Self-assembled organogels obtained by adding minute concentrations of a bile salt to AOT reverse micelles

Abstract

The two-tailed anionic surfactant, AOT is well-known to form spherical reverse micelles in organic solvents such as cyclohexane and n-alkanes. Here, we report that trace amounts (e.g., around 1 mM) of the dihydroxy bile salt, sodium deoxycholate (SDC) can transform these dilute micellar solutions into self-supporting, transparent organogels. Gels can be obtained at a total AOT + SDC concentration as low as 6 mM or about 2 mg mL⁻¹. Among all the bile salts studied, SDC is the only one that is capable of inducing organogels. The structure and rheology of these organogels is reminiscent of the self-assembled networks formed by proteins such as actin in water. In particular, both classes of gels exhibit the remarkable property of strain-stiffening, where the gel stiffness (modulus) increases with strain amplitude. Structurally, both gels are based on entangled networks of long, cylindrical filaments. We propose that SDC forms hydrogen bonds with AOT headgroups, transforming some of the spherical AOT micelles into semiflexible filaments. The average diameter of these filaments has been measured by small-angle neutron scattering (SANS), and suggests that SDC molecules are stacked together in the filament core.