Effect of light on self-assembly of aqueous mixtures of sodium dodecyl sulfate and a cationic, bolaform surfactant containing azobenzene

We report light and small-angle neutron scattering measurements that characterize microstructures formed in aqueous surfactant solutions (up to 1.0 wt % surfactant) containing mixtures of sodium dodecyl sulfate (SDS) and the light-sensitive bolaform surfactant, bis(trimethylammoniumhexyloxy)azobenzene dibromide (BTHA) as a function of composition, equilibration time, and photostationary state (i.e., solutions rich in cis-BTHA or trans-BTHA). We observed formation of vesicles in both SDS-rich and trans-BTHA-rich regions of the microstructure diagram, with vesicles present over a particularly broad range of compositions for trans-BTHA-rich solutions. Illumination of mixtures of BTHA and SDS with a broadband UV light source leads to formation of photostationary states where the fraction of BTHA present as cis isomer (75-80% cis-BTHA) is largely independent of the mixing ratio of SDS and BTHA. For a relatively limited set of mixing ratios of SDS and BTHA, we observed UV illumination of SDS-rich vesicles to result in the reversible transformation of the vesicles to micellar aggregates and UV illumination of BTHA-rich vesicles to result in irreversible precipitation. Surprisingly, however, for many mixtures of trans-BTHA and SDS that formed solutions containing vesicles, illumination with UV light (which was confirmed to lead to photoisomerization of BTHA) resulted in only a small decrease in the number of vesicles in solution, relatively little change in the sizes of the remaining vesicles, and coexistance of the vesicles with micelles. These observations are consistent with a physical model in which the trans and cis isomers of BTHA present at the photostationary state tend to segregate between the different microstructures coexisting in solution (e.g., vesicles rich in trans-BTHA and SDS coexist with micelles rich in cis-BTHA and SDS). The results presented in this paper provide guidance for the design of light-tunable surfactants systems.