Spherical assemblies

Figure 6.5 Illustrations of nanoscale spherical assemblies resulting from block copolymer phase separation in solution are shown, along with the chemical compositions that have been employed to generate each of the nanostructures (a) core crosslinked polymer micelles (b) shell crosslinked polymer micelles (SCKs) with glassy cores (c) SCKs with fluid cores (d) SCKs with crystalline cores (e) nanocages, produced from removal of the core of SCKs (f) SCKs with the crosslinked shell shielded from solution by an additional layer of surface-attached linear polymer chains (g) crosslinked vesicles (h) shaved hollow nanospheres produced from cleavage of the internally and externally attached linear polymer chains from the structure of (g)...

MESA at a liquid/liquid interface is not limited to two dimensions. Objects placed on a curved interface will assemble into a pseudo-3D system. The capillary forces between the objects are the same as those described for 2D MESA [ref. 5]. The curved interface is made by putting a drop of an immiscible liquid in another, isodense liquid the result is a sphere. The method can easily be extended to different shapes (catenoid, cylinder, cone) by distorting the shape of the drop [ref. 39]. The spherical assemblies in these experiments are made of metallic hexagons (100 pm sides 6 pm thick) with hydrophobic and hydrophilic sides [ref. 11]. Electrodeposition through lithographically defined molds fabricated metallic hexagonal rings (Fig. 4.16). Hydrophilic faces were introduced... 

Typically, synthetic capsules have a contained volume in the range 200-350 A3. In 1997, we discovered a spherical assembly consisting of [(C-methylresorcin[4]arene)6 (H20)8], 1. This assembly, Figure 1, with an enclosed volume of 1375 A3, was characterized by a single-crystal X-ray diffraction study and was found to be stable in nonpolar solvents [25]. Supramolecular assembly 1 ultimately led to the discovery of the link between 1 and the solid geometry principles of Plato and Archimedes [26], Before this discussion progresses, it is useful to examine briefly Platonic and Archimedean solids. 

We were also able to link the spherical assembly to the Archimedean solid known as the snub cube, Table 2. In a recent review, we have set forth structural classifications and general principles for the design of spherical molecular hosts based, in part, on the solid geometry ideas of Plato and Archimedes [27]. Indeed,... 

Analogous to the surfactant approach, assembly of proteins into empty virus core shells has also been modelled in case of spherical assembly, as seen in Section 9.3. We propose as a first Ansatz a simple extension to Equation (9.15) in order to account for nonspherical and/or branched structures, and at the same time to take into account endpoint defects (i.e., the endpoints do not fuUy contribute to the chemical potential) here, compared to when an aggregate would be infinite, in the same manner as done in, for example. Equation (9.11) for rods, that is, aggregates of dimension 1. Taking into account an arbitrary shape, and/or degree of branching, the dimensionality may be denoted by its fractal dimension, Df, defined by... 

Further studies by electron microscopy on some of the samples exhibiting the Pm3n cubic phase show the existence of grain bormdaries and stacking faults [118]. These are all consistent with the presence of quasi-spherical assemblies or more precisely to polyhedral-like micelles, and moreover suggest that the supramolecular spheres are deformable, interacting with one another through a relatively soft pair potential [119]. The majority of such quasi-spherical assemblies are thus distorted into an oblate shape. 

HMW biopolymers are part of the colloid-sized fraction of marine organic matter that can be visualized and enumerated with transmission electron microscopy (TEM) (Wells and Goldberg, 1991, 1993). Colloids range in size from a few nanometers up to 1 p,m in size. Very small colloids (<30 nm) are irregular in shape, while larger colloids (—30-60 nm) are more spherical assemblies of 2-5 nm sized subparticles. Concentrations of small colloids (<200 nm) range from nondetectable (<10" colloids ml ) to... 

The other chapters then lead from the simple to the more complex molecular assemblies. Syntheses of simple synkinons are described at first. Micelles made of 10-100 molecules follow in chapter three. It is attempted to show how structurally ill-defined assemblies can be most useful to isolate single and pairs of molecules and that micelles may produce very dynamic reaction systems. A short introduction to covalent micelles, which actually are out of the scope of this book, as well as the discussion of rigid amphiphiles indicate where molecular assembly chemistry should aim at, namely the synkinesis of solid spherical assemblies. Chapter four dealing with vesicles concentrates on asymmetric monolayer membranes and the perforation of membranes with pores and transport systems. The regioselective dissolution of porphyrins and steroids, and some polymerization and photo reactions within vesicle membranes are also described in order to characterize dynamic assemblies. 

The rate for homogeneous nucleation J) of spherical assemblies can be expressed by the classical nucleation equation (Gibbs, 1948 Volmer, 1939 Becker and Doring, 1935 Turnbull and Fisher, 1949) ... 

Aqueous micelles are thermodynamically stable and kinetically labile spherical assemblies. Their association-dissociation process is very fast and occurs within milliseconds. The actual order is less than shown in Figure 1. Driving forces for the formation of aqueous micelles or vesicles are the solvation of the headgroup and the desolvation of the alkyl chain ( hydrophobic effect ). Because of the rapid exchange of surfactants, the core of the micelle contains a small percentage of water molecules. Aqueous assemblies are preferentially stabilized by entropy, and reverse micelles by enthalpy [4]. The actual formation of micelles begins above a certain temperature (Krafffs point) and above a characteristic concentration (critical micelle concentration, CMC). Table 1 shows a selection of typical micelle-forming surfactants and their CMCs. 

At synthesis these are grouped in spatial deposits with sizes of around 100 nm, similar to Dahlia, Fig. 9c. This comes from the fact that the quantity of carbon atoms released by the laser pulse is large and the energy of the separate atoms is likewise significant, as a result these are dispersed in all directions and traverse a long path. The large work pressure obstructs the speedy dispersal of the atoms and facilitates deposit formation. The form of the SWNH deposit is also dependent on the nature of the atmospheric ambient. When the work gas is He or N2 the spherical assembly with a budlike shape bud[ k.Q nanohoms) is predominant 70-80 %, while for an Ar work gas ambient 95 % of the formation resembles the dahlia flower (42), (43). Their structure has fewer defects than that of the bud iike nanohoms. 

McKenna BJ, Birkedal H, Baitl MH et al (2004) Micrometer-sized spherical assemblies of polypeptides and small molecules by acid-base chemistry. Angew Chem Int Ed 43(42) ... 

He proposed packing a spherical layer of high explosives around a spherical assembly of tamper and a hollow but thick-walled spherical core. 

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