Dielectric study of KOH-doped acetone and trimethylene oxide clathrate hydrates

Abstract

Dielectric permittivities of KOH( x = 1.8 × 10 −4)-doped acetone and trimethylene oxide (TMO) structure II clathrate hydrates were measured in the 15–250 K temperature range and the 20–10 6Hz frequency range. In the acetone hydrate, the dielectric dispersion associated with water reorientation in the high temperature phase suddenly disappeared at the transition temperature (47 K) and no more dielectric dispersion was observed below that temperature. This indicates that both acetone and water molecules are ordered in the low temperature phase. On the other hand, the dielectric dispersion due to the guest reorientation in TMO hydrate was still observed below the transition at 35 K, showing that the TMO molecules are still orientationally disordered in the low temperature phase. From the analyses of the dielectric relaxation times, the activation enthalpy for the TMO reorientation in the KOH-doped hydrate was larger than that in the pure one, indicating that the TMO molecules in the low temperature phase are bound more tightly than those in the high temperature phase. The distribution of the relaxation times of the TMO reorientation was not affected significantly by the KOH-doping, both being very broad and exhibiting negative temperature dependence. For the water reorientation, the relaxation time and the associated activation enthalpy did not depend appreciably on the guest molecules while the distribution of the relaxation times was guest-dependent; the TMO hydrate had larger distribution than the acetone hydrate. It was also found that the distribution of the relaxation times observed over a wide range of temperatures extending above 200 K became broad progressively as the temperature was lowered.