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44193-AC7
Equilibrium and Dynamic Properties of a Photosensitive Surfactant System
The goal of our research is to develop a fundamental understanding of the equilibrium phase behavior and dynamics of an aqueous mixed surfactant system containing a light-sensitive surfactant. Because bulk and interfacial properties of the surfactant system can be reversibly tuned as well as spatially patterned by illumination with different wavelengths of light, this type of aqueous surfactant system has the potential to be broadly useful as a “tunable solvent” in a number of contexts, including novel separations and analytical methods.
Progress over the past year has been made on several fronts. We used light scattering and small angle neutron scattering measurements to characterize nanostructures formed in aqueous surfactant solutions containing mixtures of sodium dodecyl sulfate (SDS) and the light-sensitive bolaform surfactant bis(trimethylammoniumhexyloxy)-azobenzene dibromide (BTHA) as a function of (i) composition, (ii) equilibration time, and (iii) photostationary state (i.e., solutions rich in cis-BTHA or trans-BTHA). We observed the formation of vesicles in both SDS-rich and trans-BTHA-rich regions of the microstructure diagram, with vesicles present over a particularly broader range of compositions for trans-BTHA-rich solutions. Illumination of mixtures of BTHA and SDS with a broadband UV light source led to the formation of photostationary states where the fraction of BTHA present as cis isomer (75-80% cis-BTHA) was 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. 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 (i) only a small decrease in the number of vesicles in solution, (ii) relatively little change in the sizes of the remaining vesicles, and (iii) coexistance of the vesicles with micelles. These observations were determined to be 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 nanostructures coexisting in solution (e.g., vesicles rich in trans-BTHA and SDS, and micelles rich in cis-BTHA and SDS).
We also performed a detailed investigation of the thicknesses of vesicle bilayers formed by rigid bolaform surfactants such as trans-BTHA and several anionic surfactants (including SDS). Past studies have established that the thickness of a vesicle bilayer formed from a mixture of conventional anionic and cationic surfactants is determined by a delicate balance of factors, including electrostatic interactions, van der Waals forces, and chain packing constraints. Consequently, the bilayer thickness is not generally predictable based on knowledge of conventional surfactant molecular structures. In contrast, by using SANS, we demonstrated that use of rigid bolaform surfactants provides a simple means to design vesicles from anionic and cationic surfactants that form spontaneously and have predictable bilayer thicknesses. Results obtained with three isomeric cationic bolaform surfactants (synthesized in the Abbott group) indicate that the rigid, bolaform surfactants span the vesicle bilayers, thus constraining the bilayer thicknesses to the lengths of the bolaform surfactants. Systematic tuning of the thicknesses of the bilayers of the vesicles was demonstrated. This result suggests that use of bolaform surfactants can lead to simple rules for the rational design of surfactant-based nanostructures.
