Unraveling the Phase Behavior and Stability of Surfactant-Free Microemulsions: From Molecular Interactions to Macroscopic Properties

Surfactant-free microemulsions (SFMEs), formed in mixed ternary systems such as water/ethanol/oil, have garnered substantial interest due to their unique properties and broad applications in areas such as enzyme-catalyzed reactions and nanoparticle synthesis. In this work, we conducted an in-depth investigation of the spontaneous nucleation and stabilization mechanisms of SFMEs, employing experimental techniques, molecular dynamics (MD) simulations, and Flory-Huggins (F-H) theory. The formation of multiscale nanostructures (characteristic scales of ∼1 and ∼100 nm) and their interfacial charging characteristics in SFMEs have been revealed experimentally. MD simulations investigated the structure and stability on the microscopic scale, enhancing our understanding of molecular interactions within these microemulsions. Our theoretical analysis revealed that the stability of mesoscopic nanodroplets within SFMEs hinges on a delicate balance between mixing entropy and internal energy. Equilibrium between these energies results in stable nanodroplet solutions, showcasing a delicate balance that can be manipulated by adjusting the volume fractions of the components and their interaction parameters. This research not only advances the theoretical understanding of SFMEs but also highlights their potential in industrial applications, emphasizing the importance of integrating theoretical and experimental approaches to develop functional nanostructured materials.