Molecular Insights into the Reversible Mechanisms of CO₂-Switchable Surfactant Solubilization and Emulsification

Switchable surfactants exhibit broad application potential due to their reversible response to external stimuli. The reversible mechanism of the CO₂-switchable surfactant (N'-dodecyl-N, N-dimethyl-acetamidines, DDA) solubilization polycyclic aromatic hydrocarbons (PAHs) and the microscopic dynamic behavior of emulsification/demulsification were systematically studied using coarse-grained molecular dynamics simulations. The dynamic transition processes of protonation (DDA to DDA⁺) and deprotonation (DDA⁺ to DDA) were successfully simulated. The results show that DDA molecules self-assembled into oil droplet-like aggregates through hydrophobic interactions and electrostatic attraction under initial dispersion conditions; in the process of DDA to DDA⁺, the hydrophilic headgroup of DDA⁺ formed micelle interfaces, with DDA⁺ hydrophobic chains constituting the core, forming a typical core-shell structure. Besides, we found that the hydrophobic chain region of DDA⁺ exhibited a relatively low density, thereby providing sufficient space for the solubilization of PAHs. Our results elucidate the solubilization mechanism governed by the ordered assembly of surfactant molecules and the hydrophobic interaction with pollutants. Deprotonation (DDA⁺ to DDA) simulation showed that neutral DDA molecules, due to the loss of charge-driven effects, led to micelle dissociation and release of PAHs. However, for large-sized micelles, the release efficiency of pollutants was low due to steric hindrance within the core. Additionally, the emulsification/demulsification behaviors showed that DDA⁺ stabilized emulsions by inserting hydrophobic chains into the dodecane oil droplet core and exposing hydrophilic headgroups at the interface, while DDA molecules mixed uniformly with the oil phase, triggering emulsion instability. The results align closely with experimental observation, verifying the reliability of the coarse-grained model in the simulation of complex multiphase systems. This work laid a methodological foundation for the molecular design and process optimization of switchable surfactants.