Design of pH-responsive “on-off” emulsions using CTAB/PPA emulsifiers by simulations and experiments

Abstract

The pH-induced demulsification behaviors of “on-off” oil-in-water emulsions using cetyltrimethylammonium bromide (CTAB) and potassium phthalic acid (PPA) mixtures as emulsifiers were investigated by coarse-grained (CG) molecular dynamics (MD) simulations and experiments, aiming to design a new pH-responsive emulsion. On the one hand, we simulated the stability of emulsions of n-octane and aqueous CTAB/PPA solution, and phase diagrams of emulsion with different CTAB/PPA contents were observed at different pH. Furthermore, the demulsification processes from the low pH to high pH was investagated and clearly it began with film rupture and then accomplished with droplets fusion in few nanoseconds. Via analyzing the structure and molecular arrangment of the emulsion droplets, it was found that the reasons on pH-induced demulsification were weak hindrance effect of the surfactant film and electrostatic repulsion among emulsion droplets at high pH. On the other hand, through our experiments, the differences on stability of emulsions were verified and the reversible emulsification and demulsification processes by changing pH values were observed, which agreed well with our simulations. This work designed a new kind of pH-responsive emulsion using commercial surfactants CTAB/PPA by simulations and verified its pH-induced demulsification by experiments. It sets up a bridge between CG MD approach and experiment, which is expected to trigger further understanding and studies on stimuli responsive emulsion systems.