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

Abstract

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. Using density functional theory (DFT) and coarse-grained molecular dynamics (MD) 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 the 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.