Role of unsaturation in governing synergistic micellization and interaction energetics in ionic liquid-fatty acid...

This study is undertaken to reveal the molecular origins of non-ideal aggregation in ionic liquid (IL)-surfactant systems by investigating the micellization behaviour of a double-tailed imidazolium-based 1,3-didecyl-2-methylimidazolium chloride in binary mixtures with sodium oleate and sodium linoleate. Surface tension measurements indicate significant non-ideality, with much lower critical micelle concentrations (CMCs) and very negative interaction parameters, owing to strong synergistic interactions. Electrostatic ion pairs form upon mixing oppositely charged surfactants, providing enhanced electrostatic interactions relative to pure micelles while maintaining thermodynamic stability. This confirms that the micellization process occurs spontaneously and requires very little energy. Here, an apparent difference between the two systems contributes: with stronger interaction parameters and a lower CMC, the C₁₀C₁₀mim Cl-sodium oleate mixture demonstrates more efficient molecular packing as opposed to minor cooperativity (and therefore higher conformational disorder) in the C₁₀C₁₀mim Cl-sodium linoleate system. Frontier molecular orbital, molecular electrostatic potential, and non-covalent interaction analyses are performed using density functional theory (DFT) calculations and provide insight into the nature of these differences, which are attributed to varying levels of unsaturation in the surfactant tail, modulating the spatial distribution of electrostatic interactions and electronic softness. The oleate system exhibits relatively more localized electrostatic interactions, whereas the linoleate system shows comparatively delocalized interaction regions. These observations qualitatively support differences in molecular packing tendencies between the two systems. The structural differences directly affect solubilization behaviour: the greater conformational flexibility and larger free volume of the C₁₀C₁₀mim Cl-sodium linoleate system provide a looser environment for guest molecules, resulting in a higher drug-dissolved fraction than in the more compact C₁₀C₁₀mim Cl-sodium oleate system. Taken together, this work provides a direct link between molecular structure, interaction energetics, and aggregation behaviour: we have shown that unsaturation can couple packing efficiency to configurational freedom, thereby impacting the aggregate composition in mixed micelles.