Enthalpy–entropy compensation of ionic liquid-type Gemini imidazolium surfactants in aqueous solutions: A free energy perturbation study

Molecular dynamics simulations and free energy perturbation (FEP) were performed for studying the enthalpy–entropy compensation with a series of ionic liquid-type Gemini imidazolium surfactants (ILGIS), with different carbon atoms of the hydrophobic group or the spacer chain length, in aqueous solutions. According to the law of mass action, the thermodynamic properties of micellization in aqueous solutions for ILGIS were discussed. The results show that the solvation free energy changes calculated from the free energy perturbation are close to the Gibbs free energy calculated from the surface tension method and can be used to discriminate the tendency for micellization and predict the thermodynamic properties of ILGIS. The micellization of ILGIS in aqueous solutions is a spontaneous and entropy-driven process. It is enthalpy–entropy compensated, and the enthalpy–entropy compensation plots exhibit an excellent linearity. The compensation temperature was found to be (307 ± 2) K. As the number carbon atoms in the alkyl chains is increased, the tendency and stability of micellization both increase. At spacer length S ⩽ 6, with the spacer chain length increasing, the thermodynamic favorability and stability of the micelles decrease. However, if S > 6, thermodynamic favorability and stability increase with raising the spacer chain length.

We used the free energy perturbation method to investigate the thermodynamic properties of micellization in aqueous solutions for several ionic liquid-type Gemini imidazolium surfactants.Figure optionsDownload high-quality image (88 K)Download as PowerPoint slideHighlights
► Thermodynamic properties (ΔGmθ, ΔHmθ and ΔSmθ) of a series of ionic liquid-type Gemini imidazolium surfactants were predicted with free energy perturbation method.
► Micellization of ionic liquid-type Gemini imidazolium surfactants is entropy-driven and enthalpy–entropy compensated. The compensation temperature is (307 ± 2) K.
► The contribution of entropy to the negative values of ΔGmθ begins to decrease with raising the temperature while the contribution of enthalpy is increased.
► A longer hydrophobic chain length will enhance the formation and the stability of the micelles.
► At high S (S > 6), a longer spacer chain length favors the formation and the stability of the micelles but is unfavorable at low S.