Self-assembled amphiphiles structures

Fig. 4 Stability and permeability of self-assembled amphiphilic structures. Amphiphilic molecules such as fatty acids having carbon chain lengths of 9 or more carbons form bilayer membranes when sufficiently concentrated, a Pure bilayers of ionized fatty acid are relatively unstable but become markedly more stable as long chain alcohols are added, b Dimensions of the amphiphile also play a role. Shorter chain amphiphiles (9-10 carbons) are less able to form bilayers, while those of intermediate chain length (12-14 carbons) produce stable bilayers that also are permeable to ionic and polar solutes. Longer chain lengths (16-18 carbons) produce bilayers that are increasingly less permeable to solutes [48]...

The size of the self-assembled amphiphilic structures varies from about 10 nm to a micron scale. Micelles are smaller than the vesicles, because they do not have internal aqueous core, and the wall is monolayer, not bilayer like in the liposomes. 

The presentation in this book focuses on physicochemical concepts that form the basis of understanding colloidal and interfacial phenomena rather than on experimental methods and techniques. Because in biosciences, colloidal systems are more often than not aqneous solutions, gels of biopolymers, and self-assembled amphiphilic structures, emphasis is placed on these reversible, soft colloids. 

Packing parameter and Curvature. An important factor determining the shape of self-assembled amphiphilic structures is the size of the hydrophobic part of the amphiphile relative to the hydrophilic part. The shape can be parameterized in terms of the packing parameter A, which is given by... 

Hakanpaa J, Paananen A, Askolin S, Nakari-SetalaT, Parkkinen T, Penttila M, Linder MB, Rouvinen J, Atomic resolution structure of the HFBII hydrophobin, a self-assembling amphiphile, /Biol Chem 279 534—539, 2004. 

The first self-assembling block copolymers were PS-fe-PMPS- -PS synthesised by Matyjaszewski and Moller. They observed micellar aggregates by ATM after casting dilute dioxane solutions (a solvent selective for the PS block) of the copolymer. The observed micelles were taken to have internal PMPS cores and were measured at 25-30nm in diameter [73], The hrst self-assembling amphiphilic polysilane block copolymers to be investigated was the PMPS-PEO multi-block copolymer with normal distribution PMPS blocks and uniform low polydispersity PEO blocks. After dialysis aqueous dispersions of this copolymer formed micellar as well as vesicular structures [78, 79] as shown in Eig. 19. 

Figure 11 Illustration of typical sizes and shapes of various self-assembled amphiphilic micelle and liposome structures. Such structures spontaneously form under aqueous conditions from noncovalent interactions of the hydrophobic and hydrophilic tails of the amphiphilic molecules. (Adapted from Ref. 38. Elsevier, 2009.)...

This chapter describes supramolecular assemblies in mesoscopic dimension and their recent developments. It also compliments earlier reviews [21,22]. The mesoscopic supramolecular assemblies are defined as hierarchically self-assembled amphiphilic supramolecular structures whose ternary and the higher assembly structures are controlled through solvophilic-solvophobic interactions. Here, pairs of molecules brought by secondary interactions are designed that acquire amphiphilicity upon complexation. They become units of self-assembly and hierarchically grow into mesoscopic-scale supermolecules that are dispersed stably in aqueous or in organic media. 

The aforesaid limitations of dissolving apolar solutes in ILs can be overcome by incorporating hydrophobic domains in the IL-containing solution by the formation of microemulsions. A microemulsion is a thermodynamically stable dispersion of two immiscible liquids, a polar and an apolar phase, stabilized by an adsorbed surfactant film at the liquid-liquid interface [32,33]. Water has been conventionally used as the polar phase in microemulsions, since amphiphilic surfactants spontaneously self-assemble in water [34-A6]. In 1982, Evans and coworkers showed, for the first time, the formation of self-assembled micellar structure in the RTTL ethylammo-nium nitrate (EAN) [47]. Since then, a large number of protic and aprotic ILs have been shown to support the self-assembly of surfactants in IL-surfactant binary... 

Natural amphiphiles are often lipids. In this chapter, we focus on polar lipids with an amphiphilic character, such as phospholipids (see the next section for examples). The latter, together with proteins, make np cell membranes that are formed from self-assembled bilayer structures. A key feature of amphiphilic membranes in biological systems is that they are ordered and yet fluid, allowing the transport of material across them. The properties of membranes are considered in Section 4.11 of this chapter. 

For structures with a high curvature (e.g., small micelles) or situations where orientational interactions become important (e.g., the gel phase of a membrane) lattice-based models might be inappropriate. Off-lattice models for amphiphiles, which are quite similar to their counterparts in polymeric systems, have been used to study the self-assembly into micelles [ ], or to explore the phase behaviour of Langmuir monolayers [ ] and bilayers. In those systems, various phases with a nematic ordering of the hydrophobic tails occur. 

FIG. 1 Self-assembled structures in amphiphilic systems micellar structures (a) and (b) exist in aqueous solution as well as in ternary oil/water/amphiphile mixtures. In the latter case, they are swollen by the oil on the hydrophobic (tail) side. Monolayers (c) separate water from oil domains in ternary systems. Lipids in water tend to form bilayers (d) rather than micelles, since their hydrophobic block (two chains) is so compact and bulky, compared to the head group, that they cannot easily pack into a sphere [4]. At small concentrations, bilayers often close up to form vesicles (e). Some surfactants also form cyhndrical (wormlike) micelles (not shown). 

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