The Subcritical Preparation of Aerogels Based on Sodium Water Glass

A procedure is presented which involves working below the supercritical temperature and pressure and which conserves the structure of the wet gel. The fundamental step of the subcritical preparation of aerogels is the prevention of the condensation reaction of the ≡SisurfaceOH groups at the inner surface of the gel during drying. The reaction\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $$2* \equiv {\text{Si}}_{{\text{surface}}} {\text{OH}} \to \equiv {\text{Si}} - \equiv + {\text{H}}_{\text{2}} 0$$ \end{document} is avoided by occupation of the surface with a stable chemical group which does not condense. An esterification ≡SisurfaceOH + ROH → ≡ SisurfaceOR + H2O is a simple method of sealing the inner surface. This reaction is an equilibrium reaction. The equilibrium is shifted in favor of ≡ SisurfaceOR by removal of the water. The best esterification was obtained by use of n-butanol. Densities of the aerogels prepared under subcritical conditions lower than 0.1 g cm−3 were obtained. The changes in the structure of the gel during all procedure steps were studied by small angle X-ray scattering, transmission- and scanning electron microscopy.