Effects of pH and salinity on adsorption of different imidazolium ionic liquids at the interface of oil–water

Abstract

Adsorption and micelle formation of six short and long alkyl chain imidazolium ionic liquids (ILs), [Cnmim][X] with n = 6–8, 12, 16 and X = Cl−, Br−, were studied at toluene–water interface under different pHs (within 5.5–8.5) at 298.2 K. Further, the adsorption of long alkyl chain ILs (n = 12 and 16) was explored in presence of different uni-univalent salts at the same temperature. By the adsorption of each IL having different alkyl chain length or different anions at the interface, interfacial tension (IFT) decreased and ultimately micelles formed. Interestingly, increase in aqueous phase pH or salinity intensified the reduction in the IFT and CMC values. Experimental data, relevant to concentrations below the CMCs, were satisfactorily reproduced by Frumkin adsorption isotherm and parameters were obtained. For each IL, by rising pH, adsorption equilibrium constant and maximum interface excess increased and interaction parameter tended to positive values because of hydroxyl anion adsorption along with cationic portion of ILs. Besides, the presence of uni-univalent electrolytes caused increase the adsorption equilibrium constant and maximum interface excess to achieve due to salting out effect.