Formation mechanism of surfactant-free microemulsion and a judgment on whether it can be formed in one ternary system

What is the formation and stability mechanism of surfactant-free microemulsion (SFME)? What is the inherent relationship between three components to form SFME? These problems restrict the understanding, popularization and application of SFME. In this work, a new methyl salicylate/ethanol/water SFME is constructed as a representative for experimental and theoretical research. Results show that: the dynamic equilibrium of conservative, dissipative and random forces leads to the decrease of free energy of ternary system and promotes the formation of SFME. In oil in water (O/W) and water in oil (W/O) droplets, directional arrangement and charge redistribution of ethanol molecules in the interface layer can be induced by water. The induced charges on the O/W and W/O droplets are –48.999 and 2.697 a.u., which is conducive to the dispersion of droplets and improves the stability of SFME. When O/W droplets collide, an “electron trap” can be formed by water molecules, so that charge transfer can be carried out spontaneously. In addition, a novel parameter “ ƍ ” is defined representing the interaction between amphi -solvent, oil and water. The relationship of S (the proportion of single-phase area in ternary phase diagram) and ƍ is: S  = 67.8 – 97.2 ƍ . ƍ less than 0.7 is the requirement for SFME to be formed and the maximum of S is 70%. The proposed regulation is verified by other 44 ternary systems. Hope this work would lay a theoretical foundation for explaining the stability mechanism, and provide guidance for developing more SFMEs.