Polymorphic phase transition in liquid and supercritical carbon dioxide

We present experiments on molecular density fluctuations in liquid and supercritical (SC) CO2 using small-angle neutron scattering. Thermal density fluctuations in SC-CO2 determine susceptibility and correlation length identifying the Widom line at their maxima. Droplet formation occurs at the gas–liquid line and between 20 and 60 bar above the Widom line, the corresponding borderline identified as the Frenkel line. The droplets start to form spheres of constant radius of ≈ 45 Å and transform into rods and globules at higher pressure. Droplet formation represents a liquid–liquid (polymorphic) phase transition of the same composition but different density, whose difference defines its order parameter. Polymorphism in CO2 is a new observation stimulating interesting discussions on the topics of gas-like to liquid-like transition in SC fluids and polymorphism since CO2 represents a “simple” van der Waals liquid in contrast to water, which is the most widely studied liquid showing polymorphism in its supercooled state

Aqueous Solutions of Poly(ethylene oxide): Crossover from Ordinary to Tricritical Behavior

The aqueous associating solution poly(ethylene oxide) (PEO) of molar mass between 0.45 and 280 kDa was studied with small-angle neutron scattering (SANS) near their critical concentration in a temperature interval from 11.8 to 95.7 °C. SANS measures thermal concentration fluctuations whose susceptibility and correlation length identified mean-field, 3D-Ising, and tricritical Θ-point critical behavior. Scaling laws of the critical amplitudes of susceptibility and correlation length were observed for the samples obeying 3D-Ising and tricritical Θ-point behavior in agreement with the predicted ones for nonassociating semidilute solutions. On the other hand, chain conformation is different for both classes of criticality and is attributed to the concentrated and semidilute regime of the phase diagram. In the concentrated regime the chains are slightly more compacted than Gaussian coils. However, all PEO solutions approach ideal chain conformation at the Θ-temperature. A small amount of PEO of the order of 10–5 is forming compact PEO aggregates

Densification of Supercritical Carbon Dioxide Accompanied by Droplet Formation When Passing the Widom Line

Thermal density fluctuations of supercritical CO2 were explored using small-angle neutron scattering (SANS), whose amplitude (susceptibility) and correlation length show the expected maximum at the Widom line. At low pressure, the susceptibility is in excellent agreement with the evaluated values on the basis of mass density measurements. At about 20 bar beyond the Widom line, SANS shows the formation of droplets accompanied by an enhanced number density of the supercritical fluid. The corresponding borderline is interpreted as a Frenkel line separating gas- and liquidlike regimes

3D-ising and Lifshitz critical behavior in a mixture of a polymer blend and a corresponding diblock copolymer

Thermal composition fluctuations and the associated crossover from the 3D-Ising to the isotropic Lifshitz universality class have been studied in a three-component mixture made of a critical polymer blend and the corresponding diblock copolymer. The rather complex phase diagram and the critical exponents of the isotropic Lifshitz universality class will be presented. (C) 2000 Elsevier Science B.V. All rights reserved