Large pore MCM-41 silica with the pore diameter of 5.0 nm was chemically modified by bonding monomeric-type ligands, such as trimethylsilyl, butyldimethylsilyl, and octyldimethylsilyl, as well as polymeric-type 3-aminopropylsilyl, (hexanoyl-3-aminopropyl)silyl, and octylsilyl ligands. The obtained materials were characterized using elemental analysis, high-resolution thermogravimetry (TGA), and nitrogen adsorption at 77 K in a wide range of pressures. Surface coverages of bonded ligands were between 2.5 and 3.0 μmol/m2. It was shown that pore diameters of the samples studied decreased systematically with the increase in size of ligands. The modified materials exhibited narrow and monodisperse pore size distributions, indicating that the chemical bonding procedure did not diminish the structural ordering of the MCM-41 support. TGA data showed that the surface affinity to water was strongly dependent on the structure and functionality of the bonded species. Nitrogen adsorption data provided additional information about surface properties of the materials. A significant decrease in the amount of nitrogen adsorbed at low pressures was observed for the modified samples, especially those with long-chain alkyl groups. Low-pressure adsorption data were used to calculate adsorption energy distributions (AEDs), and peaks on these distributions were assigned to certain groups present on the silica surface or in the structure of bonded phases. It was thus demonstrated that the pore size and surface functionality of ordered mesoporous silicas can be engineered by a proper choice of the pore diameter of the support and the size and structure of bonded ligands. Nitrogen adsorption measurements including low-pressure data were shown to be a powerful tool to characterize structural and surface properties of unmodified and surface-modified novel porous materials.

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.

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