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.

Graphical abstractThe 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 full-size imageDownload 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.