Micelles, cylindrical spherical

Between 5 and 23 minutes, a continuous evolution is observed, characterised by the increase of the intensity at low q and by the q"1 slope in a log-log plot that is the signature of ID objects (figure 3). This corresponds to the transformation of the shape of the micelles from spherical to cylindrical and their continuous growth in length up to few tens of nanometers. The increase of turbidity in solution is related to the growth of the micelles, and coincides with the beginning of the condensation of silica. 

The micelles are spherical, but when the concentration of surfactant increases, the shape of the ionic micelles changes following the spherical sequence cylindrical-hexagonal-laminar [22], In the case of nonionic micelles the shape... 

As discussed by Israelachvili (1992), the shapes of surfactant aggregates can, to a first approximation, be anticipated based on the packing of simple molecular shapes (Tanford 1980). Figure 12-1 from Israelachvili illustrates this principle Conical molecules with bulky head groups attached to slender tails form spherical micelles cylindrical molecules with heads and tails of equal buUdness form bilayers and wedge-shaped molecules with tails bulkier than their heads form inverted micelles containing the heads in their interiors. A simple dimensionless molecular parameter that controls the shape of the aggregates is the molecular shape parameter here v is the volume occupied by the hydrocarbon... 

Possible candidates for aggregates can now be examined. For surfactant-water systems these have been restricted in the past to spherical micelles, non-spherical micelles (globular, cylindrical), vesicles, liposomes, bilayers, and for oil-water-surfactant systems spherical drops, normal or inverted (water in oil) or (oil in water). 

With appropriate values of the various parameters, this model yielded predictions in good agreement with experiment on such phenomena as the amounts of various pure hydrocarbons solubilized in micelles of sodium dodecyl sulfate SDS as well as the amounts of benzene and -hexane solubilized in the same micelles from various mixtures of the two hydrocarbons. It was also able to predict transformation of rodlike micelles to spherical microemulsion droplets as a result of hydrocarbon solubilization, an effect that has been observed experimentally. In the absence of hydrocarbon, films of these surfactants can attain their preferred curvatures only by forming cylindrical micelles, as micelle radius is limited to the extended length of a surfactant molecule. However, when considerable hydrocarbon is present, this constraint no longer applies, and spherical microemulsion droplets can grow until the preferred curvature is reached. 

The initial publications did not emphasize the specific action of the hydrotrope molecules in different applications. Instead they considered the structural modification of aqueous micelles by the addition of hydrotrope. Assessing the results from this point of view [61-64] the conclusion was that the reduction of electrostatic repulsion is the main cause of the modification of surfactant micelles from spherical to cylindrical shape after addition of a hydrotrope with opposite charge. 

The value of o varies not only with the structure of the hydrophilic head group, but also with changes in the electrolyte content, temperature, pH, and the presence of additives in the solution. Additives, such as medium-chain alcohols that are solubilized in the vicinity of the head groups (Chapter 4, Section IIIA), increase the value of oq. With ionic surfactants, o decreases with increase in the electrolyte content of the solution, due to compression of the electrical double layer, and also with increase in the concentration of the ionic surfactant, since that increases the concentration of counterions in the solution. This decrease in the value of ao promotes change in the shape of the micelle from spherical to cylindrical. For POE nonionic surfactants, an increase in temperature may cause a change in shape if temperature increase results in increased dehydration of the POE chain. 

Fig. 9. Self-organization structures of block copolymers and surfactants spherical micelles, cylindrical micelles, vesicles, fee- and bcc-packed spheres (FCC, BCC), hexagonaUy packed cylinders (HEX), various minimal surfaces (gyroid, F-surface, P-surface), simple lamellae (LAM), as well as modulated and perforated lamellae (MLAM, PLAM) (with permission from [5])...

Fig. C4.9 Four different microphase segregated structures which can exist in diblock-copolymer melt. Layers (lamellas), spherical micelles, cylindrical micelles, and bicontinuous phase are shown. The figure is courtesy of P.G. Khalatur.

