Thermodynamics of micellization of alkylimidazolium surfactants in aqueous solution

Alkylimidazolium salts are a very important class of compounds. So far, calorimetry has hardly been used to characterize their solution behaviour. The enthalpies obtained from indirect methods have an intrinsic large uncertainty, and nowadays it is clear that calorimetry is the most sensitive technique for directly measuring the thermodynamic properties of aggregation. In this work, isothermal titration calorimetry (ITC) was used along with conductivity to determine the thermodynamics of aggregation of 1-alkyl-3-methylimidazolium chlorides ([C(n)mim]Cl, n=8, 10, 12, and 14) in aqueous solution. The critical micelle concentrations, cmc, were obtained from conductivity and calorimetry, and the enthalpies of micelle formation, Delta H-mic. were derived from the calorimetric titrations. From conductivity, we could also derive the values for the degree of ionisation of the micelles (a), the molar conductivity (Delta(M)) of the [C(n)mim]Cl micellar species and the molar conductivity at infinite dilution (Delta(infinity)) for the [C(n)mim](+) cations. Values are therefore reported for the enthalpy (Delta H-mic), the Gibbs free energy (Delta G(mic)) and entropy (Delta S-mic) changes for micelle formation, Further, the aggregate sizes and aggregation numbers were obtained by light scattering (LS) measurements. The observed variation of the thermodynamic properties with the alkyl chain length is discussed in detail and compared with the traditional cationic surfactants 1-alkyl-trimethylammonium chlorides, [C(n)TA]CI. The difference in the values of the thermodynamic parameters for both types of surfactants is here related to the structure of their head groups. (c) 2008 Elsevier Ltd. All rights reserved.