High-pressure phase behaviour of the binary systems (propane + adamantane) and (ethane + adamantane)

(Solid+liquid) and (vapour+liquid) equilibria for the binary systems (propane+adamantane) and (ethane+adamantane) were determined according to the synthetic method in the temperature range 300–510 K and in the pressure range 0.3–10 and 0.3–180 MPa, respectively. For both systems also the three-phase curve (solid adamantane+liquid+vapour) was determined and some vapour–liquid critical points at low mole fractions of adamantane. In the case of (propane+adamantane) the three-phase curve (solid adamantane+liquid+vapour) is continuous and shows a pressure maximum at a temperature higher than the critical temperature of propane. In the case of (ethane+adamantane) the three-phase curve (solid adamantane+liquid+vapour) is split into a low-temperature branch which runs from low temperature with a positive slope dP/dT to higher temperature and ends at a first critical endpoint at T=309.55 K and P=5.17 MPa. The high temperature branch of this curve starts at the second critical endpoint T=432.65 K and P=15.85 MPa, runs to higher temperature with a negative slope dP/dT and ends at the triple point of adamantane.