Polymorphic phase transition in liquid and supercritical carbon dioxide

Vitaliy Pipich,Dietmar Schwahn

doi:10.1038/s41598-020-68451-y

Vitaliy Pipich, Dietmar Schwahn

Open Access

https://doi.org/10.1038/s41598-020-68451-y

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Abstract

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.

Highlights

We present experiments on molecular density fluctuations in liquid and supercritical (SC) CO2 using small-angle neutron scattering
This perception has changed in recent years as demonstrated in the pressure–temperature projection of the phase diagram of C O2 in Fig. 1, showing the gas–liquid phase transition line, the critical point (CP), and the Widom line in the SCF regime above CP3–5
Both curves were measured at two pressures of 100 bar and 350 bar (Widom line P W = 110 bar) at 56 °C, which is, respectively, below and above the Frenkel line [ PFr. = (173 ± 10) bar]

Summary

Introduction

We present experiments on molecular density fluctuations in liquid and supercritical (SC) CO2 using small-angle neutron scattering. Classical thermodynamics regards supercritical fluids (SCFs) as a homogeneous phase, which opens up a pathway to transform a gas phase into the liquid phase without passing any phase transition line[1,2] This perception has changed in recent years as demonstrated in the pressure–temperature projection of the phase diagram of C O2, showing the gas–liquid phase transition line (solid line), the critical point (CP), and the Widom line in the SCF regime above CP3–5. The Widom line (dashed line) is defined as the position of maximum strength of thermal density fluctuations expressed as susceptibility (i.e. the thermodynamic response function of the compressibility) and of the largest correlation length Both parameters become infinite at the gas–liquid critical point[7,8,9,10]. The main results of our experiments are (i) shape and size of the droplets, (ii) estimation of CO2 number density of droplets, as well as (iii) the characteristic of phase transition

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