Shape self-assembly

While flattened vesicles are sometimes observed with the more hydrophobic amphiphiles, recently however we have discovered a route to disc shaped self assemblies (Fig. 11.6(a)) and found that branched polymers because of their lower hydrophilic head group area give rise to disc shaped self assemblies in the presence of cholesterol. The difference in hydrophilic head group area is also evidenced by the lower CMC values exhibited by the branched polymer (Fig. 11.6(b)). A lower hydrophilic head group area would favour molecular aggregation in aqueous media as stabilisation of the individual molecules in water via hydrogen bonding would be less likely. 

Wei, B., Dai, M., Yin, P., 2012. Complex shapes self-assembled from single-stranded DNA tiles. Nature 485 (7400), 623-626. 

This inequality indicates the amphiphile adopts a shape essentially equivalent to that of a cone with basal area <3. Such cones self-assemble to fonn spheroidal micelles in solution or spheroidal hemimicelles on surfaces (see section C2.3.15). Single-chain surfactants with bulky headgroups, such as SDS, typify surfactants in this category. 

Another synthetic strategy is based on self-assembly driven by molecular recognition between complementary TT-donors and 7T-acceptors. Examples include the synthesis of catenanes and rotaxanes that can act as controUable molecular shuttles (6,236). The TT-donors in the shuttles are located in the dumb-beU shaped component of the rotaxane and the 7T-acceptors in the macrocycHc component, or vice versa. The shuttles may be switched by chemical, electrochemical, or photochemical means. 

Crystallization is an important separation proeess that purifies fluids by forming solids. Crystallization is also a partiele formation proeess by whieh moleeules in solution or vapour are transformed into a solid phase of regular lattiee strueture, whieh is refleeted on the external faees. Crystallization may be further deseribed as a self-assembly moleeular building proeess. Crystallo-graphie and moleeular faetors are thus very important in affeeting the shape (habit), purity and strueture of erystals, as eonsidered in detail by, for example, Mullin (2001) and Myerson (1999). In this ehapter the internal erystal strueture and external partiele eharaeteristies of size and shape are eonsidered, whieh are important indieators of produet quality and ean affeet downstream proeessing, sueh as solid-liquid separation markedly. Larger partieles separate out from fluids more quiekly than fines and are less prone to dust formation whilst smaller partieles dissolve more rapidly. 

Some of the more remarkable examples of this form of topologically controlled radical polymerization were reported by Percec et cii.231 234 Dendron maeromonomers were observed to self-assemble at a concentration above 0.20 mol/L in benzene to form spherical micellar aggregates where the polymerizable double bonds are concentrated inside. The polymerization of the aggregates initiated by AIBN showed some living characteristics. Diversities were narrow and molecular weights were dictated by the size of the aggregate. The shape of the resultant macroniolecules, as observed by atomic force microscopy (ATM), was found to depend on Xn. With A, <20, the polymer remained spherical. On the other hand, with X>20, the polymer became cylindrical.231,232... 

Tian, L. and Hammond, P.T. Comb-dendritic block copolymers as tree-shaped macromolecular amphi-philes for nanoparticle self-assembly, Chem. Mater., 18, 3976, 2006. 

Amphipilic polypeptides that are synthesized with appropriate ratios of hydrophilic to hydrophobic blocks can form ordered vesicular shapes. Although many polypeptides can self-assemble into vesicles when simply dissolved in the correct solvent, others require more processing steps. This section provides an overview of the techniques that have been developed to process various polypeptide and polypeptide hybrid systems into vesicles. 

Examples representing the very wide range of self-assembled protein structures obtained as just described are presented in Figure 1. These examples demonstrate that the size and shape of self-assembled nanostructures made of proteins primarily depend on the molecular mechanism effecting self-assembly and are not merely an amplified reflection of the shape and size of the starting building block. ... 

FIG. 1 Examples of shape, geometry and size of self-assembled protein made structures demonstrating that the size and shape of in vitro obtained nanostructures depend on the mechanism of self-assembly rather than on the size and shape of the building block. Note that the scale of the building blocks (left-hand side) is in nanometers, while the scale of the structures obtained (right-hand side) is in microns. 

The foregoing results demonstrate that the thickness of the capsule wall can be controlled at the nanometer level by varying the number of deposition cycles, while the shell size and shape are predetermined by the dimensions of the templating colloid employed. This approach has recently been used to produce hollow iron oxide, magnetic, and heterocomposite capsules [108], The fabrication of these and related capsules is expected to open up new areas of applications, particularly since the technology of self-assembly and colloidal templating allows unprecedented control over the geometry, size, diameter, wall thickness, and composition of the hollow capsules. This provides a means to tailor then-properties to meet the criteria of certain applications. 

Figure 4.4. Preparation of MTS materials. The diagram shows self assembly of the surfactant into micelles followed by condensation of silica around the micelles. The micelles arrange themselves into an approximately hexagonal array. After the formation of the silica around the micelles, the micelles are burnt out, leaving pores where the micelles were. The pores are an accnrate reflection of the size and shape of the micelles. This makes the pores uniformly sized and shaped.

Finally, the shape and self-assembly of these particles can also be controlled which gives rise to novel nanomaterials displaying interesting physical properties in the fields of semi-conductors, magnetism, or optics. 

Self-assembly of metal nanoparticles, of course, occurs easier the smaller the difference in size and shape of the nanoparticles is. Most of the known 3D assemblies are built of ligand-protected metal nanoparticles. From... 

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