The surface structure of ethoxylated silica gel

Abstract The surface and pore structures of ethoxylated silica gel were studied by means of infrared spectroscopy and heat of immersion in water and n -hexane. The ethoxylation of this sample was carried out at about 240°C in liquid phase for various periods of time. The infrared study of ethoxylation shows that all silanol groups on the outer surface (free silanol groups) can be ethoxylated, but most of the hydrogen-bonded silanol groups around the points of contact between primary particles, and the inner OH groups cannot be substituted. The number of ethoxy groups per unit area of fully ethoxylated silica gel is approximately 3 nm −2 , and this value is equal to the number of free silanol groups on the outer surface of silica gel. The number of silanol groups thus obtained is in good agreement with values estimated by different methods reported previously. The values of the heat of immersion of ethoxylated silica gels in water decrease in proportion to the number of ethoxy groups per unit surface area, or are proportional to the number of remaining silanol groups. The heat of immersion in n-hexane is constant and independent of the surface concentration of these groups.