Micelle formation of alkylimidazolium ionic liquids in water and in ethylammonium nitrate ionic liquid: A calorimetric study

Using isothermal titration calorimetry heats of dilution of the aqueous ionic liquid solutions [C4MIM][C8SO4], [C12MIM]Cl, [C16MIM][BF4], and [C16MIM]Cl have been measured in the concentration range of (1–80, 1–35, 0.2–2.2, and 0.1–2.6) mmol dm−3 respectively at different temperatures between 283 K and 338 K. Formation of micelles has been observed. The CMC, the equilibrium constant of micellization as well as the enthalpy of micellization has been determined from the experimental data using a mass-action model with 3–5 parameters adjusted to the experimental data points of the heat of dilution over the whole concentration range. Three modified versions of the model have been tested providing similar results with exception of [C16MIM][BF4] solutions where one of the versions failed. The consistency of the model has been tested by comparing the model parameters of our own experimental results of Na[C12SO4] solutions with those obtained from experimental data of other authors in the literature. Our evaluation process shows that the CMC decreases in the order of [C4MIM][C8SO4], [C12MIM]Cl, [C16MIM]Cl and [C16MIM][BF4]. The enthalpy of micellization changes its sign with temperature in all cases studied in aqueous solutions. It is positive at lower temperatures and becomes negative at higher temperatures. Also solutions of [C16MIM][BF4] and [C16MIM]Cl in ethylammonium nitrate (EAN) have been studied in the same way. In contrast to the aqueous solutions the enthalpy of micellization is always negative without significant dependence on temperature.