Photo-destructible Surfactants in Microemulsions

Abstract

A photo-destructible surfactant sodium 4-hexylphenylazosulfonate (C6-PAS, Scheme 1) has been employed in normal AOT-stabilized water-in-heptane microemulsions. Phase studies are consistent with initially homogeneous microemulsions, for which significant changes in stability as a function of UV irradiation time are observed. Photolysis of C6-PAS in these systems results in eventual separation of water to yield a Winsor II system. Proton NMR spectra show that C6-PAS undergoes UV-induced decomposition, to yield a mixture of 4-hexylphenol and the non-surface active hexylbenzene as main product. This photo-triggered breakdown gives rise to changes in adsorption and aggregation properties of C6-PAS, representing a unique route to induce microemulsion destabilization. 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%. Multi-contrast SANS experiments gave further insight, for example it was demonstrated that the surfactant shell thickness remained constant (∼ 9 Å). This study represents a novel example of light-induced microemulsion destabilization.