Bicontinuous cubic crystalline phases

Cr Cub, Cubv d E G HT Iso Isore l LamN LaniSm/col Lamsm/dis LC LT M N/N Rp Rh Rsi SmA Crystalline solid Spheroidic (micellar) cubic phase Bicontinuous cubic phase Layer periodicity Crystalline E phase Glassy state High temperature phase Isotropic liquid Re-entrant isotropic phase Molecular length Laminated nematic phase Correlated laminated smectic phase Non-correlated laminated smectic phase Liquid crystal/Liquid crystalline Low temperature phase Unknown mesophase Nematic phase/Chiral nematic Phase Perfluoroalkyl chain Alkyl chain Carbosilane chain Smectic A phase (nontilted smectic phase)... 

FIGURE 4.2 Schematic illustration of 2 x 2 x 2 unit cells of a lipid/water phase with gyroid cubic symmetry. In reversed bicontinuous cubic phases the lipid bilayer membrane separates two intertwined water-filled subvolumes resembling 3D arrays of interconnected tunnels. Black box (right) represents an enlargement of a part of the folded liquid crystalline lipid bilayer membrane structure. 

This chapter focuses on the fixation of lyotropic liquid crystalline phases by the polymerization of one (or more) component(s) following equilibration of the phase. The primary emphasis will be on the polymerization of bicontinuous cubic phases, a particular class of liquid crystals which exhibit simultaneous continuity of hydrophilic — usually aqueous — and hydrophobic — typically hydrocarbon — components, a property known as bicontinuity (1), together with cubic crystallographic symmetry (2). The potential technological impact of such a process lies in the fact that after polymerization of one component to form a continuous polymeric matrix, removal of the other component creates a microporous material with a highly-branched, monodisperse, triply-periodic porespace (3). 

The propensity of membranes to fuse correlates with the fraction of inverted phase-forming lipids conversely, membrane fusability is reduced with an increase of the lipid fraction that inhibits inverted phase formation. Substantial evidence suggests that the mechanism of lipid membrane fusion is related to the mechanism of lamellar/inverted phase transitions (23). The intermediates that form in membrane fusion seem to be identical to those that form during the transformations between lamellar, bicontinuous inverted cubic and inverted hexagonal lipid liquid-crystalline phases, and these transitions can be used successfully as a model for studying the lipid membrane fusion mechanism and kinetics. 

A central issue in the field of surfactant self-assembly is the structure of the liquid crystalline mesophases denoted bicontinuous cubic, and "intermediate" phases (i.e. rhombohedral, monoclinic and tetragonal phases). Cubic phases were detected by Luzzati et al. and Fontell in the 1960 s, although they were believed to be rare in comparison with the classical lamellar, hexagonal and micellar mesophases. It is now clear that these phases are ubiquitous in surfactant and Upid systems. Further, a number of cubic phases can occur within the same system, as the temperature or concentration is varied. Luzzati s group also discovered a number of crystalline mesophases in soaps and lipids, of tetragonal and rhombohedral symmetries (the so-called "T" and "R" phases). More recently, Tiddy et al. have detected systematic replacement of cubic mesophases by "intermediate" T and R phases as the surfactant architecture is varied [22-24]. The most detailed mesophase study to date has revealed the presence of monoclinic. 

Figure 23. Phase diagram of 18 with acryl amide (k = crystalline, s = smectic, cub = bicontinuous cubic, col = hexagonal columnar, i = isotropic).

Ichikawa T, Yoshio M, Hamasaki A et al (2007) Self-organization of room-temperature ionic liquids exhibiting hquid-crystalline bicontinuous cubic phases formation of nano-ion channel networks. J Am Chem Soc 129 10662-10663... 

Fig. 4. Schematic drawing of lipid-water mesophases (Lc, lamellar crystalline Lps Pp., lamellar gel L , lamellar liquid-crystalline Qn, Qn°, Qn , inverse bicontinuous cubics Hu, inverse hexagonal). The cubic phases are represented by the G, D, and P minimal surfaces, which locate the midplanes of fluid hpid bilayers.

