Extractives on fiber surfaces investigated by XPS, ToF-SIMS and AFM

Colloids and Surfaces A: Physicochem. Eng. Aspects 255 (2005) 91–103

Extractives on fiber surfaces investigated by XPS, ToF-SIMS and AFM

Pedro Fardim∗, Johanna Gustafsson, Sebastian von Schoultz,Jouko Peltonen, Bjarne Holmbom

Abo Akademi University, Porthansgatan 3, FIN-20500 Turku/Abo, Finland

Received 29 April 2004; accepted 10 December 2004Available online 21 January 2005

Abstract

The composition, nanostructure and distribution of extractives on fiber surfaces of a birch bleached kraft pulp (BKP) and a recycled deinkedpulp (DIP) was investigated using X-ray photoelectron spectroscopy (XPS or ESCA), time-of-flight secondary ion mass spectrometry (ToF-SIMS) and atomic force microscopy (AFM). Effects of different extraction methods on the fiber surfaces were also investigated, in additionto the analyses of extractives by gas chromatography (GC). Traces of surface extractives were observed by XPS, ToF-SIMS and AFM in theBKP after extraction using an acetone–phosphate method (AcP). This method improved the extraction yield of palmitic and stearic acids inD ToF-SIMSw a diametero ualitativeo for fibers©

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IP, but the removal of surface extractives was not as effective as for BKP. Contaminants, identified as siloxanes and phthalates byere observed for DIP and BKP, even after AcP extraction. Stearic acid and its calcium salts were found to form aggregates withf 100–500 nm, while oleic acid formed a uniform layer. Estimations of extractive surface coverage by XPS were consistent with qbservations by ToF-SIMS and AFM. The combinations of different techniques strengthen the value of XPS, ToF-SIMS and AFMurface investigation.2004 Elsevier B.V. All rights reserved.

eywords: Pulp fibers; Recycled pulp; Fatty acids; Nanostructure; ToF-SIMS; AFM

. Introduction

Wood extractive is a generic term for a large numberf compounds present in wood which are soluble either

n neutral organic solvents or water. They are usuallyow-molecular mass compounds that are considered to beon-structural wood constituents[1]. The composition ofxtractives is dependent on the wood species and the amount

s generally much lower than carbohydrates and lignin.owever, relatively high amounts of extractives can be

ound in heartwood and knots in softwoods[2] and in certainardwood and softwood cells[3].

Wood resin is a term frequently used in the pulp and pa-er industry for the wood extractives which are lipophilicnd soluble in neutral organic solvents, such as acetone,ichloromethane, or petroleum ether. Wood resin consists

∗ Corresponding author. Tel.: +358 2 215 42 33; fax: +358 2 215 48 68.E-mail address:[email protected] (P. Fardim).

of terpenes, resin acids, fatty acids and their esters,hols, hydrocarbons and other neutral compounds. Theis mainly located in parenchyma cells both in hardwoodsoftwoods, as well as in resin ducts in softwoods. The proused for pulp manufacture significantly affects the amand the distribution of extractives, which influence intetions in the different bleaching and papermaking proceThe surface coverage of extractives is much higher thaamount in pulp[4]. Extractives deposit onto fiber surfacduring chemical pulping[5], bleaching[6] and papermaking [7] and also cover the fiber surfaces of mechanicalchemimechanical pulps[8]. Surface extractives are believto affect, both pulp[9,10] and paper[11] strength, refinin[12] and wetting properties[13], and are an important paraeter which influences product performance.

Analyses of surface extractives are regularly carriedby X-ray photoelectron spectroscopy (XPS) and morecently with time-of-flight secondary ion mass spectrom(ToF-SIMS). XPS has been used for the estimation of su

927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
oi:10.1016/j.colsurfa.2004.12.027

92 P. Fardim et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 255 (2005) 91–103

coverage of extractives[5,14–18], while ToF-SIMS has beenused for assess the detailed surface composition in additionto surface distribution[16,19]. Applications of atomic forcemicroscopy (AFM) for characterization of fiber surfaces arerelatively recent compared to XPS. Morphological and struc-tural investigations have been applied to chemical[20,21],mechanical[22] and chemimechanical pulps[23]. Lignin hasbeen reported as patches[23,24] or globules[25,26]. Addi-tionally, granular shapes have been proposed to be formedby lignin, extractives or hemicelluloses[21]. An increase infibrillar nanostructures due to exposure of cellulose surfacesduring pulping was also reported[21,25]. The high spatialresolution obtained with AFM makes it possible to achievedetailed information about surface morphology and topogra-phy. However, roughness limits the practical scanning area.Other limitations concern convolution effects due to tip radiusand little chemical specificity without a proper developmentof modified tips.

