A liquid CO2-compatible hydrocarbon surfactant: experiment and modelling.

Area of Science:

• Physical Chemistry
• Materials Science
• Chemical Engineering

Background:

• Surfactants soluble in liquid CO2 are rare, with limited understanding of their interfacial and self-assembly behaviors.
• Polyoxyethylene (5) isooctylphenyl ether exhibits interfacial activity at the water-liquid CO2 interface.

Purpose of the Study:

• To investigate the interfacial activity and self-assembly of polyoxyethylene (5) isooctylphenyl ether at the water-liquid CO2 interface.
• To model the system using self-consistent field theory and a free-volume approach.

Main Methods:

• Pendant drop method to measure interfacial tension at the water-liquid CO2 interface.
• Self-consistent field (SCF) theory with a free-volume approach and a freely-jointed chain model.

Main Results:

• A critical micelle concentration (CMC)-like cusp was observed at ~50 mM in bulk liquid CO2.
• Partial wetting was identified at the water-vapor interface, attributed to proximity to the CO2 critical point.
• Stable, water-swollen reverse micelles (aggregation number ~100) formed in CO2-rich phases above ~40 mM.
• Compressible CO2-swollen surfactant bilayers were stable in bulk water at elevated concentrations.
• Increasing pressure in liquid CO2 increased the critical reverse micellar concentration and decreased aggregation number.

Conclusions:

• Polyoxyethylene (5) isooctylphenyl ether is effective at the water-liquid CO2 interface, enabling self-assembly into reverse micelles and bilayers.
• The study provides insights into wetting transitions and surfactant self-assembly in CO2-based systems.
• Findings are relevant for applications involving CO2 as a solvent or in supercritical fluid technologies.