Effect of porosity on the distribution and reactivity of hydroxyl groups on the surface of silica gel
The distribution and reactivity of the different hydroxyl species on the surface of silica gel are studied using Fourier-transform infrared spectroscopy with photo-acoustic detection and chemical modification with trichlorosilane. The effect of the porosity is investigated using three types of silica gel with different pore distributions. The number of free and bridged hydroxyl groups on the surface of silica gel and their reactivity towards trichlorosilane is quantitatively assessed. It is concluded that smaller pore radii favour the condensation of bridged hydroxyl groups to free ones with increasing pretreatment temperature. Trichlorosilane reacts exclusively with free hydroxyl groups at high pretreatment temperatures, and partially with bridged hydroxyl groups at low pretreatment temperatures. The reactivity of the free hydroxyl groups is not affected by the pore radius, whereas the bridged hydroxyls become less reactive towards trichlorosilane as the pore radius decreases.