Quantitative analysis using ToF-SIMS has many limita-tions caused by the effects of the electronic state of the sur-face on the secondary ion yield[27]. Imaging of fiber surfacesby ToF-SIMS has the advantage of chemical specificity, buthas limitations regarding spatial resolution and surface dam-age at small raster size. Such problems are not observed inAFM imaging. XPS quantification is also limited due to sur-f urvefi ToF-S y. Int n ofe nkedp tiono

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acetone (method Ac), 150 ml of acetone and 0.75 ml of aceticacid (method AcH), and a special phosphate method (methodAcP). The AcP extraction method was done by addition of150 mg of KH2PO4 into 150 ml of acetone:water solution(9:1) added before to the pulp sample, as described earlier[31]. The reflux period was also 4 h. After the reflux period,two aliquots of 10 ml each were put into a screw-cappedtest tube for the GC analysis. The pulp samples were fil-tered using a glass filter and washed six times with deion-ized water. All chemicals used were of proanalysis gradewith a minimum purity of 99.5%. The solutions contain-ing the extracted material were evaporated under nitrogenflux until the volume in the test tube was around 1 ml, fol-lowed by extracting twice using methyltertiary-butyl ether(MTBE). The extractives were silylated and then analyzedby GC using a short thin-film capillary column and flameionization detector (FID) as described in detail previously[32].

2.3. Surface analysis by XPS

X-ray photoelectron spectra were obtained in low and highresolution modes using a Physical Electronics Quantum 2000ESCA instrument, equipped with a monochromatic Al K�X-ray source, operated at 25 W of power. The photoelectronc ◦ ndt ts medi-a and2 rgec iong urvefi ck-g half-m ofa -l therff peakra

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ace heterogeneity, charging and overlapping in C 1s ctting [28–30]. The strengths and weaknesses of XPS,IMS and AFM techniques make them complementar

his study, the composition, nanostructure and distributioxtractives in a bleached kraft pulp and a recycled deiulp were critically investigated using GC and a combinaf XPS, ToF-SIMS and AFM analyses.

. Experimental

.1. Pulps

Samples of a bleached birch kraft pulp (BKP) and a rled deinked pulp (DIP) were obtained from a FinnishSwedish pulp mill, respectively. The BKP was bleac

sing oxygen (O), chlorine dioxide (D0, D1 and D2) and al-aline extraction reinforced with oxygen and hydrogenxide (EOP) stages in the sequence O–D0–EOP–D1–D2.he DIP was produced using office waste. The samere washed by suspending 20.0 g o.d. (oven dry)

n distilled water at 1% consistency and stirring at roemperature for 1 h. After that, the samples were filtwice through a glass fiber filter and stored in a refriger4◦C).

.2. Pulp extraction and analysis by GC

Three different extraction methods were applied u.0 g o.d. of the washed pulp. The pulp sample was pl

n a flask and refluxed for a period of 4 h using 150 m

ollection was at 45in relation to the sample surface ahe spot size was 500�m× 400�m. At least three differenpots were measured in pulp hand sheets prepared imtely after the extractions. The pass energy was 117.43.5 eV for low and high resolution, respectively. Chaompensation was applied using a combination of anun bombarding and a low energy electron flood gun. Ctting of C 1s peak was performed using a Shirley baround, Gauss–Lorentzian character and a full width ataximum (FWHM) of 0.9–1.2 eV. Binding energy (BE)ll spectra was related to C1 (CC, C H) at 285 eV. The fol

owing BE, relative to the C1 position, were employed forespective groups, 1.7± 0.2 eV for C2 (C O), 3.1± 0.3 eVor C3 (C O, O C O), and 4.6± 0.3 for C4 (O C O). Sur-ace coverage of extractives was determined using O/Catios (φext) [14] and C1 relative areas (θext) [33] of extractednd unextracted samples.

