Micelle cylindrical

Larger aggregates seldom have spherical geometry, but tend to form cylindrical micelles. In this case, the diameter of the cylinders can usually be adjusted such that the head groups can cover their optimal head group area Uq, and the interaction free energy per surfactant reduces to the constant The size distribution for cylindrical micelles is then exponential in the limit of large N,... 

At small N, correction terms come into play, which account for the ends of the cylinders. In particular, the aggregation number of cylindrical micelles in this simple picture must always be larger than M, the most probable aggregation number of a spherical micelle. Putting everything together, the expected size distribution has a peak at M which corresponds to spherical micelles, and an exponential tail at large N which is due to the contribution of cylindrical micelles. 

FIG. 10 Micelle size distribution for H2T2 surfactants within the Larson model. The dashed lines show fits to the expected form for spherical micelles (main peak) and cylindrical micelles (tail). Inset shows the tail of the distribution on a semi-logarithmic plot to demonstrate the exponential decay predicted for the cylindrical micelles. (From Nelson et al. [120].)... 

Amphiphilic molecules (surfactants) are composed of two different parts hydrophobic tail and hydrophilic head [1 ]. Due to their chemical structure they self-assemble into internal surfaces in water solutions or in mixtures of oil and water, where the tails are separated from the water solvent. These surfaces can form closed spherical or cylindrical micelles or bicontinuous phases [3,5]. In the latter case a single surface extends over the volume of the system and divides it into separated and mutually interwoven subvolumes. 

Figure 5. Hexagonal array of cylindrical micelles of cetyltrimethylammonium in MCM-41 synthesis...

In sodium bis(2-ethylhexyl) phosphate microemulsions, which are composed of cylindrical micelles in the dilute region, it has been observed that the formation of micellar clusters is characterized by a branched structure as the volume fraction (<1>) of the aggregates increases. At d> > 0.2, these clusters mutually overlap, forming a network expanded overall [283]. 

Figure 8.4 (a) Atypical molecule that behaves as lyotropic liquid crystal (b) schematic representation of a plate-shaped micelle (c) a spherical micelle (d) a cylindrical micelle. 

Time-resolved in situ Small Angle Neutron Scattering (SANS) investigations have provided direct experimental evidence for the initial steps in the formation of the SBA-15 mesoporous material, prepared using the non-ionic tri-block copolymer Pluronic 123 and TEOS as silica precursor. Upon time, three steps take place during the cooperative self-assembly of the Pluronic micelles and the silica species. First, the hydrolysis of TEOS is completed, without modifications of the Pluronic spherical micelles. Then, when silica species begin to interact with the micelles, a transformation from spherical to cylindrical micelles takes place before the precipitation of the ordered SBA-15 material. Lastly, the precipitation occurs and hybrid cylindrical micelles assemble into the two-dimensional hexagonal structure of SBA-15. 

Figure 2 Evolution of the neutron scattering intensities with time. Only spherical micelles of P123 block copolymer are present in the synthesis mixture within the first few minutes of the reaction (300 s), during the hydrolysis of the silica precursor. Then, hybrid organic-inorganic cylindrical micelles are detected (300-1400 s). The SBA-15 hexagonal phase is formed when the precipitation occurs, after 1400 s.

Fig. 10 Cross sections of (a) spherical and cylindrical micelles, and (b) cylindrical and lamellar vesicles in aqueous solution. Each surfactant molecule making up the structures has a polar head-group, depicted as a circle, and a nonpolar, hydrophobic chain, depicted as a zigzag.

Cylindrical alkaline primary cells, 3 441-449 anode, 3 443-444 cathode, 3 444-446 cell chemistry, 3 443 performance, 3 446-449 zinc oxidation mechanism, 3 444-446 Cylindrical element filters, 11 362—364 Cylindrical guided wave technique, for pipe inspection, 17 433-434 Cylindrical micelle, 24 124 Cylindrical mirror analyzer (CMA), 24 103-104... 

For some applications, it is desirable to lock the micellar structure by cross-Hnking one of the micellar compartments, as discussed previously in Sect. 2.6. Cross-Hnked core-shell-corona micelles have been prepared and investigated by several groups as illustrated by the work of Wooley and Ma [278], who reported the cross-linking of PS-PMA-PAA micelles in aqueous solution by amidation of the PAA shell. Very recently, Wooley et al. prepared toroidal block copolymer micelles from similar PS-PMA-PAA copolymers dissolved in a mixture of water, THF, and 2,2-(ethylenedioxy)diethylamine [279]. Under optimized conditions, the toroidal phase was the predominant structure of the amphiphilic triblock copolymer (Fig. 19). The collapse of the negatively charged cylindrical micelles into toroids was found to be driven by the divalent 2,2-(ethylenedioxy)diethylamine cation. 

In a very recent investigation, hydrophobic PFS (Sect. 7.1) was attached to a hydrophilic PEO block to form an amphiphilic PFSi2-[Ru]-PE07o block copolymer [331]. Rodlike micelles were observed in water for this copolymer (Fig. 24). These micelles have a constant diameter but are rather polydisperse in length, and DLS measurements indicate that they are flexible. Crystallization of the PFS in these micelles was observed and is thought to be the key behind the formation of rodlike structures. The cylindrical micelles can be cleaved into smaller rods whenever the temperature of the solution is increased or whenever they are exposed to ultrasound. 

Fig.24 TEM picture of cylindrical micelles formed by a PFSi2-[Ru]-PEC>7o metallosupramolecular copolymer in water. No contrasting agent has been added for visualization of these micelles because they contain iron atoms in the core. Reprinted with permission from [331]. Copyright (2004) Wiley...

Figure 1. Scheme for the liquid crystalline templating mechanism proposed by Kresge et al 1 for synthesis of mesoporous silica MCM-41. Formation of a hexagonal array of cylindrical micelles possibly mediated by silicate anions followed by condensation of the silicate anions from the silicate source (tetraethylorthosilicate) leads to templated framework structure. Calcination or extraction of the template produces hexagonally ordered mesoporous silica. 

Gohy JE, Lohmeijer BGG, Alexeev A, Wang XS, Manners 1, Winnik MA, Schubert US (2004) Cylindrical micelles from the aqueous self-assembly of an amphiphilic poly(ethylene oxide)-b-poly(ferrocenylsilane) (PEO-b-PES) block copolymer with a metallo-supramolecular linker at the block junction. Chem Eur J 20 4315 323... 

Figure 2.3 Representation of organized assemblies (a) bilayers, (b) cylindrical micelles, and (c) lamellar structure.

Micelles can be spherical or laminar or cylindrical. Micelles tend to be approximately spherical over a fairly wide range of concentrations above CMC (critical micelle concentration) but often they are marked transitions to larger, non spherical liquid crystal structures at high concentrations. For straight chain ionic surfactants, the number of monomer units per micelle ranges between 30 and 80. 

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