Thermal treatment of aerosil 200 silica: Induced surface porosity and surface chemistry relative to the heat of immersion in water

Abstract

Silica Aerosil 200 was thermally dehydrated in vacuo in the temperature range 20 – 900 °C. The nitrogen Brunauer-Emmett-Teller (BET) surface areas and the heats of immersion in water are estimated and discussed. The changes in the surface area and the heat of immersion are related to the water content, the number of surface silanols and the average pore radius. All these changes are dependent on the pretreatment temperature. The adsorption studies show that the BET surface area and the reciprocal of the average pore radius are harmonic functions of the pretreatment temperature. The increase in the area of the hysteresis loops in the adsorption isotherms in the temperature range 110 – 480 °C indicates that there is a successive loss of water from the surface and/or the pore system which affects the heats of immersion in water. The heats of immersion in water show two maxima, one at 200 – 300 °C and the other at 480 °C. The number of surface silanols decreases over the temperature range studied. The changes in the heats of immersion are related to the surface chemistry and to the porosity induced by the thermal treatment.