.4. Surface analysis by ToF-SIMS

Secondary ion mass spectra were obtained using acal Electronics ToF-SIMS TRIFT II spectrometer. A p

ary ion beam of69Ga+ liquid metal ion source (LIMSith 15 kV applied voltage, 600 pA aperture current anunched pulsed width of 20 ns was used in positive andtive modes. A raster size of 200�m× 200�m was used ant least three different spots were analyzed. Surface dis

ion of extractives was obtained with the best spatial resion using the ion gun operating at 25 kV, 600 pA of aperurrent and an unbunched pulse width of 20 ns. Spectracquired for 6 min with a fluence of∼1012 ions/cm2 ensur-

P. Fardim et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 255 (2005) 91–103 93

ing static conditions. Charge compensation was performedusing an electron flood gun pulsed out of phase with the iongun. The surface distribution images were processed to im-prove the contrast using Paint Shop Pro Software. The origi-nal ToF-SIMS images in gray scale were initially convertedto negative images and then smoothed by using the equalizehistogram function[34].

2.5. Atomic force microscopy

The AFM images were recorded with a Digital NanoscopeIIIa microscope equipped with the extender electronics mod-ule enabling phase imaging in tapping mode. Silicon can-tilevers with a resonance frequency of 250–300 kHz wereused. According to the manufacturer, the radius of curvatureof the used cantilever tips was 10–20 nm. The tip convolutioneffects[35] were not taken into consideration in dimensionalanalysis. Thus the real dimensions would be slightly smallerthan measured. The trace and retrace signals were set to beidentical before image capturing. A low tapping amplitude(A0) of 30 nm and damping ratio (rsp) of 0.6 were used forthe adhesion contrast studies. The principles of the analy-sis of the high and low tapping amplitude images have beendescribed earlier[25,26]. Prior to AFM imaging, the pulpsamples were dried on sample stubs covered by double-sideda ringw lacedo ien-t le toe

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3.2. Chemical microscopy of extractives on fibersurfaces

A combination of spectrometric and imaging methods wasapplied to investigate the location, morphology and distribu-tion of extractives on the fiber surfaces. Initially, exploratoryexperiments were performed by deposition of extractivesonto different substrates to observe if different compoundsform different types of aggregates. After that, extracted andunextracted BK and DIP were investigated.

3.2.1. Deposition of extractives onto cotton linters andTMP fibers

Similar amounts (7–10 mg/g) of resin acids, oleic acid andstearic acid were added to pre-extracted acetone cotton lin-ters and thermomechanical pulp (TMP) fibres. The excess ofacetone was removed by air drying. The cotton linters werethen analyzed by XPS, ToF-SIMS and AFM while the TMPhand sheets were analyzed only by ToF-SIMS. The aim ofusing two different substrates was to avoid interferences byamorphous components such as hemicelluloses and lignin onthe morphology of extractive aggregates.

The XPS O/C ratio was significantly reduced after additionof extractives on the cotton linter surfaces (Table 2). Similarreduced levels were observed for oleic acid and stearic acid.H O/Cr ervedi IMS( MP(s e ag-g bilityo ervedial ervedo -t suredb TheA arti-ca ace,wo cs oft

3X

KP( ains l,S d aspo ac-ti

dhesive tape. All images were measured in air. Filteas not used during scanning. The microscope was pn an active vibration damping table (MOD-1, JAS Sc

ific Instruments), in turn placed on a massive stone tabliminate external vibrational noise.

. Results

.1. Composition of extractives by different extractionethods and GC

The material extracted using the extraction methodscH and AcP contained saturated and unsaturated fatty aterols, long-chain alcohols and triterpenyl alcohols foramples. However, triterpenyl alcohols and resin acidsresent only in BKP and DIP, respectively (Table 1). The

argest component of the BKP extracts was betulinolowed by fatty acids and sterols. Only very small amounnsaturated fatty acids such as oleic and linoleic acidsetected. No effects of different extraction methods onxtractive content were observed if a coefficient of variaf 3% was considered for each procedure[32]. The largesomponent group in the DIP extract was saturated fatty aollowed by resin acids and sterols. The AcP method gaveer extraction of fatty acids, specifically palmitic and stecids, which are partly present in form of fatty acid salts[31].alcium salts of fatty acids were used for ink removal in

ecycling process and remained attached onto pulp fiberould not be completely removed by conventional acextraction (Table 1).

