Strut, Mesh, and Ribbon Phases

Exotic phases are found at compositions between lamellae and hexagonal cylinders (see Figs. 12-19 and 12-20). Some examples of the morphologies of these phases are shown in Fig. 12-22 these include cubic strut phases, tetragonal and rhombohedral mesh phases, and rectangular ribbon phases. 

The other phases are less exotic. The mesh phases consists of lamellae with ordered holes, while ribbon phases are deformed cylinders on a rectangular lattice (see Fig. 12-22). These phases can are usually type I phases with the tails inside the deformed cylinders or inside the hole-filled lamellae, but they could also be inverse, type II, phases. Type II mesh and ribbon phases seem not to have been reported much type II strut phases are common for two-tailed lipids, such as those in cell membranes. In fact, type II strut phases evidently serve biological functions, since they have been found to exist in cellular structures such as the endoplasmic reticulum and the mitochondrion (Seddon 1996). 

Cubic strut phases are common in the phase diagrams of two-tailed surfactants. These surfactants have a relatively high value of the vfaolc parameter, because the volume-to-length ratio v/i(. of the double tail is twice that of a single tail. A high value of v/aoic is consistent with the formation of type II bicontinuous and other inverse phases, such as the inverse hexagonal phase in Fig. 12-24. 

The values of v/aoic for which one should expect bicontinuous cubic phases can be derived by an extension of the packing argument of Israelachvili et al. (Hyde 1990, 1992 Strom and Anderson 1992). Consider a small patch of area z(0) on a curved surface. If this patch is displaced a small distance in a direction normal to the surface, then the area a( ) of the displaced surface becomes 

Now let the original surface be the dividing surface separating head and tail groups, and let a(0) = ao be the area occupied by a head group on that surface, , the tail length, and V the tail volume. Then Eq. (12-38) implies that 

---