“Figure/Ground” Study of Colloidal Silica Nanoparticles and Corresponding Microporous Xerogels

• Published in: Langmuir 1998-06, Vol.14 (12), p.3202-3209
• Main Authors: Meixner, D. Laurence, Gilicinski, Andrew G, Dyer, Paul N
• Format: Article
• Language: English
• Subjects: Chemistry; Colloidal gels. Colloidal sols; Colloidal state and disperse state; Exact sciences and technology; General and physical chemistry
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la971111n

A critical hurdle in the development of sol−gel techniques for the preparation of microporous inorganic materials is elucidating the effect of key synthesis and processing parameters on the structure of the resulting sols. Characterization of colloidal particles with diameters of less than approximately 10 nm is problematic using conventional techniques such as dynamic light scattering. Similarly, quantitative determination of pore size distribution in the resulting materials using gas adsorption isotherms is difficult in the microporous regime. The present work extends these traditional methods with the objective of obtaining information both on the particle size in an inorganic sol and on the pore size distribution in the resulting microporous xerogel. Silica sols synthesized at pH 3 and an H2O/TEOS ratio of r = 83 were shown by atomic force microscopy to have a distribution of particle diameters centered around approximately 4−6 nm. Nitrogen and molecular probe adsorption studies on xerogels derived from the same sols indicated a mean micropore diameter of approximately 1 nm. These adsorption studies also suggested nearly close packing of the silica particles in the xerogel. The pore size determined via nitrogen and molecular probe adsorption experiments was consistent with nearly close packed particles of the dimension determined by the AFM experiments. The molecular probe adsorption studies, particularly the equilibrium adsorption of N(C4F9)3, indicated clearly that the smallest micropore diameters were achieved in xerogels derived from sols prepared at pH 3. Larger particle sizes and micropore diameters were determined for xerogels derived from sols prepared at pH 5. This observation was attributed primarily to particle agglomeration in the pH 5 sol, which was less stable to gelation than the sol prepared at pH 3. These complementary “figure/ground” data on the silica particles in the sol and the interstitial microporosity in the corresponding xerogel provided a consistent picture of the microstructure formed in these silica materials.