The surface properties of silica gels. I. Importance of pH in the preparation from sodium silicate and sulphuric acid

Abstract

AbstractGels were prepared by adding sodium silicate solution to sulphuric acid until a certain pH was reached, and then the products were aged, washed and dried under controlled conditions. The surface properties of the dry gels were studied by measuring adsorption isotherms of nitrogen at – 195·8° and nitrous oxide at — 78·5°. The pH reached came within the range 3·7–5·8, as this corresponded to a decrease in setting time from over an hour to a few seconds. For gels prepared over this pH range, the isotherms of the two gases also changed profoundly, both sets showing the same general pattern. In one preparation the temperature was varied, and in another the ageing period was reduced to the minimum; in neither case could any difference in the adsorption of nitrous oxide be detected. Activation, by soaking in very dilute sulphuric acid, was shown to be more important for a gel prepared at the higher‐pH end of the range.Surface areas have been calculated from both sets of isotherms by the Brunauer, Emmett and Teller (B.E.T.) method. Good agreement is obtained, in some cases, by assuming that each nitrous oxide molecule occupies an area of 20·4 Å2 in the close‐packed monolayer, and taking the usual value of 16·2 Å2 for the nitrogen molecule. The values of residual water content and B.E.T. surface area both decreased substantially over the pH range, but there appears to be no simple relationship between them.