Surface Properties And Pore Size Analysis Of Unmodified And Modified Silica Gels
SURFACE PROPERTIES AND PORE SIZE ANALYSIS OF UNMODIFIED AND MODIFIED SILICA GELS
J. Choma1, M Jaroniec2, H. Grajek1
1Military Technical Academy, 00-908 Warsaw, Poland
2Kent State University, 44-242 Kent, Ohio, USA
Porous silica gels are commonly used as adsorbents, catalyst supports and chromatographic packings because of the easiness of controlling their specific surface area, particle morphology and porosity [1]. Their another attractive feature is high mechanical stability. In addition, silica gels exhibit high reactivity towards organosilanes, which allows one to prepare modified materials of desired surface properties. Since different silica gels can be prepared by varying synthesis conditions and post-synthesis modification, their detailed characterization is essential for many applications.
The aim of the current work is to present the methodology for an accurate characterization of unmodified and modified silica gels on the basis of nitrogen adsorption data. This methodology is illustrated by using unmodified silica gel (Prodigy from Phenomenex, Torrance, CA, USA) and modified sample with octyldimethylchlorosilane [2]. The elemental analysis of the modified sample showed 13.15% of carbon, which gave the surface coverage of 3.11 μmol/m2 [2]. Low temperature (77K) nitrogen adsorption isotherms were measured for unmodified and modified samples in the entire range of relative pressures, i.e., from 10-6 to about 1.0 [2]. The standard BET method was used to evaluate the specific surface area, which was equal to 433m2/g for unmodified silica and 251 m2/g for the sample with octyl bonded phase. The αs-plot analysis [3] was used to study the surface properties of the samples. The αs -plots (see Figure 1) were constructed by using the reference nitrogen adsorption isotherms measured for unmodified [4] and octyl-modified [5] LiChrospher Si-1000 (Merck, Germany) silicas. The αs -plots gave the following specific surface areas and pore volumes: 437 m2/g and 0.90cm3/g (unmodified silica) and 254 m2/g and 0.53 cm3/g (octyl-modified silica). The pore size analysis (see Figure 2) was performed by Dollimore-Heal method [6] using the corrected Kelvin equation [5]:
where r is the radius of pores as a function of the relative pressure p/po, γ=8.88·10-3 N/m and V1 = 34.68 cm3/mol are the surface tension and molar volume of liquid nitrogen at 77K, R = 8.314 J/(K·mol) is the universal gas constant and t(p/po) is the statistical film thickness on unmodified [4] and octyl-modified [5] reference silica gels. The maximum of the pore size distribution as well as the area under distribution decreases after modification of the silica surface with octyldimethylchlorosilane.
Figure 1. The αs -plots for the samples studied.
Figure 2. Pore size distributions for the samples studied.
It is shown that the comparative plot analysis and pore size analysis give valuable and accurate information about surface and structural properties of unmodified and modified silica gels providing that proper statistical film thickness and Kelvin relation are used.
References
1. M. Kruk, M. Jaroniec, Surfaces of nanoparticles and porous materials, J.A.Schwarz, C.I. Contescu (Eds.), Marcel Dekker, New York, 1999,p. 443-472.
2. Y. Bereznitski, M. Jaroniec, J. Liq. Chrom., 1999,22, 1945.
3. S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1991.
4. M. Jaroniec, M. Kruk, J.P. Olivier, Langmuir, 1999,15, 5410,
5. M. Kruk, V. Antochshuk, M. Jaroniec, A. Sayari, J. Phys. Chem. B, 1999, 103, 10670.
6. D. Dollimore, G.R. Heal, J. Appl. Chem., 1964,14, 109,
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