Abstract
The micellization of sodium dodecyl sulfate (SDS) in water and aqueous solutions of NaCl (0.1 M) and three ionic liquids (IL, 0.01 M) with different side-chain length, i.e. 1,3-dimethylimidazolium chloride, 1-ethyl-3-methylimidazolium chloride, and 1-butyl-3-methylimidazolium chloride was investigated in the temperature range from 288.15 to 328.15 K using isothermal titration calorimetry (ITC). Thermodynamics of micellization was studied by the fitting of the mass-action model to the ITC data. The micellization of SDS in all the studied systems is an entropy-driven at lower temperatures and an enthalpy-driven at higher temperatures. It was also found that with the increasing nonpolar character of IL the interactions between the SDS should be stronger leading to more negative Gibbs free energy and enthalpy of micellization. To obtain information about the micellar charge, the conductivity and zeta-potential measurements were performed at 298.15 K. It is assumed that the charge screening between negative sulfate heads is more efficient in the presence of ILs due to their possible incorporation into the micellar structure reflecting in less negative zeta-potential of micelles comparing to SDS in water and consequently higher degrees of micelle ionization due to the larger portion of sodium ions in solution.
Highlights
As explained already in Introduction, the impact of lower concentrations of NaCl (e.g. 0.01 M)[12] on the micellization process of sodium dodecyl sulfate (SDS) is less pronounced in terms of cmc and thermodynamic parameters comparing to investigated ionic liquids (ILs), all experiments were performed in 0.1 M NaCl solutions
Resulting cmc values are similar comparing to 10-times less concentrated solutions of ILs which emphasize the effect of organic additives on micellization process
It is evident that the values of ζ-potential after cmc in 0.01 M IL and 0.1 M NaCl solutions are less negative than in water. This can be attributed to the lower micellar charge, due to possible incorporation of IL cations, or more efficient screening around SDS micelles
Summary
Sodium dodecyl sulfate (SDS), known as sodium lauryl sulfate (SLS), is a well-known anionic surfactant, widely used in cleaning and hygiene products,[1] as a food additive[2] and in research, as a cell disruptor, denaturating agent etc.[3,4,5] It belongs to one of the most studied surfactants and several characteristics of its micellization processes in aqueous solutions as, for example, the influence of inorganic electrolytes on critical micelle concentration (cmc), the shape of micelles and thermodynamic parameters of micellization of SDS are well-known.[6,7,8,9,10,11,12] the presence of organic electrolytes usually affects these parameters in much more dramatic way if their hydrophobic chains can penetrate the micelles,[13] as was already observed for many other surfactant systems.[14,15,16,17,18,19,20] For an investigation of these effects, ionic liquids (ILs) as the most studied organic electrolytes in the last decades[21,22] seem to be the most appropriate. We estimated a degree of micelle ionization (α) which will be discussed in the light of the determined zeta-potentials (ζ)
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