Supercooling suppression in the tetrahydrofuran clathrate hydrate formation

Abstract

Generally, supercooling as large as about 25 K is necessary to form tetrahydrofuran clathrate hydrate (THF hydrate). In the present study, we have investigated how the existence of various additives affects the degree of supercooling (ΔT) in the THF hydrate formation without any external stimulus. The addition of either AgO or Ag₃PO₄ yields the best ΔT suppression, where ΔT was approximately 4 K, much smaller than 25 K. Raman spectra reveal that, in the supercooled THF aqueous solution with AgO before crystallization, the THF molecule forms a coordination bond with AgO, whereas it forms a hydrogen bond with a water molecule in the system without AgO. The oxygen atom of the THF molecules, which is a hydrogen-bonding site, was oriented to a AgO molecule in the aqueous solution. The orientation and the subsequent interruption of the hydrogen bond make it easy to form a hydration shell around the THF molecule. Scanning Transmission Electron Microscopy images by using the freeze-fracture replica method reveal that a cluster, which is the smallest structural unit of a crystal, includes Ag-containing nanoparticles formed from AgO or Ag₃PO₄, which accelerate the cluster formation. As a result, the addition of AgO or Ag₃PO₄ effectively diminishes the degree of supercooling in the THF hydrate formation.