Crystalline gels

A polymorphic behavior involving packing of chains having completely different conformations has been found also for isotactic polymers. For instance, isotactic polystyrene, under suitable experimental conditions, can produce crystalline gels in which the chains assume a nearly fully extended conformation [11,12], very close to a truns-planar, rather than the classical conformation of three-fold helix [13]. The two possible conformations proposed for the two crystalline forms of i-PS are shown in Fig. 2. 

Fig. 2 a, b. Side view and projection along the chain axis of models of isotactic polystyrene in the a) s(3/l) helical conformation b) nearly /raw-planar conformation, proposed for the crystalline gels [12]... 

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. 

Among the uncommon stmctures of stereoregular polymers determined in recent times, is that of isotactic polystyrene first observed by Keller and coworkers in crystalline gels (185) and later studied by Corradini et al. (186). In this case too, a highly stretched helix [ (6/1)] is observed, with unit height h = 5.1 A and imit twist t = 60°. The repeating unit contains two independent monomer units with rotation angles close to 180°. 

Firestone, M. A., Dzielawa, J. A., Zapol, R, Curtiss, L. A., Seifert, S., and Dietz, M. L., Lyotropic liquid-crystalline gel formation in a room-temperature ionic liquid, Langmuir, 18, 7258-7260, 2002. 

Figure 11. Possible mechanism of crystalline gel formation from aqueous amylose solutions ( left solution middle gel right crystalline gel) (42)...

This is in sharp contrast to the conditions in stratum corneum where the lipid membranes are almost impermeable to water. As a consequence of these facts, we expect the bulk of lipids that form the skin barrier to be in a crystalline (gel) state, that is, to have long carbon chains (C > 20 0) to comply with the physical requirement that the transition temperature should be higher than normal skin temperature (>35°C). A physiological mixture of ceramides, free fatty acids (FFA), and cholesterol is indeed needed for a normal barrier function. 

FIGURE 2.5 The stacked bilayers of the skin barrier are envisioned as composed of crystalline domains separated by fringes of lipids in the liquid crystalline state.38 The fringe zones may actually oscillate in a very small time scale between a liquid crystalline state and a crystalline (gel) state. Such a tentative idea would mean that the barrier is open just temporarily at a certain location since penetration must occur in the liquid crystalline areas. Thus, the action of a penetration enhancer would be to stabilize a liquid crystalline state or transform it into another type of structure, for example, a cubic phase. 

FIGURE 1.14 Neutron diffraction patterns of n-butylammonium vermiculite close to the crystalline-gel phase transition at T = 14°C and an external salt concentration of c — 0.1 M. (a) low-angle region, (b) wide-angle region... 

The formation of layered assemblies can be induced by the addition of a small amount of water to isotropic ionic liquid l(10/Br ) [52, 53], A lyotropic liquid crystalline gel consisting of l(10/Br ) and water of 16wt% has been prepared. Addition of water to l(10/Br ) induces the formation of a lamellar structure with... 

Lipids constitute a diverse and important group of biomolecules. Most lipids can behave as lyotropic liquid crystals. In the presence of water, they self-assemble in a variety of phases with different stmcture and geometry. The lipid polymorphic and mesomorphic behavior, i.e., their ability to form various ordered, crystalline, gel, or liquid-crystalline phases as a function of water content, temperature, and composition, is one of the most intriguing features of lipid-water systems. The mutual transformations between these phases and their physiologic implications are the subject of this article. 

The transition from liquid-crystalline to gel phase, which results in a marked change in the physical properties of the lipid bilayer, also strongly affects the activities of membrane proteins. Some membrane proteins, such as the Ca +-ATPase, show low activity in gel-phase bilayers because of the effects of the gel phase on the protein conformation (22). An increase in the bilayer thickness during a liquid-crystalline-gel transition has been shown also to affect the activity of membrane proteins, for example, that of the diacylglycerol kinase (18). 

Figure 6 Phase diagram of the ternary mixture distearoylphosphatidylcholine (DSPQ/dioleoylphosphatidylcholine (DOPQ/cholesterol at 23° C, showing four regions of two-phase coexistencediquid crystalline and gel (La + Lp), liquid ordered and gel (Lo + Lp), liquid crystalline and liquid ordered (L + Lo), and liquid ordered and crystals of cholesterol monohydrate also one region of three-phase coexistence exists, liquid crystalline, gel and liquid ordered (La + Lp + Lo). (Reproduced from Reference 74 with permission.)...

Highly ordered lamellar gel microstructures are formed by certain surfactants and mixtures of a surfactant and long-chain fatty alcohols in water. Using small angle X-ray scattering (SAXS), an ordered lamellar stack lattice model was proposed for the gel formed by 10% w/w cetostearyl alcohol containing 0.5% cetri-mide surfactant. In contrast, the microstructure of a Brij 96 gel depends on the surfactants concentration. A hexagonal liquid-crystalline gel structure was... 

Stable o/w creams prepared with ionic or nonionic emulsifying waxes are composed of (at least) four phases (Fig. 7.20) (f) dispersed oil phase, (2) crystalline gel phase, (3) crystalline hydrate phase, and (4) bulk aqueous phase containing a dilute solution of surfactant. The interaction of the surfactant and fatty alcohol components of emulsifying mixtures to form these stmctures (body) is critical. It is also time-dependent, giving the name self-bodying to these emulsions. The overall stability of a cream is dependent on the stability of the crystalline gel phase. 

Figure 7.20 Schematic diagram of a typical semisolid cream prepared with cetostearyl alcohol and ionic surfactant. Note the four phases (1) the dispersed oil phase (2) the crystalline gel phase containing interlamellar-fixed water (3) phase composed of crystalline hydrates of cetostearyl alcohol (4) bulk water phase.

Ikeda, T., Nakano, M., Yu, Y., Tsutsumi, O. and Kanazawa, A. (2003) Anisotropic bending and unbending behavior of azobenzene liquid-crystalline gels by light exposure. Adv. Mater., 15, 201-205. 

Dumaual AC, Jenski LJ, 8tillwell W. Liquid crystalline/gel state phase separation in docosahexaenoic acid-containing bilayers and monolayers. Biochim Biophys Acta 2000 1463 395-406. 

Figure 14.14 The gel network theory suggests that when a cream is formulated, it is composed of four phases bulk water, a dispersed oil phase, a crystalline hydrate and a crystalline gel composed of bilayers of surfactant and fatty alcohol separated by layers of interlamellar fixed water Ccourtesy of Dr. G. M. Eccleston).

Fig. 8. /".p-phase diagram of DPPC bilayers in excess water and schematic drawing of the lamellar lattice constant and lipid packing in the bilayer plane of DPPC gel phases at 23 °C [44,85]. It is noteworthy that an additional crystalline gel phase (Lg) can be induced in the low-temperatue regime after prolonged cooling. 

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