Kinetics of Trifluoromethane Clathrate Hydrate Formation from CHF₃ Gas and Ice Particles

We report the formation kinetics of trifluoromethane clathrate hydrate (CH) from less than 75 μm diameter ice particles and CHF₃ gas. As previously observed for difluoromethane and propane hydrate formation, the initial stages of the reaction exhibit a strong negative correlation of the reaction rate with temperature, consistent with a negative activation energy of formation. The values obtained for trifluoromethane, ca. -6 kJ/mol (H₂O), are similar to those for difluoromethane, even though the two molecules have different intermolecular interactions and sizes. The activation energy is lesser per mole of H₂O, but greater per mole of guest molecule, than for propane hydrate, which has a different crystal structure. We propose a possible explanation for the negative activation barrier based on the stabilization of metastable structures at low temperature. A pronounced dependence of the formation kinetics on the gas flow rate into the cell is observed. At 253 K and a flow rate of 15 mmol/h, the stage II enclathration of trifluoromethane proceeds so quickly that the overpressure, the difference between the gas cell pressure and the hydrate vapor pressure, is only 0.06 MPa.