owever, stearic acid had a much lower repeatability inatio data than oleic acid. Large aggregates were obsn the stearic acid images of TMP surfaces by ToF-SFig. 1a). A peak at 323 Da was present in the spectra of TFig. 1a) and cotton linters (Fig. 1b) indicating the calciumtearate as an impurity. Probably, the presence of largregates of stearic acid was the reason for lower repeatabserved in XPS analyses. Smaller aggregates were obs

n the ToF-SIMS images of TMP for the resin acids (Fig. 1c)nd no aggregates were observed for oleic acid (Fig. 1e). The

arge aggregates formed by stearic acid were also obsn the cotton linter surfaces by AFM (Fig. 1b). The diame

er of the nodules which compose the aggregates meay AFM was found to vary between 100 and 500 nm.FM images of resin acids showed round asymmetric ples with a diameter in the order of 30–200 nm (Fig. 1d). Oleiccid was found to form a uniform layer covering the surfithout any evidence of aggregation (Fig. 1f). Obviously, theleic acid layer was very thin because the characteristi

he cellulose microfibril underneath were also visible.

.2.2. Surface coverage of extractives in pulp fibers byPSThe low-resolution XPS spectra of unextracted B

BKP-U) and DIP (DIP-U) pulps had C and O as the murface elements (Fig. 2). The DIP-U spectrum also had Ai and Ca peaks originating from mineral particles useaper fillers, such as clay, silicates and CaCO3. An O/C ratiof 0.67 was obtained for BKP-U and after different extr

ions it increased to values close to 0.83 (Table 3), whichs the typical ratio for cellulose or xylan[36]. This indi-


Table 1Amounts of components extracted from birch ECF pulp (BKP) and deinked pulp (DIP) using acetone (Ac), acetone–acetic acid (AcH) and phosphate (AcP)extraction methods

Component Pulp sample and extraction method

BKP–Ac BKP–AcH BKP–AcP DIP–Ac DIP–AcH DIP–AcP

Fatty acidsPalmitic acid 39 40 41 282 306 321Heptadecanoic acid 10 8 10 29 23 31Stearic acid 37 33 35 335 379 380Oleic acid 4 4 4 115 121 119Linoleic acid 8 7 8 32 34 34Arachidic acid 35 33 34 12 16 14Behenic acid 34 33 34 8 10 8Lignoceric acid 9 8 9 9 11 11

Total fatty acids 176 166 175 822 900 918

Resin acidsPimaric acid – – – 26 27 28Sandaracopimaric acid – – – 16 17 18Isopimaric acid – – – 37 38 40Palustric acid – – – 15 15 4Dehydroabietic, levopimaric acids – – – 132 138 144Abietic acid – – – 92 96 78Neoabietic acid – – – 3 3 7

Total resin acids – – – 321 334 319

SterolsSitosterol 10 10 10 9 9 10Sitostanol 20 20 20 4 3 4

Total sterols 30 30 30 13 12 14

OthersDocosanol 7 7 7 20 20 20Tetracosanol 9 9 8 10 10 10Lupeol 40 40 40 – – –Betulinol 345 340 350 – – –Betula-7-prenol and betula-8-prenol 30 30 30 – – –Methyl betulinate 20 20 30 – – –

Total others 451 446 465 30 30 30

Total extractives 657 642 670 1186 1276 1281

Results expressed in�g/g. –: <3�g/g.

cated that the extractives were extensively removed fromthe region, either limited by the XPS analysis depth or theresidual concentration is below the detection limit for thistechnique[14]. Higher O/C ratios were observed in DIP-U in comparison with BK-U. After extraction, the O/C ra-

Table 2Results of XPS analyses of resin acids, oleic acid and stearic acid depositedon acetone-extracted cotton linter surfaces

Sample Measured O/C ratio Theoretical O/C ratio

Cotton linters 0.76 (0.02) 0.83a

Resin acids 0.44 (0.08) 0.10Oleic acid 0.23 (0.01) 0.11Stearic acid 0.23 (0.14) 0.11

Theoretical values are also presented.a Considering cotton linters as pure cellulose.

tios of DIP were higher than 0.83 due to the presence ofoxides.

The curve fitting of C 1s peaks of BK-U and DIP-U iden-tified C1, C2, C3 and C4 as carbon oxidation components(Fig. 3). A significant reduction of C1 area followed by anincrease in C2 and C3 due to extraction was observed forboth samples. However, relative areas of at least 4 and 18%of C1 were still observed after extraction for BKP and DIPsamples, respectively. This may have been due to a residuallayer of lignin, extractives or contaminants originating fromcontact with the environment or in the instrument vacuumchamber[15].

