Microscopic properties and stabilization mechanism of a supercritical carbon dioxide microemulsion with extremely high water content

HYPOTHESIS

Developing the supercritical carbon dioxide microemulsion with a broad water content (W₀) window can provide more possibility for designing highly efficient chemical processes, which is challenging due to the lack of comprehension about its formation mechanism. Molecular dynamics simulation method is expected to reveal the microscopic stabilization mechanism of high-W₀ microemulsions.

SIMULATIONS

All-atom molecular dynamics simulations of the ternary systems with varied W₀ stabilized by 4FG(EO)₂ surfactant were designed according to phase behavior experiments. A systematic investigation was performed concerning the self-assembling, equilibrium morphology and detailed microstructure of the microemulsion droplet. An in-depth comparative study about the distribution of both H₂O and CO₂, the interfacial behaviors of 4FG(EO)₂, as well as the microscopic interactions was conducted.

FINDINGS

For the first time, direct evidence was provided for the formation of water-in-carbon dioxide microemulsion with extremely high W₀ (80) under the effect of 4FG(EO)₂. Furthermore, a unique interfacial phenomenon, i. e. CO₂ accumulating at the interface, was revealed to be responsible for the formation and enhanced stability of the nanosized droplet with high W₀. This should set a new guiding star for synthesizing and selecting effective interfacial modifiers to create high-W₀ microemulsions.

Amphiphile self-assemblies in supercritical CO2 and ionic liquids

Supercritical (sc) CO2 and ionic liquids (ILs) are very attractive green solvents with tunable properties. Using scCO2 and ILs as alternatives of conventional solvents (water and oil) for forming amphiphile self-assemblies has many advantages. For example, the properties and structures of the amphiphile self-assemblies in these solvents can be easily modulated by tuning the properties of solvents; scCO2 has excellent solvation power and mass-transfer characteristics; ILs can dissolve both organic and inorganic substances and their properties are designable to satisfy the requirements of various applications. Therefore, the amphiphile self-assemblies in scCO2 and ILs have attracted considerable attention in recent years. This review describes the advances of using scCO2 or/and ILs as amphiphile self-assembly media in the last decade. The amphiphile self-assemblies in scCO2 and ILs are first reviewed, followed by the discussion on combination of scCO2 and ILs in creating microemulsions or emulsions. Some future directions on the amphiphile self-assemblies in scCO2 and ILs are highlighted.