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 mod ulated adiabatic scanning calorimeter (MASC). The well-known features of me lting - such as the existence of a pre-melting region and the dependence of the melting curve on the scanning rate - were observed, together with comp letely new features which, to the best of our knowledge, have never been re ported before, such as the presence of a small secondary, non-reproducible peak in the melting curve. This last result was obtained working in the sca nning operation mode, at rates lower than 3.3 x 10(-4) K s(-1). The complex melting process occurring in the sample was studied inside the melting tem perature interval, using the temperature step scanning and the adiabatic-li ke 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 character isation of the melting kinetics. The melting transition of n-dodecane is di scussed in the light of the results obtained by applying to one and the sam e 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. (C) 2000 Elsevier Science B.V. All rights reserv ed.