Rod-shaped micelles

The aggregates created by amphiphiles are usually spherical (as in the case of micelles), but may also be disc-like (bicelles), rodlike, or biaxial (all three micelle axes are distinct) (Zana, 2008). These anisotropic self-assembled nanostructures can then order themselves in much the same way as liquid crystals do, forming large-scale versions of all the thermotropic phases (such as a nematic phase of rod-shaped micelles). 

Bergstroem, M., and J. C. Eriksson. 1992. Composition uctuations promoting the formation of long rod-shaped micelles.Langmuir8 36-42. 

Fig. 4.4. A schematic picture of the dynamic protrusion of surfactant monomers from spherical and rod-shaped micelles. (By the courtesy of J. Ulmius)...

It appears that, through a combination of theoretical and experimental efforts, the kinetic methods have developed into one of the most fruitful approaches to the study of physico-chemical properties of micellar systems. One of the main problems that remains to be solved is the kinetics for systems containing rod-shaped micelles where the size distribution is very broad. There is also a problem in the interpretation of the amplitudes measured in the different kinetic experiments. 

Two other shapes of micelles may be considered, namely, the rod-shaped micelle suggested by Debye and Anacker and the lamellar micelle suggested by McBain. The rod-shaped micelle was suggested to account for the light-scattering results of cetyl trimethyl ammonium bromide in KBr solutions, whereas the lamellar micelle was considered to account for the X-ray results in soap solutions. A schematic picture of the three type of micelles is shown in Fig. 2. 

As a consequence of their tendency to form lamellar phases or rod shaped micelles at low concentration, cationic surfactants are frequently employed as the primary surfactants to thicken high salt formulas [72,73]. The viscoelastic nature of certain cationic surfactant solutions has been employed in a novel way to allow for a solution of sodium hypochlorite not to be easily diluted and therefore to remain at a higher concentration for the purpose of oxidizing clogs of human hair which form in drains [73 ]. Low concentrations of cetyl trimethyl ammonium chloride in combination with two hydro tropes form viscoelastic solutions with the values of viscosity and Tau/Go shown in Table 6.6. 

The other possible form for hn is that Hn decreases with increasing N tending to a finite limit as A oo. This form will be seen later to arise from contributions to fis from end effects, e.g., hemispherical-like ends on rod-shaped micelles, and hemicylindrical-like ends at the extremities of planar aggregates. 

One functional form for which will occur in connection with rod-shaped micelles is... 

It is of interest to note here that the behaviour of lipid aggregates for which the optimal shape is a spherical vesicle or an infinite bilayer will be quite different from that of rod-shaped micelles. One might anticipate by analogy with rods that finite disc-like bilayers could form with cylindrical rims. For such a system the free energy would take the form = n +akT/N. From section 2, for large a, the system will assemble spontaneously into infinite bilayers. Repetition of an analysis similar to that for rods leads to values of a sufficiently large so that this process always occurs. 

Kohler, H. Strnad, J. Evaluation of viscosity measurements of dilute solutions of ionic surfactants forming rod-shaped micelles. J. Phys. Chem. 1990, 94, 7628-7634. 

Mesophase structures self-assembled from surfactants (Figure 8.35) provide another class of useful and versatile templates for generating ID nanostructures in relatively large quantities. It is well known that at critical micellar concentration (CMC) surfactant molecules spontaneously organize into rod-shaped micelles [315c]. These anisotropic structures can be used immediately as soft templates to promote the formation of nanorods when coupled with appropriate chemical or electrochemical reaction. The surfactant needs to be selectively removed to collect the nanorods/nanowires as a relatively pure sample. Based on this principle, nanowires of CuS, CuSe, CdS, CdSe, ZnS and ZnSe have been grown selectively by using surfactants such as Na-AOT or Triton X of known concentrations [238, 246]. 

Other surfactants show the similar lyotropic liquid-crystal phase behavior and follow the same succession of phases, but not all of the phases are always present. Figure 8.7 shows a phase diagram for the CTAB-water binary system. CMC can also be classified CMC1 (spherical micelle) and CMC2 (rod-shaped micelle). 

