Self-assembly of CO₂-responsive surfactant solutions: from density functional theory to molecular dynamics studies

The micellar structure of CO₂-responsive surfactant systems changes upon the introduction of CO₂; however, molecular-level insights into this process are rarely reported. Therefore, this study designed a CO₂-responsive mixed system composed of long-chain tertiary amine and sodium salicylate (NaSal). Using density functional theory (DFT) and coarse-grained molecular dynamics (CGMD) simulations, we investigated the morphological transformation of micelles. We conducted a detailed analysis of reaction thermodynamics, electrostatic potential, interaction energy, radial distribution functions, and micelle statistics to characterize the self-assembly of the CO₂-responsive surfactant system. Our results demonstrate that the long-chain tertiary amine system exhibits enhanced polarity characteristics after CO₂ response. Martini 3.0.0 force field successfully reproduced experimentally observed structural changes, namely, the formation of vesicular structures prior to CO₂ response and the transition to worm-like micelles after response. In coarse-grained model optimization, we found that the stronger the hydrophobicity of the organic counterion's hydrophobic group, the more likely it is to penetrate the micelle's interior and form worm-like micelles. Additionally, hydrophobic interactions are also identified as a critical driving force in the self-assembly process of surfactants. These findings are significant for improving and designing more efficient CO₂-responsive surfactant transformation systems.