N. K. Shoniya

Adsorption of water, diethyl ether, and acetonitrile on silicas with grafted perfluorohexyl coatings

The effect of the functionality of modifiers on the adsorption of vapors of compounds capable of strong specific interactions, including water, diethyl ether, and acetonitrile, on silicas with chemically grafted perfluorohexylsilanes (RSi(CH3)3 −mClm, where m (functionality) is 1, 2, 3 and R is polyfluoroalkyl) was studied by adsorption under static conditions, gas chromatography, and IR spectroscopy. It was found that modification with monofunctional silane results in the most hydrophobic coating and a comparatively low-polarity surface with a strongly decreased contribution of specific interactions compared with other modifiers.

The Adsorption of Saturated and Aromatic Hydrocarbon Vapors on Silicas with Chemically Grafted Perfluorohexyl Groups

Regardless of the nature of the modifier (mono-, bi-, and trichlorosilanes with perfluorohexyl groups or monochlorooctylsilane), the modification of silica decreased retention volumes and adsorption values of both n-alkanes and aromatic hydrocarbons. The entropy factor can play a key role in adsorption intermolecular interactions on the surface of chemically modified silicas. The modification of the surface with bi- and trichloroperfluorohexylsilanes caused the appearance of new centers more active compared with those of the initial carrier. The most oleophobic and nonpolar coatings were obtained using monochlorosilane with perfluorohexyl groups as a modifier.

Interaction of nitrogen and hydrocarbon vapors with mesoporous silicas modified with perfluorohexylsilane

Using the static method and gas chromatography, the adsorption of nitrogen and hydrocarbons by SBA-15 silicas and commercially available silica gel (SG), as well as changes in the surface properties of these samples as a result of chemical modification with multifunctional perfluorohexylsilane ClSi(CH3)2(CH2)2(n-C6F13), are studied. The main factor that causes the high values of thermodynamic characteristics of adsorption on SBA-15 and its modified form is a relatively small pore size. The differences in the polarities of the samples testify to a high concentration of silanol groups on the initial and modified SG.

Role of the Concentration and Nature of Grafted Groups in the Adsorption of Hydrocarbon Vapors on Silica Modified by Monofunctional Polyfluoroalkylsilanes

The role of the grafting density of monofunctional polyfluoroalkylsilanes of the CnF2n − 1(CH2)mSi(CH3)2Cl 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 chromatography. 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 coatings. 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 thermodestruction of polyfluoroalkyl silica remains virtually unobserved upon heating to 523 K in an argon flow.

The nature of lyophobic coating and the adsorption of organic molecules and water on modified silicas

The protective properties of lyophobic layers of various natures chemically grafted to a silica carrier were comparatively studied. The modifiers were silanes with the compositions CF3(CH2)2Si(CH3)2Cl (CF3), C8H17Si(CH3)2Cl (C8H17), and ClSi(CH3)2[OSi(CH3)2]2Cl (OMS). The differences between the surface properties of chemically modified silicas observed in adsorption, chromatographic, and IR spectroscopic measurements were shown to be related to a nonuniform electron density distribution in CF3 grafted radicals and the special features of the structure of CF3 and OMS grafted layers caused by the possibility of interaction between the terminal groups of these radicals and the surface of the carrier. Modified silicas possessed low surface energy and were superhydrophobic materials. The sample with grafted octyl groups C8H17 had the highest stability with respect to water.

Physicochemical properties of surfaces of SBA-15 silicas, according to adsorption-static, gas-chromatographic, and IR spectroscopic data

Interaction between vapors of organic compounds and water with surfaces of mesoporous silica SBA-15 and silica SBA-15 modified with n-C6F13(CH2)2Si(CH3)2Cl via adsorption under static conditions is studied by means of gas chromatography and IR diffuse reflectance spectroscopy. It is shown that modification notably reduces the energy of disperse and specific interactions, along with the acidity of adsorption centers. Even low concentrations of the grafted groups (0.76 nm–2) allows us to obtain highly hydrophobic coatings on SBA-15 surfaces.

Adsorption of hexane and benzene on fluorinated silicas: The effect of the perfluoralkyl chain length

By means of the gas chromatography and a static adsorption method, the extent of fluorination of the carrier surface, determined by the length of the grafted radical chain, affects the adsorption properties of silicas with perfluorohexyl, perfluoropropyl, and trifluoromethyl coatings. The analysis of data on the original carrier and chemically hydrophobized silicas, including a sample with grafted octyl groups used as the reference, shows that modification leads to the decrease in both the adsorption (a at 298 K and the Henry constants (KH) in a wide temperature range for hexane and benzene. However, in contrast to silicas with perfluoropropyl or perfluorohexyl coatings, the a and KH values on trifluorometylsilica became equal (n-alkanes) or even somewhat lower (aromatic hydrocarbons) than those observed on the non-polar octylsilica surface. Irrespective of the degree of fluorination of the silica surface, the substitution and screening of silanol groups in silica lead to weakening the polarity of polyfluoroalkylsilicas as compared with the original carrier.

Physicochemical properties of the surfaces of silica species

The results from studying the physicochemical characteristics of the adsorption of n-alkanes, arenes, and acetonitrile on silicas of different origins, silica gels, and silochromes are presented. It is shown that increasing the concentration of silanol groups reduces the role of dispersion interactions accompanied by the intensification of specific interactions on silica gels, compared to silochromes. According to diffuse reflectance FTIR spectroscopy data, the acidity of silanol groups on silica gel is in this case less pronounced.

Effect of the surface condition of silicas with grafted monofunctional polyfluoroalkylsilanes on the adsorption of polar molecules

Lyophobized silicas containing relatively small numbers of grafted perfluorohexyl (1.1 nm−2) and perfluorobutyl (1.7 nm−2) groups are studied by means of gas chromatography, adsorption under static conditions, and IR spectroscopy. The results are compared to those obtained by us previously for a series of samples with dense polyfluoroalkyl monolayers (≥2.0 nm−2). Effects related to the influence of the grafting density and the size of fluorine-containing groups on the adsorption of polar compounds and the hydrophobicity of the surface are discussed.

Vapor phase adsorption of organic compounds on octyl silicas

The influence of the modification of silica by octyltrichlorosilane with the formation of an oligomeric grafted layer (sample C8(II)) and additional silanization (sample C8(III)) on the thermodynamic adsorption characteristics (TACs) of different classes of organic compounds was investigated by gas chromatography. It was shown that the modification leads to decreased adsorption values for most of the investigated compounds (with the exception of alkanes, for which TACs on sample C8(II) approach the values observed on the initial support, due probably to additional interactions with silanol groups formed in modifying the surface with octyltrichlorosilane). It was established that blocking these silanol groups during additional silanization with trimethylsilane resulted in inert surfaces whose adsorption properties with respect to many compounds (including some capable of participating in strong specific interactions) approaches to the properties of octyl-silica with a close-packed grafted monolayer.