Liquid crystal glycolipids - Carbohydrate Chemistry: Chemical and Biological Approaches - page 332

function of concentration, one columnar phase gave way to cubic, which
in turn gave way to lamellar; the process was then repeated in reverse
with lamellar giving cubic and cubic giving columnar, i.e. normal phases
gave way to inverted phases across the Gibbs' phase diagram. Thus in-
duced curvatures of the two compounds compensate as a function of
concentration, as one material induces positive curvature and the other
gives negative. 10 The phase diagram therefore resembles that often ob-
tained for lyotropic systems as a function of water and oil based solvent
systems.
4 Cubic phases - micellar and bicontinuous
There are two types of cubic phase, the micellar phase where the curva-
ture of packing is maximised, and the bicontinuous phase where curva-
ture alternates via the local structuring to minimise in value.
In the first case, if the degree of substitution is increased further over
the columnar situation discussed previously, i.e. three head groups to
one tail or three tails to one head group, the molecules will have conical
shapes, with curvature possibly occurring in three dimensions. When
such molecules pack together they will do so forming objects possessing
curvature in three dimensions, e.g. spheres or oblates etc. Figure 21 de-
picts the shapes of the molecules, and a spherical example of a supra-
molecular structure formed by the packing of the conical-shaped
molecules. The supramolecular entities are shown to pack in a simple
cubic array; however, body-centred and face-centred structures may be
possible.
In order to create glycolipids that exhibit thermotropic cubic phases, a
simple design pattern following that described earlier would be to attach
three heads to one chain or three chains to one head. Derivatives of
pentaerythritol 17, 18 and 19 show this progression through compound
18, which has two chains and two heads. 31 As expected the two materials
with opposing structures exhibit cubic phases, whereas compound 18
which has two heads and two tails exhibits a smectic A* phase, indicating
that the shape of the molecular architecture is rod-like. Similarly, mixing
17 and 19 together would result in the 50-50 mol% mixture also
Fig. 21 Amphiphiles with conical structures (a) form structures where the curvature
of packing results in the formation of a spherical supramolecular entity (b), which self-
organises to give a simple cubic mesophase (c).