Effect of guest molecule size on the thermal conductivity and heat capacity of clathrate hydrates

Using the transient hot-wire method, the authors have measured both the thermal conductivity, lambda , and the heat capacity per unit volume, rho c p , for structure II clathrate hydrats of 1,3-dioxolane (DO) and cyclobutanone (CB), and compared the results with similar previous measurements for tetrahydrofuran (THF). All three clathrate hydrates showed a glass-like temperature dependence of lambda . The magnitude of lambda was small, about the same as liquid water, and decreased in the sequence DO-THF-CB which corresponds to an increase in the size of the guest molecule. There was some evidence to support the suggestion that the dominant process in the thermal resistivity was associated with excitation of rotation vibrations of the guest molecules. In all cases, the pressure dependence of lambda was unusually small. The authors found evidence that there was a phase transition on the CB clathrate hydrate (and almost certainly also in the DO clathrate) at 135K and 1 GPa, which are essentially the same conditions under which they previously found an analogous transition for THF. In each case, the resultant presumably metastable phase may be a new type of clathrate hydrate arising from densification of the host structure. For the three clathrates, the authors results for rho c p did not depend on the nature of the guest species, and at 0.1 GPa, were the same as previously found for ice Ih.