Self-organization, amphiphilic molecule

Diverse chiral nanometric ribbons and tubules formed by self-assembly of organic amphiphilic molecules can be transcribed to inorganic... 

The most well-known example of a self-organized system is provided by proteins that fold in aqueous solution into a eompact native structure where most of the hydro-phobic residues tend to reside in the core (popularly called the hydrophobic core ) while the hydrophilic residues are mostly on the surface. Such a self-organization is possible because of the simultaneous presence of hydrophilic and hydrophobic amino acid residues along the linear peptide chain. In fact, such self-organization of molecules is fairly common in nature. In this chapter, we shall discuss how water leads to the formations of exotic structures known as micelles, reverse micelles, and microemulsions. These structures are formed by molecules known as surficants, which are long-chain molecules and amphiphilic in nature, meaning that they contain two distinct individual parts that like water (because the part is polar) and dislike water (as it consists of hydrocarbons). These two opposite parts are usually located at the two ends of the surfactant molecule, named head and tail . 

The monolayer resulting when amphiphilic molecules are introduced to the water—air interface was traditionally called a two-dimensional gas owing to what were the expected large distances between the molecules. However, it has become quite clear that amphiphiles self-organize at the air—water interface even at relatively low surface pressures (7—10). For example, x-ray diffraction data from a monolayer of heneicosanoic acid spread on a 0.5-mM CaCl2 solution at zero pressure (11) showed that once the barrier starts moving and compresses the molecules, the surface pressure, 7T, increases and the area per molecule, M, decreases. The surface pressure, ie, the force per unit length of the barrier (in N/m) is the difference between CJq, the surface tension of pure water, and O, that of the water covered with a monolayer. Where the total number of molecules and the total area that the monolayer occupies is known, the area per molecules can be calculated and a 7T-M isotherm constmcted. This isotherm (Fig. 2), which describes surface pressure as a function of the area per molecule (3,4), is rich in information on stabiUty of the monolayer at the water—air interface, the reorientation of molecules in the two-dimensional system, phase transitions, and conformational transformations. 

A (macro)emulsion is formed when two immiscible Hquids, usually water and a hydrophobic organic solvent, an oil, are mechanically agitated (5) so that one Hquid forms droplets in the other one. A microemulsion, on the other hand, forms spontaneously because of the self-association of added amphiphilic molecules. During the emulsification agitation both Hquids form droplets, and with no stabilization, two emulsion layers are formed, one with oil droplets in water (o /w) and one of water in oil (w/o). However, if not stabilized the droplets separate into two phases when the agitation ceases. If an emulsifier (a stabilizing compound) is added to the two immiscible Hquids, one of them becomes continuous and the other one remains in droplet form. 

Recently, new ordered mesoporous silicas have also been synthesized by using self-organization of amphiphilic molecules, surfactants and polymers either in acidic or basic condition. A schematic phase diagram of water-surfactant is shown in the figure. 

Of course, self-assembly of this kind occurs in water, and not, say, in ethanol. Any self-organization process must be defined in a given set of initial conditions. Initial conditions, as always in thermodynamics, determine the outcome of the process, and in particular whether the process is also under thermodynamic control or not. Aside from that, it is well known that a large series of amphiphilic molecules... 

We have already seen in Chapter 5, on self-organization, how and why amphiphilic molecules tend to form aggregates such as micelles, vesicles, and other organized structures. 

It is not yet understood how life began on Earth nearly four billion years ago, but it is certain that at some point very early in evolutionary history life became cellular. All cell membranes today are composed of complex amphiphilic molecules called phospholipids. It was discovered in 1965 that if phospholipids are isolated from cell membranes by extraction with an organic solvent, then exposed to water, they self-assemble into microscopic cell-sized vesicles called liposomes. It is now known that the membranes of the vesicles are composed of bimolecular layers of phospholipid, and the problem is that such complex molecules could not have been available at the time of life s beginning. Phospholipids are the result of a long evolutionary process, and their synthesis requires enzymatically catalyzed reactions that were not available for the first forms of cellular life. 

The first synthetic amphiphiles found to self-organize into bilayers, were quaternary ammonium salts bearing two long alkyl chains 1.13.47.48.49 it is interesting to note that these molecules did not contain a connector moiety between the polar and the apolar part, as in the case of the biolipids. While the physicochemical properties of these bilayers were found to be comparable to those of the biological membranes, the synthetic lipids were found to... 

In Nature, self-assembly to form finite assemblies often involves the non-covalent organization of molecules containing not only amphiphilic character, but also specific information needed for additional intermolecular recognition processes to occur, e.g., hydrogen... 

In the 1940s, it was demonstrated in the pioneering work of Zisman and coworkers [8] that the LB technique is not the only way to create an organized organic monolayer on a solid substrate. It was demonstrated that when a compatible substrate is exposed to a solution of an amphiphilic compound, the dissolved molecules form a self-assembled monolayer on the substrate surface. Such films maintain their structural integrity after they are removed from solution. The most common examples of such films are organosulfur films on gold substrates [9] and alkyltrichlorosilane films on silicon dioxide substrates [10]. Compared with the LB films, the self-assembled films are somewhat less ordered. On the other hand, these films are easier to prepare, since they do not require special instrumentation and can easily be deposited on both planar and non-planar substrates. Also, in many cases the amphiphilic molecules which make the self-assembled film are chemisorbed on the substrate. Such films are more stable when heated or exposed to solvents than are typical LB films, which are held to the substrate by non-covalent interactions. 

Thus far, methods of organizing small molecules into mono- and multilayers have been discussed. However, both LB and self-assembly techniques allow deposition of polymer films as well. Two approaches to preparing polymer films by LB method have been described. An amphiphilic polymer film can be... 

The ability of the above mentioned substances to self-organize into bilayer membranes is caused by their amphiphility. During the formation of the vesicles the amphiphilic molecules orient themselves in such a way that their polar heads contact aqueous phases outside and inside the vesicle, while their nonpolar tails are directed towards the interior of the bilayer as shown in Fig. 2c. Vesicles can be classified in multilamellar, small unilamellar (d = 200-500 A) and large unilamellar (d = 1000-5000 A) ones. Since these are small unilamellar vesicles that are typically used for studying PET, in further discussion the term vesicle will always refer to the vesicles of this type, unless otherwise specified. 

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