The impact of SiO2 nanoparticles on the dilational viscoelastic properties of water-nonionic surfactant-fuel interface

Surfactants in ultra-low sulfur diesel (ULSD) can improve the stability of the emulsified water droplet by forming a thin film at the water-fuel interface. Emulsified water droplets block the oil spray nozzle and break down the engine, which need to be removed. For colliding emulsified water droplets, surfactant film with high viscoelastic modulus provides an obstacle to their coalescence. Micro/nanosized-SiO2 particles were added into ULSD to interrupt the arrangement of the surfactant film. After adding with 20 nm SiO2, the interfacial tension of water-monoolein-ULSD increased from 16.0 mN/m to 21.4 mN/m, and its viscoelastic modulus decreased from 17.1 mN/m to 10.9 mN/m, a decrease of 36.3%. It proved that the addition of nanosized SiO2 into water-monoolein-ULSD system increased the interfacial tension and reduced the viscoelastic modulus of the monoolein film. However, after adding nanosized SiO2, the viscoelastic modulus of the water-PETO-B-ULSD layer had no significant changes. Therefore, we concluded that 20 nm SiO2 disturbed the dense packing of monoolein as its adsorption to surfactants and the electrostatic repulsion force between them and the hydrophobic group, resulting in a decrease in elastic modulus. However, PETO-B had a complex branched structure leading to a loose packing, which had unoccupied adsorption sites and barely affected by the intrusion of 20 nm SiO2. Therefore, this research provided a strategy to add nanosized-SiO2 into W/O emulsions containing monoolein-like surfactant to reduce its stability.