Packing efficiency of small silica particles on large latex particles: a facile route to colloidal nanocomposites.

The adsorption of small silica particles onto large sterically stabilized poly(2-vinylpyridine) [P2VP] latex particles in aqueous solution is assessed as a potential route to nanocomposite particles with a "core-shell" morphology. Geometric considerations allow the packing efficiency, P, to be related to the number of adsorbed silica particles per latex particle, N. Making no assumptions about the packing structure, this approach leads to a theoretical estimate for P of 86 +/- 4%. Experimentally, dynamic light scattering is used to obtain a plot of hydrodynamic diameter against N, which indicates the conditions required for monolayer coverage of the latex by the silica particles. Transmission electron microscopy confirmed that, at approximately monolayer coverage, calcination of these nanocomposite particles led to the formation of well-defined hollow silica shells. This is interpreted as strong evidence for a contiguous monolayer of silica particles surrounding the latex cores. On this basis, an experimental value for P of 69 +/- 4% was estimated for nanocomposite particles prepared by the heteroflocculation of a 20 nm silica sol with near-monodisperse P2VP latexes of either 463 or 616 nm diameter at approximately pH 10. X-ray photoelectron spectroscopy was used to quantify the extent of latex surface coverage by the silica particles. This technique gave good agreement with the silica packing efficiencies estimated from calcination studies.