The nature of asymmetry in fluid criticality

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 |Delta T˜|1-alpha, where DeltaT˜ = (T - Tc)/T c 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 |Delta T˜|2beta 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.