Lamellar phases bilayer solubilization

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. 

An interpretation of NMR data from solubilized alkanes in aqueous lamellar phases of a nonionic surfactant is found consistent with the presence of a layer of oil located at the center of the bilayer. This supports a similar novel conclusion derived from small angle X-ray data (1). The presence of such an oil layer will have consequences, as yet not understood, on the osmotic pressures within the bilayer and the interlayer interactions responsible for the stability of the lamellar phase. 

The lamellar spacing of a monoglyceride gel phase as a function of water content is plotted in Figure 14. The gel phase of the neutral monoglyceride has a lipid bilayer thickness of 49.5 A, and it swells to a unit layer thickness of 64 A (20). If an ionic amphiphilic substance (e.g. a soap) is solubilized in the lipid bilayer, it is possible to obtain a gel phase with high water content. As with the gel phases with infinite swelling that were discussed above, there is, however, a minimum water layer thickness which in this monoglyceride gel is about 40 A. 

The formation of extended bilayer structures (lamellar micelles) is believed to be energetically unfavorable due to edge effects [98]. Indeed, evidence for such structures in either the water (Li) or nonpolar (L2) media is scarce [346]. On the basis of SANS studies of the structure of hydrated micelles formed from the C (EO)w surfactants in oil media [347,348], it was concluded that the extent of solubilization of water (Wm x) in the L2 phase did not exceed the maximum hydration requirement of the hydrophilic EO groups. The scattering spectral analysis is also consistent with the formation of small bilayer structures that become thicker and more extended as IFincreases. At low W, the thickness of the bilayer micelles ( hanks ) was found to be less than twice the fully extended conformation of the surfactant, suggesting that the EO chains of the surfactant were interdigitated (or coiled)... 

Liposomes and vesicles are ideal systems for cosmetic applications. They offer a convenient method for solubilizing nonpolar active substances in the hydrocarbon core of the bilayer. Polar substances can also be intercalated in the aqueous layer between the bilayer. They will also form lamellar liquid crystalline phases and they do not disrupt the stratum corneum. No facilitated transdermal transport is possible thus eliminating skin irritation. Phospholipid liposomes can be used as in vitro indicators for studying skin irritation by surfactants. 

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