Elongate coexistence curve and its curvilinear diameter as factors of global fluid asymmetry

Abstract

Some inconsistencies of the conventional predictive methodologies applied in the region of vapor-liquid VLE-coexistence and its criticality are considered. As a rule, they are related to the semi-empirical concept of “rectilinear diameter” accepted in the temperature-density plane. The often curved, in practice, "rectilinear diameter” of coexistence curve (CXC) is discussable in both alternative descriptions of criticality: 1) by the Ising-based (Ib-) complete scaling phenomenology and 2) by the classical van der Waals-Maxwell-Gibbs-based (Wb-) phenomenology of VLE-transition. The latter has been essentially modified by the model of fluctuational thermodynamics (FT). The new transformation of CXC-representation based on the measurable equilibrium data obtained far away from criticality is proposed in the present work. It leads to the well-established location of critical point (CP) which corresponds to the intersection between the elongate CXC depicted in the compressibility factor-density plane and its strongly curvilinear here diameter. The universality of global fluid asymmetry (GFA)-principle introduced earlier by FT-model becomes apparent in the whole temperature range of VLE-transition. The developed predictive CP-methodology can be especially useful for the set of substances in which the direct experiment on criticality is hardly realizable.