Assessing the non-ideality of the CO2-CS2 system at molecular level: a Raman scattering study

Area of Science:

• Physical Chemistry
• Spectroscopy
• Thermodynamics

Background:

• Understanding the behavior of dense fluid mixtures is crucial for chemical engineering and materials science.
• Carbon dioxide (CO2) and carbon disulfide (CS2) mixtures exhibit complex phase behavior, including non-ideal mixing and potential phase separation.

Purpose of the Study:

• To investigate the molecular structure and solvation dynamics of dense CO2-CS2 mixtures using Raman spectroscopy.
• To identify and characterize different solvation regimes within the CO2-CS2 system as a function of CO2 concentration and pressure.

Main Methods:

• Raman spectroscopy was employed to analyze the dense phase of CO2-CS2 mixtures.
• Measurements were conducted across a range of CO2 concentrations (0.02-0.95 mole fractions) and pressures (0.5-7.7 MPa) at a constant temperature (313 K).
• Polarized and depolarized spectra of induced modes (CS2) and Fermi resonance dyads (CO2, CS2) were recorded.

Main Results:

• A novel "plateau-like" region was observed in spectroscopic observables between 0.3-0.7 mole fractions of CO2.
• The bandshape and intensity of CS2 induced modes remained similar to pure CS2 up to equimolar concentration, with subsequent variations.
• Spectroscopic evidence suggests two distinct solvation regimes: segregated CS2 clusters at high CS2 concentrations and dynamic coexistence of CO2 monomers, dimers, and hetero-dimers at higher CO2 concentrations.

Conclusions:

• The study identifies two distinct solvation regimes in CO2-CS2 mixtures, separated by a transition occurring in the observed plateau region.
• The interplay of attractive and repulsive forces explains the non-ideality, volume expansion, and the plateau phenomenon.
• The findings provide a molecular-level interpretation for the previously observed pressure-composition diagram features, challenging earlier explanations related to critical solution temperatures.