CH formation is not confirmed at -10 degrees C. This temperature dependence is closely related to the interfacial quasi liquid layer (QLL), which was detected by ice chromatographic experiments in our previous work According to the ice chromatographic study, an interfacial QLL of several nanometers in thickness emerges at a temperature higher than -2.0 degrees C and is developed as the temperature increases. The presence of this structurally perturbed thin layer is required for the facile nucleation of the THF CH on the surface of ice, and -2.0 degrees C is the critical temperature for the entire process to occur within hours.