Manipulating the fractal fiber network of a molecular gel with surfactants.

Supramolecular assembly governs the formation and properties of many soft materials. Hence, it is significant to develop convenient approaches to control the assembly process. In this work, it is demonstrated that by using surfactants consisting of a sorbitan group (either ethoxylated or not) and an aliphatic chain, as additives, the fractal fiber network structure of a π gelator (with two alkyl chains) can be engineered. The two surfactants, which have the same hydrophobic tails but different hydrophilic heads, demonstrate different effects on the fiber network of the gelator. The surfactant with a large hydrophilic head (ethoxylated sorbitan) promotes fiber tip branching and that with a smaller hydrophilic head (non-ethoxylated sorbitan) enhances fiber side branching. Fractal analysis based on the Avrami model also demonstrates enhancement of fiber branching by the surfactants. Furthermore, the fluorescence emission of the gelator is enhanced by more than 30%. The observations have significant implications in engineering a class of supramolecular materials.