Properties of hydrogels derived from cationic analogues of bile acid: remarkably distinct flowing characteristics

Abstract

A new class of cationic bile acid derivatives was synthesized to gel water and aqueous electrolyte solutions. Investigations on these hydrogels are carried out at different length scales by a combination of physical techniques. Each of these hydrogels exhibits unique characteristics, thus providing a spectrum of thermal, optical, and mechanical properties. X-ray crystallographic investigation of the single crystals of two of the gelators shows significant differences in the solid-state packing. X-ray scattering experiments indicate that the gel state consists of a different morph than that in the solid. Electron microscopic investigations of the xerogels reveal the fibrous nature of the gel structure. These fibers are associated mainly through bundling processes. A detailed rheological study reveals significant differences in the mechanical properties of the three hydrogels. The storage modulus varies in the range (0.2-2) × 10⁵ Pa at C = 2 wt % for these systems. The exponents of the scaling of the rheological parameters with the concentration for two of the systems agree well with those expected for cellular solids or strongly interacting colloidal gels. A third system exhibits a singular behavior with the energy of interaction between the colloidal flocs increasing with the concentration.