Evidence for Secondary Minimum Flocculation of Stöber Silica Nanoparticles at the Air−Water Interface: Film Balance Investigations and Computer Simulations

Abstract

The interactions of Stober silica nanospheres having diameters of ca. 40, 100, and 200 nm were studied at the water−air interface with a Wilhelmy film balance. The particle sizes and size distribution functions were determined from TEM measurements. According to in situ Brewster angle microscopy investigations and the calculated DLVO interparticle energies, the particles formed weakly cohesive films at the interface; hence, the increasing surface pressure during the compression was attributed to interparticle repulsion. The repulsion energies that were determined from the surface pressure versus surface area isotherms exceeded the calculated DLVO energies by 2−3 orders of magnitude. The extremely high interparticle repulsion was attributed to dipole−dipole interactions. Despite the high interparticle repulsion, the monolayer of the particles was in a weakly cohesive state prior to compression that was attributed to the recently recognized long-range attractions of capillary and electrostatic origin. The p...