8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Thu 811:10-14:00P8.25Enthalpy and heat capacity measurements by adiabaticscanning calorimetry of some pure alkanes: melting,solidification and supercoolingJan Leys, 1 Christ Glorieux, 1 and Jan Thoen 11 Laboratorium voor Akoestiek en Thermische Fysica, Katholieke Universiteit Leuven,Celestijnenlaan 200D 3, Leuven, BelgiumAdiabatic scanning calorimetry (ASC) [1] allows a direct determination of the enthalpy from theconstant power applied to the sample and the temperature evolution of the sample. Moreover,numerical differentiation of this temperature-time evolution leads to the heat capacity. The constantpower and consequently the slow effective scanning rate near and during phase transitionsallow a very high resolution in the temperature dependence of the enthalpy. The enthalpy ofsome alkanes between tetradecane (C14) and tetracosane (C24) is studied. Below the melting,most alkanes in this range show rotator phases before crystallizing. Some of these can only bereached by supercooling the sample [2]. Therefore, ASC was applied in heating and coolingscans at slow rates (0.5-2 K/h in the liquid phase). Information on the supercooling was obtained:the major transitions supercooled by 1-6 K, but exceptions were seen. The dependence on thecooling rate was not pronounced. The enthalpy profiles showed that the transitions are first order;the largest latent heats are for the rotator-liquid and crystal-rotator transitions. The latent heatsof the different rotator-rotator transitions are much smaller. For example, in C24, upon heating,the crystal-rotator and rotator-liquid transitions are detected, both have a large latent heat. Uponcooling, the liquid-rotator transition did only supercool by 0.1 K in this case (being an exception),but the rotator-crystal transition supercooled by some 5.5 K; in this supercooled region, tworotator-rotator transitions were observed, which were weakly first-order transitions.[1] High Resolution adiabatic scanning calorimetry and heat capacities. J. Thoen, In: Heatcapacities: liquids, solutions and vapours, E. Wilhelm, T. M. Letcher, Eds. (The Royal Society ofChemistry, London, 2010), pp. 287-306[2] E. B. Sirota, H. E. King, D. M. Singer, H. H. Shao, J. Chem. Phys. 98, 5809 (1993); E. B.Sirota, D. M. Singer, J. Chem. Phys. 101, 10873 (1994)25