Analysis of rotator phase transitions in the linear alkanes hexacosane to triacontane by adiabatic scanning calorimetry and by photopyroelectric calorimetry

The study of the nature of various phase transitions between rotator phases in several linear alkanes was performed by analyzing the hysteretic behavior of the specific heat between heating and cooling measurements. The investigations have been carried out by both adiabatic scanning calorimetry and photopyroelectric calorimetry techniques, whose combined use has provided complementary information concerning the changes occurring in the samples' structure during their temperature change. The study enabled to establish that, unlike what previously reported, the Riii-Rivand the Rii-Rivtransitions are of first order, despite them not showing any sharp peak profile in the temperature dependence of the specific heat. The first order of the Rv-Riitransition was confirmed also in the present study and, finally the Rv-Riiitransition could be observed by calorimetric detection for the first time and shown to be of second order. The obtained results were discussed in terms of order parameters power terms in the Landau free energy expansion.