Synergistic mechanism in anionic/zwitterionic and anionic/nonionic surfactant mixtures on improving the thermal stability of emulsions: An experimental and simulation study

Surfactant emulsions are crucial for stabilizing and enhancing the dispersibility of immiscible substances, enabling efficient mixing and delivery of active ingredients in various industries such as oil recovery, pharmaceuticals, food, and cosmetics. However, a comprehensive understanding of the synergistic mechanism governing surfactant-formed emulsions and their thermal stability remains elusive. Motivated by these knowledge gaps, the self-assembly and interfacial behavior of binary surfactants as well as their influence on the thermal stability of emulsions were systematically investigated. Utilizing a combination of experimental and theoretical approaches, we observed extraordinary synergistic effects in the anionic/zwitterionic and anionic/nonionic surfactant systems, leading to a significant reduction in surface tension and critical micelle concentration, quantitatively assessed by activity parameters and intermolecular interaction parameters. The microscopic mechanism responsible for the observed synergistic effects was investigated by dissipative particle dynamics (DPD) simulations, which could be attributed to steric effects, electrostatic interactions, and hydrophobic interactions. The self-assembly behavior of binary surfactants into micelles was further analyzed to substantiate these findings. Furthermore, the synergistic effects of emulsification performance and thermal stability in binary surfactant systems were quantified by water separation rate and emulsification composite index. Coarse-grained models of oil/water/surfactants were developed to demonstrate the augmented stability of interfacial films to the presence of binary surfactants. The obtained results shed lights on the inherent synergistic mechanism within binary surfactant mixtures, demonstrating their potential in enhancing emulsion thermal stability and paving the way for diverse industrial applications.