Switching Pickering emulsion stabilized by Chitosan-SDS complexes through ion competition

Pickering emulsions stabilized by chitosan (CS) have attracted a large amount of attention due to their characteristics of green, renewability, and environment friendliness. However, CS cannot stabilize an emulsion under acidic conditions because of its water-solubility resulting from the protonation of its amine groups. Through simple electrostatic interactions, a trace amount of SDS (approximately 1/100-1/10 the critical micelles concentration of SDS) can link with the protonated amine groups of CS, and thus significantly enhance the hydrophobicity and interfacial activity of CS. As a result, high-molecular-weight CS precipitates as micro-size flocslike particles and low-molecular-weight CS is transformed into a non-covalent polymer surfactant. The former forms an elastic interface film to prevent the droplets from merging, while the latter decreases the interface free energy by decreasing the oil/water interface tension. Both are favorable for stabilizing the emulsion. The resulting emulsion can be rapidly destabilized upon the introduction of hexadecyltrimethylammonium bromide (CTAB), due to desorption of SDS from CS and its combination with CTAB. The emulsion can be reversibly switched on (stabilization) and off (destabilization) over nine cycles without any deterioration, by alternately adding SDS and CTAB.