A topological approach to mass barotropic phenomena in asymmetric mixtures

Mass density inversion, a phenomenon also known as mass barotropy, corresponds to a singular behavior that directly affects the relative position of phases in heterogeneous mixtures when exposed to gravitational fields. In this contribution we introduce a topological description of the mass density inversions observed in asymmetric binary mixtures, a case that is appropriately exemplified by the evolution of the barotropy experimentally observed in carbon dioxide + hydrocarbon mixtures. From this basic example, necessary and sufficient conditions are deduced for exhibiting mass density inversions in direct relation with the global phase behavior of the mixture. Finally, the global phase diagram (GPD) predicted by the PC-SAFT equation is used for describing barotropic ranges in mixtures composed by carbon dioxide and hydrocarbons of increasing chain length, together with the specific effects that transitional mechanisms induce on the phenomenon.In excellent agreement with the available experimental data, results show that mass barotropy is present over ranges essentially involving Type III and IV systems. In addition, since the phenomenon is shown to appear also in Type II systems, it is concluded that mathematical double point (MDP) transitions do not necessarily control barotropic behavior. Finally, the occurrence of double critical end points satisfying the conditions of mass barotropy unequivocally explains the shift of barotropic points along the branch(es) of the liquid–liquid–gas line, as the alkane's chain length increases.

► Description of mass and molar barotropy in CO2 + n-alkane mixtures.
► Mass and molar barotropy are independent phenomena.
► Necessary and sufficient conditions for incipient barotropic behavior are established.
► Mass barotropy and global phase behaviors are related in global phase diagrams.
► New phase behavior exhibiting barotropy is described.