Abstract:

The aggregation behavior and thermodynamics of micellization for three Gemini surfactants with diester and rigid spacers in pure water and 10% (mass fraction) organic alcohol-water co-solvents (MA-WR, EG-WR and GL-WR) across a range of temperatures from 283.15 to 308.15 K were investigated by electrical conductivity measurements. The aggregation behavior parameters, including critical micelle concentration (cmc), degree of counter ion dissociation (α), and the thermodynamic parameters of micellization including standard Gibbs energy (ΔG_m^o), Gibbs energy of transfer (ΔG_trans^o), Gibbs energy of micellization per alkyl tail (ΔG_tail^o), standard enthalpy (ΔH_m^o), and standard entropy (ΔS_m^o) were calculated and discussed. Gemini surfactants with longer hydrophobic chain length have a lower cmc value, which was found to increase with the increasing number of hydroxyl groups of the alcohol in the organic mixed solvent. The temperature dependence of the cmc value for the Gemini surfactants was U-shaped in all the investigated systems. The micellization process is spontaneous, is endothermic at temperatures below 293.15 K, and becomes exothermic at temperatures above 293.15 K. The micropolarity of the Gemini surfactants in pure water and organic alcohol-water co-solvents were evaluated by steady-state fluorescence spectroscopy. The results showed that the hydrophobicity of the microenvironment for Gemini surfactant solutions becomes stronger with the increasing chain length of the surfactants and the number of hydroxyl groups of the alcohol in the organic mixed solvents. The compensation between the enthalpy and entropy for micellization of all the three gemini surfactants were observed in all the studied mixed media.

Key words: Gemini surfactants, Conductivity, Organic alcohol co-solvent, Thermodynamic property