Micellar shape

Since micelles are dynamic structures with a liquid core it is probably unrealistic to regard them as rigid structures with a precise shape. It is, however, instructive to consider an average micellar shape. 

The experimental determination of an unequivocal shape for small micelles such as those formed by ionic surfactants in the absence of added electrolyte and close to the CMC has not yet proved feasible. Although for the purposes of interpretation of experimental data it is usual to assume micellar sphericity, several authors [66-69] have shown from geometrical considerations that most of the common surfactants with a single unbranched hydrocarbon chain cannot 

of C atoms in hydrocarbon chain Extended chain length (nm) A o / o 25 /25 Max. aggregation number 

Although there is little disagreement on the lack of sphericity, there remains some dispute as to the exact shape of these small micelles. Tanford [69] has proposed that the distortion of the micellar shape into an ellipsoid of revolution is 

There is some controversy concerning the shape of the larger micelles formed by non-ionic surfactants and ionic surfactants at high concentration or in the presence of added electrolyte. 

---

Effects on the micellar shape are also induced by electrolyte addition. It has been observed that, in decane, the water-containing AOT-reversed micelles become more spherical upon addition of salt (NaCl, CaCli) [6]. 

The solubilization of amino acids in AOT-reversed micelles has been widely investigated showing the importance of the hydrophobic effect as a driving force in interfacial solubihzation [153-157]. Hydrophilic amino acids are solubilized in the aqueous micellar core through electrostatic interactions. The amino acids with strongly hydrophobic groups are incorporated mainly in the interfacial layer. The partition coefficient for tryptophan and micellar shape are affected by the loading ratio of tryptophan to AOT [158],... 

The effect characteristic of a multi-chain hydrophobe, that is, increase in the cmc and simultaneous decrease in the cloud point, appears to be inconsistent with the well-known HLB concept in surfactants. Tanford has pointed out that based on geometric considerations of micellar shape and size, amphiphilic molecules having a double-chain hydrophobe tend to form a bilayer micelle more highly packed rather than those of single-chain types ( ). In fact, a higher homologue of a,a -dialkylglyceryl polyoxyethylene monoether has been found to form a stable vesicle or lamellar micelle (9 ). Probably, the multi-chain type nonionics listed in... 

PS-PMMA Mw = 640-150(1.24-69% PMMA-PS PMMA Mv — 110.50% Toluene -f p-tymene (selective for PS) Micellar shape transition as a function of toluene// -cymene mixture composition. SLS Kotaka el al (1978)... 

Figure 9 shows schematic depictions of the two newly discovered structures described above. A cross section of a distorted hexagonal lattice is shown in Fig. 9a, displaying lipid micelles of an elliptical cross section and DNA molecules localized in the interstitial space. The distortion of the lattice is likely caused by the asymmetry in the micellar shape. X-ray diffraction data shows that the distortion increases with 4>MV[.BisG2-... 

New theory will be required to describe the phase diagram of block liposomes. In particular, theories have to break new ground in explaining why nanorods and nanotubes stay attached to spherical vesicles. All current theories of lipid self-assemblies (based on Helffich s theory of membranes [98]), in contrast, predict spherical, tubular, and micellar shaped liposomes but only as separate objects. In our experiments, not a single instance of an isolated rod- or tube-shaped liposome (i.e., not connected to a sphere- or pear-shaped vesicle) was found. 

Counterion specificity has been observed to be markedly more pronounced for cationic surfactants than for anionic ones. This can certainly be mainly referred to a weaker hydration of typical counter-anions. From the variation of CMC with counterion and from ion activity measurements it can be inferred that the binding to -N(CH3)3 and -NH3 headgroups follows the sequence P>NOj >Br > CP. (As an example a recent study223-1 of decylammonium salts shows the CMC to decrease from 0.064 M for the chloride to 0.038 M for the iodide). The counterion specific effects on micellar shape for -N(CH3)3 surfactants were discussed above. For cationic (as well as some anionic) amphiphiles, a marked counterion specificity is also indicated in the phase diagrams8 but systematic studies of the counterion dependence have not yet been reported. 

Although a number of infrared bands can be used to establish that a micellar shape change has occurred, it is difficult to determine the actual shape unambiguously from the spectroscopic data alone. We therefore make use of micelle aggregation numbers and solution rheological properties, which depend on micelle size and shape, for correlation with the structural information (packing) provided by the FTIR spectra. 

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... 

For completeness, we now give the equations which describe the transition micellar shapes from spheres to cylinders, and examine some of their properties. 

Micelles are dynamic species there is a constant rapid interchange of surfactant molecules between the micelle and the bulk solution. Micelles cannot, therefore, be regarded as rigid structures with a defined shape, although an average micellar shape may be considered. 

Micellar shape can be affected by changes in temperature, concentration and the presence of added electrolyte to the liquid phase. Changes in any of these factors may affect micellar size, shape, and aggregation... 

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). 

Packing parameter Surfactant Mesophase example... 

A theory of micellar structure, based upon the geometry of various micellar shapes and the space occupied by the hydrophilic and hydrophobic groups of the surfactant molecules, has been developed by Israelachvili, Mitchell, and Ninham (1976, 1977) and Mitchell and Ninham (1981). The volume Vh occupied by the hydrophobic groups in the micellar core, the length of the hydrophobic group in the core /,., and the cross-sectional area ao occupied by the hydrophilic group at the micelle-solution interface are used to calculate a packing parameter, Vn/hao, which determines the shape of the micelle. 

Trimeric and oligomeric surfactants have also been prepared (Zana, 1995 Sumida, 1998 In, 2000 Onitsuka, 2001). Their CMC values are even smaller than those of the analogous geminis. As the number of hydrophobic groups per molecule increases for gemini quaternary C12 ammonium compounds with polymethylene -(CH2) j spacers, their surface layers become more dense, their micellar microviscosity increases, and their micellar shape changes from spherical to wormlike, to... 

Fig. 8. Scheme of isometric polysoaps of different geometry, and the assumed possible micellar shapes in aqueous solution. (Reprinted with kind permission from [231], Copyright 1987 Steinkopff Verlag, Darmstadt)... 

Quantitative evidence on micellar shape and size has in the past usually been obtained by light scattering [40], or by thermodynamic methods [41], although recently Turro and his coworkers developed an ingenious method, based on fluorescence quenching, for estimating micellar size [42,43]. 

---