Optical Diagnostics of Supercritical CO2 and CO2-Ethanol Mixture in the Widom Delta

Molecules

Alexander Lazarev¹  Timur Semenov²  Fedor Potemkin²  Alexander Sviridov³  Nikita Minaev³  Vyacheslav Gordienko³  Evgenii Mareev³ 

[1] Faculty of Chemistry, M. V. Lomonosov Moscow State University, Leninskie Gory bld.1/2, 119991 Moscow, Russia;Faculty of Physics, M. V. Lomonosov Moscow State University, Leninskie Gory bld.1/2, 119991 Moscow, Russia;Institute of Photon Technologies of Federal Scientific Research Centre “Crystallography and Photonics” of Russian Academy of Sciences, Pionerskaya 2, Troitsk, 108840 Moscow, Russia;

关键词: supercritical fluid;    Raman spectroscopy;    Widom delta;    carbon dioxide;    nonlinear optics;    clusters;   

DOI  :  10.3390/molecules25225424

来源: DOAJ

【 摘 要 】

The supercritical CO₂ (scCO₂) is widely used as solvent and transport media in different technologies. The technological aspects of scCO₂ fluid applications strongly depend on spatial–temporal fluctuations of its thermodynamic parameters. The region of these parameters’ maximal fluctuations on the p-T (pressure-temperature) diagram is called Widom delta. It has significant practical and fundamental interest. We offer an approach that combines optical measurements and molecular dynamics simulation in a wide range of pressures and temperatures. We studied the microstructure of supercritical CO₂ fluid and its binary mixture with ethanol in a wide range of temperatures and pressures using molecular dynamics (MD) simulation. MD is used to retrieve a set of optical characteristics such as Raman spectra, refractive indexes and molecular refraction and was verified by appropriate experimental measurements. We demonstrated that in the Widom delta the monotonic dependence of the optical properties on the CO₂ density is violated. It is caused by the rapid increase of density fluctuations and medium-sized (20–30 molecules) cluster formation. We identified the correlation between cluster parameters and optical properties of the media; in particular, it is established that the clusters in the Widom delta acts as a seed for clustering in molecular jets. MD demonstrates that the cluster formation is stronger in the supercritical CO₂-ethanol mixture, where the extended binary clusters are formed; that is, the nonlinear refractive index significantly increased. The influence of the supercritical state in the cell on the formation of supersonic cluster jets is studied using the Mie scattering technique.

【 授权许可】

Unknown