Stimuli-responsive materials are interesting for a variety of applications including shape-memory systems, molecular machines and delivery systems. These materials change their properties in response to one or more stimuli in their local environment, such as light, pH, magnetic fields or heat.
We are interested in stimuli-responsive materials that respond to light. Light is a very effective stimulus as it can be applied selectively at different wavelengths and intensities, at controlled times and locations. Our main focus is investigating the properties of photoresponsive surfactants, which contain a light-sensitive unit buried within an amphiphilic surfactant structure. In this way, light can be used to control the many properties of the surfactant, from interfacial tension, to spreading and foaming and self-assembly.
At present we are working on photosurfactants that contain an azobenzene as the light-responsive group. Azobenzenes undergo a reversible trans-cisphotoisomerisation, which changes the shape and polarity of the molecule. This affects the interfacial properties of the photosurfactant and how they self-assemble.
Figure 1. (a) Photoisomerisation of the azobenzene group leads to a change in the shape and polarity of the unit. (b) Energy level diagram for formation and relaxation of the trans- and cis0 isomers. (Figure courtesy of Dr J. E. Houston)
Our work involves the custom design and synthesis of new surfactant structures, investigation of the physicochemical and interfacial properties and characterisation of the self-assembly of these molecules using small-angle scattering techniques. These systems are being investigated for a variety of applications including catalysis, lyotropic liquid crystal phases, and emulsions.
Figure 2. Azobenzene photosurfactants self-assemble into (a) micelles (cryo-SEM image) and (b) lyotropic liquid crystal phases (polarised optical microscopy). Photoisomerisation of the azobenzene group leads to destruction of the lamellar phase. (Figure courtesy of Dr J. E. Houston)
Unlocking Structure-Self-Assembly Relationships in Cationic Azobenzene Photosurfactants, C. Blayo, J. E. Houston, S. M. King, R. C. Evans, Langmuir,2018, accepted.