Two-step modification of silica to obtain energetically homogeneous chromatographic sorbents

Abstract

We have carried out a two-step liquid-phase modification of amorphous silica Silokhrom-80 with polyethyl hydride siloxane and hexamethyldisilazane. Using the parameters of the Langmuir equation, we have determined the concentration of residual hydrophilic sites on the surface of the modified silica, and we have identified them. We have shown that two-step modification of silica by selected reagents makes it possible to completely deactivate the most active vicinal hydroxyl groups on the sorbent surface, and to deactivate its isolated hydroxyl groups to a significant extent.It is known [1] that despite the diversity of existing methods for chemical modification of the surface of silicas by different classes of organic compounds, in practice complete blocking of the surface hydroxyl groups has not been able to be achieved. With the goal of deactivating the residual silanol groups on the silica surface, two-step modification is practiced: in the second step, small organic molecules are used that react with the residual hydrophilic sites on the surface.The completeness of modification of silicas can be assessed by physicochemical methods, in particular from heats of adsorption of polar sorbates obtained by gas chromatography. This method, due to the specific experimental conditions (exceptionally low coverage of the surface and high temperatures) is more sensitive than wetting methods and structural sorption methods carried out at room temperature [2].In this work, we have carried out two-step modification of the disperse amorphous silica (aerosil gel) Silokhrom-80 by polyethyl hydride siloxane (PEHS) according to the procedure in [3] and by hexamethyldisilazane (HMDS). We used the oligomer PEHS because, in contrast to organic compounds reacting with surface OH groups according to a localized mechanism, along with chemical shielding it also provides physical shielding of the active surface sites [4]. However, the hydrophobic film formed on the silica surface has defects: breaks in continuity. So the second step of modification of the silica using HMDS is carried out to shield the active sites at such defects.Additional modification was carried out by treatment of the PEHS-modified silica with a 0.6% toluene solution of HMDS with solid : liquid ratio 1 : 3.5, heating the suspension obtained for 2 h at 110°C in a sand bath until the excess solvent was evaporated. The air-dried samples of Silokhrom-80 + PEHS and Silokhrom-80 + PEHS + HMDS were used for further studies.The specific surface areas of the considered samples were determined on an ASAP 2000M (Micromeritics, USA) from adsorption of nitrogen, and were calculated by the BET method [5]. For the original silica Silokhrom-80, S = 86 m 2 /g, while for the modified samples the specific surface area decreases down to 50-55 m 2 /g.