Light-responsive hydrophobic association of surfactants with azobenzene-modified polymers

We examined light-triggered formation/dissociation of complexes between azobenzene-modified polymers (AMPs) and surfactants, and we found that the origin of their response to light clearly differs from that of self-assembling of micelles of photosurfactants. We studied a set of AMPs derived from polyacrylic acids with varying integration levels of azobenzene, mixed with (non-responsive) surfactants (TX 100, SDBS) that are representative of common polymer-surfactant formulae. In dilute solutions, light scattering measurements showed that surfactant-AMP association is reversibly affected by exposure to UV/visible light. Binding isotherms of the azobenzene chromophore (into micelle-like cores) and the surfactants (into AMP chains) were determined by spectrophotometry and capillary electrophoresis, respectively. Three regimes can be distinguished with increasing surfactant concentration: i) absence of binding below the CMC, ii) cooperative binding between CMC and ca. 3 x CMC, with the formation of a hydrophobic cluster containing a significant fraction (50-20 mol%) of bound azobenzene, and iii) anticooperative association in equilibrium with excess free micelles, and formation of hydrophobic clusters containing typically 8-18 mol% azobenzene. Reversible UV-triggered dissociation is achieved over the entire anticooperative regime, irrespective of AMP structure. Both too low (a few mol%) and too high (above 30 mol%) fractions of azobenzene in the mixed micelles appear deleterious to the magnitude of response to light. Altogether, our results indicate that micellelike clusters containing about 10 mol% photoanchors can easily swing between attachment/detachment upon photo-isomerisation. A model is proposed to interpret the responsiveness of AMPs with low integration levels of azobenzene.