Thermodynamic and computational study of isomerism effect at micellization of imidazolium based surface-active ionic liquids: Counterion structure

Abstract

In this work the influence of benzoate, ortho-, meta- and para-hydroxybenzoate anions, as counterions, on micellization process of 1-dodecyl-3-methyl-imidazolium cation in water is investigated by isothermal titration calorimetry (ITC) in the temeprature range between 278.15 and 318.15 K. ITC experimental data were analyzed by the help of an improved mass-action model, yielding the values of critical micelle concentration, cmc, the degree of counterion binding, β, aggregation number, n, standard heat capacity, ΔMcpθ, enthalpy, ∆MHθ, entropy, ∆MSθ, and Gibbs free energy, ∆MGθ of micellization. It was found that the investigated systems behave mainly like common ionic surfactants and already investigated SAILs: the micellization process of investigated systems is entropically driven at low temperatures, whereas at high temperatures the enthalpic contribution becomes equally important. The last is especially important in the case of ortho-hydroxybenzoate, which incorporates into micellar structure affecting also the entropy-enthalpy compensation. But evidently, the presence and position of –OH group in the counterion influence considerably the micellization process.ΔMcpθ values were further discussed in the light of the removal of water molecules from contact with nonpolar surface area upon micelle formation. All the values are negative, the most in the case of benzoate anion, which could be ascribed to the absence of –OH group.To refine the thermodynamic parameters obtained from ITC, the molecular simulations were performed. First, it is shown that the binding energies between anion and cations increase in the order from benzoate to para-, meta- and ortho-hydroxybenzoate, which coincide with ∆MHθ values. Second, it is demonstrated that water around counterions is strongly perturbed leading to differences in ΔMcpθ.