Revealing the microscopic formation mechanism and stability characteristics of anionic surfactant microemulsions using coarse-grained simulations

Microemulsion is thermodynamically stable dispersion system formed by emulsifiers such as surfactants adsorbed at the oil-water interface. However, the evolution mechanism of surfactant-stabilized microemulsions and its stability are not adequately understood. Motivated by these issues, the underlying formation mechanism and the complex interplay on the microemulsion stability were systematically investigated by dissipative particle dynamics simulations. Coarse-grained models of oil/water/surfactants were developed to demonstrate a clear Ostwald ripening mechanism for W/O microemulsion, whereas a collisional fusion mechanism for O/W microemulsion. Mutual transformation between O/W and W/O microemulsions was dominated by the change of interfacial water/oil area. As the surfactant concentration increased, smaller microemulsion formed with a higher stability. Compared to sodium lauryl sulfonate, sodium dodecylbenzene sulfonate with benzene structure could form a thicker interfacial film, thereby improving the microemulsion stability. Internal olefin sulfonate with a branched structure was able to form an interfacial film with greater strength, enduring a higher shear rate of 0.033/ps. Furthermore, longer molecular chains of surfactant could result in a stronger steric effect, leading to a higher salinity tolerance of microemulsion with aHH is 17. The obtained results are expected to provide a solid theoretical foundation for the rational design of efficient microemulsions and pave the way for the applications such as oil recovery, pollution control, and dispersion of drugs.