Structural Study of Cholesteryl Anthraquinone-2-carboxylate (CAQ) Physical Organogels by Neutron and X-ray Small Angle Scattering

Physical organogels of cholesteryl anthraquinone-2-carboxylate (CAQ) and an organic liquid have been studied by small-angle neutron and X-ray scattering techniques. The gel structure consists of long and rigid fibers of CAQ interconnected in swollen lyotropic microdomains. The nature of the “junction zones” of the three-dimensional network is dependent upon the type of liquid component. In decane, they consist of bundles of several hexagonal-like arrangements of cylindrical aggregates (diameter ca. 102 Å). In alkane/alcohol mixtures, a merging process toward a more compact lamellar-like structure occurs progressively with increasing alcohol content. In pure 1-alkanols, the aggregates may have a more rectangular cross-sectional shape (ribbons). Schlieren optical textures confirm the inhomogeneity of the orientation of thread-like structures in the birefringent materials. When the structural features of CAQ gels are compared to those of other low-molecular weight gelators, the nature of the link between the cholesterol and anthraquinone moieties of CAQ appears to influence the packing in the condensed state while the aromatic part determines the aggregation mechanism which leads to organogels.