Supercritical Ethanol-CO2 Mixtures Exhibit Microscopic Immiscibility: A Combined Study Using X-ray Scattering and
Jingcun Fan1, Taekeun Yoon1, Guillaume Vignat1
1Department of Mechanical Engineering, Stanford University, Stanford, California 94305, United States.
The Journal of Physical Chemistry Letters
|July 3, 2025
Summary
Supercritical ethanol (EtOH) and carbon dioxide (CO2) mixtures show unexpected microscopic immiscibility. Molecular dynamics and X-ray scattering reveal EtOH aggregates, challenging type-I fluid classifications.
Area of Science:
- Chemical Engineering
- Materials Science
- Physical Chemistry
Background:
- Supercritical fluids are widely used in various industrial applications.
- Ethanol-carbon dioxide (EtOH-CO2) mixtures are classified as type-I, indicating complete macroscopic miscibility.
- Microscopic behavior may differ from macroscopic observations due to molecular interactions.
Purpose of the Study:
- To investigate the microscopic structure and phase behavior of supercritical EtOH-CO2 mixtures.
- To understand the influence of molecular polarity on mixture heterogeneity.
- To challenge conventional views of type-I fluid behavior.
Main Methods:
- Small-angle X-ray scattering (SAXS) experiments to probe mixture structure.
- Molecular dynamics (MD) simulations with optimized force fields for atomistic insights.
- Cluster analysis to identify preferential compositions and aggregation.
Main Results:
- SAXS data revealed nonlinear, composition-dependent structure factors with significant local density fluctuations.
- MD simulations showed ethanol forming hydrogen-bonded aggregates, while CO2 remained dispersed.
- Microscopic analysis indicated EtOH-rich clusters, contradicting macroscopic miscibility.
Conclusions:
- Supercritical EtOH-CO2 mixtures exhibit significant microscopic heterogeneity and immiscibility.
- Molecular polarity and interactions drive the formation of EtOH aggregates.
- Findings necessitate a revision of type-I fluid classifications and consideration of microscopic effects.
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