Adsorption and thermogravimetric studies of silica-based amide bonded phases

A series of chemically modified porous silicas, which can be used in high-performance liquid chromatography (HPLC), were prepared by a two-step modification process. Aminopropyl groups were first attached to the silica surface and subsequently converted to amides via the reaction with appropriate acid chlorides. Changes in the surface properties of the silica caused by the chemical modification were monitored using elemental analysis and high-resolution thermogravimetry. In addition, complete nitrogen adsorption isotherms were measured at 77 K in the relative pressure range from 10(-6) to 0.99. Standard adsorption characterization of the materials included the evaluation of the specific surface area, total pore volume, and pore size distribution. Moreover, adsorption energy distribution functions were calculated from submonolayer adsorption data using the regularization method. It was estimated that about 40% of the available surface silanols were converted to amine groups and about 60% of amines were converted to amides. It was shown that although the chemical modification of the surface does not significantly alter the porous properties of the starting silica, it significantly affects the interaction of nitrogen probe molecules with the surface. Moreover, it was demonstrated that nitrogen adsorption is quite sensitive to the presence or absence of various functional groups on the silica surface and that the information obtained from the adsorption measurements can be utilized for the characterization of silica-based materials used in chromatographic applications.