Amphiphiles, self-organization

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. 

Baker, G. A. Pandey, S. (2005). Amphiphilic self organization in ionic liquids. ACS Symposium Series. Rogers, R. D. and Seddon, K R. 901, 234-243. 

A typical biomembrane consists largely of amphiphilic lipids with small hydrophilic head groups and long hydrophobic fatty acid tails. These amphiphiles are insoluble in water (<10 ° mol L ) and capable of self-organization into uitrathin bilaycr lipid membranes (BLMs). Until 1977 only natural lipids, in particular phospholipids like lecithins, were believed to form spherical and related vesicular membrane structures. Intricate interactions of the head groups were supposed to be necessary for the self-organization of several ten thousands of... 

Vesicles [10, 11] these aggregates of insoluble natural or artificial amphiphiles in water can have various shapes (spherical, cylindrical). Depending on the preparation conditions, small unilamellar or large multilamellar vesicles can be produced. The structures meet the self-organization criterion, because they are, albeit on a long time scale, dynamic and not in thermodynamic equilibrium, which would in many cases be a macroscopically phase separated lamellar phase. 

Kirstein, S. and Daehne, S. (2006). J-aggregates of amphiphilic cyanine dyes self-organization of artificial light harvesting complexes. Int. J. Photoenergy 5, 1-21. 

Self Organized Nanostructures of Amphiphilic Block Copolymers II... 

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. 

Up to now only amphiphilic dendrimers with soft cores and soft shells were used. Unfortunately the shape and the size of this type of dendrimer also show a dependence on the factors mentioned above [75-77]. By using dendrimers with a hard inner core it would be possible to study the self-organization of amphiphilic dendrimers as related to shape and size of the core. 

Fig. 6.1 Self-organization of amphiphilic block polymers in water (cmc = critical micelle concentration).

Gayathri SS, Patnaik A (2006) Electrical rectification from a fullerene[60]-dyad based metal-organic-metal junction. Chem Commun (Cambridge, UK) 1977-1979 Matino F, Arima V, Piacenza M et al (2009) Rectification in supramolecular zinc porphyrin/ fulleropyrrolidine dyads self-organized on gold(lll). Chemphyschem 10 2633-2641 Acharya S, Song H, Lee J et al (2009) An amphiphilic Cgo penta-addition derivative as a new U-type molecular rectifier. Org Electron 10 85-94... 

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. 

The observation of single channel currents may suggest the successful self-organization of supramolecular channels. This process may require several steps (1) incorporation of the amphiphilic carboxylate-ammonium ion pair into the bilayer lipid membrane (2) molecular recognition of the relatively polar oligoether chain from the surrounding hydrophobic lipid components to induce domain formation of molecular level and (3) interlayer connection of these hydrophilic domains existing in different lipid layers. 

Amphiphilic D03A derivatives with long alkyl chains at the 10-N atom display micellar self-organization leading to an increase of rR, and therefore also of the relaxivity [81,82]. 

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