Interfacial Molecular Behavior and Organization of Low-Fluorine Surfactants at the sc-CO₂/Water Interface

Understanding the molecular organization of surfactants at the sc-CO₂/water interface is essential for the rational design of CO₂-philic interfacial formulations with reduced fluorine content. In this work, three low-fluorinated anionic surfactants (LF1-LF3) with different hydrophobic-tail architectures were investigated together with a fluorine-free branched co-surfactant (ZBJ) using density functional theory (DFT) calculations and molecular dynamics (MD) simulations. DFT calculations revealed a clear amphiphilic segregation of solvent affinity: the fluorinated tails preferentially interacted with CO₂, whereas the sulfonate headgroups exhibited stronger affinity for water. MD simulations showed that the surfactants, initially positioned at the sc-CO₂/water interface, underwent interfacial reorganization and orientational ordering, with their fluorinated tails extending into the sc-CO₂ phase and their hydrophilic headgroups oriented toward the aqueous phase. Tail branching strongly affected interfacial packing and structural stability; the three-tailed, claw-like LF3 formed the thickest and most thermodynamically stable monolayer, with an interfacial formation energy of -330.30 kJ/mol. In the LF3/ZBJ mixed system, ZBJ occupied packing voids between LF3 molecules, further stabilizing the mixed monolayer and yielding an interfacial formation energy of -342.07 kJ/mol. These results clarify how tail architecture, CO₂-philic/hydrophilic partitioning, counterion coordination, and steric complementarity govern the interfacial organization and stabilization of low-fluorine surfactants, providing molecular-level guidance for the rational design of low-fluorine surfactant formulations for sc-CO₂/water systems.