Small-Angle Neutron Scattering Study of the Micellization of Photosensitive Surfactants in Solution and in the Presence of a Hydrophobically Modified Polyelectrolyte

The self-assembly behavior of a light-sensitive azobenzene-based surfactant, both in pure surfactant solutions and in the presence of a hydrophobically modified, water-soluble polymer, has been investigated using small-angle neutron scattering (SANS), light scattering, and UV−vis absorption techniques. The surfactant undergoes reversible photoisomerization upon exposure to the appropriate wavelength of light, with the trans form predominant under visible light being more hydrophobic than the cis isomer under UV-light. As a result, the trans form exhibits a lower critical micelle concentration than does the cis form of the surfactant, allowing photoreversible control of micelle formation. The SANS measurements reveal that micelle formation in pure surfactant solutions with the trans surfactant proceeds as commonly observed in traditional alkyl-based surfactants. Fully developed micelles were observed with aggregation numbers >50, whereas the micelle shapes are consistent with triaxial ellipsoids with axes R a, R b, and R c approximately equal to 20, 30, and 30−35 Å, respectively. In contrast, with the surfactant in the cis conformation disk-shaped premicellar aggregates were observed at low surfactant concentrations with aggregation numbers