Conversion of a surfactant-based microemulsion to a surfactant-free microemulsion by CO₂

A microemulsion with a CO₂ response was prepared by mixing the surfactant sodium oleate (NaOA), the co-surfactant isopropyl alcohol (IPA), oil phase oleic acid (HOA) and water. This surfactant-based microemulsion (SBME) shows a CO₂ responsive behavior, and the introduction of CO₂ can breakdown the microemulsion. Through the research in this paper, it is found that the content of IPA has a direct impact on the CO₂ response behavior of SBME. It was found that the lower the IPA content (22.73 wt%), the more obvious the CO₂ response behavior of SBME. Conversely, when the concentration of IPA is high (54.05 wt% and 63.83 wt%), the introduction of CO₂ does not directly lead to the demulsification of the microemulsion. NaOA can be converted to HOA under the action of CO₂, which is why SBME shows CO₂ response behavior. By comparing the effects of CO₂ on the (pseudo-)ternary phase diagrams of SBME and surfactant-free microemulsion (SFME), we found evidence that SBME shows different CO₂ response behaviors. When CO₂ was bubbled into the SBME system with a low IPA content, IPA cannot stabilize the excessive HOA and water in the system and eventually break the microemulsion. The situation is different when CO₂ is applied to the SBME system with a high IPA content. IPA as an amphiphilic solvent can stabilize the HOA and water in the system to form SFME. In this process, SBME can be demulsified (low IPA content) or can be converted to SFME (high IPA content) in the presence of CO₂.