Micelle shape elongated

The shape and size of self-assembled micellar systems depend on the conditions for a given system. Changes can be induced, e.g., by addition of cosurfactant or salt or by high surfactant concentrations. An aqueous solution of cetyltrimethylammonium chloride (CTAC) exhibits a transition from spherical micelles to elongated cylinders upon addition of chlorate anions, which is shown, for instance, by a strong increase in viscosity [25]. This system was experimentally studied by TRLQ, with results in good agreement with theory, as shown in Fig. 4. 

Molecular dynamics simulations of gemini surfactants in aqueous solutions accounted for the change of micelle shape from spheroidal to elongated upon decreasing spacer carbon number [115]. They also predicted the formation of branched thread-like micelles. Oligomeric surfactants are capable of forming such structures because the different alkyl chains of such a surfactant can take different relative orientations in the micelles. 

As the concentration is increased, the micelles may remain spheroidal or grow and become oblate (disklike) or prolate (or elongated, cylindrical, or rodlike), with the prolate shape much more often encountered than the oblate shape. The micelle shape is determined by the value of the surfactant packing parameter P given by ... 

An alternative proposal might be that bile salts elongate in ribbonlike structures with increased counterion concentration. These micelles would be very asymmetric. However, in all previous studies where the shape of the bile salt micelles has been studied (11) it appeared to be almost spherical. This tends to rule out an elongated ribbon-like structure. 

The exact shapes of the micelles are unknown, and this subject is open for discussion. Possible micellar structures could be spherical or nearly spherical over a wide range of concentrations not too far from the c.m.c. In a highly concentrated solution, the micellar shape is elongated and forms larger, nonspherical (i.e., cylindrical or lamellar) liquid structures, as illustrated in Figure 4.21. The size of a spherical micelle is determined by the length of the hydrocarbon chain in the... 

In highly concentrated solution, a gradual change in micellar shape is thought to occur with many ionic systems, the micelles elongating to form cylindrical stmctures (see Fig. 6.27). 

It is also known that additives may change the size and shape of micelles.At a certain point, as the surfactant or additive concentrations change, ionic micelles may change shape from spherical or nearly spherical to rodlike or other elongated forms. This may also affect the solubilization of the additive. It appears that alkane solubilization increases as the micelles become large, rodlike aggregates, whereas for polar additives like alcohols the solubilization decreases. ... 

Fig. 6 Cryo-TEM micrographs show the shape of the aggregates in the course of the vesicle-to-micelle transformation in the MGO/CTAB mixture. (A) Coexistence of globular micelles, elongated micelles that are probably ribbon-like, and vesicles (lipid mole fraction 0.47). (B) Perforated vesicles (lipid mole fraction 0.64). Reproduced from Ref. [30] with permission of the American Chemical Society.

---