Light-responsiveness of C12E6/polymer complexes swollen with dodecane

The association behavior of light-responsive azobenzene modified poly(sodium acrylate)s (AMPs) with C(12)E(6) (hexa-oxyethyleneglycol n-dodecyl ether) surfactant micelles swollen with dodecane was investigated using dynamic light scattering, UV spectrophotometry, and capillary electrophoresis techniques. AMPs complexes with oligoethyleneglycol n-alkyl ether show promising properties as emulsifiers for the light-triggered control of inversion of emulsions and the present work aims at giving new insights with respect to the nature of their photoresponse. Depending on the dodecane amount, the size of the spherical surfactant micelles was varied with radii ranging from 4 to 8 nm. AMPs can be viewed as long PAANa chains bearing several randomly distributed azobenzene groups. First, the binding behavior of the AMPs chains to the micelles swollen with various amounts of oil was thoroughly studied under dark-adapted conditions, which means that most azobenzene groups are in their trans conformation (less polar than the cis conformation obtained under UV irradiation). The binding of azobenzene to surfactant micelles, which leads to the formation of AMPs/surfactant complexes, is controlled by the energy of transfer of the azobenzene moiety from water to the micelle core and by the energy of loops formation since multiple attachments of azobenzene to a single micelle are expected with long AMPs chains. We show that the change in the energy of transfer of the azobenzene group between water and micelles upon increasing the amount of dodecane within the core of micelles was quite weak (not exceeding 0.7 kT). Within the investigated range of curvature, we observed that the energy of loops formation, which decreases with increasing micelle size (decrease of curvature or increase of oil amount) was similarly weak. The effect of the presence of dodecane on the photoresponse of the complex formation was investigated. It is shown that exposure to UV light markedly weakens the association of the AMPs with surfactant within a domain of surfactant concentrations much larger for swollen micelles than for pure surfactant micelles. Consequently, we suggest that emulsion inversion triggered by light could be due to the photomodulation of the binding of AMPs to colloidal objects with various and/or specific curvatures including surfactant mesophases or small size emulsion droplets.