Azobenzene photosurfactants (AzoPS) can isomerise between cis and trans forms upon illumination with light of certain wavelengths (Fig. 1a). Photoisomerisation changes the surface activity, polarity and shape of the surfactant on demand, which has led to the exploration of AzoPS in applications from photofoaming, to transport in microfluidic devices.The interplay between these properties also affects the concentration-dependent self-assembly of AzoPS into micelles or even higher order structures such as lyotropic liquid crystals. Despite the myriad of applications, there is limited unstanding of the structure-function-self-assembly relationships in this family of materials, particularly when probed out of equilibrium. Here, we report a new approach using integrated UV/Vis absorption spectroscopy with small-angle neutron scattering (SANS) that enables the time-resolved self-assembly of AzoPS to be monitored. We have investigated a model neutral AzoPS, C8AzoOC8E4(tetraethylene glycol mono(4′,4-octyloxy, octyl-azobenzene)), using this technique, which allows the scattering profile to be correlated with the extent of photoisomerisation in real-time. It was observed that the AzoPS could switch between wormlike micelles (WLMs, blue light) and fractal aggregates (UV light), with the time to change structure lagging behind the time taken to reach the photostationary state. The formation of photoresponsive WLMs in a single component system is in itself unusual, and rheology studies have shown that these WLMs result in a shear-thinning AzoPS solution, which can be reversibly switched to a low viscosity, Newtonian fluid using UV light. We anticipate that this ability to simultaneously monitor and control photoisomerisation and track structural changes in-situ will aid in the development of tailored AzoPS structures that will enhance their uptake into light-responsive surfactant-based technologies.

Dynamic self-assembly and rheological behaviour of light-responsive surfactants

2019 Fall National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.