Kinetics of silica polymerization

Abstract

The polymerization of silicic acid in dilute aqueous solutions was studied experimentally and theoretically. Two basic processes were studied: the formation of colloidal particles by homogeneous nucleation, and their further growth by deposition of dissolved silica upon them. It was found that the state of ionization of the silica surface controls the rate of polymerization. The rate of deposition of dissolved silica on the surface of amorphous silica is proportional to the surface density of ionized silanol groups. The extent of surface ionization also determines the value of the surface tension, and thus also the rate of homogeneous nucleation. Added salts accelerate both molecular deposition and homogeneous nucleation by increasing the extent of surface ionization and decreasing the solubility of silica. Except for fluoride, salt ions were found not to have any specific catalytic effect. Fluoride was confirmed to be a powerful catalyst. Aluminum and boron were found to inhibit the reaction at pH 8. A successful quantitative theory of the homogeneous nucleation of colloidal silica particles was developed. The Lothe-Pound factor was shown to be about 3.34 × 10 25 (kg H 2O) −1. The nucleation model and results of our data analysis are incorporated in a computer code which models the homogeneous nucleation and growth of colloidal silica particles. This code is able to reproduce much of our data to within experimental error, and may be used to generate useful predictions for conditions that are typical of geothermal brines. The concentration of “adsorbed silica” on the surface of amorphous silica was determined, and was found to increase with the concentration of dissolved silica in the ambient solution. It is probably a reaction intermediate between dissolved silica and solid amorphous silica.