Reversible phase separation of CO2 responsive microemulsion: Judgment on whether and how regulated by ratio of ion to responsive surfactant concentration

CO2 responsive microemulsion is a promising solvent with diverse applications in various fields including oil exploitation, material synthesis, and metal recovery. The implementation of reversible phase separation becomes important in enabling the reuses of CO2 responsive microemulsion and facilitating the separation of products in these applications. Earlier reports have established that changes in ion concentration within CO2 responsive microemulsion result in reversible phase separation. Ion in CO2 responsive microemulsion is derived from protonation of CO2 responsive substance which is commonly used as surfactant. An increase in ion concentration leads to a decrease in surfactant concentration. In microemulsion, adsorption of surfactants and ions at the oil-water interface is correlated with interface performance, which influence the ability to achieve efficient phase separation. Interfacial tension (IFT) is an important parameter for interface performance. In this work, ratio of ion to CO2 responsive surfactant concentration was defined as f. The relationship between IFT and f was investigated through a combination of experiments and molecular dynamic simulation to explore mechanism of reversible phase separation. The results of present study indicated that f was found to play a vital role in either inhibiting or promoting the formation of hydrogen bond network by hydrophilic groups in surfactants. The changes of hydrogen bond network resulted in increase or decrease in IFT, leading to reversible phase separation. This relationship between IFT and f was used as a theoretical guidance to optimize N2/CO2 bubbling time to achieve reversible phase separation. The optimization improved the efficiency of CO2 responsive microemulsion in extracting active components from Rehmannia glutinosa Libosch. leaves, and enhanced sustainability of the microemulsion. The findings of this study could be used to advance the scientific understanding of reversible phase separation, offering theoretical guidance for green and efficient utilization of CO2 responsive microemulsion in extraction of natural products. Ultimately these results could encourage the broader applications of CO2 responsive microemulsion across diverse fields.