Phase behavior and microstructure of azobenzene ionic liquids based photo-responsive microemulsions

At present, it is still a challenge for reversible regulation of phase behavior and microstructure of microemulsions with less energy input and no composition change of the system. Photo-responsive microemulsions provide a potential way to solve these problems. In this work, 10 kinds of photo-responsive ionic liquids (ILs) with azobenzene group were used as photo-responsive surfactants to form microemulsions with water and some alcohols. Factors affecting the phase behavior and sub-regions of microemulsions, such as chemical structure of the ILs, temperature, alcohols properties and UV irradiation , were studied by conductivity, small-angle X-ray scattering, dynamic light scattering and cryo-TEM techniques. It was found that azobenzene-based ILs with a more hydrophilic head group and a shorter alkyl spacer length were favor of the formation of the microemulsions. The phase behavior of the microemulsions only slightly changed after UV irradiation, however, the area of O/W and bi-continuous sub-regions of the microemulsion could be regulated by UV irradiation. At the same time, droplets size increased after UV light irradiation , and could be restored close to the initial value by alternative vis-light irradiation.