Liquid crystal glycolipids - Carbohydrate Chemistry: Chemical and Biological Approaches - page 320

of these glycolipids are dichotomous in a number of ways, e.g. hydro-
phobic/hydrophilic, polar/non-polar, rigid/flexible etc., and it appears
that the degree of dichotomy is important in stabilizing the formation of
condensed phases.
The effect of the rigidity of the head group on the stability of liquid
crystal phases can be evaluated by comparing the open chain systems
with their cyclic (pyranose and furanose) analogues. When comparing
a- and b-substituted pyranose and furanose forms of octyl glucoside, 13
pyranose forms exhibit higher clearing temperatures, with the a form
being higher than the b for the pyranoside, whereas the order is reversed
for the furanosides. However, this trend was not found to be a general
rule, and neither the cyclic/acyclic nor cycle size nor the anomeric con-
figuration were considered as essential elements for developing property/
structure correlations for the thermotropic properties of substituted
sugars.
In a systematic study of the effect of the position of substitution of
an aliphatic chain in x-O-dodecyl a, b- D -glucopyranoses, 2,4,5 it was
demonstrated that the effect on smectic A thermal stability, as a dodecyl
chain is moved sequentially from one position to the next, gave no
obvious correlation either.
So far the studies reported have involved varying the sugar unit, while
keeping the aliphatic chain the same. In this section the chain is varied
in length and the sugar unit kept constant. Thus, the systematic study of
the liquid crystal properties in the alkyl b- D -glucopyranosides observed as
a function of aliphatic chain length reveals some interesting com-
parisons. Evaluation of the thermotropic properties shows that the
clearing points rise steeply as the chain length is increased (see Fig. 10)
whereas the melting points remain relatively constant. 4 Furthermore, as
the chain length is increased the number of crystal phases rises.
Throughout the series of compounds the only liquid crystal phase
observed is the lamellar phase. When the materials are investigated for
their lyotropic phase behaviour, mesophases other than the lamellar
phase are introduced into the phase sequences as the head groups
became swollen with the addition of water. It is, however, interesting to
note that very few studies have been performed in which the solvent is
organic, such as an oil.
It is interesting to now compare glycolipids composed of one head
and one tail, where the tail is moved sequentially around the head
group and varied in length. One such study was performed on the methyl
x-O-(2-hydroxyalkyl)-a- D -glucopyranosides, see structures (6 to 9) 14 ; the
transition temperature as a function of the even chain lengths (n) are
given in Fig. 11.