The surfactant templated synthesis of mesoporous ceramics was first reported in 1992 [1], and since that time there has been a veritable explosion in the number of papers in the area. There has been particular interest in the functionalization of mesoporous ceramics [2-S]. As outlined in Figure 1, the original synthesis employed rod-shaped micelles composed of cationic surfactant molecules as the pore template (more recently, this methodology has been extended to a wide variety of other surfactant systems and reaction conditions). When exposed to routine sol-gel conditions, the cationic micelles undergo an anionic metathesis with silicate anions, resulting in a glass-coated log which... 

The minimum in the interfacial free energy predetermines three kinds of geometry in nature spheres, cylinders and planes. Correspondingly, the most stable amphiphile aggregation structures are i) spherical (Hartley) micelles, ii) rod-shaped micelles and anisotropic middle phases, iii) disk-shaped micelles and lamellar mesophases. They exist as aggregates in a water continuum with a hydrocarbon core surrounded by hydrated polar groups (the normal type) and as aggregates in a hydrocarbon continuum (the reverse or inverted type) where water and... 

Most of the emulsifiers used form associations in water of the closed type N lies between 20 and 100. The soap molecules here are so arranged that the polar groups are facing outward and the hydrocarbon units inward. The exact geometric form of the micelles is not known in detail in particular, there has been discussion of spherical and rod-shaped micelles. 

Figure 13 Schematic representative transition of the morphoiogy of micelle from aggregates of nanosized sphere-shaped micelles to micrometer rod-shaped micelles. Reprinted from Zhang, W. Zhang, W. Zhou, N. et at. J. Polym. ScL, Part A Polym. Chem. 2009,47.6304. ...

Simple surfactants with relatively small head gronps, or ionics in the presence of large amounts of electrolyte Relatively large cylindrical or rod-shaped micelles... 

There are two types of lipid-water phase diagrams. The first type, discussed above, is obtained from polar lipids, which are insoluble in water (i.e. the solubility is quite small, monolaurin for example has a solubility of about 10 m). Fig. 8.12 illustrates the principles of phase equilibria in this type of lipid-water system. The second type of binary system is obtained when the lipid is soluble as micelles in water. Examples of such lipids are fatty acid salts and lysolecithin. An aqueous soap system is illustrated in Fig. 8.13. When the lipid concentration in the micellar solution is increased, the spherical micelles are transformed into rod-shaped micelles. At still higher lipid concentrations the lipid cylinders are hexagonally arranged and the liquid-crystalline phase Hi is formed. The lamellar liquid-crystalline phase is usually formed in the region between Hi and the anhydrous lipid. Excellent reviews of the association behaviour of amphiphiles of this type have been published (Wennerstrom and Lindman, 1979 Lindman and Wennerstrom, 1980). 

Figure 8.39 Dielectric constant values as a function of the volume fraction of the aqueous micellar core in the L2-phase of the system sodium octanoate/decanol/water. The regions I-III correspond to the premicellar solution, the region of spherical micelles and the region with rod-shaped micelles respectively (Sjoblom et aL, 1983).

The structure in the micellar region of the phase diagrams of potassium soaps has been analysed by Reiss-Husson and Luzzati (1969) using X-ray methods. A common feature in soaps of saturated fatty acids is that spherical micelles exist at low concentrations, and at increased concentrations a transition into rod micelles occurs. Sodium oleate, however, was found to give rod-shaped micelles at all concentrations. The micellar association and phase behaviour have been reviewed by Wenner-strom and Lindman (1979) and Lindman and Wennerstrom (1980). 

At intermediate concentrations considerably above the CMC, rod-shaped micelles often form. At very high concentrations, lamellar aggregates and other condensed phases form. In organic solvents, some surfactants form spherical inverse micelles with the head groups facing small pools of trapped water, with hydrocarbon tails facing the bulk solvent phase (Fig. 2). 

Fig. 5 Influence of the concentration of ferrocene (Fc) on D measured by cyclic voltammetry at a glassy carbon electrode in 0.15 M CTAB/0.1 M tetraethylammonium bromide. Points are experimental line is best fit to Eq. (12) with n = 3 by nonlinear regression, giving Di = 3.3 X 10 cm s for the rod-shaped micelles. (Reproduced with permission from Ref. [4], Copyright 1994 by Marcel Dekker.)...

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