Structural Evolution of Supercritical CO₂across the Frenkel Line

Abstract

The supercritical state is currently viewed as uniform and homogeneous on the pressuretemperature phase diagram in terms of physical properties. Here, we study structural properties of the supercritical carbon dioxide, and discover the existence of persistent medium-range order correlations which make supercritical carbon dioxide non-uniform and heterogeneous on an intermediate length scale, a result not hitherto anticipated. We report on the carbon dioxide heterogeneity shell structure where, in the first shell, both carbon and oxygen atoms experience gas-like type interactions with short range order correlations, while within the second shell oxygen atoms essentially exhibit liquid-like type of interactions with medium range order correlations due to localisation of transverse-like phonon packets. We show that the local order heterogeneity remains in the three phase-like equilibrium within very wide temperature range. Importantly, we highlight a catalytic role of atoms inside the nearest neighbor heterogeneity shell in providing a mechanism for diffusion in the supercritical carbon dioxide on an intermediate length scale. Finally, we discuss important implications for answering the intriguing question whether Venus may have had carbon dioxide oceans and urge for an experimental detection of this persistent local order heterogeneity.PACS numbers: 05.20. Jj,5.70.Fh, 05.70.+a Unlike the long-range order of ideal crystalline structures, local order is an intrinsic characteristic of soft matter materials and often serves as the key to the tuning of their properties and their possible applications [1][2][3][4][5][6][7][8]. Liquid materials often exhibit medium and/or short range order, such as nanoscale structural motifs, clustering, or local ordering at the level of the atomic coordination spheres. Describing this local order of soft matter systems and supercritical fluids in particular is crucial for understanding the origins of their material properties [9][10][11][12][13][14].

Widom Lines in Binary Mixtures of Supercritical Fluids

“…The Widom line is commonly associated with the thermodynamic transition of a pure fluid, and has been studied for a number of single species, such as CO 2 17, 32, 34–36 , O 2 16 , Ar 10, 29 , and H 2 O 37, 38 . However, most practical applications, such as carbon capture or supercritical fluid injection, require the consideration of mixtures of multiple species.…”

Path Integral Molecular Dynamics Simulation on Atomic Distribution in Amorphized Ice Ic

“…Namely, the original ice Ic structure is retained at molar volume 15. for HDA at 10 K is extremely flat. Surprisingly, such uniform profile is similar to RDFs of supercritical fluids in general [32] [33]. This is a consequence of both the quantum dispersion of atoms and the amorphous structure of the whole system.…”

Pressure/temperature fluid cell apparatus for the neutron powder diffractometer instrument: Probing atomic structure in situ

“…This second experiment was motivated by the limited number and relatively low resolution of experimental results 9-15 available on the local structure of supercritical or subcritical CO 2 . Obtained results can be compared with the increasing amount of theoretical work [16][17][18][19][20] relevant to non-ambient CO 2 structure and behavior, and can also be used as bulk CO 2 reference structures for future studies on CO 2 -water mixtures (as CO 2 -bearing fluids) and CO 2 -water-mineral interactions.…”

“…Further data interpretation can be made according to a number of theoretical studies completed for bulk supercritical CO 2 . [16][17][18]20 Although the simulations depend on the particular choice of interaction potentials and thermodynamic conditions, the following structural correlation features appear to be rather general. 11,[16][17][18]20 (1) In the supercritical states, the closest neighbor CO 2 molecules tend to lie in the equatorial plane of the central CO 2 oriented as a T-shaped interaction between any two CO 2 molecules (inset plot in Figure 5(c)).…”