Equilibrium Critical Phenomena in Binary Liquid Mixtures

Measurement and correlation of the (pressure, density, temperature) relation of carbon dioxide II. Saturated-liquid and saturated-vapour densities and the vapour pressure along the entire coexistence curve

After a review of the history and an assessment of the current status of the subject, we present light-scattering data to determine the critical correlation-function exponents for an aqueous electrolyte solution yielding the experimental values γ=1.238±0.012, ν=0.629±0.003, η=0.032±0.013. We conclude the paper with some comments concerning the temperature dependence of non-asymptotic effective critical-exponent values of fluids.

Experimental Critical-Exponent Values for Fluids

From Van der Waals' equation to the scaling laws

Neutron diffraction experiments for supercritical CO2 have been carried out over a wide range of Q (0.018≤Q≤30 A−1) at ρ*=ρ/ρc=1.5, 1.2, 0.77, and 0.34 along an isotherm at 310 K (T*=T/Tc=1.02). The measurement enabled us to obtain quantitatively reliable radial distribution functions of the fluid including both short‐ranged structure and long‐ranged density fluctuation. The structure factor and radial distribution function showed the structural change from the attraction‐predominant gaslike structure to the repulsion‐predominant liquidlike one with increasing fluid density. With respect to the long‐ranged structure, almost linear Ornstein–Zernike–Debye plots were obtained for S(Q) of the fluids at all densities. A plot of correlation length against reduced density seems to have a maximum at the critical density. A ratio, α(r), of the density fluctuation produced by the correlation within r to that to the infinity presented a new aspect of the density fluctuation. Molecular dynamics simulation has also been performed in order to investigate a molecular basis of the experimental radial distribution functions. The calculated GN(r)’s were in good agreement with experimental ones at each density. In laboratory space, a variety of orientational coordination structures are almost equally found in the fluid at all densities examined, while T‐shaped structure is preferentially present in polar‐angle space.

Joseph H. Lunacek

Papers

Long-Range Correlation Length and Isothermal Compressibility of Carbon Dioxide Near the Critical Point