Molecular Interactions and Responsive Locations of CO2-Responsive Surfactants in an Oil–Water–Surfactant System: Molecular Dynamics Simulation and Free Energy Perturbation

Understanding the mechanism of a CO₂-responsive surfactant is essential for enhancing its industrial applications. Conventional experimental methods face challenges in pinpointing the exact location of proton transfer within the system and in accurately describing the impact of intermolecular and intramolecular interactions on the CO₂ responsiveness of such substances. To address this gap, this study employs molecular dynamics simulations and free energy perturbation methods to investigate the proton transfer process between a CO₂-responsive cationic surfactant N'-dodecyl-N,N-dimethylacetamidinium (DMAAH⁺) and its counterion bicarbonate ion at the oil-water interface and micelle surface and in the bulk aqueous phase. Molecular dynamics simulations identified potential locations for the proton transfer process within the system and elucidated the types of interactions contributing to changes in Gibbs free energy. Subsequently, free energy perturbation was employed to calculate Gibbs free energy changes associated with proton transfer at different locations. The respective contributions of various intramolecular and intermolecular interactions were then compared and analyzed. It has been revealed that the deprotonation process is not thermodynamically spontaneous at all three responsive locations. The proton transfer occurs more frequently at the oil-water interface than at the micelle surface and is less common in the bulk aqueous phase. The findings enhance our understanding of the fundamental mechanisms governing the responsiveness of CO₂-responsive surfactants and provide valuable insights for their practical application in industrial processes.