Cubic phase dispersion

Fig. 6.31 Sequence of grating pairs and prism pairs for the compensation of quadratic and cubic phase dispersion. LL and MM are two phase-fronts. The solid line represents a reference path and the dashed line illustrates the paths for the wave of wavelength X, which is diffracted by an angle p at the first grating and refracted by an angle a against the reference path in a prism [689]...

Johnsson, M., Barauskas, J. Tiberg, F. (2005). Cubic phases and cubic phase dispersions in a phospholipid-based system. Journal of the American Chemical Society, 127(4), 1076-7. 

It can be seen from Figure 2 that BaTiOj whose major phase is cubic phase is prepared at 973K far below that required for conventional solid-state reaction over 1373K. TEM morphology of BaTiOj powders shown in Figure 3 reveals that besides a little conglobation, there are many well-dispersed single particles which are spherical in form and 25 60 nm in diameter. 

Inverted micellar cubic phases have been observed mainly in mixtures of double-chain polar lipids with fatty acids or diacylglycerols but also in some single-component dispersions of glycolipids (61). The most frequently observed inverted micellar cubic phase in lipids is of space group Fd im). For medium-chain lipids (>16C atoms), it typically forms via an Hn-Q transition however, the Lp gel phase of diC19-xylopyranosyl has been found to melt directly into the Fd im cubic phase (61). 

Tenchov B, Koynova R, Rapp G. Accelerated formation of cubic phases in phosphatidylethanolamine dispersions. Biophys. J. 1998 75 853-866. 

At higher temperatures and water concentrations, the system may shift into the cubic mesophase structure (see Figure 15). The water is present as spheres totally surrounded by monoglyceride. This phase has a high viscosity and is sometimes called viscous isotropic in the hterature the two terms refer to the same structure. In the presence of more water than can be accommodated in the internal spherical phase, one obtains a mixture of lumps of this cubic structure dispersed in excess water. With a saturated monoglyceride such as GMS, the lamellar structure is the main mesophase found under practical conditions, while with unsaturated monoglycerides this cubic phase is the predominant one at lower temperatures. At lower water concentrations, the spherical water micelles are farther apart, so the viscosity of the mixture becomes lower, approaching that of melted pure surfactant. This is the fluid isotropic mesophase, sometimes referred to as the L2 phase. 

Surfactants are amphiphilic molecules which, when dispersed in a solvent, spontaneously self-assemble to form a wide variety of structures, including spherical and asymmetric micelles, hexagonal, lamellar, and a plethora of cubic phases. With the exception of the lamellar phase, each of these phase structures can exist in both normal and reverse orientations with the hydrophobic chains on the exterior of the aggregate, in contact with solvent or vice versa orientation. The range of structures a particular surfactant forms and the concentration range over which they form, depends upon the molecular architecture of the surfactant, its concentration, and the solvent in which it is dispersed. For example, some solvents such as ethanol do not support the formation of aggregates. As most pharmaceutical systems use water as their solvent, this entry will concentrate on aqueous-based systems, although other solvent systems, particularly other non-aqueous polar systems, will be mentioned where appropriate. 

A liquid crystal is a general term used to describe a variety of anisotropic structures formed by amphiphilic molecules, typically but not exclusively at high concentrations. Hexagonal, lamellar, and cubic phases are all examples of liquid crystalline phases. These phases have been examined as drug delivery systems because of their stability, broad solubilization potential, ability to delay the release of encapsulated drug, and, in the case of lamellar phases, their ability to form closed, spherical bilayer structures known as vesicles, which can entrap both hydrophobic and hydrophilic drug. This section will review SANS studies performed on all liquid crystalline phases, except vesicles, which will be considered separately. Vesicles will be considered separately because, with a few exceptions, generally mixed systems, vesicles (unlike the other liquid crystalline phases mentioned) do not form spontaneously upon dispersal of the surfactant in water and because there have been many more SANS studies performed on these systems. 

When a bicontinuous cubic lipid-water phase is mechanically fragmented in the presence of a liposomal dispersion or of certain micellar solutions e.g. bile salt solution), a dispersion can be formed with high kinetic stability. In the polarising microscope it is sometimes possible to see an outer birefringent layer with radial symmetry (showing an extinction cross like that exhibited by a liposome). However, the core of these structures is isotropic. Such dispersions are formed in ternary systems, in a region where the cubic phase coexists in equilibrium with water and the L(x phase. The dispersion is due to a localisation of the La phase outside cubic particles. The structure has been confirmed by electron microscopy by Landh and Buchheim [15], and is shown in Fig. 5.4. It is natural to term these novel structures "cubosomes". They are an example of supra self-assembly. 

The cubic phases can also be dispersed by amphiphilic proteins. Caseins, for example, which also are very effective as emulsifiers, can disperse the cubic phase just as do simple surfactants, such as bile salts. The mechanism is exactly the same as the solubilisation of bilayers by detergents at the cmc - the mixed system has a different surfactant parameter, i.e. local curvature, and the system takes on a different structure. 

The molecules that formed the first most primitive form of life had to self-assemble in the "primeval soup". Besides the obvious requirement of selfreproduction, there must have been an encapsulating membrane, separating inside from outside. The cubosome (a dispersed bicontinuous cubic phase, cf. Chapter 5) provides a number of remarkable properties that make it a candidate as an organisational assembly for the earliest forms of life. 

There has been one lattice dynamics study of leucite by inelastic neutron scattering (Boysen 1990). The low-energy dispersion curves were measured for the high-temperature cubic phase along a few symmetry directions in reciprocal space. The results... 

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