Thermodynamics of micellization of cationic surfactants based on O-alkyl and O-perfluoro-N,N′-diisopropylisourea: Effect of the counter ion nature

A thermodynamic study on the micellization process of two series of cationic surfactants, viz. the O-dodecyl-N,N′-diisopropylisourea hydrochloride, hydrobromide, and hydroiodide and the O-tridecafluorooctyl-N,N  ′-diisopropylisourea hydrochloride, and hydrobromide, is reported. In order to explain the effect of the counter ion nature in the micellization process, thermodynamic parameters to include the standard free energy (ΔGmic∘), enthalpy (ΔHmic∘), entropy (ΔSmic∘), and heat capacity (ΔCp,mic∘) of micellization have been discussed. These parameters are evaluated from the variation of the critical micelle concentration (CMC) with temperature by fitting these values to expressions derived on the basis of a micellization thermodynamic model. The heat capacities of micellization (ΔCp,mic∘) were determined from the temperature dependence of ΔHmic∘. For both surfactant series, the free energy of micellization is weakly temperature dependent and becomes more negative with increasing the size of the counter ion, indicating a more favored micellization. The enthalpy and entropy of micellization decrease strongly with increasing temperature. The heat capacities of micellization (ΔCp,mic∘) were found to be negative. The enthalpy–entropy compensation phenomenon was observed for all the surfactants studied.

The pseudo phase thermodynamic model was applied to study the process of micellization of O-dodecyl- and O-tridecafluorooctyl-N,N′-diisopropylisourea hydrohalogenides. Free energy of micellization is weakly temperature dependent and decreased slightly with the size of the counterion, indicating a more favored micellization. Enthalpies and entropies of micellization decrease strongly with increasing temperature. The enthalpy–entropy compensation phenomenon was observed for all the surfactants studied.Figure optionsDownload as PowerPoint slideHighlights
► The pseudo phase thermodynamic model was applied to study the process of micellization of O-dodecyl- and O-tridecafluorooctyl-N,N′-diisopropylisourea hydrohalogenides.
► Free energy of micellization is weakly temperature dependent and decreased slightly with the size of the counterion, indicating a more favored micellization.
► Enthalpies and entropies of micellization decrease strongly with increasing temperature.
► The enthalpy–entropy compensation phenomenon was observed for all the surfactants studied.