Micelle shape rodlike

Upon increasing the amphiphile concentration an evolution toward more asymmetric shapes (rodlike or disklike) and decreasing surface/volume ratio is observed. Eventually cylindrical (capped) micelles, bilayers (extended open sheet with rounded edges), and closed vesicles are formed. 

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 ... 

They are repulsive and work perpendicular to the interface. Repulsive interactions induce micelle ordering at values of the surfactant volume fraction and with symmetry that depend on the micelle shape.Spherical micelles close pack into a cubic array, forming the discontinuous cubic phase I. Rodlike micelles pack into a hexagonal array, forming the hexagonal phase H. Bilayers pack into the lamellar phase An important point is that this simple approach leads one to predict for a surfactant that forms spherical micelles (P < 1/3) the following sequence of phases as the surfactant concentration is increased ... 

Figure 4.6. Five of the proposed micelle shapes, as interpreted from experimental data (a) spherical (b) lamellar (c) inverted (or reversed) (d) disk (e) cylindrical or rodlike.

Micellar structure has been a subject of much discussion [104]. Early proposals for spherical [159] and lamellar [160] micelles may both have merit. A schematic of a spherical micelle and a unilamellar vesicle is shown in Fig. Xni-11. In addition to the most common spherical micelles, scattering and microscopy experiments have shown the existence of rodlike [161, 162], disklike [163], threadlike [132] and even quadmple-helix [164] structures. Lattice models (see Fig. XIII-12) by Leermakers and Scheutjens have confirmed and characterized the properties of spherical and membrane like micelles [165]. Similar analyses exist for micelles formed by diblock copolymers in a selective solvent [166]. Other shapes proposed include ellipsoidal [167] and a sphere-to-cylinder transition [168]. Fluorescence depolarization and NMR studies both point to a rather fluid micellar core consistent with the disorder implied by Fig. Xm-12. 

Above 0.45 NaCl a rodlike shape has to be assigned to the micelles and Eq. 19 must be used ... 

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). 

Here, V is the volume of the hydrocarbon chain(s) of the surfactant, the mean cross-sectional (effective) headgroup surface area, and 4 is the length of the hydrocarbon tail in the all-trans configuration. Surfactants with Pcone-shaped and form spherical micelles. For l/3truncated-cone-shaped, resulting in wormlike micelles (the term wormlike is preferred over rodlike to highlight the highly dynamic nature of these micelles). 

In the presence of sodium tosylate (NaTos), both the rate constant kexp and the viscosity of the solution show maxima at the same electrolyte concentration. The dramatic variation in 17 shown in Figure 8.10a suggests that the sodium tosylate alters the shape of the micelles, first producing rodlike structures that subsequently break up into more compact structures. The complicated phase diagrams of surfactants make this a plausible explanation. Effects such as this clearly complicate the picture not only of inhibition, but also of micellar catalysis in general. 

The size and shape of micelles also are affected by fluonnation Whereas hydrocarbon surfactants usually form spherical micelles, linear fluorocarbon surfactants tend to produce larger rodlike species [31, 32] This is attributed to two inherent characteristics of the (CFj), chain (1) it adopts a helical rather than a linear zigzag conformation [dd 34, 35,36], and (2) it is much suffer than the (CH2) chain J5 37, 38] The relatively stiff, helical (CF2) chains thus prefer cylindrical to spherical packing... 

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. ... 

Toernblom, M. and Henriksson, U. (1997) Effect of solubilization of aliphatic hydrocarbons on size and shape of rodlike Cl 6TABR micelles studied by 2H NMR relaxation. /. Phys. Chem. B, 101, 6028-6035. 

The Dill-Flory model may be considered as a more rigorous version of the Hartley model (30). Both models are readily applied to other shapes of micelles, such as rods, discs, bilayers, and vesicles. Also, it follows that diameters of spherical, rodlike, and disclike micelles cannot exceed the total length of two hydrocarbon chains in all-trans conformation. The number of entities in one micelle, i.e. the aggregation number s, is therefore readily estimated for any given chain length r. Assuming equal densities p (= 0.777 g/cm ) for micelles and solid n-alkanes, r may be obtained from the volume v and the constant cross section A (= 2.385 x 10 cm ) of alkane chains ... 

Due to the d3mamic nature and to the small micellization enthalpies (25), micelles and other aggregates of amphiphilic molecules are sensitive in shape and size to various additives. It has been known for a long time that the addition of salt to solutions of ionic spherical micelles induces the formation of rodlike aggregates (144), but also the solubilization of alcohols, alkanes, and aromatic liquids (36,145-147) has consequences on aggregation numbers and on shapes of micelles. 

In aqueous solutions of surfactants at concentrations above the critical micelle concentration (CMC), the molecules self-assemble to form micelles, vesicles, or other colloidal aggregates. These may vary in size and shape depending on solution conditions. In addition to surfactant molecular structure, the effects of concentration, pH, other additives, cosolvents, temperature, and shear affect the nanostructure of the micelles. The presence of TLMs or cylindrical, rodlike, or wormlike micelles at concentrations > CMCii are generally believed to be necessary for surfactant solutions to be drag reducing [Zakin et al., 2007]. 

A cubic phase of space group Pmln is usually observed in type I systems [164]. Several structures have been suggested for the Pmln phase [168-171]. It is now agreed that it contains two types of micelles [172] two quasi-spherical micelles packed on a body-centered cubic lattice and six slightly asymmetrical micelles arranged in parallel rows on opposite faces of the unit cell. The asymmetrical micelles are assumed to be disklike [173] or rodlike with rotational disorder around one of the short axes [174,175]. In order to pack space completely, each asymmetrical micelle, together with the water that surrounds it, takes the shape of a... 

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