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.

The synthesis and surface activity of gemini surfactants with ester bond as spacer are described. Their critical micelle concentrations (CMC) are much lower than that of conventional surfactants, i.e., 1.62×10 5 , 1.38×10 5 , 1.28×10 5 mol L 1 for gemini , and respectively. Surface tension at the CMC of gemini , and were 36.4, 38.5, 41.2 mN m 1 . The physico-chemical properties such as Krafft points, foaming abilities and emulsifying power were also investigated. It is found that the title compounds have low Krafft points and show good solubility in water. The gemini surfactants synthesized also exhibit good foaming properties and excellent emulsifying power toward toluene. The foaming abilities and emulsifying power increase with the increase in carbon number of hydrophobic chain, and this might be caused by the sheer viscosity of different gemini surfactant solution.

Four anionic gemini surfactants of the sulfate type C 12 C n C 12 , where n is the spacer chain length (n ¼ 3, 4, 6, and 10) were synthesized. The structures of these surfactants were confirmed by FT-IR, 1 H NMR, ESI mass spectra (ESI-MS), and elemental analysis. The surface-active properties of these compounds were investigated by means of surface tension, electrical conductivity, and fluorescence measurements. Premicellar aggregations were found for the four gemini surfactants, as revealed by the conductivity measurement. The formation of premicellar aggregates may account for the discrepancy between the critical micelle concentration (cmc) obtained by the surface tension and conductivity measurement. The cmc values of these gemini surfactants were much lower than that of sodium dodecylsulfate (SDS) and decreased monotonously with the increase of spacer chain length from 3 to 10. The effect of spacer chain length on the performance properties like foaming, emulsion stability, and lime soap dispersing ability were also studied and discussed.Practical applications: Alkyl sulfate surfactants are one of the most widely used surfactants. The new alkyl sulfate gemini surfactants synthesized in our study are more surface-active than sodium dodecylsulfate. These gemini surfactants possess low critical micelle concentrations, high emulsion stability, and excellent lime soap dispersing ability. They have potential applications in the fields of cosmetics, detergents, etc.