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

A systematic investigation of influence of the length of the side alkyl chain and the counter-ions on thethermodynamics 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 ([C(10)mim] Cl), 1-dodecyl-3-methylimidazolium ([C(12)mim] Cl), 1-tetradecyl-3-methylimidazolium ([C(14)mim] Cl) and 1-hexadecyl-3-methylimidazolium ([C(16)mim] Cl) chlorides, whereas the influence of counter-ion was studied at the micellization of 1-dodecyl3-methylimidazolium chloride ([C(12)mim] Cl), bromide ([C(12)mim] Br), iodide ([C(12)mim] I), acetate([C(12)mim] OAc), methanesulfonate ([C(12)mim] OMs), toluen sulfonate (C(12)mim] OTs), trifluromethane sulfonate ([C(12)mim] OTf), trifluoro acetate (C(12)mim] TFA) and salicylate ([C(12)mim] Sal) in water. From ITC experimental data, the corresponding standard thermodynamic parameters of micellization(enthalpy, Delta MH0; Gibbs free energy, Delta MG(0); entropy, Delta MS0; heat capacity change, Delta Mc(p)(0)) were estimatedby 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 functionswas observed, however the entropy-enthalpy compensation turned out as a matter of fact, arising fromthe relationship Delta G = Delta H -T Delta S.Delta(M)c(p)(0)values were further discussed in terms of the removal of solventaccessible surface areas of SAILs and counter-ion from the contact with water after micellization. (C) 2017 Elsevier B.V. All rights reserved.