Monodisperse, nonporous, spherical silica particles

Merck Monospher 250, which comprises monodisperse porous silica particles ≈250 nm in diameter, was subjected to acid-base titration at 20°C in 0.01 M KNO3 and in 0.01 M Et4NNO3 (with no K+ present). The specific proton charge at pH 9 amounted to −55 C g−1 in 0.01 M KNO3, but only −8 C g−1 in 0.01 M Et4NNO3. This difference indicates significant microporosity. Samples of Monospher were heated in air at 800°C for 3–36 h. After heat treatment the particles were still spherical, but slightly smaller, as judged by scanning electron microscopy and dynamic light scattering. The specific charge of the heated samples determined by titration under the same conditions as before was −2 C g−1 for both counterions, which shows that the silica was no longer microporous. The change in the specific charge in 0.01 M Et4NNO3 from −8 to −2 C g−1 suggests that the mesoporosity was also lost during the heat treatment. The specific charge of Aerosil OX-50 was not affected by heat treatment. The surface charge densities of Aerosil and heated Monospher were both between 0.12 and 0.17 C m−2. For monosphere, a slow increase of the titrated charge took place after prolonged contact with water. After ageing for 2 months at pH 5, about 25% of the charge before heating was re-established; after ageing at pH 9, the proton charge increased to about 35% of the original charge. Nonporous, monodisperse spherical silica particles produced by this method have potential uses in theoretical studies and synthetic applications, at least for limited periods of contact with aqueous solutions.