n-Dodecane melting studied by the combined use of different calorimetric modes

The solid–liquid phase transition of n-dodecane has been studied with a modulated adiabatic scanning calorimeter (MASC). The well-known features of melting — such as the existence of a pre-melting region and the dependence of the melting curve on the scanning rate — were observed, together with completely new features which, to the best of our knowledge, have never been reported before, such as the presence of a small secondary, non-reproducible peak in the melting curve. This last result was obtained working in the scanning operation mode, at rates lower than 3.3×10−4 K s−1. The complex melting process occurring in the sample was studied inside the melting temperature interval, using the temperature step scanning and the adiabatic-like enthalpy step operation mode of MASC. A very slow rate of attainment of equilibrium was observed when the liquid and solid phases were co-existing. Temperature modulation, in both the continuous scanning mode and the quasi-isothermal step-scanning mode, was also used to obtain a clearer characterisation of the melting kinetics. The melting transition of n-dodecane is discussed in the light of the results obtained by applying to one and the same sample different operation modes. The transition enthalpy values measured are compared to the data in the literature. The two peaks observed in the melting curve of n-dodecane are analysed and possible explanations for the second peak are discussed.