On the effects of surfactant charge on interfacial stability in nano-emulsions

Nano-emulsions, with their small droplet size, uniform dispersion, and long-term stability, show better viscosity reduction and oil recovery performance than conventional coarse emulsions. In this study, oil-in-water nano-emulsions with different charges were prepared using nonionic surfactant (fatty alcohol polyoxyethylene ether, AEO-7), cationic surfactant (dodecyltrimethylammonium bromide), and anionic surfactant (sodium dodecyl sulfate, SDS). The microscopic structure, droplet size distribution, zeta potential, and dynamic stability of different nano-emulsions were systematically characterized. In addition, the heavy oil-washing performance and interfacial stability mechanisms of nano-emulsions were analyzed through sand surface morphology and elemental variations, as well as interfacial adhesion/desorption forces between nano-emulsion and oil–sand using a piezoelectric ceramic device and a surface tensiometer. The results show that the AEO-7/SDS nano-emulsion (NE-AS) exhibits the highest oil-washing efficiency (95.44%), compared with NE-A (80.76%) and NE-AD (33.30%), with narrow droplet size distribution, high negative zeta potential, and excellent dynamic stability. This is attributed to the synergistic stabilization of the oil–water interface by AEO-7 and SDS, as well as electrostatic repulsion between negatively charged droplets and oil–sand surfaces, which promotes oil peeling and enhances oil–sand separation. Moreover, the NE-AS exhibits lower interfacial adhesion/desorption forces (183.227 μN/84.427 μN) than NE-A and NE-AD with the same hydrophobicity of sand surfaces, as measured by a piezoelectric ceramic device. Notably, the piezoelectric ceramic device demonstrates higher force values, precision, and stability than the surface tensiometer under the same conditions, making it a more reliable tool for interfacial force measurement. This study provides novel interfacial mechanical insights for developing high-performance nano-emulsion systems.