Bile Salts: Natural Surfactants and Precursors of a Broad Family of Complex Amphiphiles

Bile salts (BSs) are naturally occurring rigid surfactants with a steroidal skeleton and specific self-assembly and interface behaviors. Using bile salts as precursors, derivatives can be synthesized to obtain molecules with specific functionalities and amphiphilic structure. Modifications on single molecules are normally performed by substituting the least-hindered hydroxyl group on carbon C-3 of the steroidal A ring or at the end of the lateral chain. This leads to monosteroidal rigid building blocks that are often able to self-organize into 1D structures such as tubules, twisted ribbons, and fibrils with helical supramolecular packing. Tubular aggregates are of particular interest, and they are characterized by cross-section inner diameters spanning a wide range of values (3-500 nm). They can form through appealing pH- or temperature-responsive aggregation and in mixtures of bile salt derivatives to provide mixed tubules with tunable charge and size. Other derivatives can be prepared by covalently linking two or more bile salt molecules to provide complex systems such as oligomers, dendrimers, and polymeric materials. The unconventional amphiphilic molecular structure imparts specific features to BSs and derivatives that can be exploited in the formulation of capsules, drug carriers, dispersants, and templates for the synthesis of nanomaterials.

Characterizing Viscoelastic Modulations in Biopolymer Hydrogels by Coherence-Gated Light Scattering

pH-responsive hydrogels are of great interest for the controlled release of drugs. However, the changes in the structural and mechanical properties of hydrogels during the pH-responsive swelling/contraction process remains largely unknown. In this article, we demonstrate that coherence-gated dynamic light scattering can be used to in situ characterize the structural dynamics of chitosan (CS) hydrogels at different pH values and show that the CS hydrogels undergo viscoelastic modulations during the swelling/contraction/recovery process induced by pH changes. The conditions for the CS hydrogels to undergo these modulations are found by continuously monitoring the nonequilibrium, long-term dynamical process. Our findings are in a close correspondence to the macroscopic observations made at time points where the CS hydrogels are at equilibrium.

Phase Transitions in Diffusion of Light

It has been a long time belief that, with increasing the scattering strength of multiple scattering media, the transport of light gradually slows down and, eventually, comes to a halt corresponding to a localized state. Here we present experimental evidence that different stages emerge in this evolution, which cannot be described by classical diffusion with conventional scaling arguments. A microscopic model captures the relevant aspects of electromagnetic wave propagation and explains the competing mechanisms that prevent the three-dimensional wave localization. We demonstrate that strong evanescent-field couplings hinder the localization of wave resonances and, therefore, impede the slowing down of diffusion. The emerging out of equilibrium steady-state process resembles the diffusion of classical particles in spatially correlated random potentials and the thermalization of matter waves due to atomic collisions.