Quantitative determination of surface area of silica gel particles by near infrared spectroscopy and chemometrics

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Abstract

Surface areas of silica gel particles have been quantitatively determined by using near infrared spectrometry and chemometrics. Surface areas of six silica gel samples with varying surface area (300–750 m2/g) have been determined by the common Brunauer–Emmett–Teller (BET) method and used in preparing mixtures of silica gel samples with varying surface area in the ranges 300–474 and 500–750 m2/g. The near infrared spectra of the mixtures were measured and the spectral profiles calibrated against the calculated surface area of the mixtures as dependent variable using partial least squares (PLS) technique. The models obtained from the calibrations were then used in predicting the surface area of unknown silica gel samples.

The error estimate in the surface area predicted for the unknown samples lie in the same range as the error limits in the BET surface area determination. The technique provides a fast, easy and inexpensive method for the determination of surface area of silica gel particles.

Introduction

Porous materials are generally characterised by several physical parameters such as porosity, surface area, mean pore size and pore size distribution. Silica gel, the amorphous form of silica is porous and therefore characterised by the same parameters. Amorphous silica is an important material in the field of catalysis, separation science and polymers. In the field of separation science silanol groups in silica gel are chemically modified to give stationary phases with a variety of different functional groups and the concentration of these functional groups on the surface is a measure of the effectivity of a chromatographic column. One of the parameters that are needed for the determination of this stationary phase effectivity is the surface area of the silica gel particles.

The surface areas of silica gel samples are measured by Brunauer–Emmett–Teller (BET) method or by light scattering using a light scattering particle size distribution analyser (LSPDA) [1]. However, the BET method is the most common. The method employs the adsorption of nitrogen at the temperature of liquid nitrogen (77 K). The experimental nitrogen adsorption isotherm for the relative pressures (p/p0) between 0.05 and 0.35 is fit to a two term BET equation (Eq. (1)) describing a theoretically obtained isotherm for a multilayer nitrogen adsorption.pXa(p0p)=1XmC+(C1)XmCp0pXa is the adsorbed nitrogen in mol/g adsorbant, Xm is the specific monolayer capacity of nitrogen in mol/g adsorbent and C is a constant.

The slope and intercept of the straight line fit of the data are then used to calculate the specific monolayer capacity, Xm. The surface area according to BET (SBET) is then determined by the multiplication of Xm by the cross-sectional area, Am of a nitrogen molecule and the Avagadros number N (Eq. (2)).SBET=XmAmN×1018m2/gThe reproducibility of the surface area determined by BET method is around ±5% [1].

Infrared spectroscopy both in the mid-IR region [2], [3], [4], [5], [6], [7], [8] and in the near-IR region [9], [10] has been extensively used in the study of silica particles. A lot of attention was focused on the study of the silanol groups on the surface.

This is because of the strong absorption bands arising from the OH vibrations. The silanol groups OH stretching vibrations absorb in the region 4000–700 cm−1and the overtones and combination frequencies of the silanol groups absorb in the region 4000–11,000 cm−1. The peak assignments of the absorptions of the surface silanol groups in the mid- and near-infrared region are given in Table 1.

There have been several reports on the determination of silanol groups using physical and chemical methods [11], [12], [13], [14]. Even though, there are differences in the silanol number determined by these methods, there is a general understanding that the concentration of silanol groups on the silica surface is constant.

The silanol groups are polarised and can form hydrogen bondings with molecules that are polar. Water molecules are readily absorbed by silica gel particles because of this nature and silica gel is regarded as one of the best drying agent in industrial applications. When dry silica gel particles are exposed to air, water molecules are adsorbed on to the surface by forming hydrogen bonds with the silanol groups.

The amount of water that is adsorbed is obviously depends on the concentration of silanol groups on the surface. If the concentration of silanol groups, silanol number, is constant then the mass of the water molecules that a silica sample adsorb should be proportional to its surface area. Furthermore, the infrared spectrum of the silica gel sample should contain absorption bands with their intensities of absorptions proportional to the water molecules adsorbed. These facts suggest that the computation of the surface area from the intensity of one of the bands in the infrared spectrum is possible if the concentration of the silanol number and the mass of the water molecules that form a monolayer on the silica gel surface is known. However, the practical approach to do this is rather difficult because of the difficulties in determining the mass of the monomolecular layer of water adsorbed on the silica gel surface from the infrared spectrum.

Another approach to the problem is to find a correlation between the surface area and the infrared spectra of silica gel samples in their hydrated form. The correlation can then be used in determining surface area of an unknown sample. This approach naturally requires a multivariate approach so that a model describing the correlation can be built. The partial least squares (PLS) technique is an excellent tool in this respect and the applications involving partial least squares calibrations are numerous.

The aim of this paper is to demonstrate that the surface area of silica gel particles can be determined with a set of samples with known surface area and their near infrared spectra. This is the first time that such an approach is being reported in the literature.

Section snippets

Samples, determination of surface area and preparation of mixtures for analysis

Six silica gel samples with varying surface areas from 300–750 m2/g were purchased from Sigma Alridch. The determination of surface area of the silica gel samples was carried out by Brunauer, Emmett and Teller (BET) technique with a TriStar 3000 gas adsorption analyser (Micromeritics Instrument Corporation, USA). These six samples were then used to prepare two sets of silica gel samples of varying surface areas. One set of samples containing 17 different mixtures in the range 300–474 m2/g and the

Results and discussion

The near infrared spectra of a few silica gel samples with varying surface area are shown in Fig. 2. The behaviour of the second derivated near infrared profiles of silica gel samples with increasing surface area is illustrated in Fig. 3. Near infrared spectra of a silica gel sample before and after drying at 200 °C under vacuum are shown in Fig. 4.

The PLS calibration loading plots for the calibration models are shown in Fig. 5. The Plots showing the measured and predicted surface area of the

Conclusion

In this paper we have demonstrated for the first time how infrared spectrometry in the near-infrared region could be used in combination with multivariate data analysis to determine the surface area of silica gel samples. The procedures used in the determinations are relatively cheap, simple, time saving and use no toxic chemicals.

The methodology demonstrated in this paper for the determination of surface area of silica gel samples removes the technical expertise one needs to determine the

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