Role of the concentration and nature of grafted groups in the adsorption of hydrocarbon vapors on silica modified by monofunctional polyfluoroalkylsilanes

Abstract

The role of the grafting density of monofunctional polyfluoroalkylsilanes of the C n F 2n -1 (CH 2 ) m Si(CH 3 ) 2 Cl general formula (where n = 3, 4, and 6; and m = 2 and 3) and their composition in intermolecular interactions of the molecules of saturated and aromatic hydrocarbons with a surface of chemically modified silica is studied by means of IR spectroscopy and adsorption static and gas chromato graphy. It is shown that the higher the concentration and the shorter the length of the grafted chain, the greater (by a factor of 2 to 25) the drop in the adsorption values of hydrocarbons as a result of modifications, due to an increase in the degree of oleophobization of surface upon the formation of polyorganofluorine coat ings. The high specificity of the surface with respect to benzene, which is due to the active participation of the polar fragment of a grafted chain in adsorption process, is related to the features of a relatively low density sample with a concentration of grafted perfluorobutyl groups of 1.7 nm -2 . It is shown that the thermodestruc tion of polyfluoroalkyl silica remains virtually unobserved upon heating to 523 K in an argon flow.