The effect of gas pressure on the melting behavior of compounds

The melting and crystallization behavior of pure compounds under gas pressure was experimentally investigated using a scanning transitiometer, which is a sensitive twin calorimeter combined with a computer driven PVT control. This technique allows to determine different thermodynamic effects occurring with changes of state. While one of the three state variables pressure P, volume V, or temperature T is kept constant, another one is changed with time, both the enthalpy effect and the change of the remaining third state variable can simultaneously be measured with high precision. Thermodynamic equilibrium can be achieved during the entire process because the state is changed very slowly. The solid–liquid phase change of dimethylterephthalate (DMT) and biphenyl was investigated under He, CO₂, and N₂O pressure up to 30 MPa, and the SLG three phase lines were determined. The solid–liquid phase change appeared unexpectedly as a pair of two different heat signals.