Surface properties of silica gels. IV. Influence of acid concentration in the preparation from sodium silicate and acetic acid

Abstract

Silica gels were prepared by adding sodium silicate to acetic acid, or acetic acid‐sodium acetate buffer, until the pH reached 4.64, the addition of silicate being made rapidly (over 1–2 min.) or slowly (over about 30 min.). Adsorption isotherms of nitrogen were determined on the outgassed samples at − 195.8° and used for the calculation of surface areas and pore volumes. The gels prepared either by rapid addition of silicate to acid, over the concentration range 2–30%, or from the buffer (initial pH 3.78), were all microporous and had very similar surface properties. On the other hand, the surface properties of those gels prepared by the slow addition of silicate depended on the concentration of acid used, the surface area of the sample prepared from 30% acetic acid being half that of the one from 10% acid. It is suggested that this effect is associated with the loss of some of the smaller primary particles, which occurs as the concentration of polysilicic acid is increased and the pH allowed to change slowly during the reaction.