Bicontinuous phases mesophases

In addition to the cubic and/or inverse cubic forms described previously, further transitional forms exist between the lamellar phase and the hexagonal meso-phase (cubic, type II) or inverse hexagonal mesophase (cubic, type III). In contrast to the discontinuous phases of types I and IV, cubic mesophases of type II and type III belong to the bicontinuous phases (Fig. 4F). A range of lyotropic mesophases are possible, depending on the mesogen concentration, the lipophilic or hydrophilic characteristics of the solvent... 

In the case of a balanced relation between the different molecular parts a SmA phase is observed. Excessive space demand of one of these parts results in the formation of a columnar mesophase or, in extreme situations, even in discontinuous cubic mesophases. Bicontinuous cubic mesophases may occur in the area between the SmA and the columnar regions. 

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. 

Figure 16.19. Local/global phase diagrams for a range of observed bicontinuous cubic mesophases, the P, D and G (gyroid) phases (cf. Figure 16.8) (a) Type VI (b) Type V2 (cf. Figures 16.7, 16.15, 16.17)...

Sponge mesophases are characterized by flow birefringence (giving anisotropic optical textures), yet they are isotropic at rest. They are typically viscous, though less so than bicontinuous cubic mesophases. Their mesostructures are closely related to the bicontinuous cubics. They often form at high (water) dilution, usually in regions of the phase diagram intermediate to lamellar and bicontinuous cubic mesophases. 

In the presence of water, surfactants and lipids give rise to a variety of phases referred to as lyotropic phases or mesophases.i The most important of these phases are the lamellar, hexagonal, cubic micellar, and cubic bicontinuous phases denoted by L, H and V, and Q, respectively (see Figure 1.11 in Chapter 1). The subscripts 1 or 2 attached to these phase symbols indicate that the phase is direct (water continuous) or inverse (discontinuous water domains). Many other lyotropic phases have been identified that differ from the main ones by the state of the alkyl chain (crystalline or disordered) and of the head group arrangement (ordered or disordered). In the particular case of the lamellar phase, additional variations come from the possible different orientations adopted by the alkyl chains with respect to the plane of the lamellae (angle of tilt of the chain) and also from the state of the surface of the lamellae that can be planar or rippled. Numerous detailed descriptions have been given for the equilibrium state of the various phases that surfactants and lipids can form in the presence of water. 

Bicontinuous phases. Surfactant aggregate stmctures related to liquid crystalline phases or mesophases that exhibit bicontinuous (two interwoven continuous phases) behavior. The most common is the cubic bicontinuous strucmre, often referred to today as cubosome, although other stmctures are possible. 

Figure 5.3. Schematic structures of some of the principal mesophases of amphiphiles (a) rod-shaped micelles (b) hexagonal close-packed rods (c) bilayer or multilayer sheets (d) a cubic bicontinuous phase (e) reversed hexagonal rods.

Like biological systems, however, the vesicle is by nature compartmentalized, which makes it amenable to the inclusion of additives in the three phases present the external aqueous phase, the hydrophobic interior of the bilayer, and the aqueous internal phase. That availability of carrying capacity has made vesicles natural candidates as delivery systems in pharmaceutical, cosmetic, and various other industrial applications. Who hasn t heard of the magical liposomes that make the wrinkles of aging disappear As will be seen below, other newer mesophases, such as the cubic bicontinuous phase or cubosomes, are now beginning to join the corps of amphiphile aggregate workhorses. 

Cyclic carbohydrates with two alkyl chains (e.g. 1,2-dialkyl (or 1,2-diacyl) glycerol 8 a (sug=Glcp, Galp) present structural similarities with glycerophospho-lipids. They form complex mesophases such as bicontinuous cubic phases, inverted hexagonal phases or myelin figures [58-61]. Other dialkyl derivatives... 

Most of the one-pot syntheses of organically functionalized mesoporous silicates have been done under basic conditions. Only hexagonal phases (2d, p6m) were reported so far, except one very recent example of a phenyl-functionalized cubic phase [17], analogous to the bicontinuous MCM-48 phase (la3d) [8]. The cubic phase prepared under acidic conditions from PTES and TEOS is indeed related to a different type of cubic mesophases, micellar mesophases, reported in the literature for various surfactant/solvent systems [23] as well as for lipid-containing systems [24]. 

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)... 

While there have been efforts to polymerize other surfactant mesophases and metastable phases, bicontinuous cubic phases have only very recently been the subject of polymerization work. Through the use of polymerizable surfactants, and aqueous monomers, in particular acrylamide, polymerization reactions have been performed in vesicles (4-8). surfactant foams ), inverted micellar solutions (10). hexagonal phase liquid crystals (111, and bicontinuous microemulsions (121. In the latter two cases rearrangement of the microstructure occured during polymerization, which in the case of bicontinuous microemulsions seems inevitable b ause microemulsions are of low viscosity and continually rearranging on the timescale of microseconds due to thermal disruption (131. In contrast, bicontinuous cubic phases are extremely viscous in genei, and although the components display self-diffusion rates comparable to those... 

Figure 4.11 Plot of the approximate compositions for which surfactant/water mixtures can form monolayers versus the surfactant parameter of the surfactant. This plot is for chain lengths of 14A, which corresponds to hydrocarbons made up of about 12 carbon atoms. The notation for various mesophases is as follows Vi, V2 are bicontinuous cubic phases (the former containing two interpenetrating hydrophobic diain networks in a polar continuum, the latter polar networks in a hydrophobic continuum). Hi and H2 denote normal and reversed hexagonal phases. La denotes the lamellar phase, and Li and L2 denote isotropic micellar and reversed micellar phases (made up of spherical micelles).

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. 

The relative stability of mesh and IPMS structures is still unclear. For example, the Ri mesophase (of rhombohedral symmetry) in the SDS-water system transforms continuously into the neighbouring bicontinuous cubic phase (Fig. 4.14) [20]. This suggests that this mesophase is a hyperbolic (reversed) bilayer Ijring on a rhombohedral IPMS. Indeed, the rhombohedral rPD surface is only marginally less homogeneous than its cubic counterparts, the P- and D-svu-faces. 

---