Self-Assembly and Aggregation

A more recent SANS study revealed a 4% increase in dendrimer volume with decreasing pH (Chen et al. 2007). Although these stmctural variations were significantly less than predicted by the MD studies, this study confirmed the occurrence of both counterion condensation and water penetration of the dendrimer interior. 

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While self-assembled and aggregated chirogenic systems are mainly based on monomeric porphyrinoids, a further increase of the pigment multiplicity, despite raising the overall complexity of supramolecular structures, may lead to novel functional properties and new applications. Therefore, in the last section we shall show a few examples of self-assembled and aggregated systems constructed from dimeric and oligomeric porphyrins. 

Nanoscale assembly Self-assembling micellar structures [113,114] Bio-self-assembly and aggregation [115,116] Nanomanipulation [117,118] Soft lithography [119,120] Molecular imprinting [121,122] Layer-by-layer electrostatic deposition [123,124] Chemical vapor deposition [125]... 

X. Zhang, T. Chen, Q. Chen, L. Wang, L. J. Wan (2009) Self-assembly and aggregation of melamine and melamine-uric/cyanurie aeid investigated by STM and AFM on solid surfaces. Phys. Chem. Chem. Phys., 11, 7708-7712. 

Star-shaped hexaphenylbenzenes and their derivatives, such as hexabenzocoronenes (HBCs), show remarkable properties such as self-assembly and aggregation in solutions, high charge carrier motilities, and high thermal stability (Muller et al., 1998 Ito et al., 2000). These conjugated systems are also useful in materials science in that they can form the core structures for discotic liquid... 

The reasons for self-assembly and the mechanisms necessary conditions for the aggregation into micelles, mono- or bilayers, structure of aggregates, distribution of aggregation numbers, etc. 

The threshold concentration above which a free amphi-phile will self-assemble and spontaneously form micelles. Below this concentration, no micelles are detectable. Note that micelle formation is not equivalent to a phase separation For most systems, the concentration range associated with the monomer-to-micelle transition is fairly narrow, and c.m.c. values can be obtained from graphical procedures. The symbol for c.m.c. is Cm. Whenever c.m.c. values are reported, the method for the determination should be clearly stated, because different physical techniques may be more or less sensitive to changes in the amphiphile s aggregation state. See Micelle Formation... 

The last of the important concepts that we will consider is self-assembly. Most chemists have, at some time in their careers, wondered why molecules cannot just make themselves. The process by which molecules build themselves is termed self-assembly and is a feature of many supramolecular systems. If the molecular components possess the correct complementary molecular recognition features and their interaction is thermodynamically favourable then simply mixing them could result in the specific and spontaneous self-assembly of the desired aggregate. This assumes that there is no significant kinetic barrier to the assembly process. The recognition features within the components may be any of the intermolecular bonding processes mentioned above, but we will be concerned with interactions between transition metal ions and polydentate ligands. 

We saw in Figure 1.2c that supramolecular chemistry is not just about solid state or solution host-guest chemistry but increasingly emphasises self-assembly and the construction of multi-nanometre scale devices and ultimately materials based on nanometre-scale components (a nanometre is 10 9 of a metre). Strict supramolecular self-assembly (Chapter 10) involves the spontaneous formation of a multi-component aggregate under thermodynamically controlled conditions based on information encoded within the individual building blocks (referred to as tectons ) themselves. The aggregate might comprise only one kind of molecule (as in the multiple copies of the same protein that comprise... 

There has been considerable recent interest in the self - assembly and surface activity of amphiphilic polymers and copolymers. Their interfacial and bulk solution properties have shown a rich pattern of behavior, and the ability to tailor their properties offers a wide range of potential applications. Their bulk aggregation behavior make them candidates, for example, for dye transportation and drug delivery whereas their surface properties make them useful as colloid stabilisers, anti -foaming agents and emulsifiers. This behavior can be illustrated in Fig. 3.24. 

These examples serve to highlight that supramolecular self-assembly and topo-chemical diacetylene polymerizations are a perfect match. Topochemical diacetylene polymerizations are an advantageous means of covalent capture for the reasons outlined above. The required order may, on the other hand, be provided by supramolecular self-assembly, which extends the scope beyond singlecrystalline monomers. This aspect becomes particularly important in the case of functional monomers in order to address specific applications. However, in contrast to previous investigations, the targeted preparation of hierarchically structured poly (diace tylene)s with a defined, finite number of strands required the presence of equally well-defined, uniform supramolecular polymers [106] with the propensity to form predictable superstructures, instead of micellar or vesicular ID aggregates. 

When collagen IV prepared from the EHS tumor is incubated at 37°C, it self-assembles, forming aggregates containing polygonal structures. These lateral associations are stabilized by the interaction between the domain NCI and sites that occur along the triple helix and are separated from one another by about 100 nm (Tsilibary and Charonis, 1986). The additional possibility of lateral aggregation of the molecules would lead to a much more complex three-dimensional structure. 

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