title: Small-angle neutron scattering study of the micellization of photosensitive surfactants in solution and in the presence of a hydrophobically modified polyelectrolyte.
abstract: 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 A, respectively. In contrast, with the surfactant in the cis conformation disk-shaped premicellar aggregates were observed at low surfactant concentrations with aggregation numbers <10, thicknesses of 6-10 A, and radii of 10-20 A whereas elevated cis-azoTAB concentrations eventually gave rise to fully developed micelles akin to the trans micelles. This stark difference between the self-assembly behavior of the two azobenzene isomers is ascribed to the different geometries of the surfactant in the trans (planar) and cis (bent) conformation. In the presence of the hydrophobically modified polymer, however, both surfactant isomers resulted in well-developed micelles at the respective critical aggregation concentrations (cac's), presumably because of the effect of the dodecyl side chains attached to the polymer on the conformation of the mixed alkyl-azobenzene micelles.
authors.affiliation: Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. tedlee@usc.edu
authors.forename: C Ted
authors.initials: CT
authors.lastname: Lee
authors.forename: Kenneth A
authors.initials: KA
authors.lastname: Smith
authors.forename: T Alan
authors.initials: TA
authors.lastname: Hatton
mesh_terms: D001391:Azo Compounds; D057927:Hydrophobic and Hydrophilic Interactions; D007536:Isomerism; D008027:Light; D008823:Micelles; D008968:Molecular Conformation / Q000528:radiation effects; D033363:Neutron Diffraction; D011108:Polymers; D053838:Scattering, Small Angle; D012996:Solutions; D013501:Surface-Active Agents / Q000737:chemistry* / Q000528:radiation effects; D014466:Ultraviolet Rays
chemical_list: D001391:Azo Compounds; D008823:Micelles; D011108:Polymers; D012996:Solutions; D013501:Surface-Active Agents; C009850:azobenzene