Anomalous behaviour of the specific heat in the urea/n-heptadecane inclusion compound around the phase transition

Abstract

The specific heat of urea/n-heptadecane was measured by adiabatic and AC calorimetry from 80 K to 330 K. The experimental curves show that the phase transition undergone by this crystal is characterised by a double peak anomaly centred around T = 158.9 K. The experimental resolution around this temperature has been improved by numerous heating thermograms with rates varying from 0.1 °C/h to 1.9 °C/h. Below 1 °C/h the same double peak is found but, as the heating rate is lowered, the profile of the specific heat curves progressively changes and exhibits two additional small anomalies at T = 158.35 K and T = 158.85 K. Moreover, a small thermal hysteresis (<0.25 °C) which affects the two peaks is also observed, suggesting the first order character assigned to this phase transition from birefringence measurements and by the study of phase boundaries. In addition to these results, the specific heat is found to depend on the thermal history. Our experimental study thus leads to the conclusion that the ordering of the alkane molecules in the low temperature phase takes place in successive steps and is associated with long thermal relaxation times. Finally, the enthalpy and the entropy of the phase transition have been determined.