Effect of particle hydrophobicity on the formation and collapse of fumed silica particle monolayers at the oil–water interface

Monolayers of fumed silica particles at the octane–water interface have been studied by compression–expansion experiments in a Langmuir trough and simultaneous microscope observations. Particle hydrophobicity has been varied in a broad range, from hydrophilic to very hydrophobic, and its effect on the formation, surface pressure and collapse of monolayers has been investigated. A strong influence of particle hydrophobicity on the degree of particle attachment to the oil–water interface has been found in accord with the existing theory for the effect of particle hydrophobicity (wettability) on the free energy of particle attachment. It is shown that fumed silica particle monolayers collapse by buckling which begins in the region of fastest increase of the surface pressure, just after the inflection point of the surface pressure–area curves. This unusual buckling behaviour has been attributed to the ability of fumed silica particles to form network structures as a result of the capillary attraction typical for particles with non-spherical shape and irregular (undulated) three phase contact line. An interesting coexistence of ripples with two very different wavelengths is observed in collapsed monolayers which has not been reported before. Surface pressure–area isotherms for monolayers of particles with different hydrophobicity are compared and information about the effect of particle hydrophobicity on particle interactions at an oil–water interface is obtained.