Abstract: In the present investigation, two samples of hydrophobic silica gel have been prepared, and heated at different temperatures up to 600 or 800 ℃.The water vapor adsorption isotherms, adsorption amount of methylene blue and the TG-DTAcurves were determined.Some conclusions were obtained as follows: (1) In water vapor adsorption, only "free" hydroxyls on silica gel surface interact with water molecules; (2) When temperature is raised to 500 ℃, the methylated surface of hydrophobic silica gel is destroyed partly, and completely destroyed at 600 ℃; (3) The heat stability of silica gel treated with trimethyl-chlorosilane is greater than that treated with dimethyldichlorosilane.

Key words: Hydrophobia silica gel, Heat-treatment, Water vapor adsorption