Mesophases inverted

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... 

There are no continuous water regions in mesophases of type 2 F. Their structure resembles that of inverted micelles. Their conductivity is low, and their properties are lipophilic. 

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... 

The minimum in the interfacial free energy predetermines three kinds of geometry in nature spheres, cylinders and planes. Correspondingly, the most stable amphiphile aggregation structures are i) spherical (Hartley) micelles, ii) rod-shaped micelles and anisotropic middle phases, iii) disk-shaped micelles and lamellar mesophases. They exist as aggregates in a water continuum with a hydrocarbon core surrounded by hydrated polar groups (the normal type) and as aggregates in a hydrocarbon continuum (the reverse or inverted type) where water and... 

Cubosomes, hexosomes, and micellar cubosomes are colloidal aqueous dispersions of surfactantlike lipids with confined inner nanostructures. The formation of these dispersed submicron-sized particles with embedded inverted-type mesophases that display nanostructures closely related to those observed in biological membranes is receiving much attention in pharmaceutical, food, and cosmetical applications. Owing to their unique physicochemical characteristics, they represent an interesting colloidal family that has excellent potential to solubilize bioactive molecules... 

Figure 4.1 Hypothetical lyotropic binary phase diagram where phase transitions can be induced by varying water content or temperature. The indicated mesophases are L(,-lamellar, laid, Pn3m or Imlm-direct and inverted bicontinuous cubic (V and Vn, respectively), Fd3m-direct and inverted...

The lamellar structure does not possess any intrinsic curvature and is considered as the midpoint of an ideal, symmetrical LLC phase progression (Fig. 12.1) [4, 5]. The current chapter is mainly focused on the inverted (W/O) mesophases (cubic and hexagonal), representing an important class of nanostructures for potential pharmaceutical applications. 

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. 

Only a few synthetic amphiphiles can mimic the behavior of biological lipids and form inverted mesophases. The unsaturated monoglycerides (monoolein and monolinolein) belong to this category [10, 64]. GMO is the preferred amphiphile for formulating LC phases for scientific research and drug delivery. This is a nontoxic, biocompatible, and biodegradable lipid, which possesses low water solubility, but swells and forms several LC phases in excess water. 

Although liquid crystal (LC) theory predicts the existence of as many as 18 distinct structures for a given molecular composition and structure. Nature appears to have been kind in that only four of those possibilities have been identified in simple, two-component surfactant-water systems. The four LC phases usually associated with surfactants include the lamellar, hexagonal (normal and inverted), and cubic (Fig. 15.3). Of the four, the cubic phase is the most difficult to define and detect. It may have a wide variety of structural variations including a bicontinuous or interpenetrating structure that involve components of the other mesophases. The remaining types are more easily characterized and, as a result, better understood. 

In a similar way, the effects of curvature of packing have been demonstrated, but more fully, in a family of amphiphilic polyhydroxy compounds shown in Figure 31 where the structure has been varied in a partially systematic way. This family of materials has been designed to have three hydroxy substituted chains and one aliphatic chain to three aliphatic chains and one hydroxy snbstituted chain. Thns, an almost full range of mesophases is exhibited from columnar (Figure 31e) through lamellar to inverted... 

Several examples that may be considered mechanical molecular devices have been reported including die rotaxane based molecular shuttle [16], the chir-optical molecular switch [17], light-activated bianthrone molecules [18], mole-eular brakes [19], molecular ratchets [20], molecular propellers [21], and eonformationally invertable columnar mesophases [22]. 

Similarly, meshes can be viewed as punctured bilayers, where ordered square and hexagonally patterned arrays of punctures result in the T and R mesophases, respectively. Inverting that argument leads to the conclusion that taking account of the diffraction peaks in the scattering pattern only allows one to reconstruct those spatially correlated domains in the mesostructure a bicontinuous membrane could diffract as a smectic or hexagonal lattice, and yet its mesostructure is far from that of the classical lamellar or hexagonal mesophases. 

The rich lyotropic phase behavior exhibited by membrane lipids is well known. The lyotropic phase behavior of membrane lipids whose structure can be described as diacylglucosylglycerols can be classified as sugar fatty acid esters, and have been studied by Mannock et al. [111]. These types of surfactants often exhibit lamellar phases at low temperature, and a transition to a different inverted nonlamellar mesophase, often reverse hexagonal (Hn) or reverse micellar cubic (Qn) phase. In this particular study acyl chains with different terminus, based on stearic and palmitic acid, were studied. Only the shorter chained derivatives tended to form a Qn phase the remainder formed Hn phases over a range of temperatures above 70 °C. 

---