Estimations of surface coverage, done by using relativeC1 areas of unextracted and extracted samples, did not showsignificant differences between the Ac, AcH and AcP meth-ods for either pulp. This result indicated that in spite of the


Fig. 1. ToF-SIMS spectra, AFM and ToF-SIMS ion images of fatty and resin acids deposited onto TMP and cotton linters. The ToF-SIMS and AFM rastersizes were 200�m× 200�m and 3�m× 3�m, respectively.

Table 3XPS results of bleached kraft pulp (BKP) and deinked pulp (DIP) before and after extractions using acetone (Ac), acetone–acetic acid (AcH) and phosphatemethod (AcP)

Sample O/C φext C1 C2 C3 C4 θext

BKP-U 0.67 (0.02) 18.5 (3.0) 64.3 (2.3) 15.3 (0.3) 1.6 (0.2)BKP–Ac 0.82 (0.04) 20 (1.0) 5.3 (1.0) 73.8 (0.2) 18.9 (1.2) 1.9 (0.3) 15 (4.0)BKP–AcH 0.83 (0.02) 21 (1.0) 4.4 (0.2) 73.6 (1.6) 19.9 (1.0) 2.0 (0.4) 16 (4.0)BKP–AcP 0.83 (0.02) 21 (1.0) 4.3 (0.6) 74.6 (1.1) 19.5 (0.7) 1.6 (0.4) 16 (4.0)DIP-U 0.77 (0.04) 33.9 (2.3) 53.6 (1.2) 10.5 (1.0) 2.0 (0.3)DIP–Ac 1.02 (0.08) a 17.9 (3.0) 64.3 (2.0) 14.7 (0.8) 3.1 (1.0) 21 (5.0)DIP–AcA 1.06 (0.08) a 20.0 (2.4) 62.4 (2.5) 13.9 (0.1) 3.6 (0.8) 19 (5.0)DIP–AcP 1.13 (0.11) a 18.2 (2.3) 65.5 (1.4) 14.2 (1.8) 2.1 (2.0) 21 (5.0)

Surface coverage of extractives estimated using O/C ratios (φext) and relative areas of C1 (CC), C2 (C O), C3 (C=O and O C O) and C4 (O=C O) bycurve fitting of C 1s peak. Surface coverage of extractives was also estimated using the C1 relative areas (θext).

a Surface coverage using O/C ratio was not estimated due to presence of oxides.


Fig. 2. XPS low-resolution spectra of unextracted bleached kraft pulp (BKP-U) and deinked pulp (DIP-U).

higher bulk extraction yield for the DIP when the AcP methodwas used, the same was not observed for the surface. Theθextfor both pulps was also very similar. The estimation ofφextusing O/C ratios was only possible for sample BKP becauseof presence of oxides in DIP. Theφext results using O/C werecomparable as obtained by using C1 areas, but analytical vari-

F anda

ations in the latter were higher due to the curve fitting process.A tentative explanation for this difference is the ill-conditionof C 1s peaks in heterogeneous samples, i.e. in pulp samplesusing even FWHM of 0.9–1.2 eV, there is a clear overlap be-tween the peaks C1 and C2 due to the presence of multipletypes of C O bonds either as part of aromatics, aliphaticsor in carbohydrates (Fig. 2). This factor broadens the rangeof chemical shifts at a core level and makes estimation ofrelative areas more scattered[28].

3.2.3. Composition of extractives on fiber surfaces byToF-SIMS spectrometry

The composition of extractives on the unextracted fibersurfaces were analyzed by ToF-SIMS spectrometry. Onlypositive secondary ion spectra were used in ToF-SIMS be-cause of the low intensity observed for the characteristic ex-tractive peaks in the negative mode. Extractive secondary ionsin positive mode were quasimolecular ions of type [M + H]+,but dimers and trimers leading to [2M + H]+ and [3M + H]+

ions were also observed. Neutral losses from quasimolecularions were the most prominent fragmentation, especially elim-ination of small molecules such as H2O, HCOOH, HCOH,CO and CnH2n [37]. Sodium and calcium salts were detectedin positive mode due to formation of quasimolecular ion ofthe type [M − H + Na]+ and [M − H + Ca]2+, respectively.

i 27,4 Da),p acid( 283,

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ig. 3. XPS high-resolution spectra of unextracted (BKP-U, DIP-U)cetone–phosphate extracted (BKP–AcP, DIP–AcP) pulps.

