Temperature effect on adsorption of imidazolium-based ionic liquids at liquid–liquid interface

• The synthesized imidazolium-based ILs act as cationic surface active agent at the interface.
• Surface active properties of the used ILs enhance with their alkyl chain length.
• The CMC of each IL shows a minimum value at around 303 K.
• The experimental data fit quite well with the Frumkin adsorption isotherm.
• Adsorption tendency, effectiveness and interaction exhibit specific variations with temperature.

Interfacial tension variation and micelle formation by two amphiphilic synthesized ionic liquids (ILs), 1-alkyl-3-methylimidazolium chloride ([Cnmim][Cl], n = 7 and 8), are investigated at conventional temperatures, within 293.2–313.2 K. These ILs significantly reduce the interfacial tension of toluene–water system and the influence is more effective for the IL with n = 8 because of the relevant more hydrophobicity character. The relevant critical micelle concentrations (CMCs) depend on temperature with a minimum appeared around 303 K, for both the ILs. The Frumkin adsorption isotherm predicts quite well the interfacial adsorption of the ILs for concentrations less than CMC. Accordingly, the adsorption tendency reveals increasing with temperature. However, the saturated interface excess and absolute electrostatic repulsion get their maximum values at around the same temperature of 303 K.

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