Interfacial and aggregation behavior of dicationic and tricationic ionic liquid-based surfactants in aqueous solution

• Two classes of tricationic IL-based surfactants were synthesized for the first time.
• Aggregation behavior of dicationic and tricationic IL-based surfactants was examined.
• Surfactant aggregation characterized by surface tension, conductivity, and fluorescence.
• Electrolytes enhanced the aggregation of IL-based surfactants at air/water interface.
• CMC values of tricationic surfactants were lower than linear trimeric analogs.

The interfacial and aggregation behavior of three dicationic and two tricationic ionic liquid-based (IL) surfactants in aqueous solutions have been carried out. The studied dicationic IL-based surfactants were 1,12-di(3-butylimidazolium)dodecane bromide ((C4Im)2C12-2Br), 1,12-di(3-decylimidazolium)dodecane bromide ((C10Im)2C12-2Br), and 1,12-di(3-hexadecylimidazolium)dodecane bromide ((C16Im)2C12-2Br). Two classes of trigonal tricationic surfactants containing triethylamine (Am) and 1,3,5-trimethylbenzene (Bn) cores were synthesized for first time and characterized, specifically, 3,3′,3″-octyl-1,1′,1″-(2-ethylamino) imidazolium bromide ((C8Im)3Am-3Br), and 3,3′,3″-octyl-1,1′,1″-(1,3,5)tris(methylene) benzene imidazolium bromide ((C8Im)3Bn-3Br). The critical micelle concentration (CMC) values and other aggregation parameters, including adsorption efficiency (pC20), surface tension at the CMC (γcmc), degree of dissociation (α), and the degree of counterion binding in the micelle (β) were obtained using a variety of techniques including surface tension, conductivity, and fluorescence measurements. For the first time, the aggregation numbers (Nagg) were also obtained for this group of surfactants. A study into the influence of acetonitrile on the stability and aggregation behavior of dicationic and tricationic IL-based surfactants was also undertaken. The CMC values of the dicationic surfactants clearly decrease as a function of the side chain, when the same linker is maintained. The CMC values of the tricationic surfactants were lower than analogous linear trimeric surfactants. For the novel tricationic surfactants, the effect of sodium salts (C6H5COONa, NaBr, and NaI) on the surface activity was also investigated. It was also observed that the surface activity parameters (pC20 and CMC/C20) of tricationic surfactants increased significantly in the presence of added electrolytes.

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