Micellization and antimicrobial properties of surface active ionic liquids containing cleavable carbonate linkages

Abstract

Imidazolium-based ionic liquids (ILs) containing cleavable carbonate linkages, 1-alkyloxycarbonyloxyethyl-3-methylimidazolium chlorides with alkyl chains of 10, 12 and 14 carbon atoms, were synthesized and their self-assembly behavior and antimicrobial activity investigated. Differential scanning calorimetry and polarized optical microscopy studies reveal that carbonate-functionalized ILs form stable thermotropic smectic liquid crystalline phases in a wide range of temperatures. Surface activity and aggregation behavior of these new ILs were investigated by tensiometry, conductometry, potentiometry and spectrofluorimetry. The size of aggregates was examined by dynamic light scattering (DLS). Carbonate-functionalized ILs display higher adsorption efficiency and lower critical micelle concentration (cmc) than simple alkyl chain substituted ILs. The insertion of a carbonate ester moiety in the alkyl side chain favors adsorption at the air-water interface and micellization in the bulk solution when compared to non-functionalized ILs. DLS measurements show that small micelle-like aggregates are spontaneously formed above the cmc. Furthermore, carbonate-functionalized ILs were examined for their antimicrobial activity against a panel of clinically relevant microorganisms. Biological activity was found to increase with hydrophobicity. The presence of a carbonate ester moiety significantly enhances the antimicrobial efficiency as compared to non-functionalized ILs, being particularly remarkable the susceptibility of Staphylococcus sp. towards the action of these compounds. It has been demonstrated that the functionalization of the alkyl side chain of the imidazolium salts can modify not only the aggregation behavior but also lead to differences in both efficiency and spectrum of antimicrobial activity of amphiphilic ILs.