The Nature of Asymmetry in Fluid Criticality

Title of dissertation: THE NATURE OF ASYMMETRY IN FLUID CRITICALITY Jingtao Wang, Doctor of Philosophy, 2006 Dissertation directed by: Professor Mikhail A. Anisimov Institute for Physical Science and Technology and Department of Chemical and Biomolecular Engineering This dissertation deals with an investigation of the nature of asymmetry in ‡uid criticality, especially for vapor-liquid equilibra in one-component ‡uids and liquid-liquid equilibra in binary ‡uid mixtures. The conventional mixing of physical variables in scaling theory introduces an asymmetric term in diameters of coexistence curves that asymptotically varies as e T 1 , where e T = (T Tc) =Tc is the relative distance of the temperature T from the critical temperature Tc. "Complete scaling" implies the presence of an additional asymmetric term proportional to e T 2 in diameters which is more dominant near the critical point. To clarify the nature of vapor-liquid asymmetry, we have used the thermodynamic freedom of a proper choice for the critical entropy to simplify "complete scaling" to a form with only two independent mixing coe¢ cients and developed a procedure to obtain these two coe¢ cients, responsible for the two di¤erent singular sources for the asymmetry, from mean-…eld equations of state. By analyzing some classical equations of state we have found that the vapor-liquid asymmetry in classical ‡uids near the critical point can be controlled by molecular parameters, such as the degree of association and the strength of three-body interactions. By combining accurate vapor-liquid coexistence and heat-capacity data, we have obtained the unambiguous evidence for "complete scaling" from existing experimental and simulation data. A number of systems, real ‡uids and simulated models have been analyzed. Furthermore, we have examined the consequences of "complete scaling" when extended to liquid-liquid coexistence in binary mixtures. The procedure for extending "complete scaling" from one-component ‡uids to binary ‡uid mixtures follows rigorously the theory of isomorphism of critical phenomena. We have shown that the "singular" diameter of liquid-liquid coexistence also originates from two di¤erent sources. Finally, we have studied special phase equilibria that can only be described by including non-linear mixing of physical …elds into the scaling …elds. Based on scaling and isomorphism, an approach is presented to represent closed-loop coexistence curves and expressions to describe the critical lines near a double critical point (DCP) are derived. The results demonstrate the practical signi…cance of applying scaling and isomorphism theory to the treatment of phase equilibria in chemical engineering. THE NATURE OF ASYMMETRY IN FLUID CRITICALITY