Membranes crystal-liquid crystal

The theory had never been tested on a logical model system. Let us consider in detail one representative case, the superimposable stacking of the two benzene rings, one from each triplet diphenylcarbene molecule. These are considered to represent idealized modes of dimeric interaction of the aromatic ring parts of open-shell molecules in ordered molecular assemblies like crystals, liquid crystals and membranes. 

Recently very interesting results have been reported on the behaviour of polymer/liquid crystal membrane consisting of bisphe-nol A polycarbonate (PC) and of N-ethoxybenzylidene A -n-butyl aniline (EBBA). This substance shows a crystal-nematic transition at 304° K and nematic-to-isotropic phase transition at 355° K. 

Some final remarks concerning living membranes an important feature of biomembranes is the strongly heterogeneous lipid composition of the liquid crystal membrane matrix. Studies of mixed lipid monolayers demonstrate that the variation of the surface potential as a function of the area differs from that of pure monolayers depending on the composition, dAV/dA could either be enhanced or weakened. 

CRYSTAL-LIQUID CRYSTAL PHASE TRANSFORMATION OF SYNTHETIC BIMOLECULAR MEMBRANES... 

The crystal-liquid crystal phase transition behavior and the membrane structure in the artificial amphiphile/ water system were investigated on the basis of the thermal analysis, and wide- and small-angle x-ray analysis. 

S. H. Yoon, M. Kang, W. I. Park, and H. J. Jin, Electrically conductive polymeric membranes by incorporation of carbon nanotubes. Mol. Crystals Liquid Crystals 424,103-685 (2007). 

Forty years have passed since the publication of three famous books bearing the same title—Colloid Science. The first was by A. E. Alexander and P. Johnson, the second by J. W. McBain, and the third by H. R. Kruyt. Since then, colloid science has generated a number of new but related fields, such as studies of polymers, semiconductors, liquid crystals, membranes, and vesicles. Micellar aggregates have served as an important bridge between microscopic and macroscopic chemical species in the development of new technologies. The importance of colloid science is now fully recognized, and it has become established as the basic foundation of nearly all fields of solution science. 

The principles described in this paper are of importance for polymerizations not only in the solid state, but also in ordered systems in general. Thus, to a greater or lesser extent they must be relevant to reactions in liquid crystals, membranes, surface films, and so on, and thus for both natural and synthetic systems. The polymers produced by such processes may be expected to have unusual, and even possibly unique, chemical, physical, and mechanical properties, particularly as long as maintained in the parent matrix. Philosophically, these reactions in chiral aggregates are of interest as one possible mode of abiotic formation of chiral polymers. 

Special structures, theories for micellization Complex phase behaviour, prediction of CMC (Micro)gels, polymers in solutions and surfaces, liquid crystals Membranes, cell adhesion, drug delivery... 

Hydrated bilayers containing one or more lipid components are commonly employed as models for biological membranes. These model systems exhibit a multiplicity of structural phases that are not observed in biological membranes. In the state that is analogous to fluid biological membranes, the liquid crystal or La bilayer phase present above the main bilayer phase transition temperature, Ta, the lipid hydrocarbon chains are conforma-tionally disordered and fluid ( melted ), and the lipids diffuse in the plane of the bilayer. At temperatures well below Ta, hydrated bilayers exist in the gel, or Lp, state in which the mostly all-trans chains are collectively tilted and pack in a regular two-dimensional... 

At present moment, no generally feasible method exists for the large-scale production of optically pure products. Although for the separation of virtually every racemic mixture an analytical method is available (gas chromatography, liquid chromatography or capillary electrophoresis), this is not the case for the separation of racemic mixtures on an industrial scale. The most widely applied method for the separation of racemic mixtures is diastereomeric salt crystallization [1]. However, this usually requires many steps, making the process complicated and inducing considerable losses of valuable product. In order to avoid the problems associated with diastereomeric salt crystallization, membrane-based processes may be considered as a viable alternative. 

Caffrey, M. Structural, Mesomorphic and Time-Resolved Studies of Biological Liquid Crystals and Lipid Membranes Using Synchrotron X-Radiation. 151, 75-109 (1989). 

Fig. 27 a and b. Schematic representation of the molecular structure of a side chain polymeric liquid crystals b polymer model membranes studied by 2H NMR... 

The three classes of liquid crystals differ in the arrangement of their molecules. In the nematic phase, the molecules lie together, all in the same direction but staggered, like cars on a busy multilane highway (Fig. 5.49). In the smectic phase, the molecules line up like soldiers on parade and form layers (Fig. 5.50). Cell membranes are composed mainly of smectic liquid crystals. In the cholesteric phase, the molecules form ordered layers, but neighboring layers have molecules at different angles and so the liquid crystal has a helical arrangement of molecules (Fig. 5.51). 

Papahadjopoulos, D., and Watkins, J. C. (1967). Phospholipid model membranes. II. Permeability properties of hydrated liquid crystals, Biochim. Biophys. Acta. 135. 639-652. 

The thickness of the membrane phase can be either macroscopic ( thick )—membranes with a thickness greater than micrometres—or microscopic ( thin ), i.e. with thicknesses comparable to molecular dimensions (biological membranes and their models, bilayer lipid films). Thick membranes are crystalline, glassy or liquid, while thin membranes possess the properties of liquid crystals (fluid) or gels (crystalline). 

Phospholipids, which are one of the main structural components of the membrane, are present primarily as bilayers, as shown by molecular spectroscopy, electron microscopy and membrane transport studies (see Section 6.4.4). Phospholipid mobility in the membrane is limited. Rotational and vibrational motion is very rapid (the amplitude of the vibration of the alkyl chains increases with increasing distance from the polar head). Lateral diffusion is also fast (in the direction parallel to the membrane surface). In contrast, transport of the phospholipid from one side of the membrane to the other (flip-flop) is very slow. These properties are typical for the liquid-crystal type of membranes, characterized chiefly by ordering along a single coordinate. When decreasing the temperature (passing the transition or Kraft point, characteristic for various phospholipids), the liquid-crystalline bilayer is converted into the crystalline (gel) structure, where movement in the plane is impossible. 

Guizard, C., Bac, A., Barboiu, M. and Hovnanaian, N. (2000) Organic-inorganic hybrid materials with specific solute and gas transport properties for membrane and sensors applications. Molecular Crystals and Liquid Crystals, 354, 91-106. 

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