Scaling models of thermodynamic properties on the coexistence curve: Problems and some solutions

Models consistent with the scaling theory of critical phenomena and capable of describing the thermodynamic properties F of substances on the coexistence curve, such as the density of the liquid ρ _l , density of the gas ρ _g , order parameter f _s , mean coexistence curve diameter f _d , and saturation pressure P _s are discussed. The models are presented in the form of equations F = (τ, D, C), where τ = (T _c ? T)/T _c , and D = (α, β, T _c , ρ _c , P _c , ...) are the critical characteristics, such as T _c , ρ _c , and P _c (temperature, density, and pressure, respectively), α and β are the scaling exponents, and C are adjustable coefficients. The authors developed combined models f(τ, D, C) for describing the indicated properties of a number of compounds (CH₄, NH₃, SF₆, water, methanol, ethanol, diethyl ether, and freons R134a, R143a, and R236ea). The coefficients C were determined based on experimental data over a wide temperature range, including the critical point. The equations derived are used to perform practical calculations, including estimates of the first and second derivatives of the saturation pressure with respect to the temperature in the critical region.