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

ALFRED A. CHRISTY

Department of Mathematics and Sciences, Agder University College, Kristiansand, Norway

Points to discuss

1. Silica gel Silica gel and its characteristics Silica gel and its importance Silanol number and surface area of the silicagel particles

2. Determination of surface area 3. Experimental 4. Results and discussion 5. Conclusion

Silica gel and its characteristics

Silicagel is prepared by the reaction of sodium silicate with hydrochloric acid

Dry silica has a surface area of 200-800 m2/g Has a pore volume 0.7 cm3/g and average pore diameter of

80-150 Å The surface of the silica gel particles contain silanol

groups (Si-OH) and siloxane groups (Si-O-Si) Some silanol groups lie buried in the bulk On heating above 200o C causes loss of OH groups OH groups are completely annihilated at 1200o C

Silica gel and its characteristics

Silica gel and its importance

The silanol groups can form strong hydrogen bonds with water molecules

The presence of silanol groups make silica an important material in separation science and in the field of catalysis

The silanol groups can be chemically modified-different stationary phases for chromatographical applications

Concentration of the 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.

Silanol number and surface area of the silicagel particles The properties of silicagel is connected to the Number of silanol

groups per nm2 area on the surface The methods used in the determination of silanol number

1. Theoretical calculation

2. Chemical methods

3. Physical methods

All these methods conclude that the number of silanol groups on the silicagel surface is a constant with 4.55 OH/nm2

Water molecules are adsorbed onto the surface by forming hydrogen bonds with silanol groups.

The number of water molecules-mass of water molecules adsorbed per square metre silicagel surface is proportional to the surface area

Determination of surface area of silicagel particles The surface area of silicagel samples are measured

by Brunauer-Emmett-Teller (BET) method or by light scattering by using a light scattering particle size distribution analyser (LSPDA).

The BET method is the most common.

The method employs the adsorption of nitrogen at the temperature of liquid nitrogen (77K).

Determination of surface area of silicagel particles The experimental nitrogen adsorption isotherm for the relative pressures (p/po)

between 0.05 and 0.35 is fit to a two term BET equation (eqn. 1) describing a theoretically obtained isotherm for a multi layer nitrogen adsorption.

p/{Xa(po-p)}= (1/ XmC) +{ (C-1)/ Xm C}(po/p) (1)

{Xa= adsorbed nitrogen in moles/gram adsorbant, Xm =specific monolayer capacity of nitrogen in moles/gram adsorbent and C is a constant}

The slope and intercept of the straight line fit of the data is then used to calculate the specific monolayer capacity, Xm. Determination of 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 (eqn. 2).

SBET = Xm.Am.N.10-18 m2/g (2)

Water adsorption on Silica gel

Experimental-Samples

1. Six silica gel samples with varying surface area from 300-750 m2/g were purchased from Sigma Alridch.

2. The surface area-determined by Brunauer, Emmett and Teller (BET) technique with a TriStar 3000 gas adsorption analyser

1. The samples were used to prepare two sets of silica gel samples of varying surface areas.

2. 17 different mixtures in the range 300-474 m2/g

3. 30 mixtures in the range 500-750 m2/g

4. In addition some independent samples in the regions mentioned above were prepared for the prediction of surface area.

Experimental-NIR measurements

The near infrared measurements were made using a Perkin Elmer Spectrum One NTS FT- NIR spectrometer equipped with a Perkin Elmer transflectance accessory and a DTGS detector

The glass vial containing the samples were placed directly on the crystal of the transflectance accessory and the near infrared spectra were measured in the region 10,000-4000 cm-1 at a resolution of 16 cm-1.

A total of 20 scans were made on each sample and the average spectrum was finally saved as the representative spectrum of the sample.

Experimental-NIR measurements

Experimental-Multivariate calibration

Near infrared spectral profiles transferred to SIRIUS program for data handling

Near infrared spectral profiles put in a matrix with rows representing the spectra of the mixtures

The spectral profiles in the matrix double derivated to eliminate baseline effects

Multivariate calibration using PLS technique carried out with spectral profile data as independent variables and calculated surface area of the respective mixtures as dependent variable

Experimental-Multivariate calibration

Near Infrared spectra arranged in a Matrix

Results and discussion- NIR spectra of samples with varying surface area

Table 1 Mid-IR and NIR Band assignments for water adsorbed silicagel samples.

Peak (cm-1)Band assignment

3700-3550 OH stretch silanol groups

3400-3500 OH stretch hydrogen boded silanol groups

4540 Combination band OH stretching and bending silanol groups

5265 Combination band OH stretching and bending water molecules

6800-7300 Overtones of OH stretch silanol groups and hydrogen bonded silanol groups

Results and discussion-Silicagel sample before and after drying

Results and discussionLoading plot-PLS analysis

Results and discussionCorrelation between measured and predicted

surface area

Results and discussionPrediction of surface area of unknown

samples

Cal. set Surface area predicted m2/g Surface area calculated m2/g

Set. 1

405 414

436 434

458 457

Set. 2

533 525

555 553

719 730

731 741

Conclusion 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 remove the technical expertise one needs to determine the surface area by BET technique.

Once the preliminary area measurements of the samples were made, the calibration can be carried anywhere or in any laboratory equipped with a near infrared spectrometer and the required accessory for measuring the near infrared spectra of the samples.