Amphiphilic molecules, molecular

Polarity of molecules ist Amphiphilic molecules, molecular bilayers... 

Phosphorus-containing surfactants are amphiphilic molecules, exhibiting the same surface-active properties as other surfactants. That means that they reduce the surface tension of water and aqueous solutions, are adsorbed at interfaces, form foam, and are able to build micelles in the bulk phase. On account of the many possibilities for alteration of molecular structure, the surface-active properties of phosphorus-containing surfactants cover a wide field of effects. Of main interest are those properties which can only be realized with difficulty or in some cases not at all by other surfactants. Often even quantitative differences are highly useful. 

Fig. 15—Sketch of preparation of L-B films (a) spread amphiphilic molecules on water surface, (b) compress the molecules using the barrier to get close packed and ordered molecular film, (c) transfer the film onto a substrate through the vertical immerse/retreat process, (d) transfer the film by horizontal lifting.

In the case of amphiphilic molecules, characterized by the coexistence of spatially separated apolar (alkyl chains) and polar moieties, both parts cooperate to drive the intermolecular aggregation. This simple but pivotal peculiarity makes amphiphilic molecules soluble in both polar and apolar solvents and able to realize, in suitable conditions, an impressive variety of molecular aggregates characterized by spatially separated apolar and polar domains, local order at short times and fluidity at long times, and differences in size, shape (linear or branched chains, cyclic or globular aggregates, extended fractal-like molecular networks), and lifetime. 

An enormous literature has been produced in recent decades in the field of molecular aggregation of amphiphilic molecules in liquid systems, emphasizing the extremely wide variety of accessible structures and dynamics. Among these molecular aggregates, in this chapter our attention will be restricted to those formed by some amphiphilic molecules (surfactants) in apolar solvents called reversed micelles [1]. 

The concept of micelles consists of aggregation of amphiphilic molecules that contain polar and non-polar moieties, which associate in a manner that minimizes hydrophobic and lipophilic interactions. However, a cascade molecule consisting of an internal lipophilic framework and a external hydrophilic surface would effectively be a unimolecular micelle [59] capable of hosting molecular guest(s). 

Pohorille, A. Benjamin, I., Structure and energetics of model amphiphilic molecules at the water liquid-vapor interface. A molecular dynamic study, J. Phys. Chem. 1993, 97, 2664-2670... 

The out-of-plane orientation of chromophores can be more easily controlled in LB films as compared with the in-plane orientation. Many chromophores are known to show anisotropic orientation in the surface normal direction. The molecular structure of chromophores and their position in amphiphile molecules, the surface pressure, the subphase conditions are among those affect their out-of-plane orientation. The out-of-plane orientation has been studied by dichroic ratio at 45° incidence, absorbance ratio at normal and 45° incidence, and incident angle dependence of p-polarized absorption [3,4,27,33-41]. The evaluation of the out-of-plane orientation in LB films is given below using amphipathic porphyrin (AMP) as an example [5,10,12]. 

This means that the partial molar area may directly be determined from the change in molecular area, when an amphiphilic molecule is introduced into a host liquid crystalline pattern. Of course, this area is the change of area per molecule at the introduction of one molecule of the substance in question and may be influenced by the interaction between the host molecules and the guest molecules. Since this interaction is an essential part of the present problem, it appears obvious that the method exactly meets the requirements. 

The measurements of n versus A isotherms generally exhibit, when compressed, a sharp break in the isotherms that has been connected to the collapse of the mono-layer under given experimental conditions. The monolayer of some lipids, such as cholesterol, is found to exhibit an unusual isotherm (Figure 4.7). The magnitude of FI increases very little as compression takes place. In fact, the collapse state or point is the most useful molecular information from such studies. It has been found that this is the only method that can provide information about the structure and orientation of amphiphile molecules at the surface of water (Birdi, 1989). 

The structure and orientation of the deposited amphiphile molecules have been found to be governed by the angle of contact between the monolayer and the solid surface. The deposited monolayers, in general, have been characterized as X- Y-, and Z -type, and their molecular arrangements can be described as follows. 

Figure 14.10 Self-assembly of peptide-amphiphiles into nanofibers (a) a peptide amphiphile molecule with five distinct regions designed for hydroxyapatite mineralization, (b) a schematic of molecular self-assembly, and (c) a negatively stain transmission electron microscopy image of the nanofibers. Reprinted from Hartgerink et al. (2001). Copyright 2001 American Association for the Advancement of Science.

Most molecular assemblies are usually formed by amphiphilic molecules (also called surfactant or detergent molecules) consisting of a polar head and of one or more nonpolar tails . Head groups can be cationic, anionic, non-ionic... 

Some amphiphilic molecules such as oleic acid and hexadecyl alcohol containing an alkyl chain and a polar head group form monolayers on the surface of water. The polar head groups of these molecules are attracted to and are in contact with water while their hydrocarbon tails protrude above it (Figure 15). The term monolayer implies the presence of a uniform mono-molecular film on the surface of water. Monolayer films can be classified as gaseous, liquid, or solid depending upon the degree of compression and the effective area per molecule. Clearly the liquid phase of a monolayer film and, more so, the solid represent constrained environments for individual molecules of amphiphiles. Monolayers, just like micelles, are dynamic species. 

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