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Structure of Liquid Crystalline Phases

The above-mentioned unlimited self-assembly structures in ID, 2D or 3D are referred to as liquid crystalline structures. They behave as fluids and are usually highly viscous. At the same time, X-ray studies of these phases yield a small number of relatively sharp lines that resemble those produced by crystals [6]. Since they are fluids they are less ordered than crystals, but because of the X-ray lines and their high viscosity it is also apparent that they are more ordered than ordinary liquids. Thus, the term liquid crystalline phase is very appropriate for describing these self-assembled structures. A brief description of the various liquid crystalline structures that can be produced vith surfactants is given below, and Table 3.1 shows the most commonly used notation to describe these systems. [Pg.59]


NMR Self-Diffusion of Desmopressin. The NMR-diffusion technique (3,10) offers a convenient way to measure the translational self-diffusion coefficient of molecules in solution and in isotropic liquid crystalline phases. The technique is nonperturbing, in that it does not require the addition of foreign probe molecules or the creation of a concentration-gradient in the sample it is direct in that it does not involve any model dependent assumptions. Obstruction by objects much smaller than the molecular root-mean-square displacement during A (approx 1 pm), lead to a reduced apparent diffusion coefficient in equation (1) (10). Thus, the NMR-diffusion technique offers a fruitful way to study molecular interactions in liquids (11) and the phase structure of liquid crystalline phases (11,12). [Pg.256]

In an earlier review [3], mixed micelles formed by bile salts were classified into those with (i) non-polar lipids (e.g., linear or cyclic hydrocarbons) (ii) insoluble amphiphiles (e.g., cholesterol, protonated fatty acids, etc.) (iii) insoluble swelling amphiphiles (e.g., phospholipids, monoglycerides, acid soaps ) and (iv) soluble amphiphiles (e.g., mixtures of bile salts with themselves, with soaps and with detergents) and the literature up to that date (1970) was critically summarized. Much recent work has appeared in all of these areas, but the most significant is the dramatic advances that have taken place in our understanding of the structure, size, shape, equilibria, and thermodynamics of bile salt-lecithin [16,18,28,29,99-102,127, 144,218,223,231-238] and bile salt-lecithin-cholesterol [238,239] micelles which are of crucial importance to the solubihty of cholesterol in bile [1]. This section briefly surveys recent results on the above subclasses. Information on solubilization, solubilization capacities or phase equilibria of binary, ternary or quaternary systems or structures of liquid crystalline phases can be found in several excellent reviews [5,85,207,208,210,211,213,216,217] and, where relevant, have been referred to earlier. [Pg.388]

XRD is the most powerful experimental technique to determine the structure of liquid-crystalline phases (Seddon, 1998 Templer, 1998). X-rays interact with the electron cloud of the material. The various scattered wavelets from the different atomic sites combine and undergo constructive or destmctive interference, depending on the relative phases of the different wavelets, thus on their path difference. This can be expressed by Bragg s law ... [Pg.22]

For more than two thousand years liquid-crystalline phases of soaps have been used for cleaning. The physical structure of these phases was revealed only recently. This was mainly due to the opinion that soap molecules should be regarded as rigid rods. In an infrared absorption spectroscopy study of anhydrous soaps, Chapman (1958) concluded that a high-temperature phase transition was caused by melting of the chains. Later Luzzati and co-workers (1960) conclusively demonstrated the liquid nature of the hydrocarbon chains as a fundamental feature in the structure of liquid-crystalline phases. They were also able to determine the structures of the most common liquid-crystalline phases. [Pg.327]

Relations between shape of lipid molecules and structure of liquid-crystalline phases... [Pg.333]

Ekwall, P, Composition, properties, and structures of liquid crystalline phases in systems of amphiphilic compounds, in Advances in Liquid Crystals, vol. 1, Brown, G.H. (ed.). Academic Press, New York, 1975, p. 1. [Pg.232]

A. Douy, R. Mayer, J. Rossi and B. Gallot, Structure of liquid crystalline phases from amorphous block copolymers. Mol. Cryst. Liq. Cryst. 7 103 (1969). [Pg.260]

The interaction of liquid crystals with neighbor phases (gas, liquid, solid) is a very interesting problem relevant to their electrooptical behavior. The structure of liquid crystalline phases in close proximity to an interface is different from that in the bulk, and this surface structure changes boundary conditions and influences the behavior of a liquid crystal in bulky samples. The nematic phase is especially sensitive to external agents, in particular, to surface forces, and the majority of papers devoted to the surface properties of mesophases have been carried out on nematics. In addition, the nematic phase is of great importance from the point of view of applications in electrooptical devices. Thus, in this chapter, we will concentrate on surface properties of nematics, though the properties of the other phases will not be skipped either. [Pg.97]

In the same maimer, the structures of liquid crystalline phases in the gap between two surfaces can be probed by stud5dng surface forces as first demonstrated by Horn et al. [4]. Since then, the relation between the structural forces in concentrated lyotropic Uquid crystalline systems trapped between two solid supports has been determined and the perturbing effect of the surface has been clearly demonstrated [5]. The surface may induce a surfactant phase at the solid-liquid interface that is different than that found in the bulk. Related phenomena, induced by the preferential interaction between the surface and one of the components in the environment, are capillary condensation [6], capillary evaporation [7] (an important mechanism behind some of the reports concerned with long-range hydrophobic interactions [8]), and surface-induced phase separation in polymer mixtures [9]. [Pg.636]

