ADSORPTION AND THERMOGRAVIMETRIC STUDIES OF SILICA-BASED AMIDE BONDEDPHASES

A series of chemically modified porous silicas, which can be used in h igh-performance liquid chromatography (HPLC), were prepared by a two-s tep modification process. Aminopropyl groups were first attached to th e 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 e lemental analysis and high-resolution thermogravimetry. In addition, c omplete nitrogen adsorption isotherms were measured at 77 K in the rel ative pressure range from 10(-6) to 0.99. Standard adsorption characte rization of the materials included the evaluation of the specific surf ace area, total pore volume, and pore size distribution. Moreover, ads orption energy distribution functions were calculated from submonolaye r adsorption data using the regularization method. It was estimated th at 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 signi ficantly alter the porous properties of the starting silica, it signif icantly affects the interaction of nitrogen probe molecules with the s urface. Moreover, it was demonstrated that nitrogen adsorption is quit e sensitive to the presence or absence of various functional groups on the silica surface and that the information obtained from the adsorpt ion measurements can be utilized for the characterization of silica-ba sed materials used in chromatographic applications.