Alkyl chain length influence of hydrotrope on the pH responsiveness of surfactant aggregates based on dynamic imine bond

Abstract

• Ionic hydrotropes with different chain length based on imine bond is prepared. • The pH responsiveness of solution by mixing CTAB and hydrotrope is explored. • The mechanism of different pH responsiveness of mixture is analyzed. The combination of pH-responsive hydrotropes and cationic surfactants is a novel way to construct pH-responsive surfactant aggregates. Herein, in order to investigate the chain length influence of hydrotrope on surfactant aggregate, three hydrotropes were formed from p-hydroxybenzaldehyde (HB) and added amine (hexylamine (HA), octylamine (OA), dodecylamine (DA)) respectively based on dynamic imine bonds. Then the surfactant aggregates were constructed by adding cetyltrimethyl ammonium bromide (CTAB). The molar ratio of CTAB: HB: HA/OA/DA was fixed at 100:55:55. The formation of imine bonds was proved by means of 1 H NMR measurement. Rheological measurements, and cryo -TEM were used to explore the pH responsiveness of the surfactant aggregates. The results showed that as pH increased from 6.0 to 12.0, the CTAB/HB/HA system changed from a low viscosity fluid to a viscoelastic fluid, as the CTAB/HB/OA system transformed from aqueous liquid via a transparent gel solution to a turbid fluid. On the other hand, the CTAB/HB/DA system can achieve four state transitions, from a non-viscosity fluid, via a viscoelastic fluid, turbid fluid and finally to a viscoelastic fluid. With the change of pH, the CTAB/HB/HA system turned from spherical to wormlike micelles, while CTAB/HB/OA experienced changes from spherical micelles via wormlike micelles to vesicles. Finally, the CTAB/HB/DA system was the most complex one, which converted from spherical micelles, wormlike micelles via vesicle, into final wormlike micelles. In conclusion, the sensitivity of pH responsiveness among these three systems gradually improved by increasing the chain length of the hydrotrope formed by HB and amine. This can be attributed to the different growth rates of the packing parameter. This work demonstrated the feasibility of constructing pH responsive surfactant aggregates by introducing hydrotropes to cationic surfactants.