Abstract: The stability mechanism of 5%(w/w) lambda-cyhalothrin O/W emulsion prepared with octenyl succinic anhydride(OSA)-modified starch as emulsifier was studied. For this purpose, the effects of OSA-modified starch content, inorganic ions, pH value and temperature on the Zeta potential of oil droplets and surface loads(Γ) of modified starch were investigated. When the OSA-modified starch content was 7%(mass fraction), Zeta potential could reach the maximum, and Γ appear to reach saturation level; Na+, Mg2+ and Al3+ were able to decrease Zeta potential by compressing the electronic double layer thickness of oil droplet surface, which could lead to the enhancement of molecular flexibility due to the weakened electrostatic exclusion and make Γ enlarger. With the increasing of ionic strength, the capacity of compressing the electronic double layer thickness of oil droplet surface was subsequently enhanced by Na+, Mg2+ and Al3+, and the degrees of Zeta potential reduction and Γ increment were obvious in the sequence of Na+, Mg2+and Al3+. As the pH affected the ionization of OSA-modified starch in water, in the range of alkaline, strong electrostatic repulsive force was provided by more ionized carboxylic ions and the Zeta potential increased while the Γ decreased. As temperature increased, the solubility or escaping tendency of OSA-modified starch increased, so the Zeta potential and Γ decreased, furthermore the difference was not obvious at low temperatures, but at higher temperature it was, the less the Zeta potential and surface load(Γ) was. The OSA-modified starch that was adsorbed at the oil droplets surface was able to stabilize the emulsion by very strong electrostatic exclusion and steric-hindrance double effect.

Key words: lambda-cyhalothrin, O/W emulsion, OSA-modified starch, Zeta potential, surface loads(&, Gamma, )