Alkyl chain length, counter anion and temperature effects on the interfacial activity of imidazolium ionic liquids: Comparison with structurally related surfactants

• Long-chain imidazolium ionic liquids strongly adsorb at toluene–water interface.
• Alkyl chain length, anion kind and temperature have influence on interfacial tension.
• Micelle formation depends on the number of carbons in alkyl chain and the kind of anion.
• The Frumkin adsorption isotherm can successfully reproduce the experimental data.
• Interfacial activity of ionic liquids is superior to conventional related surfactants.

Adsorption of three long-chain imidazolium ionic liquids (ILs) at toluene–water interface was studied along with their micelle formations at temperatures within (293.2–313.2) K. The used ILs act as excellent surfactants and under the same conditions, their interfacial activity is reasonably appeared in the order of [C16mim][Br] > [C16mim][Cl] > [C12mim][Cl]. The experimental data were satisfactorily reproduced by the Frumkin adsorption isotherm for concentrations less than critical micelle concentration (CMC). Accordingly, the highest maximum interface excess and adsorption equilibrium constant were for [C16mim][Br] because of its long alkyl chain as well as more polarizable anion. In this regard, the balance between electrostatic repulsion and van der Waals attraction provides a stronger attractive interaction for this IL. On the other hand, by rising temperature, the interface excess concentration, adsorption tendency and electrostatic repulsion between adsorbed molecules increase. The results show that, within the same concentration ranges, the maximum interfacial tension reduction were 72% for [C12mim][Cl] and [C16mim][Br] ILs, compared with 68% for similarly structure DTMAC and CTAB surfactants. Also, these ILs, respectively, have the CMC values about 81% and 63% of related surfactants.

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