Effect of aquo-organic solvents on the aggregation of hydroxyethyl-based dimeric surfactants

The self-assembly behavior of Gemini surfactants i.e. alkanediyl-α,α-bis(hydroxyethylmethylhexadecylammonium bromide) (16-s-16 MEA, 2Br−; s = 4 and 6) was investigated in pure water and in water–organic mixtures (10 and 20 % v/v) of acetonitrile (ACN), dimethyl sulfoxide (DMSO), and dimethylformamide (DMF) by conducticty and surface tension method at 300–320 K. The critical micelle concentration (CMC), degree of micellar ionization (α), surface excess concentration (Γmax), minimum molecular area at the interface (Amin), surface pressure at the CMC (πCMC), and thermodynamic parameters i.e. standard Gibbs free energies of micellization (ΔG°m), Gibbs free energy of adsorption (ΔG°ads), Gibbs free energy of transfer (ΔG°trans), standard Gibbs free energy of micellization per mole of surfactant monomer (ΔG°tail), enthalpy (ΔH°m), and entropy (ΔS°m), of micellization of the Gemini surfactant have also been determined. The CMC value increases with increasing spacer of Gemini surfactant (16-4-16, MEA 2Br−< 16-6-16, MEA 2Br−), volume percentage of solvents and temperatures. Acetonitrile shows the higher CMC values as compare to other solvents. The surface excess concentration value increases with inceasing volume perentage of solvents. The negative free energy of adsorption shows a greater propensity for adsorption at the air-solution interface than for micellization.