Syntheses and characterization of new photoresponsive surfactants, N-(azobenzene-4-oxy-2-hydroxypropyl)-N-(alkyloxy-2-hydroxypropyl) aminopropyl sulfonic acid sodium salt

Plot of 1/CMC versus the mole fraction of the cis isomer for the GnAZAS series. (a) G6AZAS, ◼; (b) G8AZAS, ●; and (c) G12AZAS, ▲. [Display omitted] This study fabricated N-(azobenzene-4-oxy-2-hydroxypropyl)-N-(alkyloxy-2-hydroxypropyl) aminopropyl sulfonic acid sodium salt (GnAZAS, n = 6, 8, and 12), a new photoresponsive anionic surfactant, through a simple, high-yield process involving ring-opening reactions of alkylglycidyl ether containing azobenzene and epoxide. The fabricated trans-GnAZAS can be transformed into 92.5% cis upon irradiation with 350 nm UV light, and was converted reversibly into 74% trans under irradiation with 450 nm visible light. Photoisomerization causes changes in the surface activity of surfactants. The critical micelle concentration (CMC) of the trans isomer of G12AZAS was 1.45 × 10−4 mol/L and the value of the 92.5% of cis isomer is 2.84 × 10−4 mol/L. Surface activity can be controlled by light in the range between the two CMC values, and the CMC difference for cis and trans forms of GnAZAS increased proportionately to the alkyl chain length. The minimum average area per molecule (Amina/w) of trans-G12AZAS is 1.14 nm2. This becomes 1.20 nm2 when converted to the cis form. The Amina/w is determined by structural differences between trans and cis isomers. In addition, this study examined the fabricated azobenzene surfactant in terms of emulsion stability, foaming power, and Turbiscan Stability Index (TSI), and compared it to commercialized surfactants LAE-9 and SLES-3.