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INTERPRETATION OF XPS COMPOSITION DATA FOR EXTRACTIVE-FREE PULP SURFACES
Derek G. Gray , Morag Weller, Nilgun Ulkem and Agnès Lejeune

Keywords: X-ray photoelectron spectroscopy (XPS); Surface analysis; Softwood kraft pulp.

ABSTRACT
X-ray photoelectron spectroscopy (XPS) has been widely applied to the technologically important problem of assessing the surface composition of lignocellulosic materials. The surface compositions of solid wood products, pulps, paper and board are critical to their end use performance. However, the materials are chemically and morphologically complex, and it is difficult to determine the amounts of cellulose, hemicelluloses, lignin, extractives and additives at the surface of these materials. While experimental XPS methods for surface analysis of pulp samples have improved, their interpretation in terms of surface lignin and extractives is still moot. We present a simplified approach, based on the original XPS equations of Dorris and Gray, that allows correction for the ubiquitous carbon contamination. The method will be illustrated using XPS results for a series of extracted softwood kraft pulps.

INTRODUCTION
The photoelectron effect, proposed by Einstein in 19051, (and for which he was awarded the Nobel Prize in 1921) states that if a material is exposed to a photon source, of energy hn, then photoelectrons will be emitted with a kinetic energy Ee given by Ee = hn - Eb , where Eb is the binding energy of the electron. The energy of the photoelectron thus depends on the frequency, but not the intensity, of the incident radiation. The binding energy is characteristic of every orbital in every atom in the periodic table, and thus it may be used as an analytical indicator if the photoelectron energy (typically 0-1500 eV) can be measured with sufficient accuracy. 

Suitable spectrometers for X-ray photoelectron spectroscopy (XPS) were developed by Kai Siegbahn2 (for which he was awarded the Nobel Prize in 1981). Because of the relatively low energy of the emitted photoelectrons, the spectrometers work in very high vacuum to avoid collisions between electrons and gas molecules, and elastic photo-emission from solids only occurs from very close to the sample surface.

XPS, also known as Electron Spectroscopy for Chemical Analysis (ESCA), has been widely applied to the technologically important problem of assessing the surface composition of lignocellulosic materials. The materials are chemically and morphologically complex and it is difficult to determine the amounts of cellulose, hemicelluloses, lignin, extractives and additives at the surface. Here, we illustrate the interpretation of the XPS spectra of some solvent-extracted kraft pulps, and present a correction for the ubiquitous carbon-rich surface contamination.

The XPS method on solids gives, in essence, the elemental composition of a thin surface layer.2 An introduction to the method as applied to paper surfaces was first given by Dorris and Gray.3 They measured the XPS spectra for filter paper and samples of bleached kraft and sulfite papers, and for isolated lignins. The results were interpreted in terms of the ratio of oxygen atoms to carbon atoms, (No/Nc), in the surface region, and the observed chemical shifts of the carbon 1s XPS peaks.

Under suitable circumstances, the experimental XPS oxygen-carbon ratio (No/Nc), and the components of the carbon 1s peak after deconvolution4 may be used to estimate the surface composition in terms of the individual wood components. 

For example, lignin and especially extractives have a higher proportion of carbon atoms relative to oxygen than cellulose, and so should have much lower No/Nc values than cellulose, which has 5 oxygen atoms for every 6 carbon atoms. This suggested an approximate way to estimate how much lignin was on the surface of handsheets made from mechanical pulps.5 The method was extended to some sulphite pulps; both oxygen-carbon and sulphur-carbon ratios indicated an excess of lignin on the fibre surfaces.6 The measurements required careful extraction of the sheets to remove carbon-rich resin and fatty acids from the sheet surface, and results were compared to a reference cellulose sample of acetone-extracted filter paper. In principal, XPS can also quantify the surface extractives content. An attempt using stearic acid as a model has been reported7, but in general quantification of extractive coverage is difficult.

