Ribbon phases

Figure 5.39 Circular dichroism spectra of equimolar mixture of DCg PC and DNPC (2 mM total lipid concentration). Top panel In nanotubule phase at (a) 4°C and (b) 20°C. (c) Also shown is spectra from pure DCg PC microtubules in water at 20°C. Bottom panel (d) In nanombule phase at 25°C, (e) in twisted ribbon phase at 26°C, (/) in microtubule phase at 38°C, and (g) completely melted at 40°C. Curves have been offset vertically for display. Reprinted with permission from Ref. 153. Copyright 2001 by the American Chemical Society.

Due to the rigidity of Rp-chains, in fluorinated LCs the transition from smectic to columnar organization often takes place via modulated smectic phases (ribbon phases, c2mm, p2gg, and Colob see Fig. 20, left) which completely or partly replace the bicontinuous cubic phases at the Sm-to-Col cross-over. Similarly, in columnar phases the circular columns can be deformed to an elliptic, rectangular, or square... 

Hence, there are two options for molecules with unlike end-chains either the incompatible chains mix which retains the lamellar structure, or these chains segregate and layer modulation with formation of ribbon phases (columnar phases)... 

In a related series of compounds with 5-heptyl-2-phenylpyrimidines replacing the 4 -cyanbiphenyls of compounds 208-210, the sequence SmC-SmA-Colrec was observed on increasing the number of phenylpyrimidine units [367]. This indicates an even stronger distortion of layer curvature by these rod-like units as hexagonal columnar and cubic phases are removed and replaced by a rectangular columnar ribbon phase. 

Exotic phases are found at compositions between lamellae and hexagonal cylinders (see Figs. 12-19 and 12-20). Some examples of the morphologies of these phases are shown in Fig. 12-22 these include cubic strut phases, tetragonal and rhombohedral mesh phases, and rectangular ribbon phases. 

Figure 12.22 Types of phases that occur between hexagonal and lamellar phases (a) Strut phase, the left image is the minimal P surface over which surfactant is draped, and the right is the topology of the two strut networks on either side of the minimal surface (b) tetragonal mesh phase, (c) ribbon phases containing cylinders with ellipsoidal cross sections.

The other phases are less exotic. The mesh phases consists of lamellae with ordered holes, while ribbon phases are deformed cylinders on a rectangular lattice (see Fig. 12-22). These phases can are usually type I phases with the tails inside the deformed cylinders or inside the hole-filled lamellae, but they could also be inverse, type II, phases. Type II mesh and ribbon phases seem not to have been reported much type II strut phases are common for two-tailed lipids, such as those in cell membranes. In fact, type II strut phases evidently serve biological functions, since they have been found to exist in cellular structures such as the endoplasmic reticulum and the mitochondrion (Seddon 1996). 

The Al-Sc quasicrystalline phase was unstable, and after 300 days holding at room temperature ribbon phase composition transformed to a-Al+Al3Sc. Ribbon hardness increased with growing the content of the intermetallic Al3Sc phase to x=9 and practically stabilized at the level of about 2.3 GPa (Fig. 3). The appearance of the quasicry stal component did not increase this level. 

Aside from exhibiting a rich range of conventional smectic phases, members of the m.OnO.m family also exhibit novel modulated hexatic phases [155] in which the tilted hexatic smectic monolayers have a periodic modulation in their smectic structure, analogous to the Sm C ribbon phase. Two such phases have been identified and have been termed the Sml and Sm2 phases. The Sml phase is exhibited by members of the m.OnO.m family with... 

The columnar phase of amphiphilic liquid crystals is most often of a hexagonal disordered type. Only a very few examples are known for rectangular [173-175] or monoclinic [173] columnar phases or for ribbon phases [176]. 

Ribbon phases have been the most comprehensively studied of the intermediate phases. They occur when the surfactant molecules aggregate to form long flat ribbons with an aspect ratio of about 0.5 located on two dimensional lattices of oblique, rectangular (primitive or centred), or hexagonal symmetry. Ribbon phases were first proposed by Luzzati [68,69] in aqueous sur-... 

Figure 41 Arrangement of polar groups (spheres) within ribbon phases.

It is dearly seen from Figure 17, at low DP, the nematic stmcture mostly appears, whereas at higher DP, more ordered smectic A stmcture and the so-called reentrant nematic Nre, as well as smectic ribbon phase SmC arise. These facts are very important because neglecting the influence of the DP on the types of mesophases, especially if nonfractionated samples are used for physical studies, will lead to erroneous interpretation of the experimental data. 

The tilted antiphase (or ribbon phase) SmC [75] arises from an asymmetric 2-d lock in of the wavevectors. The denomination C emphasizes the fact that both the layers and the antiferroelectric modulations are tilted with respect to the director n. 

The experimental-theoretical study of mesophase formation in amphiphilic systems emphasizes the basic chemical, physical, and materials science aspects of the systems. The most commonly discussed mesophases, beyond the simple micelles discussed in Chapter 4, are lamellar aggregated micellar (packed in various cubic and hexagonal close-packed arrays), columnar or ribbon phases (rod-shaped micelles stacked in a two-dimensional hexagonal or rectangular array) microemulsions, and the cubic bicontinuous mesophases. The experimental techniques normally used to identify these mesophases are NMR Uneshape analysis, diffusion measurements, smaU-angle neutron and X-ray scattering, and optical texture analyses. In addition, reconstraction of electron density profiles and very low temperature transmission electron microscopy (TEM) have been used to elucidate the details of these mesostractures. 

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