Formation mechanism of surfactant-free microemulsion and a judgment on whether it can be formed in one ternary system
Ying Han, Ning Pan, Deqiang Li, Deqiang Li, Shuhui Liu, Bin Sun, Jinling Chai, Dejie Li, Dejie Li
Abstract
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.