A perspective of microemulsions in critical metal separation and recovery: Implications for potential application of CO2-responsive microemulsions

Study of Phase Separation and Reversibility in CO2-Responsive Superamphiphile Microemulsions

Phase separation of microemulsions occupies a key position in many applications, such as oil recovery, nanomaterial synthesis, and chemical reactions. Achieving an intelligent response is crucial to microemulsion development and application. For this reason, in this study, CO2-responsive superamphiphilic molecules were developed as rapidly switchable oil-in-water microemulsions. These superhydrophilic molecules with linear structures were produced by electrostatic interaction of stearic acid and aminotrimethyltriazine COSM-1 in a 1:1 molar ratio. The introduction of n-butanol as a cosurfactant in the CO2-responsive microemulsion system resulted in the spontaneous formation of stable microemulsions. The effect of the addition of n-butanol on the carbon dioxide-responsive microemulsion system was investigated, and the optimum amount of n-butanol was determined by optimization. After exposure to CO2 for 30 s, the superhydrophilic molecules decomposed into inactive components at the interface, leading to a complete phase separation of the microemulsion into oil and water phases. The system was purged with N2 at 60 °C for 10 min to remove the CO2, and the phase separation system was transformed into a clear microemulsion, which was then evaluated for its properties. The rapid response and complete demulsification of these surface CNFS superamphiphiles to CO2 suggest that they have promising applications in product separation, microemulsion recovery, and enhanced crude oil recovery.

Microstructure-dependent CO2-responsive microemulsions for deep-cleaning of oil-contaminated soils

CO2-responsive microemulsion (ME) is considered a promising candidate for deep-cleaning and oil recovery from oil-contaminated soils. Understanding the responsive nature of different microstructures (i.e., oil-in-water (O/W), bicontinuous (B.C.) and water-in-oil (W/O)) is essential for unlocking the potential and mechanisms of CO2-responsive emulsions in complex multiphase systems and providing comprehensive guidance for remediation of oil-contaminated soils. Herein, the responsiveness of microstructures of ME to CO2 trigger was investigated using experimental designs and coarse-grained molecular dynamic simulations. MEs were formed for the first time by a weakly associated pseudo-Gemini surfactant of indigenous organic acids (naphthenic acids, NAs are a class of natural surface-active molecules in crude oil) and tetraethylenepentamine (TEPA) through fine tuning of co-solvent of dodecyl benzene sulfonic acid (DBSA) and butanol. The O/W ME exhibited an optimal CO2-responsive character due to easier proton migration in the continuous aqueous phase and more pronounced dependence of configuration on deprotonated NA ions. Conversely, the ME with W/O microstructure exhibited a weak to none responsive characteristic, most likely attributed to its high viscosity and strong oil-NA interactions. The O/W ME also showed superior cleaning efficiency and oil recovery from oil-contaminated soils. The results from this study provide insights for the design of CO2-responsive MEs with desired performance and guidance for choosing the favorable operating conditions in various industrial applications, such as oily solid waste treatment, enhanced oil recovery (EOR), and pipeline transportation. The insights from this work allow more efficient and tailored design of switchable MEs for manufacturing advanced responsive materials in various industrial sectors and formulation of household products.