Cationic silicon-based gemini surfactants: Effect of hydrophobic chains on surface activity, physic-chemical properties and aggregation behaviors

Abstract

Abstract A series of cationic silicon-based gemini surfactants (C m -PSi-C m ) having different hydrophobic chains length (m = 8, 10, 12, 14, 16, 18) were synthesized. Subsequently, surface parameters including equilibrium surface tension (γ CMC ), critical micelle concentration (CMC), surface pressure at CMC ( Π CMC ) and maximum surface excess ( Γ max ) were calculated according to the surface tension measurements. The importance of hydrophobic chains length on the surface activity of C m -PSi-C m was revealed in detail. With the increasing hydrophobic chains length, the equilibrium surface tension (γ CMC ) values follow the order of C 8 -PSi-C 8 > C 10 -PSi-C 10 > C 12 -PSi-C 12 14 -PSi-C 14 16 -PSi-C 16 18 -PSi-C 18 . The thermodynamic parameters of micellization process, namely, standard Gibbs free energy (ΔG 0 m ), enthalpy (ΔH 0 m ) and entropy (ΔS 0 m ), were derived from conductivity measurements at different temperatures. The aggregation of the cationic silicon-based gemini surfactants (C 12 -PSi-C 12 ) in aqueous solution at different concentrations was studied by DLS and TEM measurements. The formation of micelles and vesicles in C 12 -PSi-C 12 solutions was observed at a concentration well above the CMC. Moreover, the morphology of vesicles changed from sphere to rod-like, dumbble-like and finally string dumbbell-like with the increasing of the surfactant concentration.