Molecular diffusion coefficients in ethanol/water/carbon dioxide mixtures

Area of Science:

• Chemical Engineering
• Physical Chemistry
• Mass Transport Phenomena

Background:

• Supercritical and liquid mixtures of ethanol/water/carbon dioxide (CO2) are utilized for non-toxic, rapid solute extraction from various matrices.
• Understanding mass transport properties, specifically diffusion coefficients, is crucial for optimizing these extraction processes.

Purpose of the Study:

• To investigate the diffusion coefficients of benzene, anthracene, m-cresol, and p-nitrophenol in ethanol/water/CO2 mixtures.
• To evaluate the impact of mixture composition and temperature on solute diffusion.

Main Methods:

• Experimental measurement of diffusion coefficients for four model solutes in ethanol/water/CO2 mixtures.
• Systematic variation of CO2 concentration and temperature (25-60°C) in a fixed ethanol/water ratio (0.61/0.39 mole ratio).

Main Results:

• Diffusion coefficients of all four solutes increased notably with the addition of 30 mol% CO2 or a temperature increase from 25 to 60°C.
• Experimental diffusion coefficients frequently exceeded predictions from Stokes-Einstein and Wilke-Chang models.
• The Eyring relationship effectively correlated diffusion coefficient variations with temperature across tested mixtures.

Conclusions:

• Ethanol/water/CO2 mixtures exhibit favorable mass transport characteristics for solute extraction.
• Existing theoretical models may underestimate diffusion in these enhanced-fluidity systems.
• The Eyring model provides a reliable framework for understanding temperature-dependent diffusion in these ternary mixtures.