Competition between hydrogen bonding and electrostatic repulsion in pH-switchable emulsions

Abstract

Electrostatic interactions and hydrogen bonding play important roles in the stabilization of self-assembled systems. However, the competition between hydrogen bonding and electrostatic repulsion in emulsions hasn’t been reported. Herein, we report on electrostatic repulsion overcoming hydrogen bonding in oil-in-water (O/W) emulsions prepared by the mixture of escin and hydrophilic silica nanoparticles triggered by pH, in which the location of the nanoparticles is reversibly transformed from being adsorbed at the oil–water interface to being dispersed in the continuous phase. Escin adsorbs on silica particles in alkaline solution by its hydroxyl groups to endow the particles with surface activity, yielding a stable Pickering emulsion. However, such adsorption via hydrogen bonding is reversed by electrostatic repulsion between carboxylate ions and negatively charged silica particles after escin was converted to sodium aescinate. Demulsification occurred after silica particles reverted to being hydrophilic, which could further co-stabilize the oil-in-dispersion (OID) emulsions with sodium aescinate through electrostatic repulsion after re-homogenization. Pickering or OID emulsions with high internal phase volume fraction could also be obtained. This strategy is universal for O/W emulsions stabilized by nanoparticles and similarly charged surfactants with a polar headgroup near the nonionic functional group, e.g. silica plus polyoxyethylene lauryl ether carboxylate. This work aids in understanding the role of electrostatic repulsion and hydrogen bonding in stabilizing emulsions and sheds light on the design of smart surfactants for practical applications.