Molar isopycnicity in heterogeneous binary mixtures

Abstract

Heterogeneous mixtures are said to be in molar isopycnic phase equilibrium when – at least – two of the constituent phases exhibit the same molar volume v or, equivalently, the same molar density ρ. Consequently, the molar densities of the constituent phases experiment inversion (or molar barotropy), thus yielding an interesting singularity that may be observed in both super- and sub-critical equilibrium conditions.This contribution aims to theoretically characterize the molar isopycnic behavior observed in binary mixtures. Accordingly, theoretical predictions and strategically guided molecular simulations of the Lennard–Jones fluid are performed to describe the phase behavior and the interfacial properties of mixtures characterized by molar density inversions. Necessary and sufficient conditions are then deduced to detect molar isopycnicity in binary mixtures and, finally, these limiting conditions are related to the various classes of global phase behavior.Our results indicate that molar isopycnicity is present in mixtures exhibiting Types II, III, IV and V behavior. It follows then that molar density inversions seem to be more common than previously believed and that liquid phase immiscibility provides a natural condition for its existence. It is also observed that the phenomenon is controlled by differences in molecular size and critical properties. Particularly, molar isopycnicity becomes observable in ranges where stable fluid phases can be observed as the constituents’ molecular size ratio increases. Differences in critical properties, in turn, promote wider temperature ranges of molar density inversions. The main classes and mechanisms that constraint molar isopycnic equilibria are illustrated and exemplified by considering binary van der Waals mixtures.