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Synthesis, Characterization and Thermodynamic Parameters of Anionic Gemini Surfactants with Different Spacer Groups

Abstract The aim of this work is to investigate the micellization and adsorption thermodynamics (ΔmicH°, ΔmicG°, ΔmicS°, ΔadsH°, ΔadsG°, ΔadsS°) for three novel anionic Gemini surfactants with the same structure except the spacer length of the polymethylene chain have been synthesized based on nonylphenol (NP). The surface tension at cmc (γcmc) and critical micelle concentration (cmc) of the surfactants in aqueous solutions have been investigated. C20 values (surfactant concentration in the solution phase that will reduce the surface tension of the solvent by 20 mNm−1) were also obtained. The critical micellar concentration (cmc) and the minimum average area per surfactant molecule (Amin) at the air-water interface of Gemini surfactant decrease with increasing number of methylene moieties in the spacer and, consequently, with the enhanced hydrophilic character of the molecule. The results were discussed with particular emphasis on the effect of the length spacer on the hydrophilic character of the spacer group. The surface studies of the molecules confirm that the exothermic values of the differential enthalpy of micellization (ΔmicH°) decrease as the methylene moieties are lengthened. Moreover, the values of the free energy (ΔmicG°) and entropy (ΔmicS°) of micellization indicate an entropy driven phenomenon for all surfactants. The effect of length spacer on surfactant adsorption for Gemini surfactants was utilized to calculate such thermodynamic parameters as the free energy of adsorption (ΔadsG°) and the heat of adsorption (ΔadsH°). The low value of ΔadsH° is an evidence for the physical adsorption of anionic surfactants. The relatively large value of entropic contribution (TΔadsS°) indicates that the adsorption of anionic surfactants is entropically governed.