The XPS method has subsequently been applied in many laboratories to pulp and paper samples 8,9,10,11 but there have been continuing questions regarding both the reproducibility of XPS measurements on lignocellulosics, and the validity of interpretations.12

NEW XPS FINDINGS
Recently, the results of a set of XPS measurements in four laboratories in Scandinavia and Canada on identical paper samples have been reported.13,14 The overall findings were that the experimental results were in reasonable accord, providing care is taken to avoid or correct carbon-rich contaminants on cellulose-rich surfaces. However, methods of interpretation in terms of surface lignin and extractives used by different laboratories gave somewhat scattered results.

The interpretation of XPS data in terms of surface composition involves several problems. Fibrous lignocellulosic surfaces are rough and chemically heterogeneous, both across the paper surface and in the depth direction. The main XPS observables are the ratio of oxygen atoms to carbon atoms, and the deconvolution of the carbon peak into components with different chemical shifts resulting from the numbers of oxygen atoms that are attached to the carbons. Interpretation of this data in terms of the molecular composition of the surface layer sampled by XPS required a number of assumptions.5

(i) The samples contained only carbohydrate and lignin (wood extractives were presumed to have been removed by suitable solvent treatments).
(ii) The composition of the analysed volume of material close to the surface was uniform.
(iii) The polysaccharide component of the surface layer (cellulose + hemicellulose) was represented by the empirical composition, C6O5.
(iv) The empirical formula for the lignin component was that for Freudenberg lignin, namely C9.92O3.32.

In addition to these simplifications, it is necessary to deal with the observation that surface contamination with carbon-rich materials is often observed. While precautions can be taken to minimize this experimentally, the contamination is ubiquitous, leading to No/Nc values for pure cellulose paper that are somewhat lower than the theoretical value of 5/6 = 0.833.

Experimental:  Black spruce chips, selected for uniformity, were pulped under typical Kraft conditions (Active alkali, 18%; Sulphidity, 30%; Temperature, 172oC; time to temperature, 90mins; Liquor/wood, 4.5:1; H-factors, 1065, 1260, 1510, 1760 and 2050). Five pulp samples of decreasing kappa numbers were obtained. For ESCA analysis, two small hand-sheets were made from each sample. The handsheets were extracted with acetone and pairs of the sheets were placed between Whatman filter papers. The inside contacting faces of the pulp sheets were used for XPS analysis. Two Whatman filter papers were treated in the same manner and their surface composition was analyzed along with the pulp samples.

The ESCA measurements were performed with a Kratos Ultra electron spectrometer (Kratos Analytical) using monochromated Al Ka X-ray source (15 kV, 15 mA). The low-resolution survey scans were taken with a 1 eV step and 160 eV analyzer pass energy; high-resolution spectra were taken with a 0.1 eV step and 40 eV analyzer pass energy. The analysis area was less than 1mm2 and measurements were taken at two different locations on the each of the touching faces of the hand-sheets.  

Results and discussion:  The measured No/Nc values for a series of unbleached softwood kraft pulps, cooked to a range of kappa numbers, are shown in (table 1). The corresponding measured value for a sheet of extracted filter paper was No/Nc = 0.74.

To relate the observed oxygen carbon ratio, No/Nc , to the surface composition, we first follow the assumption5 that there are S anhydroglucose or sugar units and L lignin phenylpropane segments per unit volume in the (uniform) volume sampled by XPS at the surface of the fibres. Hence, from the empirical formulae for S and L, the numbers of oxygen and carbon atoms in this volume are No = 5S + 3.32L and Nc = 6S + 9.92L, respectively. The segment mole fraction of lignin in the surface, SL, is thus L/(L + S). From the empirical formulae for S and L, the value for SL is derived in terms of the measured No/Nc

xps1(1)

The weight fraction of lignin, WL, may be calculated from the segment mole fraction and the molar masses of the segments:

xps2(2)

