Tailoring thermotropic cubic mesophases: amphiphilic polyhydroxy derivatives

Novel amphiphilic polyhydroxy compounds [N-(3,4-dialkoxybenzoyl)-1-amino-1-deoxy-D-glucitols (glucamides), N-(3,4-dialkoxybenzoyl)-1-deoxy-1-methylamino-D-glucitols, N-(3,4,5-trialkoxybenzoyl)-1-deoxy-1-methylamino-D-glucitols (N-methylgucamides), 1-benzoylaminopropane-2,3-diols,2-benzoylaminopropane-1,3-diols, 2-(3,4,5-tridodecyloxybenzoylamino)-2-(hydroxymethyl)propane-1,3-diol and (3,4,5-tridodecyloxybenzoyl)bis(2,3-dihydroxypropyl)amine] have been synthesized. Their thermotropic liquid crystalline phases were investigated by means of polarizing microscopy, differential scanning calorimetry and X-ray diffraction. Depending on the chain length, and the size of the hydrophilic polyhydroxy units, different mesophases have been found: smectic A phases (S-A), inverted bicontinuous cubic phases (Cub(V2), Ia3d), hexagonal columnar phases (Col(H2)) and micellar cubic mesophases (Cub(12), Pn3m or P43n). In strong analogy to lyotropic systems, the type of thermotropic mesophase depends on the ratio between the volume of the lipophilic moiety and the surface area of the hydrophilic moiety at the hydrophilic-lipophilic interface. The crossing from zero interface curvature (S-A phase) to the finite negative curvature of the inverted cylindrical aggregates of the columnar mesophase takes place via bicontinuous cubic mesophases. The cylindrical aggregates of the columnar mesophase are stable over a rather broad range of variation of the structural parameter. At a certain degree of the size of the lipophilic moiety in respect to the surface area of the hydrophilic group, however, the transition from the hexagonal columnar to a micellar cubic mesophase takes place. On the basis of proton conductivity measurements and from packing considerations we propose that this cubic lattice is built up by eight closed micelles per unit cell which have a rod-like shape and represent small segments of extended columns. Therefrom we can propose a model for the transformations between these different thermotropic mesophases.