Model of a Reversed Phase Grafted on Silica Gel

A model is proposed for the surface structure of reversed phase sorbents obtained via the chemical modification of silica gel with alkyldimethylchlorosilanes. The structure of the surface is consistent with the known data on the properties of silica gel and n-alkanes. The model notes that the coupling of silicon-oxygen tetrahedra (silica gel structural units) is such that the tetrahedra alternate in the order top up/top down. At the same time, the number of surface silanol groups on the model’s surface, which can be obtained for the orthorhombic structure of cristobalite, is close to the experimental value (slightly less than five groups per nm2). By comparing the parameters of such a surface and those of alkane packing in the solid phase, it is concluded that all surface silanol groups can be subjected to derivatization according to a hydride scheme. However, due to steric reasons (the presence of two methyl groups in the anchor group), only half the surface groups can be replaced upon silylating the surface of silica gel with alkyldimethylchlorosilane. At the same time, alkyl groups of sorbates or planar molecules can easily be introduced into the graft phase with no need for the conformational rearrangement of grafted alkyl radicals.