A series of novel cationic gemini surfactants with diethylammonium headgroups and a diamido spacer were synthesized, and their surface and bulk properties were investigated by surface tension, electrical conductivity, fluorescence, viscosity, dynamic light scattering (DLS), and transmission electron microscopy (TEM) measurements. An interesting phenomenon, that is, the obvious decline in surface tension upon increasing concentration above the critical micelle concentration (cmc), was found in these gemini surfactant solutions, and two explanations were proposed. This surface tension behavior could be explained by the rapid increase in the counterion activity in the bulk phase or the continued filling of the interface with increasing surfactant concentration above the cmc. More interestingly, not only vesicles but also the surfactant-concentration-induced vesicle to larger aggregate (spongelike aggregate) transition and the salt-induced vesicle and spongelike aggregate to micelle transition were found in the aqueous solutions of these gemini surfactants. The spongelike aggregate that is first reported in the cationic gemini surfactant-water binary system is probably caused by the adhesion and fusion of vesicles at high surfactant concentration.

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Three novel anionic sulfonate gemini surfactants, sodium 4,4'-(10,19-dioxo-9,11,18,20-tetraazaoctacosane-9,20-diyl) dibenzenesulfonate (Surfactant I), sodium 4,4'-(12,21-dioxo-11,13,20,22-tetraazadotriacontane-11,22-diyl) dibenzenesulfonate (Surfactant II), and sodium 4,4'-(14,23-dioxo-13,15,22,24-tetraazahezatriacontane-13,24-diyl) dibenzenesulfonate (Surfactant III), with different lengths of hydrophobic tail have been synthesized, and their assembly behavior in the presence of bovine serum albumin (BSA) has been studied using spectral methods and molecular modeling methods at physiological pH and 298 K. Critical micelle concentrations (CMCs) of the three surfactants have been determined by surface tension measurements. Despite the obvious decrease of CMC with the increase of tail length, fluorescence spectra have shown much closer CAC in the presence of BSA. Surfactant II shows the highest CAC of 3.19 × 10(-5) mol L(-1) compared with the other two. The polarity of the microenvironment in BSA-surfactant systems has been investigated using pyrene as the probe. In addition, far-UV CD spectra studied the change of the secondary structure content of BSA caused by the three surfactants. The features of the assembly behavior were discussed by three concentration regions. Surfactant II could unfold the protein much more efficiently than the other two surfactants at low concentration, but at high concentration, the change of the secondary structure and the formation of hydrophobic microenvironment show a direct relationship to the length of the hydrophobic tail with the increase of the surfactant concentration.

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