Design and synthesis of an azobenzene-betaine surfactant for photo-rheological fluids

Hypothesis: Morphology of surfactant self-assemblies are governed by the intermolecular interactions and packing constraints of the constituent molecules. Therefore, rational design of surfactant structure should allow targeting of the specific self-assembly modes, such as wormlike micelles (WLMs). By inclusion of an appropriate photo-responsive functionality to a surfactant molecule, light-based control of formulation properties without the need for additives can be achieved. Experiments: A novel azobenzene-containing surfactant was synthesised with the intention of producing photo-responsive wormlike micelles. Aggregation of the molecule in its cis and trans isomers, and its concomitant flow properties, were characterised using UV-vis spectroscopy, small-angle neutron scattering, and rheological measurements. Finally, the fluids capacity for mediating particle diffusion was assessed using dynamic light scattering. Findings: The trans isomer of the novel azo-surfactant was found to form a viscoelastic WLM network, which transitioned to inviscid ellipsoidal aggregates upon photo-switching to the cis isomer. This was accompanied by changes in zero-shear viscosity up to 16,000x. UV-vis spectroscopic and rheo-SANS analysis revealed pi - pi interactions of the trans azobenzene chromophore within the micelles, influencing aggregate structure and contributing to micellar rigidity. Particles dispersed in a 1 wt% surfactant solution showed a fivefold increase in apparent diffusion coefficient after UV-irradiation of the mixture. Crown Copyright (C) 2021 Published by Elsevier Inc. All rights reserved.

Automated summary

The morphology of surfactant self-assemblies is controlled by intermolecular interactions and packing constraints. Rational design of surfactant structure can target specific self-assembly modes, such as wormlike micelles. A novel azobenzene-containing surfactant was synthesized to produce photo-responsive wormlike micelles, showing changes in structure and diffusivity upon UV-irradiation.