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 ΔfusH of +(12.4±0.3) kJ mol−1 at Tfus=543±2 K was found for this compound. The specific volume of the vapour phase was extrapolated to 52.7±1.5 cm3 g−1 at triple point conditions. With this information, a more general description of the p–T diagram for adamantane is put forward.