Self-assembly of bile acid-PEG conjugates in aqueous solutions.

Thermoresponsive characteristics of oligo(ethylene glycol) derivatives of lithocholic acid (LCA) depend on the hydrophilic/hydrophobic balance of the compounds. Below a threshold temperature (∼30 °C), one of the derivatives, LCA(EG(4))(2), self-assembles in water into hollow nanotubes that form thixotropic gels at high concentrations. Two concentration regimes were observed in the plateau moduli (G(0) ∼ G'). At high concentrations, the mobility of the nanotubes is restricted at the ordered microdomains acting as cross-links. In semidilute aqueous solutions, the linkers between the aggregated domains in the fragile gel rupture easily, and this effect depends strongly on dilution. Organic hydrophobic additives perturb the self-assembling process, leading to changes in the two transition temperatures and poorer mechanical properties of the gel.