Surface, interfacial and aggregation properties of sulfonic acid-containing gemini surfactants with different spacer lengths.

Four sulfonic acid-containing gemini surfactants 9BA-m-9BA (m=2, 3, 4, 6), 6,6'-(ethane-1,2-diylbis(oxy)) bis(3-nonylbenzenesulfonic acid) (9BA-2-9BA), 6,6'-(propane-1,3-diylbis(oxy)) bis(3-nonylbenzenesulfonic acid) (9BA-3-9BA), 6,6'-(butane-1,4-diylbis(oxy)) bis(3-nonylbenzenesulfonic acid) (9BA-4-9BA), and 6,6'-(hexane-1,6-diylbis(oxy)) bis(3-nonylbenzenesulfonic acid) (9BA-6-9BA), were synthesized and characterized by Fourier transform infrared (FTIR), (1)H NMR, elemental analysis, and melting temperature measurements. Their ability to lower the water surface tension and hexadecane/water interfacial tension was measured and correlated with the hydrophobicity and length of their alkyl spacer chain. Their aggregates in aqueous solutions were investigated using dynamic light scattering and transmission electron microscopy. Spherical vesicles could be found in aqueous solutions of four gemini surfactants with an apparent hydrodynamic radius (Rh) of approximately 100 nm. The critical micelle concentration (cmc) of four gemini surfactants evaluated by surface tension measurements was 1 order of magnitude smaller than that of conventional single-chain surfactant sodium dodecylsulfonate (SDS). And their C20, the gemini surfactant concentration required for lowering the surface tension of water by 20 mN/m, was about 2 orders of magnitude smaller than that of SDS, showing excellent efficiency at reducing the surface tension of water. In addition, the hexadecane/water interfacial tension could be less than 1.0 mN/m after using pure 9BA-m-9BA in water. Using 9BA-6-9BA the hexadecane/water interfacial tension was reduced to 0.21 mN/m.