Study Of Silica With The Grafted
STUDY OF SILICA WITH THE GRAFTED
PHENILSILOXANE LAYER BY
ADSORPTION AND IR SPECTRA METHODS
T. M. Roshchina, H. K. Shonia, L. E. Kitaev,
K. B. Gurevich, A. A. Kazmina
M.V. Lomonosov State University of Moscow, Russia
Materials that contain phenyl groups have been successfully used in the sorption, concentration and different variants of chromatography for a long time. Nevertheless the questions connected with the research of surface chemistry, structural and adsorption silica properties with chemically grafted phenyl groups are not sufficiently covered in the literature.
This work is devoted to study of chemical phenyltrichlorosilane modification effect on the adsorption silica properties by means of various physical- chemical methods.
It is shown that the silica modification by phenyltrichlorosilane leads to practically complete disappearance of adsorption band (ad. b.) 3745sm-1 from the spectrum (which was stipulated by isolated groups SiOH oscillation); after H-D exchange its corresponding ad.b. of Si-OD groups 2770sm-1 didn’t reveal itself. Simultaneously the wide asymmetric ad.b.~ 3645sm-1 remained, which was stipulated by the OH (or interglobular groups), connected by hydrogen bonds. As a result of vapour D2O processing partial deuterium-hydrogen exchange of these groups took place and a wide ad.b. with the maximum under 2698 sm-1 revealed itself. Besides, the modified model spectra (before and after H-D exchange) displayed ad.bands of 3065 and 3082sm-1,as well as ad.b. of 1600 and 1433sm-1, which can be attributed, respectively, to valence oscillations of C-H and C-C phenyl nuclei bonds. Additional silanization by N, N-dimetiltrimetilsililamine resulted in occurrence of phenylsilica ad.b. 2970sm-1 in the spectrum, which was caused by valence oscillations of C-H methyl groups bonds. Hence, the modification by phenylsilane and additional silanization allows us to link practically completely or screen the surface silica groups.
The adsorption water vapour data showed that a rather homogeneous modificator polymer film is formed on phenyl silica (non-modified silica
surface areas are absent). This film screens the remaining silanolic groups rather well and provides practically complete hydrophobization of the sample surface, which is perfectly proved by the results of IR spectroscopic analysis.
The thermodynamic adsorption characteristics (TAC) of different classes of organic compounds on initial and modified silica in small surface filling areas were determined by gas adsorption chromatography method.
The contribution of different groups or structural molecular fragments to the heats of adsorption q was estimated for more detailed analyses of organic compounds interaction mechanism with different material surface on the base of the additive scheme. It is shown that the high contribution to q carbonic groups (especially CH2) relates to the unusual phenylsilica properties. This contribution represents the dispersion measure influence. Contributions of CH2 in q on phenylsilica are practically equal to the values, obtained on the non-polar porous polymeric sorbent Chromosorb 101, and surpass the values, observed on the graphitized soot, initial carrier and especially on phenylsiloxane liquid phases. The surface modification as a rule, doesn’t lead to the decrease of functional groups contributions into adsorption heat, except ether ketone groups, which may be connected with the higher energy of dispersion forces on phenyl silica in comparison with the initial carrier. The good correlation between the calculated and experimental data witnesses that the method of contribution calculation of separate molecular fragments in TAC can be useful for both surface properties comparative characteristics of different materials and for data prediction of their keeping.
This calculation method is used to obtain the additional information of molecules interaction with the residual surface silanolic groups. It is shown that the molecular size decrease is accompanied with the natural increase of the residual silanolic groups space availability. Besides, the lower is the temperature, the higher is the role of interaction of these groups hydrogen bonds type. Therefore considering the possibility of phenyl silica application in the analytical practice, one should take into account that this adsorbent rather suits for research of molecules with relatively large sizes and/or under high temperature.
Summing up the results, obtained by means of different physical- chemical methods, one may come to the conclusion that the gas chromatography is the most sensitive method of the residual surface heterogeneity of the chemically modified silica. It is evident, that while obtaining phenyl silica, a small number of silanolic groups remains. These groups not always can be found out by means of IR spectroscopy of static condition adsorption.
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