In 1978, Bryan [11] reported on crystal structure precursors of liquid crystalline phases and their implications for the molecular arrangement in the mesophase. In this work he presented classical nematogenic precursors, where the molecules in the crystalline state form imbricated packing, and non-classical ones with cross-sheet structures. The crystalline-nematic phase transition was called displacive. The displacive type of transition involves comparatively limited displacements of the molecules from the positions which they occupy with respect to their nearest neighbours in the crystal. In most cases, smectic precursors form layered structures. The crystalline-smectic phase transition was called reconstitutive because the molecular arrangement in the crystalline state must alter in a more pronounced fashion in order to achieve the mesophase arrangement [12]. [Pg.141]

HOST COMMON STRUCTURES OF LIQUID CRYSTALLINE STATIONARY PHASES Yi (CiH4) X(C H4) Y2 with n 1 or acre. Y and X are usually para substituents. [Pg.971]

Polar lipids form different kinds of aggregates in water, which in turn give rise to several phases, such as micellar and liquid crystalline phases. Among the latter, the lamellar phase (La) has received the far greatest attention from a pharmaceutical point of view. The lamellar phase is the origin of liposomes and helps in stabilizing oil-in-water (O/W) emulsions. The lamellar structure has also been utilized in creams. We have focused our interest on another type of liquid crystalline phase - the cubic phase... [Pg.249]

Both low molecular weight materials [145] and polymers [146,147] can show liquid crystallinity. In the case of polymers, it frequently occurs in very stiff chains such as the Kevlars and other aromatic polyamides. It can also occur with flexible chains, however, and it is these flexible chains in the elastomeric state that are the focus of the present discussion. One reason such liquid-crystalline elastomers are of particular interest is the fact that (i) they can be extensively deformed (as described for elastomers throughout this chapter), (ii) the deformation produces alignment of the chains, and (iii) alignment of the chains is central to the formation of liquid-crystalline phases. Because of fascinating properties related to their novel structures, liquid-crystalline elastomers have been the subject of numerous studies, as described in several detailed reviews [148-150]. The purpose here will be to mention some typical elastomers exhibiting liquid crystallinity, to describe some of their properties, and to provide interpretations of some of these properties in molecular terms. [Pg.365]

The bis(2-ethylhexyl) sodium sulfosuccinate system was initially investigated because its structure of liquid crystalline solution phases and mechanism of solubilization with water had been reported by Rogers and Winsor (10). In our studies, we substituted methanol for water. Table I lists critical micelle concentrations for bis(2-ethylhexyl) sodium sulfosuccinate, triethylammonium linoleate and tetradecyldimethylammonium linoleate in methanol and 2-octanol at 25°C. Literature references for critical micelle concentrations in methanol are sparse, and it has even been suggested that in polar solvents such as ethanol, either micellization does not occur or, if it does, only to a small degree (4). The data of Table I show that micellization occurs in methanol at low concentrations. [Pg.285]

Figure 4 Self-assembling structures and liquid crystalline phase behavior observed in solutions of Pu-2 in water with increasing peptide concentration c (log scale). Electron micrographs (a) of ribbons (c = 0.2mM), (b) and (c) of fibrils (c = 6.2mM), and (d) fibers. The curves in (e) were calculated with the generalized model described in the text (see also Figure 5d). The polarizing optical micrograph (f) shows the thick thread-like texture observed for a solution with c = 3.7mM in a 0.2 mM pathlength microslide ... Figure 4 Self-assembling structures and liquid crystalline phase behavior observed in solutions of Pu-2 in water with increasing peptide concentration c (log scale). Electron micrographs (a) of ribbons (c = 0.2mM), (b) and (c) of fibrils (c = 6.2mM), and (d) fibers. The curves in (e) were calculated with the generalized model described in the text (see also Figure 5d). The polarizing optical micrograph (f) shows the thick thread-like texture observed for a solution with c = 3.7mM in a 0.2 mM pathlength microslide ...
The adaptations that thwart inappropriate phase changes have been presented as theories of homeophasic adaptation (McElhaney, 1984) and dynamic phase behavior (Hazel, 1995). In essence, both theories stress the same primary points the lipid composition of the bilayer must be modified in the face of temperature change to conserve the appropriate phase structure. The liquid-crystalline phase must be conserved at low temperatures. At higher temperatures the propensity to form the hexagonal II phase must not become too great. Thus, Tm and Th must be adjusted during adaptation. [Pg.358]

Figure 3.17 Patterns of liquid crystalline phases of surfactants under the polarisation microscope (a) hexagonal phase the typical fan-like structure can be seen, (b) lamellar droplets with typical Maltese crosses and (c) lamellar phase. Figure 3.17 Patterns of liquid crystalline phases of surfactants under the polarisation microscope (a) hexagonal phase the typical fan-like structure can be seen, (b) lamellar droplets with typical Maltese crosses and (c) lamellar phase.

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