Thermodynamic studies of solids with non-negligible vapour pressure: T-v and p-T diagrams of the dimorphism of adamantane

A differential scanning calorimetry method, designed to determine the melti ng enthalpy of compounds with non-negligible vapour pressure, is applied to adamantane. This method is based upon the linear dependence of the melting -related thermal effect with the specific volume, i.e. the thermodynamic va riable v considered for heterogeneous matter. This variable, regarded as th e ratio between the inner volume of the DSC pan and the initial mass of sol id sample, is used to construct T-v diagrams through which melting enthalpi es at the triple point conditions are obtained. A T-v diagram for adamantane is presented and Delta H-fus of +(12.4 +/- 0.3 ) kJ mol(-1) at T-fus = 543 +/- 2 K was found for this compund. The specifi c volume of the vapour phase was extrapolated to 52.7 +/- 1.5 cm(3) g(-1) a t triple point conditions. With this information, a more general descriptio n of the p-T diagram for adamantane is put forward. (C) 2001 Elsevier Scien ce B.V. All rights reserved.