Thermodynamic study for micellization of imidazolium based surface active ionic liquids in water: Effect of alkyl chain length and anions

Abstract

A systematic investigation of influence of the length of the side alkyl chain and the counter-ions on the thermodynamics of the micellization process at imidazolium based surface active ionic liquids (SAILs) in aqueous solutions was carried out by isothermal titration calorimetry (ITC) in a broad temperature range. The effect of alkyl chain length on the micellization process in water has been investigated on 1-decyl-3-methylimidazolium ([C10mim]Cl), 1-dodecyl-3-methylimidazolium ([C12mim]Cl), 1- tetradecyl-3-methylimidazolium ([C14mim]Cl) and 1-hexadecyl-3- methylimidazolium ([C16mim]Cl) chlorides, whereas the influence of counter-ion was studied at the micellization of 1-dodecyl-3-methylimidazolium chloride ([C12mim]Cl), bromide ([C12mim]Br), iodide ([C12mim]I), acetate ([C12mim]OAc), methanesulfonate ([C12mim]OMs), toluen sulfonate ([C12mim]OTs), trifluromethane sulfonate ([C12mim]OTf), trifluoro acetate ([C12mim]TFA) and salicylate ([C12mim]Sal) in water. From ITC experimental data, the corresponding standard thermodynamic parameters of micellization (enthalpy; Gibbs free energy; entropy; heat capacity change) were estimated by help of the mass-action model. It was found that SAILs behave mainly like common cationic surfactants, where cmc values are decreasing with the length of the side alkyl chain and exhibit U-shape dependence on temperature. Remarkable influence of counter-ions on cmc and all thermodynamic functions was observed, however the entropy-enthalpy compensation turned out as a matter of fact, arising from the relationship ΔG = ΔH − TΔS. values were further discussed in terms of the removal of solvent accessible surface areas of SAILs and counter-ion from the contact with water after micellization.