Effect of surfactant compounding on emulsion stability and acid-base resistance using optical tweezers

Abstract In industries, emulsions must adapt to various acid-base environments, making it crucial to study their properties and acid-base resistance. We prepared an oil-in-water (O/W) emulsion by adding oleic acid and polyether amine D-230 to water, with tetradecane as the oil phase. The stability of the emulsion was assessed by measuring the interaction forces between droplets using optical tweezers. Our results demonstrated that the amphoteric surfactant could significantly improve the stability of the emulsion. The results show that in the presence of amphoteric surfactant, the maximum repulsion force between droplets can reach 18.9 pN and ζ- potential is -65.16 mV, while in the absence of surfactant, the maximum repulsion is only 1.2 pN and ζ- potential is -25.54 mV. Further stabilization was achieved with the addition of 10 mmol/L anionic surfactant sodium dodecyl sulfate (SDS) or 1 mmol/L cationic surfactant cetyltrimethyl ammonium bromide (CTAB). With SDS, the maximum repulsive force was 40.8 pN and ζ- potential was 132.44 mV at the drop distance of 1.93 µm. With CTAB, the maximum repulsive force of 48.2 pN and ζ- potential of + 42.28 mV were measured at the drop distance of 3.12 µm, indicating that adding surfactants can effectively improve the stability of the emulsion. At the same pH value, the combined emulsion can measure the interaction force at a longer drop distance than the emulsion containing only amphoteric surfactants. When pH = 8, the interaction force of 1 mmol/L CTAB and 10 mmol/L SDS can be measured at the drop distance of 2.51 µm and 2.78 µm, respectively. Only the emulsion containing amphoteric surfactant can be measured at the drop distance of 2.26 µm, and the results show that it can enhance the acid-base resistance of the emulsion. These emulsions can be used to make good anticorrosive coatings and improve various properties of the original coatings.