Differential scanning calorimetry studies of clathrate hydrate formation

Differential scanning calorimetry was applied to investigate the formation of tetrahydrofuran (THF) and cyclopentane (CP) clathrate hydrates. An emulsion technique was developed to determine the hydration number and the enthalpy of CP hydrate dissociation to liquid water and liquid cyclopentane, which were found to be 16.8 +/- 0.7 and 82.3 kJ/mol CP, respectively. The hydration number corresponds to the theoretical number of 17 if all of the large cavities in the structure II hydrate are occupied. Experiments with water-in-CP emulsions did not identify homogeneous nucleation for the hydrate since ice and hydrate always formed together on supercooling. With water-in-heptane emulsions, for which no hydrate forms, the homogeneous ice supercooling limit was -37.8 degreesC. With the CP emulsion, ice and hydrate formed close to this temperature. For bulk THF/H2O solutions, even at the hydrate stoichiometric molar ratio of 1:17, ice and hydrate were found to form in the same crystallization event upon cooling, which impeded the identification of the homogeneous nucleation temperature of the hydrate. To overcome this, a method based on emulsification of stoichiometric THF/water solutions in an immiscible fluorocarbon fluid enabled determination of the homogeneous nucleation of THF hydrate without the formation of ice. The homogeneous THF hydrate nucleation temperature was -32 degreesC. We believe this is the first measurement of homogeneous nucleation of a clathrate hydrate. Our experimental results suggest that ice is a good nucleating agent for heterogeneous formation of clathrate hydrates, whereas the hydrates are not efficient nucleators for ice.