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

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 (