Harnessing Light-Responsive Structural Control in Surfactant Assemblies

Abstract

The ability to manipulate nanoscale structure using light creates the potential for the next generation of smart, responsive materials. To create functional materials, lyotropic liquid crystals (LLCs) are ideal candidates due to their hierarchically-ordered nanostructures that are highly sensitive to their environment. LLCs are formed from the self-assembly of surfactants on the addition of a solvent and can be made light-responsive by incorporating photoswitchable chemical groups to form photosurfactants (PS). On irradiation with light, PS undergo a change in shape and polarity, which has been shown previously to affect the self-assembled LLC nanostructure. However, the mechanisms through which these structural changes occur are not fully understood and their full potential as functional materials is yet to be unlocked. In this thesis, new, light-responsive LLC systems are designed and tested for applications such as controlled delivery vectors, solar-energy storage materials, and gas diffusion membranes. First, LLC dispersions are used to entrap and release molecules on-demand using light. The release is induced by a shape change in the PS, which squeezes out the entrapped molecules. Next, UV-induced structural disordering in LLCs is harnessed to contribute to solar-energy storage. Then, a new set of arylazopyrazole photosurfactants, which have high stability in the charged state, are characterised for their self-assembly and light-response. Here, a light-induced hexagonal-to-cubic mesophase transition displayed by these PS is exploited to selectively control gas diffusion across a membrane. To further understand the dynamic light-response in these systems, a method for in-situ light irradiation during small-angle X-ray scattering (SAXS) experiments is developed. This system is then used to determine the real-time structural effects induced by light and X-rays on PS assemblies. Finally, SAXS with in-situ light irradiation is combined with small-angle neutron scattering (SANS) to uncover the mechanisms of how and why structural changes occur in light-responsive LLCs.