Surface properties of silica gel modified by trifunctional polyfluoroalkyl silanes according to data of adsorption–structural and gas chromatography studies

Abstract

Surface properties of silica gels modified by trifunctional polyfluorosilanes n-C₆F₁₃(CH₂)₂SiCl₃ and iso-C₃F₇(CH₂)₂SiCl₃ are studied using the methods of adsorption and gas chromatography with respect to nitrogen, hydrocarbon, and water molecules. It is shown that the branched structure of the grafted group and high concentration of grafted groups (2.7 nm^–2) provide better screening of both residual silanols of the support and of additional hydroxyl groups formed in the course of synthesis as a result of hydrolysis. Comparison with the properties of silica gel with grafted monofunctional silane n-C₆F₁₃(CH₂)₂Si(CH₃)₂Cl shows that, in the case of an overall decrease in the adsorption values and considerable surface lyophobization, modification by trifunctional silanes irrespective of the modifier chain length allows obtaining adsorbents containing more polar centers than those in silica gel due to the possible participation in adsorption of additional silanols and a polar fragment of the grafted chain of–CH₂–CF₂–. A considerable difference in the course of isotherms of nitrogen and hexane adsorption–desorption is found in the range of capillary condensation hysteresis related to liophobicity of polyfluoroalkyl layers, a significant decrease in the pore wetting and the corresponding increase in wetting angle θ by hexane. Adsorption of water on fluorinated silicas is negligible and capillary condensation is not observed due to nonwettability of the pore surface by liquid adsorbate (θ > 90°).