Stöber silica particles having diameters of ca. 100 and ca. 200 nm were prepared and silylated using trimethylsilyl N,N-dimethylcarbamate, achieving a range of surface coverage by trimethylsilyl groups by varying the amounts of silylating agent used. The efficacy of silylation was characterized in terms of hydrophobicity of the particles which was assessed by film balance investigations at water−air interfaces and additionally by imaging the long- and short-range structures of silica particulate layers at water−air interfaces and on mica supports by using Brewster-angle and atomic force microscopies.

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.

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