Bicontinuous bilayer phases

Ci2Es-waterKlecane. L + O is microemulsion with excess oil and L + W is the microemulsion and excess water and L is the microemulsion L is the lamellar phase and L3 is the bicontinuous bilayer phase. Figures are adapted from ref [113] and the data taken for 2a from ref [113], 2b from ref [131] and 2c from ref [113] with the decane data in 2c coming from ref [123]. 

More recently, it has been emphasized that inhomogeneous distribution or phase separation in fluid multicomponent lipid membranes could cause structural transformations from lamellar to intermediate nonlamellar phases or to bicontinuous bilayer phases of nonuniform interfacial curvature [30]. An example of a lipid mesophase with nonperiodic organization is the sponge (L3) phase [31,32]. The sponge (L3) mesophase appears to be an isotropic fluid devoid of... 

In the latter the surfactant monolayer (in oil and water mixture) or bilayer (in water only) forms a periodic surface. A periodic surface is one that repeats itself under a unit translation in one, two, or three coordinate directions similarly to the periodic arrangement of atoms in regular crystals. It is still not clear, however, whether the transition between the bicontinuous microemulsion and the ordered bicontinuous cubic phases occurs in nature. When the volume fractions of oil and water are equal, one finds the cubic phases in a narrow window of surfactant concentration around 0.5 weight fraction. However, it is not known whether these phases are bicontinuous. No experimental evidence has been published that there exist bicontinuous cubic phases with the ordered surfactant monolayer, rather than bilayer, forming the periodic surface. 

It is believed that the Gaussian bending modulus k controls the membrane topology. In particular, a negative value of this constant is needed for stable bilayers. A positive value will induce nonlamellar topologies, such as bicontinuous cubic phases. Therefore, it is believed that k is negative for membranes. 

FIGURE 4.2 Schematic illustration of 2 x 2 x 2 unit cells of a lipid/water phase with gyroid cubic symmetry. In reversed bicontinuous cubic phases the lipid bilayer membrane separates two intertwined water-filled subvolumes resembling 3D arrays of interconnected tunnels. Black box (right) represents an enlargement of a part of the folded liquid crystalline lipid bilayer membrane structure. 

Engblom, J. The bicontinuous cubic phase—A model for investigating the effects on a lipid bilayer due to a foreign substance illustrated by the skin penetration enhancer Azone. Chemistry and Physics of Lipids S4(2) 155-164, 1996. 

The relative stability of mesh and IPMS structures is still unclear. For example, the Ri mesophase (of rhombohedral symmetry) in the SDS-water system transforms continuously into the neighbouring bicontinuous cubic phase (Fig. 4.14) [20]. This suggests that this mesophase is a hyperbolic (reversed) bilayer Ijring on a rhombohedral IPMS. Indeed, the rhombohedral rPD surface is only marginally less homogeneous than its cubic counterparts, the P- and D-svu-faces. 

Other detailed structural analyses of cubic phases of systems involving monoolein have been reported [14], and space groups observed that correspond to "asymmetric" or "unbalanced" surfaces of nonzero mean curvature, related to the "balanced" IPMS. If the channel systems on each side are different, or if the lipid bilayer contains constituent monolayers of different curv ature, an as5mtimetric (bicontinuous) cubic phase results. There are thus three other asymmetric (or imbalanced) Cd Cp or Cg structures. 

According to electron micrographic evidence (principally comparison with bicontinuous cubic phases), phases are an accurate representation of so-called "non-bilayer" conformations of membranes. It must be stressed, however, that this conformation is a true bilayer. 

Lipid-water or surfactant-water bicontinuous cubic phases of the reversed type, treated in Chapters 4 and 5, consist of h)q>erbolically curved bilayers,... 

Systems with fluorinated alkyl chains were also investigated [42]. A bilayer smectic phase, SmA, was reported for didodecyldimethylammonium-benzene-hexacarboxylate [35] while a bicontinuous cubic phase was observed for 1,3,5-tri-alkoxybenzyltrimethylammonium tetrafluoroborate [49]. 

Bicontinuous cubic phases can be described by periodic minimal surfaces that are well-known in differential geometry [165]. In the case of inverted bicontinuous cubic phases, the periodic minimal surfaces lie along the middle of the bilayer. They are saddle surfaces with mean curvature zero everywhere (i.e., positive and negative curvatures of the bilayers forming the rods balance each other at every point [162] and with negative Gaussian curvature [138,157,159]. 

In the bilayer continuous structures occurring in the bicontinuous cubic and L3 phases, the diffusion can be described by essentially the same equations. Finally, we note that Eqs. (4)-(6) have been applied to the analysis of self-diffusion data from a number of bicontinuous microemulsions, L3 phases, and bicontinuous cubic phases [34-36]. 

The diffusion behavior implies a rapid, but continuous, change in structure from discrete oil droplets to bicontinuous to discrete water droplets with increasing temperature. The bicontinuous structure appears to be well described by the constant mean curvature surface structures of low mean curvature rather than a tubular structure. The same applies to the bilayer phases, often denoted L3, L4, or sponge phases, also included in Fig. 6. 

Other examples of bilayer structures already mentioned are the sponge phase and bicontinuous cubic phases. The sponge phase has been most studied for nonionic surfactants and is related to common microemulsions. Bilayers may also easily close on themselves to form discrete entities including unilamellar vesicles and multilamellar liposomes. Vesicles are of interest because of the division into inner and outer aqueous domains separated by the bilayer. Vesicles and liposomes are normally not thermodynamically stable (although there are exceptions) and tend to phase separate into a lamellar phase and a dilute aqueous solution. Lipid bilayers are important constituents of living organisms and form membranes, which act as barriers between different compartments. Certain surfactants and lipids may form reversed vesicles, i. e. vesicles with inner and outer oleic domains separated by a (reversed) amphiphile bilayer the bilayer may or may not contain some water. 

Surfactant micelles and bilayers are the building blocks of most self-assembly structures. One can divide the phase structures into two main groups [1) (1) those that are built of limited or discrete self-assemblies, which may be characterized roughly as spherical, prolate or cylindrical. (2) Infinite or unlimited self-assemblies whereby the aggregates are connected over macroscopic distances in one, two or three dimensions. The hexagonal phase (see below) is an example of onedimensional continuity, the lamellar phase of two-dimensional continuity, whereas the bicontinuous cubic phase and the sponge phase (see later) are examples of three-dimensional continuity. Figure 3.8 illustrates these two types schematically. 

---