The Influence of Ionic Liquids on Micellization of Sodium Dodecyl Sulfate in Aqueous Solutions

The micellization of sodium dodecyl sulfate (SDS) in water and in aqueous solutions of three imidazolium based ionic liquids with different side-chain length, i.e. 1,3-dimethylimidazolium chloride ( [C(1)mim] Cl), 1-ethyl-3-methylimidazolium chloride ([C(2)mim] Cl), and 1-butyl-3-methylimidazolium chloride ([C(4)mim] Cl) was investigated by isothermal titration calorimetry (ITC) in the temperature range from 288.15 to 328.15 K. For comparison, the micellization of SDS in the presence of NaCl was studied also. ITC experimental data were analysed by the two-state mass-action model, yielding the values of critical micelle concentration (cmc), aggregation number (n), standard heat capacity (Delta(M)c(p)degrees), enthalpy (Delta H-M degrees), entropy (Delta S-M degrees), and Gibbs free energy (Delta(M)G degrees) of the micellization process. It was found that the micellization of SDS in all the studied systems is an entropy-driven at lower temperatures and an enthalpy-driven at higher temperatures. In addition, it was assumed that with the increasing nonpolar character of IL, the interactions between the SDS are stronger, leading to more negative values of Delta H-M degrees and Delta(M)G degrees. To obtain more information about the micellar charge, the conductivity and zeta-potential measurements were performed at 298.15 K. Presumably the micellar charge is more positive in the presence of ILs due to their stronger interaction and possible incorporation into the micellar structure. This reflects in less negative zeta-potential comparing to SDS in water and consequently higher degrees of micelle ionization due to the larger portion of sodium ions in solution.