Light-sensitive microemulsions.

A photodestructible surfactant, sodium 4-hexylphenylazosulfonate (C6-PAS), has been introduced to AOT-stabilized water-in-heptane microemulsions. Proton NMR spectra show that C6-PAS undergoes UV-induced decomposition, to yield a mixture of 4-hexylphenol and hexylbenzene. The photostationary state was determined by 1H NMR, indicating that nearly 90% of the initial photosurfactant had been destroyed, yielding non-surface-active hexylbenzene as the main product. This phototriggered breakdown gives rise to changes in adsorption and aggregation properties of C6-PAS, representing a novel route to induce microemulsion destabilization. When a series of microemulsions containing different amounts of C6-PAS were exposed to UV light, part of the dispersed water phase-separated. Small-angle neutron scattering (SANS) was used to follow the resulting UV-induced shrinkage of the water nanodroplets: a maximum volume decrease was found to be in the order of 60-70%. Kinetic SANS studies were also carried out in order to follow the changes in aggregation as a function of UV irradiation time. Multicontrast SANS experiments gave further insight; for example, it was demonstrated that the shell thickness remained constant. This study represents the first example of light-induced microemulsion destabilization.