Mesophases can be locked into a polymer network by making use of polymerizable LCs [59]. These molecules contain moieties such as acryloyl, diacety-lenic, and diene. Self-organization and in situ photopolymerization under UV irradiation will provide ordered nanostmctured polymers maintaining the stable LC order over a wide temperature range. A number of thermotropic liquid crystalline phases, including the nematic and smectic mesophases, have been successfully applied to synthesize polymer networks. Polymerization of reactive lyotropic liquid crystals also have been employed for preparation of nanoporous polymeric materials [58, 60]. For the constmction of nanoporous membranes, lyotropics hexagonal or columnar, lamellar or smectic, and bicontinuous cubic phases have been used, polymerized, and utilized demonstrated in a variety of applications (Fig. 2.11). 

Liquid crystals (LCs) combine order and mobility on a molecular level and are important both in material and life science [1-9]. Different liquid crystalline phases have provided new methods for the design of supramolecular materials [10-13]. Nematic phases have found widely commercial applications as displays for computers and telecommunications [14], lamellar, coltrmnar, micellar, and bicontinuous cubic phases and even more complicated new phases have also found wide applications as advanced materials [12, 15-17]. 

However, even without structural studies, Friberg et al. [32], Shinoda [33], and others noted that the broad existence range with respect to the water/oil ratio could not be consistent with a micellar-only picture. Also, the rich polymorphism in general in surfactant systems made such a simplified picture unreasonable. It was natural to try to visualize microemulsions as disordered versions of the ordered liquid crystalline phases occurring under similar conditions, and the rods of hexagonal phases, the layered structure of lamellar phases, and the minimal surface structure of bicontinuous cubic phases formed a starting point. We now know that the minimal surfaces of zero or low mean curvature, as introduced in the field by Scriven [34], offer an excellent description of balanced microemulsions, i.e., microemulsions containing similar volumes of oil and water. 

Many bicontinuous cubic liquid crystalline phases have been clearly identified. 

Monoolein, a common food emulsifier, gives rise to a bicontinuous cubic liquid crystalline phase when added to water, as illustrated in Fig. 4. If a triglyceride oil is introduced into the monoolein-water system, a microemulsion (L2) phase is formed above about 10 wtVo oil as discussed in Sect. II.B. If lecithin is added, the cubic phase is preserved up to about 30 wt% lecithin, beyond which a lamellar (L ) phase is formed [8]. Adding a bile salt, e.g., sodium taurocholate, in sufficient amount will convert the cubic phase to a micellar solution (L,) [9]. 

Figure 4 A phase diagram of a system composed of monoolein and water. One representative structure of a bicontinuous cubic liquid crystalline phase is shown to the right. L, lamellar liquid crystal Hn, reverse hexagonal liquid crystal and Li, reverse micellar phase.

Figure 1.1 Partial phase diagram of QF EOio/water binary system. W stands for the micellar solution, and Hi, Vi, and U stand for hexagonal, bicontinuous cubic, and lamellar liquid crystalline phases. The arrows in the W domain show the direction of...

Fig. 12.5 Temperature-composition phase diagram of the monoolein/water system (up to 50 wt% water). A cartoon representation of the various phase states is included in which colored zones represent water. The mesophases are as follows Lc-crystalline lamellar, La-lamellar, la3d-gyroid inverted bicontinuous cubic, Pn3m-primitive inverted bicontinuous cubic, Hn-inverted hexagonal, and Fl-reverse micelles fluid phases (Taken from [8])...

The phase diagram of monoolein/water as shown in Hg. 12.5 revealed complex structural behavior. At room temperature the following phase sequence existed upon increasing hydration lamellar crystalline phase (Lc) in coexistence with a 1 phase, lamellar mesophase (L ), and the inverted bicontinuous cubic mesophases-gyroid Ia3d and diamond Pn3m. Upon heating, at about 85 °C, the cubic phase is transformed into the Hn mesophase, followed by the micellar phase. 

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