Fluid mixtures at high pressures VII. Phase separations and critical phenomena in 12 binary mixtures containing ammonia

Abstract

Critical p( T) curves have been measured for the twelve mixtures: helium+, hydrogen+, nitrogen+, argon+, carbon monoxide+, sulfur hexafluoride+, ethene+, propene+, methylpropene+, benzene+, cyclohexane+, and methanol + ammonia. The six mixtures of (a hydrocarbon + ammonia) and of (methanol + ammonia) exhibit an uninterrupted critical curve; and the other six mixtures, an interrupted critical curve. As is expected (sulfur hexafluoride + ammonia) displays barotropic behaviour of the two liquid phases on the llg curve shortly before the (liquid + liquid) upper critical end point (ll-UCEP) is reached. This is always the case, when the substance with the lower critical temperature in a binary mixture with an interrupted critical curve has a higher density than that with the higher critical temperature. Another barotropic effect occurs for (sulfur hexafluoride + ammonia), and for (argon + ammonia) in the high-temperature and high-pressure range. That can always be expected, when the more volatile substance with the higher liquid density attains liquid-like densities at supercritical temperatures, if the pressure is raised until the vapour-phase density of the mixture becomes greater than the liquid-phase density.