Amphiphilic mesophases lamellar

Figure 5. Formation of the hexagonal, lamellar, and cubic amphiphilic mesophases by the interactions between fibrous, lamellar, and globular micelles,...

The lyotropic behavior of these carbohydrate derivatives has been studied very carefully [96]. It follows the classical sequence of mesophases established for the ionic detergents such as soaps. In contact preparations of l-0-octyl-/3-D-glucopyranoside (10 e) with water at room temperature three types of lyomesophases are observed (from high to low amphiphile concentration) lamellar, cubic and columnar hexagonal, their principal structures are depicted in Fig. 12. 

Lecithins and glycolipids are naturally occurring chiral compounds, which can create a big variety of mesophases (lamellar, hexagonal, cubic, ripple, gel phases, etc.). Classical chiral phases like cholesteric or ferroelectric smectic phases are hitherto not reported and, if they were found, they would not be very typical for this class of compounds. Some of the bicontinuous cubic phases may have enantiomeric pure chiral space groups. Further, lyotropic cholesteric and blue phases might be formed (see below). Ordered tilted lamellar phases have been reported for lecithin and other amphiphilic compounds, but the special effects of chirality, like ferroelectric properties or helical order, are unknown. 

This parameter corresponds to cylindrical packing shapes. Surfactants and amphiphiles falling in this range often produce planar bilayers and lamellar mesophases. Such cylindrical building blocks also contribute to many... 

Note 5 The lyotropic equivalent of a smectic A mesophase is known as a lamellar mesophase where layers of amphiphilic molecules are separated by layers of solvent, normally water, or by oil in an inverse lamellar mesophase. 

At higher concentrations, micelles assemble in turn, to form hexagonal or cubic phases while longer chains or multi-chain compounds afford lamellar phases in which the amphiphilic derivative is arranged in parallel bilayers, separated by water. The succession of mesophases depending on temperature and concentration of the amphiphile can be visualized in a phase diagram (Fig. 3 c). 

Formation of the Vi and V2 Mesophases and of the Non-Amphiphilic Cubic Mesophase "Smectic D . In Figures 1 and 5, the formation of the amphiphilic cubic mesophases Vi and V2 is attributed to transitional globular micellar forms which arise intermediate between the indefinitely extended fibrous (M) and lamellar (G) forms which constitute the middle and neat mesophases, respectively. With a few non-amphiphilic mesogens (19, 20, 21, 22) a cubic mesophase "smectic D is found intermediate in the thermal succession of mesophases between smectic A... 

The appearance of a sharp central peak in the deuteron NMR spectrum for unaligned lyotropic liquid crystalline samples has been observed by several authors (30, 33, 34). This has been interpreted in terms of phase inhomogenities (35) or isotropic motion (36). However, recently Wennerstrom et al. showed for a lamellar amphiphile-water mesophase that this could be referred to double quantum transitions (37). It is expected that double quantum transition peaks appear in NMR spectra... 

The lamellar phase exists by virtue of the fact that amphiphilic molecules assemble into such a phase under given circumstances. One challenge is to relate properties of such a mesophase to macroscopic relevant properties, which was addressed in Section 9.2. In Section 9.3 a scheme has been given that was introduced by Israe-lachvili, which is useful in describing why certain amphiphiles form certain meso-phases. This scheme has been successfully applied to many amphiphilic systems. One of the challenges addressed in Section 9.3 is to see how that scheme may be also applied to the assembly of proteins, as another type of amphiphile, in particular how to explain recently observed phenomena of protein fibrillisation. 

Amphiphilic lipopeptides with a hydrophobic paraffinic chain containing from 12 to 18 carbon atoms and a hydrophilic peptidic chain exhibit lyotropic meso-phases and good emulsifying properties. The X-ray diffraction study of the mesophases and of dry lipopeptides showed the existence of three types of mesomorphic structures lamellar, cylindrical hexagonal and body-centred cubic. Two types of polymorphism were also identified one as a function of the length of the peptidic chain and the other as a function of the water content of the mesophases. The emulsifying properties of the lipopeptides in numerous pairs of immiscible liquids such as water/ hydrocarbons and water/base products of the cosmetic industry showed that small amounts of lipopeptides easily give three types of emulsions simple emulsions, miniemulsions and microemulsions. 

For non-ionic polysoaps the situation is quite different. Whereas in the case of oligoethyleneoxide head groups thermotropic mesophases seem to be absent [87, 121-124, 126, 231, 403, 409], polysoaps with liposaccharide [230,240, 300] surfactant fragments or with lipopeptide [244, 400-402] ones frequently show lamellar mesophases (e.g. of smectic A type) and even nematic ones (Fig. 39). It should be emphasized that the thermotropic mesophases here are not the result of mesogenic groups being present, but are the consequence of the amphiphilic character of the polysoaps and the resulting microphase separation. 

In spite of the extremely wide variation in composition of lamellar mesophase D, a curve could be drawn through all the values of the various characteristic quantities (Figure 20). The change in the interplanar distance with volume fraction of amphiphilic substance, VampV varied in phase D in a way consistent with a one-dimensional swelling, where the water is inserted between coherent double layers of amphiphilic substance. The variation in the thickness of the amphiphilic layer, da, is small it increases slowly with the decanol content from 22 to 24 A. The area per hydro-... 

The minimum in the interfacial free energy predetermines three kinds of geometry in nature spheres, cylinders and planes. Correspondingly, the most stable amphiphile aggregation structures are i) spherical (Hartley) micelles, ii) rod-shaped micelles and anisotropic middle phases, iii) disk-shaped micelles and lamellar mesophases. They exist as aggregates in a water continuum with a hydrocarbon core surrounded by hydrated polar groups (the normal type) and as aggregates in a hydrocarbon continuum (the reverse or inverted type) where water and... 

Lyotropic liquid crystals are principally systems that are made up of amphiphiles and suitable solvents or liquids. In essence an amphiphilic molecule has a dichotomous structure which has two halves that have vastly different physical properties, in particular their ability to mix with various liquids. For example, a dichotomous material may be made up of a fluorinated part and a hydrocarbon part. In a fluorinated solvent environment the fluorinated part of the material will mix with the solvent whereas the hydrocarbon part will be rejected. This leads to microphase separation of the two systems, i.e., the hydrocarbon parts of the amphiphile stick together and the fluorinated parts and the fluorinated liquid stick together. The reverse is the case when mixing with a hydrocarbon solvent. When such systems have no bend or splay curvature, i.e., they have zero curvature, lamellar sheets can be formed. In the case of hydrocarbon/fluorocarbon systems, a mesophase is formed where there are sheets of fluorocarbon species separated from other such sheets by sheets of hydrocarbon. This phase is called the La phase. In the La phase the molecules are orientationally ordered but positionally disordered, and as a consequence the amphiphiles are arranged perpendicular to the lamellae. The La phase of lyotropics is therefore equivalent to the smectic A phase of thermotropic liquid crystals. 

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