Insight into relation between optically-switched foam stability and isomerization kinetic from azobenzene-based sulfate surfactant

Abstract

The optical-switching foam can meet the different foam property requirements in industrial process. Anionic photo-responsive surfactant is more likely to produce the stable foam and to achieve a larger range of foam stability regulation. However, the foam half-life (t1/2) of photo-responsive foam agent upon the light irradiation is difficult to be predicted, limiting its accurate control at an untested condition. An optically-switched azobenzene surfactant, 4-butoxy-4′-sulfate ethoxy azobenzene (BSEAzo), is designed and synthesized for utilization as a foam controller. Insights to bulk and interface isomerization kinetics (k), the surface activities and foam properties are presented. The maximum surface tension difference (Δγ) between trans and cis isomer reach 12 mN m−1. BSEAzo exhibits rapid isomerization and excellent cycle stability by alternating light irradiations, and molecular adsorption is varied remarkably at the interface. Foam half-life under visible (Vis) and ultraviolet (UV) lights is recorded, showing a wide foam stability regulation (from 141 s to 34 s). Interestingly, a proportional relation between foam half-life and 1/k is established in the range of 0.5–2.5 g L−1, due to interface isomerization is as a determination factor of foam stability control. It is possible to provide a promising theoretical calculation to guide the foam controllability accurately.