Micelle shape wormlike

Regioselective crosslinking of the core domain of cylindrically shaped, wormlike micelles composed of poly[(butadiene)45-b-(ethylene oxide)55] and assembled in aqueous solution at < 5% block copolymer concentrations, was performed using radical coupling of the double bonds throughout the poly(butadiene) phase [27] (Figure 6.3b). This resulted in a 13% reduction in the core diameter, from 14.2 to 12.4 nm, as measured by small-angle neutron scatter-... 

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 aqueous solution, amphiphilic molecules aggregate into micelles above the critical micelle concentration. Such solutions have been the object of research for many years, with special interest in shape and size of these micellar aggregates [37]. Size and shape (spherical, wormlike, or disklike micelles) depend strongly on the molecular structure of the amphiphilic molecule. 

The appearance of the local minimum in the concentration dependency of the relaxation time of the slow process for concentrated solutions can be connected also with formation of non-spherical micelles [140]. Actually, it is well-known that micelles change their shape with increasing concentration. The increase of the concentration of some counterions can lead to the formation of giant wormlike micelles (living polymers) [141]. In this case the equilibrium size distribution of micelles changes entirely and is described by an exponential law [141]... 

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]. 

Recently, Miller and Cacciuto explored the self-assembly of spherical amphiphilic particles using molecular dynamics simulations [46]. They found that, as well as spherical micellar-type structures and wormlike strings, also bilayers and faceted polyhedra were possible as supracolloidal structures. Whitelam and Bon [47] used computer simulations to investigate the self-assembly of Janus-like peanut-shaped nanoparticles and found phases of clusters, bilayers, and non-spherical and spherical micelles, in accordance with a packing parameter that is used conventionally and in analogy to predict the assembled structures for molecular surfactants. They also found faceted polyhedra, a structure not predicted by the packing parameter (see Fig. 8). In both studies, faceted polyhedra and bilayers coexist, a phenomenon that is still unexplained. 

One may observe monolayers, spherical (globular or rotund) micelles, cylindrical (tubelike or wormlike) micelles, and bilayers. (The more ordered liquid crystals are treated in the following.) The size and shape distributions of surfactant aggregates in solution... 

Very large micelles may also form in binary surfactant systems. These are long wormlike micelles that become entangled at higher concentrations, giving rise to rheological properties similar to those in polymer solutions. Such systems have been examined by H band shape analysis [52,53]. The protons of the surfactant hydrocarbon chain form a very large dipolar coupled spin system with an essentially continuous distribution of transverse relaxation rates. The distribution of relaxation rates is related to the distribution of order... 

Extensive studies have been reported by Kunieda s group regarding the formation of worm-like micelles and micellar transient networks in water-surfactant-cosurfactant systems. However, for applications, it is also relevant to know the effect of additives on systems containing worm-hke micelles. It is reported that oils induce a rod-sphere transition in surfactant micellar solutions, leading to a reduction in viscosity [32]. Kunieda s group studied the solubilization of different oils in wormlike micellar solutions [19, 33]. The amount of solubilized oil, its location within the micelle, and its effect on micellar shape and size demonstrated to strongly depend on the nature of the oil and its interactions with the surfactants. 

When P < 1/3, individual molecules are conically shaped, as shown in Figure 10. This results in the formation of spherical micelles in solution. As the volume of the hydrophobic tail is increased, P increases. For 1/3 < P < 1/2, nonspherical (cylindrical) micelles are formed. As P increases further, bilayer structures are formed. At P > 1, inverted structures are formed. This packing parameter can be used to rationalize why sodium dodecyl sulfate (SDS) forms spherical micelles in solution, while lysolecithin forms wormlike micelles. 

In combination with Equation 9.9 it is now possible to calculate the dimensions of the rod-shaped micelles from the minimum of G Tco). Kern et al. performed systematic studies of the average micellar lengths of wormlike aggregates. Typical values are of the order of several micrometers, but extreme values of more than 0.1 mm were also sometimes observed. These data are in fairly good agreement with results of cryo-TEM, which also show the presence of entangled wormlike micelles with dimensions as long as several micrometers. 

The phase diagrams of solutions of diblock copolymers A-B may be quite complex and depend both on the chemical nature of two blocks and on the solvent. > In a selective solvent, good for the B-block and poor for the A-block, intermolecular aggregates are formed in the dilute regime so that the number of unfavorable contacts A-S is limited. The shape of aggregates (e.g, spherical or wormlike micelles, vesicles), their size and polydispersity depend very much on chain composition and length. In more concentrated solutions, aggregates order in space and form mesophases, i.e., ordered microdomains rich in A (in B) (e.g., lamellar. 

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