Forces and physical properties of the Langmuir monolayers of TiO2 particles at air/water interfaces after collisions by a particle in water

Abstract

The effect of a microsphere colliding with a particle stabilized emulsion was investigated by using a Langmuir monolayer of TiO₂ particles at an air/pH 2 water interface and a TiO₂ particle attached to a cantilever (probe) in the subphase. TiO₂ particles with diameters (D) of 75 nm, 300 nm, 3 μm and 10 μm were used to determine the effect of the particle size on the physical properties of the interface. The Monolayer Particle Interaction Apparatus was used to measure the surface pressure–area/particle isotherms of the monolayers of the particles and the forces between the monolayers at different surface pressures and a 3 μm diameter TiO₂ particle (probe) in the subphase, which acted as the colliding particle. The adhesion between the monolayer and the probe tended to decrease with a surface pressure increase. As the TiO₂ particles are positively charged in pH 2 water, this result was explained by the increase in the proportion of the particle covered areas at the water surface, which would increase the charge density of the monolayer and therefore also the repulsive force. The stiffness of the monolayer tended to decrease as the surface pressure increased for the monolayers with the D ≤ 3 μm particles, rationalized by the decrease in the interfacial tension that accompanies a surface pressure increase. The stiffness, however, increased with a surface pressure increase for the D = 10 μm particles. This was explained by the strong capillary attractions that act between closely packed large particles at an air/water interface.