Micelles Gemini surfactants

Oda, R., Panizza, P., Schmutz, M., Lequeux, F. Direct evidence of the shear-induced structure of wormlike micelles gemini surfactant 12-2-12. Langmuir 1997, 75(24), 6407-6412. 

Furthermore, Oda et al. pointed out that there are two topologically distinct types of chiral bilayers, as shown in Figure 5.46.165 Helical ribbons (helix A) have cylindrical curvature with an inner face and an outer face and are the precursors of tubules. These are, for example, the same structures that are observed in the diacetylenic lipid systems discussed in Section 4.1. By contrast, twisted ribbons (helix B) have Gaussian saddlelike curvature, with two equally curved faces and a C2 symmetry axis. They are similar to the aldonamide and peptide ribbons discussed in Sections 2 and 3, respectively. The twisted ribbons in the tartrate-gemini surfactant system were found to be stable in water for alkyl chains with 14-16 carbons. Only micelles form... 

Xu J, Han X, Liu H L, Hu Y (2005) Synthesis of monodisperse gold nanoparticles stabilized by gemini surfactant in reverse micelles. J Dispers Sci Technol 26 473-476... 

A standard free energy change upon micellization for ionic gemini surfactants with two hydrophilic groups and monovalent counterions can be calculated (Zana, 1996) by taking into account the degree of binding (1 a) of the counterions to the micelle,... 

Fig. 2 Cryo-TEM micrograph of a 0.62 wt% solution of the gemini surfactant 12-2-12 that shows spherical micelles (black dots) and thread-like mieelles. A branched thread-like micelle can be seen at the right side of the micrograph. Bar=100 nm. Reproduced from Ref L14J with permission of the American Chemical Society.

For conventional surfactants with a long alkyl chain (m > 16) and dimeric (gemini) surfactants with m>8, the entry of a surfactant in a micelle is slower than for a diffusion-controlled process. The surfactant residence time and the micelle lifetime can become long with respect to the values found for conventional surfactants. 

Micelle shape and effect on rheology of solutions of gemini surfactants... 

Arguably the most important parameter for any surfactant is the CMC value. This is because below this concentration the monomer level increases as more is dissolved, and hence the surfactant chemical potential (activity) also increases. Above the CMC, the monomer concentration and surfactant chemical potential are approximately constant, so surfactant absorption at interfaces and interfacial tensions show only small changes with composition under most conditions. For liquid crystal researchers, the CMC is the concentration at which the building blocks (micelles) of soluble surfactant mesophases appear. Moreover, with partially soluble surfactants it is the lowest concentration at which a liquid crystal dispersion in water appears. Fortunately there are well-established simple rules which describe how CMC values vary with chain length for linear, monoalkyl surfactants. From these, and a library of measured CMC values (35-38), it is possible to estimate the approximate CMC for branched alkyl chain and di- (or multi-) alkyl surfactants. Thus, most materials are covered. This includes the gemini surfactants, a new fashionable group where two conventional surfactant molecules are linked by a hydrophobic spacer of variable length (38). 

Staying in the outer shell of hydrophobic regions of micelles due to their hydrophobicity and large volumes, and hence reduce the curvature. Gemini surfactant C22-12-22 is very efficient in directing MCM-48 mesostructure even at room temperature. 

---