Limiting conditions for the critical pressure step point behavior in the Global Phase Diagram of binary mixtures composed by spherical molecules of equal sizes

Although many complex phase diagrams have been clearly systematized by means of the Global Phase Diagram (GPD) approach, no previous analysis has been devoted to establishing the limiting conditions of the critical pressure step point (CPSP) behavior for the case of binary mixtures. The CPSP mechanism implies a stationary inflection in the critical pressure of a mixture which can be observed along the PT projection of the gas–liquid critical line, although in ranges where (usually) a single critical phase exists. Due to this latter reason the CPSP cannot be detected using a phase stability analysis approach, which is the common tool used to delimiting mechanisms of phase behavior, but by clear geometric singularities of other key properties of fluid mixtures, such as the enthalpy. The scope of this work is to characterize the influence of the CPSP mechanism on the topology of phase diagrams over the whole range where the quoted mechanism exists and persists, in order to elucidate some gaps that still exist regarding the global phase behavior of binary mixtures. Accordingly, two new parametric regions of the GPD are described in this contribution, namely the goblet and the crown regions, where the effect of the CPSP on the fluid phase behavior can be clearly observed in systems that exhibit negative deviation from ideal behavior (mainly Type I and V systems) and in mixtures that are in the vicinity of the shield region.