Experimental study of the structure I clathrate hydrate of trimethylene oxide by adiabatic calorimetry

Abstract

The structure I clathrate hydrate of trimethylene oxide (TMO), of composition 6TMO:46H2O, has been studied by adiabatic calorimetry over the temperature range 85–270 K. A thermal anomaly, with the appearance of a classical λ-transition, was observed between about 140–170 K. This anomaly was reproducible, provided precautions were taken against supercooling. No evidence for such a transition has been observed in previous studies using dynamic relaxation techniques (dielectric and nmr relaxation). The temperature range of this anomaly is close to the melting point of a known TMO-rich eutectic, but evidence is presented which argues against interpretation of the present observations in these terms. The previous relaxation measurements indicated a transition involving mobility of the TMO guest, centred at about 100 K. In the present work, the only evidence of this transition consisted of a much worse (order of magnitude) precision in the measured heat capacities in this temperature range. Thermal equilibration is known to be extremely slow in this system. The transition observed in the present work at higher temperatures must therefore correspond to some restructuring of the water host, and this may be connected with anomalous results obtained previously for the far infrared absorptions of water in the TMO clathrate hydrate.