Realization of Low Interfacial Tension and High Stability in Polybutadiene Emulsions via Branched-Zwitterionic Surfactant Mixtures: A Molecular Dynamics Study

The rational design of surfactant systems that simultaneously achieve low interfacial tension (IFT) and exceptional interfacial stability represents a critical challenge in emulsion polymerization, particularly for highly exothermic processes such as polybutadiene latex synthesis. This study employs all-atom Molecular Dynamics (MD) simulations to investigate the synergistic interfacial behavior of a branched anionic surfactant (sodium 2-butyloctyl sulfate, BOS) and a zwitterionic surfactant (dodecyl carboxylic betaine, BS12) at the polybutadiene-water interface, aiming to surpass the performance of traditional linear surfactants such as SDS. The results indicate that while the branched surfactant (BOS) exhibits optimal interfacial tension (IFT) reduction efficiency attributable to its double-tail architecture, its performance is compromised by inferior thermal stability. By incorporating the zwitterionic surfactant BS12 to construct an equimolar binary system, the formulation retains robust IFT reduction capability and significantly enhances interfacial stability. Thermal perturbation simulations revealed that the critical thermal desorption temperature was elevated by approximately 26 K compared to the traditional SDS system. Mechanistically, BS12 molecules act as "molecular wedges" that intercalate into the interstitial voids of the branched BOS network, thereby increasing film compactness without compromising interfacial coverage. This synergistic reinforcement is driven by an "electrostatic locking" mechanism, wherein the dipolar headgroups of BS12 effectively screen the electrostatic repulsion between anionic BOS headgroups, stabilizing the interface through strong attractive complexation. Furthermore, hydration dynamics analysis elucidates a critical trade-off between interfacial tension reduction and film stability. Although headgroup complexation induces partial dehydration, it fosters the construction of a substantially more robust interfacial hydrogen bond network. These discoveries clarify the microscopic mechanisms governing interfacial properties, offering theoretical support for the rational design of high-performance emulsion polymerization formulations.