Structural Characteristics of Silica Gel Modified with Organosilicon Compounds According to the Data on the Adsorption of Water and n-Hexane Vapors

The adsorption of water, n-hexane and nitrogen vapors on the mesoporous silica gel with the grafted aminopropyl, mercaptopropyl and dimercaptodipropyl groups is investigated. It is shown that aminopropyl groups are oriented in parallel, whereas the mercaptopropyl ones are either vertical or tilted to the initial sorbent surface. The surface area of silica gel that is free of modifiers, as well as the number of vicinal and isolated OH groups are estimated. It is established that hydrogen bonds between the NH₂ groups of aminopropyl radical and isolated OH groups of silica gel are ruptured after the saturation of sorbent with water vapors at a relative pressure of about 0.5. Based on adsorption data, it is concluded that the most of 3-aminopropyltriethoxysilane interacts with the isolated OH groups of silica gel with a stoichiometric coefficient equal to unity.