Self-assembly structures vesicles

Polymerisable surfactants constitute another important area of polymerisation in disperse media. Over the past fifteen years, a number of studies have been made of surfactant assemblages possessing a polymerisable group. The aim of these studies is to fix the structure of the initial assemblages in such a way as to obtain stable aggregates with controllable size, rigidity and permeability. These systems could be applied where the non-polymerisable assemblages had proven inadequate as a result of their limited lifetime. The studies concerned two types of self-assembled structures vesicles and micellar systems. 

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). 

The lipid molecule is the main constituent of biological cell membranes. In aqueous solutions amphiphilic lipid molecules form self-assembled structures such as bilayer vesicles, inverse hexagonal and multi-lamellar patterns, and so on. Among these lipid assemblies, construction of the lipid bilayer on a solid substrate has long attracted much attention due to the many possibilities it presents for scientific and practical applications [4]. Use of an artificial lipid bilayer often gives insight into important aspects ofbiological cell membranes [5-7]. The wealth of functionality of this artificial structure is the result of its own chemical and physical properties, for example, two-dimensional fluidity, bio-compatibility, elasticity, and rich chemical composition. 

A certain type of lipid (or lipid-like) molecules are found that when dispersed in water tend to make self-assembly structures (Figure 4.13). Detergents were shown to aggregate to spherical or large cylindrical-shaped micelles. It is known that if egg phosphatidylethanolamine (egg lecithin) is dispersed in water at 25°C, it forms a self-assembly structure, which is called liposome or vesicle. 

During the end of the 20th century, a surge in the development of significantly advanced techniques has advanced nanoscience and technology in the development of self-assembly structures—micelles, monolayers, vesicles—biomolecules, biosensors, and surface and colloidal chemistry. In fact, the current literature indicates that there is no end to this trend regarding the vast expansion in the sensitivity and level of information. 

In the past decades, it has become more and more obvious that students and scientists of chemistry and engineering should have some understanding of surface and colloid chemistry. The textbooks on physical chemistry tend to introduce this subject insufficiently. Modern nanotechnology is another area where the role of surface and chemistry is found of much importance. Medical diagnostics applications are also extensive, where both microscale and surface reactions are determined by different aspects of surface and colloid chemical principles. Drug delivery is much based on lipid vesicles (self-assembly structure) that are stabilized by various surface forces. 

Fig.l Illustration of the self-assembled structures of block copolymers in solution spherical and cylindrical micelle and vesicle (from left to right)... 

Glucose-grafted PB85- -PS351 (Fig. 4c) were found to self assemble into vesicles or glycosomes in organic and aqueous media [28], In aqueous media, the vesicles measured about 240 nm in diameter (apparent value measured by DLS) and should be built of a polystyrene bilayered membrane and a glucose corona. However, the true structure of the vesicle membrane is not known yet. 

Nonionic block copolypeptides made of PEGylated L-lysine and L-leucine residues, PELLys- -PLLeu (Fig. lOh) have also been described [52], The copolymers adopted a rod-like conformation, due to the strong tendency of both segments to form a-helices (CD spectroscopy), and produced a variety of self-assembled structures in aqueous solution. Micrometer vesicles and sheet-like membranes could be obtained for copolymers with fractions of the hydrophobic leucine ranging from 10 to 30mol%. Conventional uncharged block copolymers of this composition would be expected to form spherical or cylindrical micelles. The assembly into bilayers was related to a secondary structure effect, as illustrated in Fig. 12. Accordingly, samples with the same composition but nonhelical chain conformation (CD),... 

In an aqueous medium, the apolar parts of the amphiphilic molecules group together, whereas in an apolar environment, the polar parts do. The resulting self-assembled structures may be of different types. The most common structures are spheres and rods (usually referred to as micelles ), and bilayer structures in a planar form (lamellae) or in a closed spherical shape (vesicles). Some of these structures are depicted in Figure 11.4. The structures shown in Figure 11.4 are cartoon-like representations in reality they are less ordered, that is, more fluid. 

A class of self-assembled structures that deserves special attention is the bilayer. This is a lamellar structure composed of two molecular layers of amphiphilic molecules. Amphiphiles having a J/v close to unity usually assemble into bilayers in which (in aqueous media) the apolar parts of the molecules are directed toward each other. Free-floating bilayers do not exist it is too unfavorable to expose the hydro-phobic edges to water. The bilayer closes into a spherical geometry, the so-called vesicle, or its edges are embedded in a nonaqueous environment. See Figure 11.13. 

It is to be noted that not only water-soluble polymers can be used to complex DNA, amphiphilic polymers, which depending on the relative ratio of hydrophilic to hydrophobic block, can also form various self-assembled structures, from spherical micelles to vesicles (polymersomes). This review will be restricted to micelleforming polymers and will exclude polymersomes, which can both encapsulate (in their aqueous interior) and complex DNA [24, 25]. 

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