Lamellar structures stabilized

Polar lipids form different kinds of aggregates in water, which in turn give rise to several phases, such as micellar and liquid crystalline phases. Among the latter, the lamellar phase (La) has received the far greatest attention from a pharmaceutical point of view. The lamellar phase is the origin of liposomes and helps in stabilizing oil-in-water (O/W) emulsions. The lamellar structure has also been utilized in creams. We have focused our interest on another type of liquid crystalline phase - the cubic phase... 

Liquid crystals stabilize in several ways. The lamellar structure leads to a strong reduction of the van der Waals forces during the coalescence step. The mathematical treatment of this problem is fairly complex (28). A diagram of the van der Waals potential (Fig. 15) illustrates the phenomenon (29). Without the liquid crystalline phase, coalescence takes place over a thin liquid film in a distance range, where the slope of the van der Waals potential is steep, ie, there is a large van der Waals force. With the liquid crystal present, coalescence takes place over a thick film and the slope of the van der Waals potential is small. In addition, the liquid crystal is highly viscous, and two droplets separated by a viscous film of liquid crystal with only a small compressive force exhibit stability against coalescence. Finally, the network of liquid crystalline leaflets (30) hinders the free mobility of the emulsion droplets. 

MCM-50, a stabilized lamellar structure, exhibits an x-ray diffraction pattern consisting of several low-angle peaks that can be indexed to (hOO) reflections, that is, the material is pillared-layered consisting of two-dimensional sheets [117]. 

For neutral bilayers, there are no long-range doublelayer forces which, coupled with the van der Waals attraction, could explain the stability of the lamellar structure. At small separations, the required repulsion is provided by the hydration force, which was investigated both experimentally6-8 and theoretically.9,10 However, it was experimentally observed that the lipid bilayers could be swollen in water up to very large interlayer distances,11 where the short-range exponential hydration repulsion becomes negligible. 

The stratum corneum intercellular lipids exist as a continuous lipid phase occupying about 20% of the stratum corneum volume and arranged in multiple lamellar structures. They are composed of cholesterol (27 /o) and ceramides (41 /o), together with free fatty acids (9 /o), cholesteryl esters (10 /o) and cholesteryl sulfate (2 /o) (Table 1). Phospholipids, which dominate in the basal layer, are converted to glucosylceramides and subsequently to ceramides and free fatty acids, and are virtually absent in the outer layers of the stratum corneum. Eight classes of ceramides have been isolated and identified in human stratum corneum but the functions of the individual ceramide types are not fully understood. Similarly, the exact function of cholesterol esters within the stratum corneum lamellae is also elusive but it is theoretically possible that cholesterol esters may span adjacent bilayers and serve as additional stabilizing moieties. 

Fig. 1 Photomicrographs of typical emulsions. (A) A liquid paraffin-in-water emulsion stabilized by 1.89% Span 40 and 1.62% Tween 80. The polydispersity of the oil droplets before homogenization is clearly seen. (B) A liquid paraffin-water cream stabilized by a cationic emulsifying wax. Note the lamellar structures surrounding the oil droplets. (C) A multiple w/o/w emulsion. Water droplets can clearly be seen within the larger oil droplets.

Nesper and co-workers [219, 248] synthesized nanotubules of alkylammonium intercalated VO by hydrothermal means. The vanadium alkoxide precursor was hydrolyzed in the presence of hexadecylamine and the hydrolysis product (lamellar structured composite of the surfactant and the vanadium oxide) yielded VO, nanotubes along with the intercalated amine under hydrothermal conditions (Figure 8.27(a) and (b)). The interesting feature of this vanadium oxide nanotube is the presence of vanadium in the mixed valent state, thereby rendering it redox-active. The template could not be removed by calcination as the structural stability was lost above 250 °C. Nevertheless, it was possible to partially extract the surfactant under mildly acidic conditions. These workers have later shown that the alkylamine intercalated in the intertubular space could be exchanged with other alkylamines of varying chain lengths as well as a,co-diamines [248]. The distance... 

The small angle X-ray data (1) gave little indication of penetration of the hydrocarbon chains into the amphiphilic layer. In fact, the observed increase in interlayer spacing, d, was too large to be accounted for even by allowing the surfactant molecules to adopt a fully extended conformation at the additon of hydrocarbon. Hence, the X-ray results were interpreted as the increased spacing being due to the formation of an oil layer in the hydrocarbon part of the lamellar structure. It is difficult to conceive a reason for stability of such a layer, and, if this interpretation is correct, it raises questions about the stability of the lamellar structure. 

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