Bicontinuous aggregate structures

Figure 16.1 Relationship between molecular shape, aggregate structure in dilute dispersions, phase behavior and packing parameter. Micellar phase (L,), cubic micellar phase (I), hexagonal phase (H), bicontinuous cubic phase (Q), La lamellar phase. Subscripts I and II indicate normal and inverted phases, respectively. From M. Scarzello, Aggregation Properties of Amphiphilic DNA-Carriers for Cene Delivery, Ph. D. Thesis University of Groningen, p 6, 2006...

The cubic phases are also known as viscous isotropic phases - because they are As the name implies, these phases have structures based around one of several possible cubic lattices, namely the primitive, face-centred and body-centred. There are two very distinct aggregate structures, i.e. one comprised of small micelles, normal or reversed, and one based on three-dimensional bicontinuous aggregates. The normal and reversed structures that occur for both make a total of four classes. It is still not certain exactly which structures can occur for the different classes, but the overall picture has become much clearer during the past few years (46, 49-57), with more and more structures being identified. The first set of structures comprised of small globular micelles is labelled I , while the second group, the bicontinuous three-dimensional (3-D) micellar network, is labelled V . 

Figure 5.2.4 shows a network surface structure of a VA/AA-based emulsion with 6.6 wt% soiids. The repeat unit in the network structure is between 5 and 10 jtm. The formation of the network or bicontinuous surfactant structure is well established in relatively high molecular weight ethylene oxide/propylene oxide segmented block copolymer nonionic surfactants, and it has been ascribed to the formation of liquid crystalline macromolecular assemblies. Upon dilution, the emulsion showed 5-10 ttm spherical domains that could form aggregates up to about 60p.m in size (Fig. 4.4.5). In Fig. 5.2.5, bubble surfaces are shown with the network surfactant structure, even in the diluted emulsion. Apparently, the surfactant macromolecules tend to concentrate on bubble surfaces and form a more viscous and probably elastic polymer surface layer. 

Figure 4.24 Normal aggregate structures and an amphiphillic bilayer (a) spherical micelle, (b) cylindrical micelle, (c) bilayer, (d) saddle surface, one half of the bilayer of a bicontinuous cubic structure...

In an intermediate concentration range, where the materials cannot decide if the water or the amphiphile aggregate structure should be the continuous matrix, often so-called bicontinuous structures form. In such bicontinuous cubic phases the interfaces have saddle-splay fype sfructures characterized by nonzero negative mean curvature and negative Gaussian curvature. The most common bicontinuous cubic phase is called gyroid... 

It is generally accepted that surfactant-polymer interactions may occur between individual surfactant molecules and the polymer chain (i.e., simple adsorption), or in the form of polymer-surfactant aggregate complexes. In the latter case, there may be a complex formation between the polymer chain and micelles, premiceUar or submiceUar aggregates, liquid crystals, and bicontinuous phases—that is, with any and all of the various surfactant aggregate structures described in Chapters 4 and 5. Other association mechanisms may result in the direct formation of what are sometimes called hemimicelles along the polymer chain. The term hemimicelle may be defined, for present purposes, as a surfactant aggregate formed... 

Two system-dependent interpretative pictures have been proposed to rationalize this percolative behavior. One attributes percolation to the formation of a bicontinuous structure [270,271], and the other it to the formation of very large, transient aggregates of reversed micelles [249,263,272], In both cases, percolation leads to the formation of a network (static or dynamic) extending over all the system and able to enhance mass, momentum, and charge transport through the system. This network could arise from an increase in the intermicellar interactions or for topological reasons. Then all the variations of external parameters, such as temperature and micellar concentration leading to an extensive intermicellar connectivity, are expected to induce percolation [273]. 

The shapes of these self-assemblies are as varied as the capacity of the molecules to weave through space will allow. The accessible interfacial geometries span a rich range of structures from spheres and planes to highly intercormected bicontinuous honeycombs. So how are the structures of these complex liquid crystalline and disordered assemblies best described and understood Typically, the problem is tackled by recourse to thermodynamic principles. A complete statistical mechanical treatment is out of the question. (The difficulty is a fundamental one. We do not yet know how to write down a partition faction that describes the full ensemble of possible aggregate shapes and their associated free energies.)... 

From the results of self-diffusion, Lindman et al. (71) have proposed the structure of microemulsions as either the systems have a bicontinuous (e.g. both oil and water continuous) structure or the aggregates present have interfaces which are easily deformable and flexible and open up on a very short time scale. This group has become more inclined to believe that the latter proposed structure of microemulsion is more realistic and close to the correct description. However, no doubt much more experimental and theoretical investigations are needed to understand the dynamic structure of these systems. 

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