pH-Responsive Emulsions with Supramolecularly Assembled Shells

Herein, we describe a novel and simple pH-switchable oil-in-water emulsion system prepared by intermolecular assembly of an amino-amide and a trace amount of citric acid. The resultant supramolecular complex, which demonstrated highly pH-responsive properties, was characterized with various techniques, including attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, ultraviolet-visible (UV-vis) spectroscopy, zeta potential, dynamic light scattering (DLS), surface tension, and interfacial tension. The switchable cyles between emulsification-demulsification states via adjusting the pH through the addition of acid or base were demonstrated with several types of oils. While emulsion was very stable at a pH of 2.2; a rapid phase separation was observed at and above pH of 5.5. Upon adjusting pH back to 2.2, stable emulsion could be re-assembled via dynamic intermolecular interactions. Such pH-triggered emulsification and demulsification cycles could be switched at least five times. The intriguing properties of these pH-switchable emulsions indicate their potential in several industrial applications such as enhanced oil recovery and cosmetics industry.