Extremely pH-sensitive fluids based on a rationally designed simple amphiphile

Abstract

In this paper, we report on an extremely pH sensitive fluid based on a simple ammonium surfactant synthesized in our lab. The fluid was obtained simply by the addition of NaOH into a 100 mM N-cetyl-N,N-dihydroxyethylammonium bromide (CDHEAB) aqueous solution. The viscosity of this aqueous solution may change 10⁶-fold within a narrow pH range of 4.97 to 5.78. The extremely pH-sensitive rheological performance was attributed to the critical molecular structure that allows the combination of strong hydrophobic interactions and effective hydrogen bonding upon protonation. Based on the understanding of the molecular origin of this superior rheological behavior, we are able to manipulate the responsive threshold value of the pH, which provides great advantage in the fine tuning and design of pH responsive fluids with desired pH values. Our results demonstrate that easy-to-process, extremely pH sensitive fluids can be produced by proper molecular design which covers the delicate balance between hydrophobic interactions and the electrostatic effect.