The interactions of Sto ¨ber 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 particle size dependent particle-particle (p-p) distances at the secondary energy minimum of total pair-interaction versus p-p distances curve were also interpreted in terms of the newly recognized interactions. A computer simulation-assisted method was proposed to estimate the error of assuming a hexagonal array of monodisperse particles, which was then taken into account in the calculation of the p-p distances determined from the pressure-area isotherms.

No abstract

The aggregation of silanized glass spheres (75±5 μm diam) was studied experimentally at liquid–air (water–air, aqueous surfactant solution–air, and aqueous glycerol solution–air) interfaces from a kinetic point of view. The number, the size, and the polydispersity of clusters was investigated as a function of time. Particles having water contact angles of ≈30° (lower hydrophobic sample) and ≈82° (higher hydrophobic sample) were prepared and used in the aggregation experiments. In the early stage of aggregation the kinetics was found to be of the second order. The experiments revealed that the increasing particle hydrophobicity increased the rate constants in every case, which could be attributed to the increasing particle–particle attractions and the decreasing hydrodynamic resistance of particles (clusters) to motion. Moreover, the lower hydrophobicity of particles manifested itself in a more important polydispersity of clusters and an unexpected cross-over during the growth. The cluster–cluster aggregation was succeeded by a particle- (large) cluster aggregation after the first (initial) part of the growth. An off-lattice computer simulation of cluster-cluster aggregation, based on molecular dynamics, was also developed for the better understanding of the interfacial aggregation. The simulations supported well the conclusions derived from the real experiments, and gave an indispensable possibility for the study of the effect of single parameters on the complex phenomenon.

No abstract