Journal of Physical Chemistry B, Vol.109, No.25, 12270-12276, 2005
Self-assembled networks of ribbons in molecular hydrogels of cationic deoxycholic acid analogues
Aqueous gels derived from three cationic 24-nor 3,12-dihydroxy cholane (DC) derivatives with N-methyl-2-pyrrolidinone (NMP), N-methylmorpholine (NMM), and 1,4-diazabicyclo [2.2.2] octane (DABCO) at the side chain positions have been exhaustively characterized by small-angle neutron-scattering experiments. Although the molecular structures differ slightly by the heterocycle grafted to the steroid core, the derived gels exhibit a range of structural behaviors at the nanoscale that depart from those observed with simple deoxycholate systems. The NMM-DC aggregates are ribbons with a bimolecular thickness of t = 37 angstrom and an anisotropy of the section b/a approximate to 0.1. DABCO-DC exhibits a remarkable transition from ribbons (t = 29.5 angstrom, b/a = 0.18) to thicker cylindrical fibers (R approximate to 59 angstrom), involving four original ribbons, upon a concentration increase. The NMP-DC system forms thick cylindrical fibers (R approximate to 68 angstrom) with steroid molecules organized in a specific morphology. Bilayered or interdigited structures are formed and favored by the presence of multiple polar interaction centers in the DC molecules. Secondary aggregation mechanisms are invoked in the formation of bundles having a lower cross-sectional anisotropic symmetry and exhibiting Bragg peaks corresponding to molecular length periodicities. The relations between the structural information and the rheological properties are discussed.