Emulsions stabilized by a CO2 - switchable surfactant based on rigid rosin with or without charged nanoparticles

•The raw material for this surfactant was natural rigid rosin secreted from pine trees.•This surfactant can be switched between nonionic and cationic with CO2 / N2.•The described emulsion can be switched between layered and stabled with CO2 / N2.•After demulsification, this surfactant can be recovered and emulsified other oil.•The stabilizing mechanism of emulsions stabilized by nonionic surfactant with charged nanoparticles was analyzed. A CO2– switchable surfactant based on natural rigid rosin had been synthesized and it can be switched between cationic and nonionic with bubbling CO2 / N2 to the solution. Both the cationic and the nonionic had excellent surface activity. The emulsifying property of the cationic was much better than that of nonionic because of electrical repulsion among droplets. The emulsion stabilized by cationic can be demulsified completely with bubbling N2 and a new emulsion can be reobtained with bubbling CO2 followed by homogenization. Moreover, this surfactant can be recovered to emulsify the new oil. The stability of emulsion stabilized by whether cationic or nonionic can be enhanced once added positivelycharged nanoparticles, and the amount of surfactant required to stabilize emulsion can be reduced dramatically. The mechanism was analyzed according to the Zeta potential, surface tension, microscope, and SEM. The surfactant whether cationic or nonionic adsorb at the oil-water interface whereas the positively charged nanoparticles are dispersed in the aqueous continuous phase among droplets. The stability of emulsion was improved by the electrostatic repulsion between surfactant and nanoparticles or between nanoparticles and nanoparticles. This stabilization mechanism is universal as it occurs with different surfactant or different nanoparticles.