The Nature of Asymmetry in Fluid Criticality

Abstract

This dissertation deals with an investigation of the nature of asymmetry in fluid criticality, especially for vapor-liquid equilibra in one-component fluids and liquid-liquid equilibra in binary fluid mixtures. The conventional mixing of physical variables in scaling theory introduces an asymmetric term in diameters of coexistence curves that asymptotically varies as |ΔT|1-α, where ΔT=(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 |ΔT|2β 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 coefficients and developed a procedure to obtain these two coefficients, responsible for the two different singular sources for the asymmetry, from mean-field equations of state. By analyzing some classical equations of state we have found that the vapor-liquid asymmetry in classical fluids 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 fluids 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 fluids to binary fluid 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 different sources. Finally, we have studied special phase equilibria that can only be described by including non-linear mixing of physical fields into the scaling fields. 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 significance of applying scaling and isomorphism theory to the treatment of phase equilibria in chemical engineering.