Synthesis and surface-active properties of sulfobetaine-type zwitterionic gemini surfactants

Sulfobetaine-type zwitterionic gemini surfactants of 1,2-bis[N-methyl-N-(3-sulfopropyl)-alkylammonium]ethane (2CnSb, where n represents hydrocarbon chain lengths of 6, 8 and 10) were synthesized by the reaction of N,N′-dimethylethylenediamine with alkyl (hexyl, octyl or decyl) bromide, followed by the reaction with 1,3-propane sultone. The physicochemical properties of 2CnSb were characterized by equilibrium and dynamic surface tension, dynamic light-scattering (DLS) and pyrene fluorescence spectroscopy techniques. 2CnSb showed poor solubility in water at 25 °C. The values of critical micelle concentration (cmc) and surface tension at the cmc for 2CnSb became small with increasing hydrocarbon chain length. The dynamic surface tension measurements indicated that the longer the hydrocarbon chain length of 2CnSb becomes, the slower the adsorption rate to the air/water interface is. The fluorescence intensity ratio and the scattering intensity by DLS changed sharply at the concentrations around the cmc. The apparent hydrodynamic diameter obtained from DLS for 2C8Sb and 2C10Sb micelles was determined to be 1.2 ± 0.7 and 2.2 ± 1.0 nm, respectively, suggesting the formation of very small micelles.