Molecular dynamics simulations are consistent with calculations based on the critical packing parameter p, which indicate that the structure of the surfactant controls the shape of the micelle at the cmc. Esselink et al. [16] show that the surfactants / 2/5, hihts, and h thts form bilayers, cylindrical micelles, and spherical micelles, respectively, as expected. However, /14/4, expected to form micelles of low curvature based on p, instead forms sphere-like structures due to the coiling of the headgroup. If this increased effective headgroup area is accounted for in the calculation of the packing parameter, then a spherical shape is predicted, in agreement with the result of the simulations. 

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... 

Under certain conditions, cylindrical micelles or even vesicles can be formed instead of spherical symmetric micelles. Cylindrical micelles are usually formed as a consequence of a delicate balance between the different terms, most notably governed by the chain stretching in the micellar core. Compared to spherical micelles, the core radius of a cylinder for an equivalent area or volume can easily be evaluated to be 2/3 smaller. Hence, in a cylindrical micelle, the amount of chain stretching is expected to be less pronounced than in a spherical one. On the other hand, chain interactimis in the corona of a cylindrical micelle is expected to be more severe due to the smaller area available for each chain. All terms must thus be included and a more detailed thermodynamic evaluation should be performed. For vesicles, the bending modules is additionally expected to be important [46]. 

Figure 25 Assembly of DNA-brush copolymers into micelles with spherical or cylindrical morphologies. Amphiphile structures are represented as cones for each respective motphology, with the hydrophobic domain highlighted in red. TEM images of (a) 25-nm spherical micelles assembled from initial DNA-brush copolymers (b) cylindrical morphology formed following DNAzyme addition to spheres (c) spherical micelles (green) formed after the addition of hi to cylinders. Reproduced with permission from Chien, M.-P. Rush, A. M. Thompson, M. P. etal. Angew. Chem. Int. Ed. 2010,49, 5076-5080. 2...

The viscosity data in this range indicate that the micelles are spherical in shape and, in the range C = 1.2 (2nd CMC) to C = 1.8 (3rd CMC), the data indicate that there is a transition from spherical to cylindrical micelles. In this range the viscosity increases with concentration according to ... 

Fig. 6. Phase diagrams with shape transition boundaries for block copolymers as a function of (a) hydrophilic fraction f (from Ref 46, with permission from ACS Publication Division) and (b) water content in dioxane/water solutions (from Ref 39, with permission from ACS Publication Division). B bilayers/vesicles C cylindrical micelles S spherical micelles.

Figure 2 Organization of block copolymers into spherical micelles, cylindrical micelles, and membranes and representative cryo-TEM images of these morphologies [(a) and (d) vesicles, (b) and (e) cylindrical micelles, and (c) and (f) spherical micelles]. (Reproduced from Ref. 4. Royal Society of Chemistry, 2011.)...

Micelles are spherical aggregates of surfactant molecules that can be represented by fig. 1.2a. The concentration at which micelles form in solution is known as the critical micelle concentration (CMC). The concentration at which surfactants aggregate at surfaces to form monolayer-level surface coverage (see fig. 1.2b) is referred to as the surface aggregation concentration (SAC). The SAC is usually very similar to the CMC, although the SAC is usually lower due to interactions with immobile lattice atoms. Other aggregate structures such as bilayers and cylindrical micelles can also form above the CMC or SAC. 

Spherical micelle Cylindrical micelle Planar bilayer... 

Figure 7 The transformation spherical micelles cylindrical micelles —> linear assem-...

The analysis presented above focused on spherical intrachain micelles. Cylindrical intrachain micelles may occur when the amphiphilic comonomers form cylindrical micelles in their free state. However, this geometry is thermodynamically stable only for short spacer chains. Spherical intrachain micelles are favored when n increases because Fcarona of starlike coronas is lower. 

Fig. 29 The schematic representation of the reversible transformation of cylindrical micelles into spherical micelles. Reprinted with permission from [79]. (2007) American Chemical Society...

---