Particle behaviour at horizontal and vertical fluid interfaces

Silica particle monolayers at horizontal and vertical silicone oil–water and air–water interfaces at ambient and low pH have been studied by microscopy. Very long-range repulsion between adsorbed particles has been detected. The two-dimensional particle sedimentation observed in the case of vertical monolayers has been analysed and reasonable values for the surface charge density at the particle–air (oil) interface have been obtained. It is found that the charge density at the particle surface in contact with air is about two times smaller than that in contact with silicone oil, in accord with the results obtained by others for a similar system but using a totally different approach.