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 melting 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 variable v considered for heterogeneous matter. This variable, regarded as the ratio between the inner volume of the DSC pan and the initial mass of solid sample, is used to construct T-v diagrams through which melting enthalpies 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 specific volume of the vapour phase was extrapolated to 52.7 +/- 1.5 cm(3) g(-1) at triple point conditions. With this information, a more general description of the p-T diagram for adamantane is put forward. (C) 2001 Elsevier Science B.V. All rights reserved.