Hexagonal inverted cylinder

Fig. 1.9 Structures adopted by phospholipids in aqueous media. Lpand (gel and liquid crystalline) states exist at low and intermediate temperatures, respectively, and the inverted cylinder (hexagonal) HM state is found at elevated temperatures [93].

The hexagonal liquid crystal is a high-viscosity fluid phase composed of a close-packed array of cylindrical assemblies of theoretically unlimited size in the axial direction. The structures may be normal (in water) in that the hydrophilic head groups are located on the outer surface of the cylinder, or inverted, with the hydrophilic group located internally. 

Likewise, the acyl chain order parameters of phos-phatidylethanolamines collectively decrease when going from the L to the Hu phase, due to the greater motional freedom of the chains in the inverted hexagonal structure [27,28,20]. Geometrical models have been recently proposed that relate chain order parameters with the cylinder curvature in a Hi.rphase [20,29]. It has been noted that similar considerations may not be valid for the Hi phase due to the possibility of chain backbending [20]. 

As the concentration increases the spherical domains become cylinders which are hexagonally packed. At even higher concentrations the domains become two-dimensionally continuous alternating lamellae of A and B. As concentration in B decreases the domain structure inverts and B domains show the same series of geometries. 

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