Photoresponsive Surfactants Exhibiting Unusually Large, Reversible Surface Tension Changes under Varying Illumination Conditions

We report on a new class of nonionic, photosensitive surfactants consisting of a polar di(ethylene oxide) headgroup attached to an alkyl spacer of between two and eight methylene groups, coupled through an ether linkage to an azobenzene moiety. Structural changes associated with the interconversion of the azobenzene group between its cis and trans forms as mediated by the wavelength of an irradiating light source cause changes in the surface tension and critical micellization concentrations (cmc's). Differences in saturated surface tensions (at concentrations above their cmc's) were as high as 14.4 mN/m under radiation of different wavelengths. The qualitative behavior of the surfactants changed as the spacer length changed; this behavior is attributed to the different orientations adopted by the different surfactants depending on their isomerization states. Both the azobenzene and the intermediate ether group play important roles in the control of surfactant conformation and hence surface tension.