Thermal treatment of silica aerosil 200: Interrelated surface areas and heats of immersion in cyclohexane

Abstract

Silica Aerosil 200 was thermally dehydrated at different temperatures in the range 20–1000 °C. The effects of thermal treatment on water loss and content, specific surface areas (as measured by cyclohexane vapour adsorption) and heats of immersion in liquid cyclohexane are presented and discussed. Brunauer-Emmett-Teller (BET) surface areas and the reciprocal of the average pore radius vary similarly when they are presented graphically as functions of the pretreatment temperature. The integral heat of immersion increases up to 300 °C and then decreases with a further rise in temperature to 1000 °C. This early increase in the integral heat of immersion (to 300 °C) is interpreted in terms of the removal of physisorbed water and an increase in cyclohexane areas in the range 110–380 °C. The heat of immersion calculated on the basis of unit area also has a maximum value at 300 °C and then decreases with a further rise in temperature. The measurable pore narrowing noted in the temperature range 110–380 °C explains qualitatively the early increase in the heat of immersion. The lyophobic surface fraction was calculated from the difference between the nitrogen and the methanol areas, or from cyclohexane areas, divided by the nitrogen BET surface area. As indicated from the present investigation, the lyophobic fraction calculated from the difference between the nitrogen and the methanol areas appeared to be a reliable measure of hydrophobic surfaces. This was revealed in more consistent changes with the average pore radius when the two parameters were represented graphically as functions of the pretreatment temperature.