Silica gel has been modified with y-arninopropyltriethoxysilane under varying conditions, controlling the influence of water in the different modification stages. Diffuse reflectance infrared Fourier-transform (DRIFT) spectra revealed the influence of surface water in the reaction stage and of air humidity in the curing stage. These results were confirmed and refined by 29Si and ' 3C cross-polarisation magic-angle-spinning nuclear magnetic resonance (CPMASNMR) spectroscopy. Combining the results of both techniques, four modification structures present on the silica surface are proposed, depending on the conditions used.

An estimate of the relative distribution of the free and bridged surface hydroxyl groups is made, using a surface modification with trichlorosilane according to the following main reaction ESi-OH + HSiCI, + =Si-O-SiHCI, + HCIThe amount of chemisorbed chloride groups on the substrates is compared with data on the total amount of surface hydroxyl groups and with hydroxyl group information derived from infrared spectra. The interpretation of these data gives an indication of the reactivity, distribution and concentration of the different types of hydroxyl groups on the surface of silica gel.Trichlorosilane reacts mainly with bridged hydroxyl groups on silica gel, pretreated at low temperatures and exclusively with free hydroxyls at high pretreatment temperatures.Extrapolation of the reactivity of trichlorosilane towards the surface hydroxyl groups demonstrates that bridged hydroxyl groups are present up to pretreatment temperatures of 973 K.

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

The temperature-programmed desorption (TPD) of pyridine from silica gel has been analysed with the variable heating, constant coverage procedure proposed by Richards and Rees (R. E. Richards and L. V. C. Rees, Zeolites, 1986, 6, 17). This results in curves of activation energies of desorption as a function of coverage. A weak heterogeneity of the silica gel surface is observed in contrast with other pyridine sorption studies. Two adsorption sites, the free and bridged hydroxyl groups, contribute to the surface heterogeneity. The relative amounts of the two sites on the surface are deduced from the calculated activation energy vs. coverage profiles. An attempt is made to extrapolate the absolute concentrations of free and bridged hydroxyls from the relative site coverages

The fundamental free hydroxyl group vibration of silica gel has been studied using high-resolution Fouriertransform infrared photo-acoustic spectroscopy (FTIR-PAS). The position of the absorption maximum shifts according to the pretreatment temperature of the silica gel. The hydroxylic environment of the free hydroxyl group probably influences the peak position. This was modelled to estimate the relative surface coverages of the free and bridged hydroxyl groups.Silica gel is a polycondensation product of silicic acid, Si(OH), . Inefficient polymerisation of the primary silica sols results in a random distribution of hydroxyl groups over the surface. The structure of silica gel, therefore, is highly porous and amorphous.' Two types of hydroxyl group exist on the surface: free and bridged hydrogen-bonded hydroxyl group^.^,^ These surface hydroxyl groups induce hydrophilic proper tie^.^ The silica gel surface is covered with adsorbed water at ambient temperature. Progressive thermal treatment dehydrates and dehydroxylates the silica gel surface. Dehydration and dehydroxylation operate in a different temperature range, although the processes are not fully separated. It is believed that the silica gel is dehydrated at 200°C under atmospheric conditions and even at lower temperatures in vacuum.' Infrared spectroscopy has been widely applied for the characterisation of hydroxyl groups on silica gel, especially in their fundamental absorption region 3800-3200 cm-'. A characteristic infrared feature in that region is the presence of only one sharp band (CQ. 3745 cm-') of free hydroxyl groups. At low and moderate pretreatment temperature, the infrared spectra also reveal a broad band (3650-3300 cm-') due to the absorption of bridged hydroxyl