Phospholipids lyotropic mesophases

Hydrophobically modified polybetaines combine the behavior of zwitterions and amphiphilic polymers. Due to the superposition of repulsive hydrophobic and attractive ionic interactions, they favor the formation of self-organized and (micro)phase-separated systems in solution, at interfaces as well as in the bulk phase. Thus, glasses with liquid-crystalline order, lyotropic mesophases, vesicles, monolayers, and micelles are formed. Particular efforts have been dedicated to hydrophobically modified polyphosphobetaines, as they can be considered as polymeric lipids [5,101,225-228]. One can emphasize that much of the research on polymeric phospholipids was not particularly focused on the betaine behavior, but rather on the understanding of the Upid membrane, and on biomimicking. So, often much was learnt about biology and the life sciences, but little on polybetaines as such. 

Figure 6. Complex lyotropic mesophase at interface of cholesterol (solid phase), phospholipid (amphiphile), and water (solvent)...

In dependence on how the liquid crystalline phase is formed, lyotropic and thermotropic mesophases can be distinguished. Thermotropic mesophases are induced by temperature and require an anisometric shape of the mesogen. In contrast, amphiphilic molecules (e.g. surfactants) are able to form lyotropic mesophases in the presence of suitable solvents. The formation of lyotropic mesophases depends on the concentration of the mesogen as well as on the temperature. Furthermore, certain molecules may form both thermotropic and lyotropic mesophases (e.g. phospholipids ). 

A typical compound, which forms a succession of the lyotropic mesophases in water solutions, is the amphiphilic phospholipid lecythine (phosphatidyl choline)... 

The amphiphilic molecules can occur in lyotropic and thermotropic mesomorph, and the organization of lipid bilaycrs such as liposomes can exist in a temperature range where all the mesophases from gel state to liquid crystal state are favored. In the gel state the phospholipid acyl chains are closely packed and the molecular movements are deteriorized, while in the liquid crystalline slate, the fatty-acid moieties are in a more fluid stale and are able to move more freely. 

Synthetic phospholipids mixtures were also shown to form lyotropic liquid crystals with interesting properties. For example ternary lipid mixtures comprising of two lipids, dioleoylphosphatidylcholine (DOPC), dioleoylglycerol (DOG), and cholesterol, of molar ratios 1 2 1 and 1 2 2, in excess water induced formation of three dimensional (3D) hexagonal mesophases [12], Other synthetic such as polyoxyethylene-10-oleyl ether [13] also can form LLC but since these structures were less studied and have less potential to serve as delivery vehicles they will not be discussed in this chapter. 

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