Polymorphic phase transition in liquid and supercritical carbon dioxide.

We present experiments on molecular density fluctuations in liquid and supercritical (SC) CO using small-angle neutron scattering. Thermal density fluctuations in SC-CO 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 CO is a new observation stimulating interesting discussions on the topics of gas-like to liquid-like transition in SC fluids and polymorphism since CO represents a "simple" van der Waals liquid in contrast to water, which is the most widely studied liquid showing polymorphism in its supercooled state.