Please cite this article in press as: M.T. Garcia, et al., Self-assembly and antimicrobial activity of long-chain amide-functionalized ionic liquids in aqueous solution, Colloids Surf. B: Biointerfaces (2014), http://dx.doi.org/10.1016/j.colsurfb.2014.09.033 ARTICLE IN PRESS G Model COLSUB-6634; No. of Pages 8 Colloids and Surfaces B: Biointerfaces xxx (2014) xxx–xxx Contents lists available at ScienceDirect Colloids and Surfaces B: Biointerfaces j o ur nal ho me pa ge: www.elsevier.com/locate/colsurfb Self-assembly and antimicrobial activity of long-chain amide-functionalized ionic liquids in aqueous solution M. Teresa Garcia a,∗ , Isabel Ribosa a , Lourdes Perez a , Angeles Manresa b , Francesc Comelles a a Department of Chemical and Surfactant Technology, Institute of Advanced Chemistry of Catalonia IQAC-CSIC, Jordi Girona 18-26, 08034 Barcelona, Spain b Facultat de Farmacia, Universitat de Barcelona, Joan XXIII, 08028 Barcelona, Spain a r t i c l e i n f o Article history: Received 20 June 2014 Received in revised form 4 September 2014 Accepted 15 September 2014 Available online xxx Keywords: Functionalized ionic liquids Surface activity Aggregation behaviour Antimicrobial activity a b s t r a c t Surface active amide-functionalized ionic liquids (ILs) consisting of a long alkyl chain (C 6 C 14 ) connected to a polar head group (methylimidazolium or pyridinium cation) via an amide functional group were syn- thesized and their thermal stability, micellar properties and antimicrobial activity in aqueous solution investigated. The incorporation of an amide group increased the thermal stability of the functionalized ionic liquids compared to simple alkyl chain substituted ionic liquids. The surface activity and aggrega- tion behaviour in aqueous solution of amide-functionalized ionic liquids were examined by tensiometry, conductivity and spectrofluorimetry. Amide-functionalized ILs displayed surface activity and their criti- cal micelle concentration (cmc) in aqueous media decreased with the elongation of the alkyl side chain as occurs for typical surfactants. Compared to non-functionalized ILs bearing the same alkyl chain, ionic liq- uids with an amide moiety possess higher surface activity (pC 20 ) and lower cmc values. The introduction of an amide group in the hydrophobic chain close to the polar head enhances adsorption at the air/water interface and micellization which could be attributed to the H-bonding in the headgroup region. The antimicrobial activity was evaluated against a panel of representative Gram-negative and Gram-positive bacteria and fungi. Amide-functionalized ILs with more than eight carbon atoms in the side chain showed broad antimicrobial activity. Antibacterial activities were found to increase with the alkyl chain length being the C 12 homologous the most effective antimicrobial agents. The introduction of an amide group enhanced significantly the antifungal activity as compared to non-functionalized ILs. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Ionic liquids are currently receiving increasing interest as a new class of non-molecular, ionic solvents in organic synthesis, catalysis, separation processes and preparation of nanostructured materials [1–6]. Most widely studied ILs are comprised of bulky, asymmetric N-containing organic cations in combination with any wide variety of anions, ranging from simple inorganic anions to more complex organic species [1]. Due to their negligible vapour pressure and non-flammability they are regarded as environmen- tally friendly alternatives to volatile organic solvents. In addition, their properties such as hydrophilicity/hydrophobicity and misci- bility with organic solvents or water can be tuned by changing the N-alkyl substituents and/or the anion [7,8]. Moreover, the incor- poration of functional groups as a part of the cation and/or anion ∗ Corresponding author. Tel.: +34 34006100; fax: +34 32045904. E-mail address: teresa.garcia@iqac.csic.es (M.T. Garcia). can impart a particular capability to the ionic liquid for specific purposes [9]. For example, long-chain ether-functionalized ionic liquids have been reported as novel media for specific homogenous enzymatic reactions [10] and a hydrophobic amide-functionalized ionic liquid has been used for selective extraction of tryptophan with higher extraction efficiency [11]. Imidazolium and pyridinium ionic liquids with alkyl side chains longer than four are generally regarded as amphiphilic com- pounds and behave as typical cationic surfactants in aqueous solution [12–16]. The self-organization of ionic liquids in aque- ous solution may have significant effects on a variety of processes such as the extraction of compounds from IL-containing sys- tems, the synthesis and purification of bulk ILs, the formation of dispersed or phase separated systems, etc. [17]. On the other hand, it can be expected that the incorporation of a specific func- tional group in the alkyl side chain attached to the cation affects the surface properties of amphiphilic ILs. Recent studies have reported that COOH-functionalized imidazolium ILs [18], long- chain -hydroxy--alkyloxy-N-methylimidazolium ILs [19] and http://dx.doi.org/10.1016/j.colsurfb.2014.09.033 0927-7765/© 2014 Elsevier B.V. All rights reserved.