Size and shape matter for micellar catalysis using light-responsive azobenzene surfactants

The micellar catalysis of a model Claisen-Schmidt aldol condensation reaction using heterogeneous nanoreactors based on cationic azobenzene trimethylammonium bromide (AzoTAB) photosurfactants is investigated. Under UV irradiation, AzoTABs undergo a trans - cis photoisomerisation, which changes not only the critical micelle concentration, but also the shape and size of the micelle. The effect of surfactant structure (tail and spacer lengths), concentration and temperature on the reaction yield were investigated. Monitoring of the zeta potential during the reaction indicated that it proceeds at the micelle/water interface for AzoTABs, with the enolate intermediate stabilised in micelle/water interface ( i.e. the Stern layer). The reaction yield was found to correlate directly to micellar shape and size, with smaller, more spherical micelles typical of cis -AzoTABs favouring higher reaction efficiencies. We show that the reaction conversion efficiency of a Claisen-Schmidt aldol condensation under micellar conditions in water is controlled by the shape and size of the micelle nanoreactors.