Thus, knowing the values for No/Nc, the weight fraction of lignin in the sample surface may be estimated. However, the value for No/Nc may contain a contribution from some carbon-rich contaminant, since the measured No/Nc value for a pure cellulose sheet (0 .74) was significantly lower than the predicted value of 0.83. We propose the following way to correct for the excess carbon signal observed in a series of XPS samples containing only lignin and carbohydrate components, where a pure cellulose sample has been run under identical conditions as the unknown lignocellulosic. The basic assumption is that the excess carbon signal which results in the value less than 5/6 for (No/Nc) for the pure cellulose sample is in the same proportion to the total signal (No + Nc) for the unknown samples, measured under the same spectrometer conditions. 

xps3(3)
 
where the excess carbon signal due to the contaminating carbon is Nc*. (Note that here we ignore the possibility that the contaminants may contain oxygen as well as carbon; the more complex situation has been considered by Li and Reeve.15) Quantification of differences in individual XPS peaks is not appropriate, but after some algebra, it may be shown that the corrected value for the peak ratio, ( No/Nc )Pulp, Corrected , is given by

xps4(4)
 
A value for No/Nc , corrected for the carbon-rich contaminant, may thus be calculated from the measured No/Nc for the sample and the measured value of No/Nc for a pure cellulose sheet, measured under the same conditions. Corrected values for the unbleached kraft pulps are included in Table 1.

In accord with many previous measurements, the surfaces of these pulps appear to be richer in lignin than the bulk. In our case, it appears that the surface excess of lignin is proportionally greater for pulps with high bulk lignin content.

Acknowledgements: We thank Paprican, NSERC Canada and FQRNT Quebec for funding.

kappa number

 Wt% bulk lignin (0.147 x kappa)

NO/NC measured

NO/NC  corrected (from eq 4)

Wt%
surface lignin

45

6.5

0.631

0.715

17.5

37

5.4

0.651

0.738

13.9

32

4.7

0.670

0.762

10.3

25

3.6

0.681

0.775

7.8

21

3.1

0.699

0.797

5.2



Table 1 XPS measurements of surface lignin content for softwood Kraft pulps.

LITERATURE CITED:
1.Einstein, A., Ann. Physik (Leipzig), 17, 132-148 (1905).
2.Siegbahn, K., Hamrin, K., Hedman, J., Johansson, G., Bergmark, T., Karlsson, S.-E., Lindgren, I. and Lindbert, B. ESCA: Atomic, Molecular and Solid State Structure by Means of Electron Spectroscopy; Almquist and Wiksells: Uppsala, Sweden, 1967.
3.Dorris, G.M. and Gray, D.G., Cellulose Chem. Technol., 12, 9-23 (1978).
4.Gray, D.G., Cellulose Chemistry and Technology, 12, 735-743 (1978).
5.Dorris, G.M. and Gray, D.G., Cellulose Chem. Technol., 12, 721-734 (1978).
6.Takeyama, S. and Gray, D.G., Transactions of the Technical Section of the CPPA, 6, TR61-TR64 (1980).
7.Takeyama, S. and Gray, D.G., Cellulose Chemistry and Technology, 16, 133-142 (1982).
8.Laine, J., Stenius, P., Carlsson, G. and Strom, G., Cellulose, 1, 145-160 (1994).
9.Johansson, L.-S., Campbell, J.M., Koljonen, K. and Stenius, P., Appl. Surf. Sci., 144-145, 92-95 (1999).
10.Gustafsson, J., Ciovica, L. and Peltonen, J., Polymer, 44, 661-670 (2002).
11.Hulten, A.H., Basta, J., Larsson, P. and Ernstsson, M., Holzforschung, 60, 14-19 (2006).
12.Li, K. and Reeve, D.W., Cellulose Chemistry and Technology, 38, 197-210 (2004).
13Johansson, L.-S., Campbell, J.M., Fardim, P., Hulten, A.H., Boisvert, J.-P. and Ernstsson, M., Surface Science, 584, 126-132 (2005).
14.Fardim, P., Hultén, A., Boisvert, J.-P., Johansson, L.-S., Ernstsson, M., Campbell, J.M., Lejeune, A., Holmbom, B., Laine, J. and Gray, D., Holzforschung, 60, 149-155 (2006).
15.Li, K. and Reeve, D.W., Journal of Wood Chemistry and Technology, 24, 183-200 (2004).

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