The surface activity and aggregation behavior of catanionic mixtures of imidazolium- or pyridinium-based surface-active ionic liquids (SAILs) and sodium N-lauroyl sarcosinate (Na-LS) in aqueous solution were investigated. The effects of the alkyl chain length, the polar head group and functional groups in the SAIL molecule on the interfacial properties and self-assembly of the catanionic mixtures were evaluated by surface tension and dynamic light scattering measurements. In addition, the toxicity of the catanionic surfactant mixtures against bacteria and fungi was studied. The SAIL-LS mixed systems reduced water surface tension with high effectiveness and efficiency, and had a lower critical aggregation concentration compared to the individual components. In all the catanionic systems investigated, synergistic effects caused by strong electrostatic and hydrophobic interactions resulted in negative interaction parameter values, which increased with the alkyl chain length and in the order of non-functionalized < amide-functionalized < ester-functionalized SAILs. The structure of the mixed aggregates in the SAIL-LS catanionic systems depended on the hydrophobicity of the SAIL, the micelle-vesicle transition being driven by the alkyl chain length. The catanionic systems exhibited similar microbial toxicity to that of individual SAIL components and behaved like broad-spectrum antimicrobials.

The authors thank the Spanish Ministry of Science and Innovation (CTQ2017-88948-P) for financial support.

Peer reviewed