Global thermodynamic behavior of fluids and fluid mixtures in the critical region

Fluids near liquid-vapor critical points are assumed to belong to the universality class of three-dimensional Ising-like systems, i.e. systems with a scalar order parameter and with short-range interactions. Asymptotically close to a critical point, the thermodynamic properties of fluids and fluid mixtures exhibit scaling laws with universal critical exponents and with universal scaling functions. Far from the critical point, the thermodynamic properties are described with the aid of classical equations, such as the van der Waals equation, cubic equations, or modified cubic equations. The scaling laws are only valid in a very limited range of temperatures and densities near the critical point. On the other hand, critical effects on the thermodynamic properties of fluids are observed in reality in a large range of temperatures and densities around the critical point. These facts motivated us to construct a thermodynamic free energy for fluids and fluid mixtures in the critical region that incorporates the crossover from asymptotic singular behavior near the critical points to regular classical behavior far away from the critical points. Specifically, in this dissertation, we formulate such a crossover model on the basis of the renormalization-group theory of critical phenomena. We first formulate this crossover theory for one-component fluids and apply it to represent experimental thermodynamic-property data for carbon dioxide and ethane. We then generalize the theory to binary mixtures by introducing a new field variable {dollar}zeta{dollar} that represents the ratio of fugacities of the corresponding pure components, and compare it with the experimental thermodynamic-property data for mixtures of carbon dioxide and ethane.