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Kinetics of reaction between silicic acid and amorphous silica surfaces in NaCl solutions

Journal of Colloid and Interface Science
Publication Date
DOI: 10.1016/0021-9797(86)90351-6


Abstract The condensation polymerization between silicic acid and silica surfaces has been studied in the absence of nucleation, by seeding silicic acid solutions with colloidal amorphous silica particles of known surface area. Experiments were performed at 25–50°C, 0–1 M NaCl, and 4 ≦ pH ≦ 8 in unbuffered solutions. The reaction falls into two kinetic regimes, limited at high silicic acid concentration by polymerization-dissolution events, but at lower concentration by a process whereby deposited silicic acid condenses further to silica. The overall rates of both events appear directly proportional to the silica surface area A s . Polymerization is first-order in both silicic acid concentration, C, and in the surface concentration of ionized hydroxyl groups (the surface charge) on amorphous silica, [SiO −]. Thus, −dc dt ∝ k 0A s [ SiO −]C . Solution pH influences silica surface charge but not the rate constant, while salt concentration influences both. The salt-free polymerization rate constant is k 0 = 1.03 ± 0.12 M −1 s −1 at at 25°C, and exhibits an Arrhenius temperature dependence with an apparent activation energy of 13.1 ± 0.9 kcal/mole in the range 25–100°C, based upon analysis of the present data combined with two studies from the literature. Polymerization is not diffusionally limited under these conditions. Freshly deposited silica exhibits an enhanced solubility roughly twice the equilibrium level; this means that polymerization approaches this pseudoequilibrium point rather than approaching amorphous silica solubility. This also means that silica dissolution rates vary with the surface concentration of freshly deposited silicic acid molecules; thus, the overall kinetics of silicic acid concentration decline are determined by both surface and solution concentrations of silicic acid. A rate law is proposed which reproduces these and other recent literature data; this model appears applicable to silica dissolution data as well. Conflicting reports of both the reaction stoichiometry and the effect of silica surface charge have been resolved.

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