The ToF-SIMS spectrum for the BKP-U (Fig. 4) showedons due to methyl betulinate (457 Da), betulinol (443, 411 Da), sitosterol (415, 397 Da), sitostanol (417, 399almitic acid (257, 239, 221, 207 Da), heptadecanoic271, 253 Da), stearic acid (285, 267 Da), oleic acid (

ig. 4. ToF-SIMS spectra of unextracted (BKP-U), acetoxtracted (BKP–Ac), acetone–acetic acid extracted (BKP–AcH),cetone–phosphate extracted (BKP–AcP) bleached kraft pulp.


265 Da), linoleic acid (281, 263 Da), arachidic acid (313,299 Da), behenic acid (341, 327 Da), and lignoceric acid(369, 355 Da). Calcium salts of palmitic acid (295, 551 Da)and stearic acid (325, 606 Da) were observed. Sodium salt oflignoceric acid was also present (391, 413 Da). After extrac-tion with acetone (BKP–Ac), peaks of fatty acid calcium salts(palmitic and stearic), fatty acids (palmitic, linoleic, stearic)and sterols were still observed in the ToF-SIMS spectrum. Ex-traction with acetone–acetic acid (BKP–AcH) removed fattyacid calcium salts, but peaks due to fatty acid were still ob-served, especially linoleic and lignoceric acids. After extrac-tion using the phosphate method (BKP–AcP) the ToF-SIMSspectrum showed extractive secondary ion peaks of very lowintensity, close to the background level (Fig. 4).

The ToF-SIMS spectrum for the DIP-U (Fig. 5) was muchmore complex than the spectra of BKP. Inorganic com-ponents such as CaCO3, clay and talc were identified inthe low-mass region. Peaks of Na, Mg, Al, Si, K and Cawere present in the high-intensity region. The spectral re-gion of extractives showed secondary ions from sitosterol(415, 397 Da), sitostanol (417, 399 Da), palmitic acid (257,239, 221, 207 Da), heptadecanoic acid (271, 253 Da), stearicacid (285, 267 Da), linoleic acid (281, 253 Da), arachidic acid(313, 299, 295 Da), behenic acid (341, 327, 323 Da), lig-noceric acid (369, 355 Da) and resin acids (303, 302 Da).C arica 91,4 tone( aks

F ne-ea

due to calcium salts of palmitic and stearic acids as wellas sodium salts of lignoceric acid was observed, but linoleicacid peaks were also present. Extraction with acetone–aceticacid (DIP–AcH) gave a similar spectrum as observed forDIP–Ac, but with lower peak intensities. After extractionwith the phosphate method (DIP–AcP), peaks due to fattyacid calcium salts, particularly from palmitic and stearicacids, and sodium lignocerate were still detected. A peak ofsodium–potassium lignocerate was also observed at 429 Da.The latter was a cationization product in consequence of ad-dition of K+ ions in the AcP method (Fig. 5).

Overall, the same components identified by extraction andGC were present in the ToF-SIMS spectra of unextractedsamples. Furthermore, typical peaks of siloxane (207, 221and 281 Da) and phthalates (77, 91, 149 and 161 Da) wereobserved in ToF-SIMS spectra for BKP and DIP, before andafter extraction. These contaminants are commonly presentin industrial samples[20]. Their peak intensities were signif-icantly reduced after different extractions for BK, but onlyslightly for the DIP, where they can probably originate fromcomponents of printing residuals or the contact with plasticsin the recycling process.

3.2.4. Distribution of extractives by ToF-SIMS imagingToF-SIMS imaging was employed to assess the distribu-

t tri-b andoi im-p erec anyi efully[ ina ntm ogra-p entsd sed.T KPa spotsw

isp edf waso factsa age.T talcw XPS.T rigino anyp e in-d sent.E nts( ed int ag-

alcium salts of palmitic acid (295, 551 Da) and stecid (325, 606 Da) and sodium salt of lignoceric acid (313 Da) were identified. Even after extraction with aceDIP–Ac), the spectrum changed. A dominance of pe

ig. 5. ToF-SIMS spectra of unextracted (DIP-U), aceto
xtracted (DIP–Ac), acetone–acetic acid extracted (DIP–AcH), andcetone–phosphate extracted (DIP–AcP) deinked pulps.
g ob-s TL.

ion of extractives in unextracted BKP and DIP. The disution of extractives (EXT), fatty acids (FA) and sterolsthers (STL) for BKP are presented inFig. 6. A total ion

mage (TI) was also taken. Artifacts due to effects of theact angle of the primary ion gun in the sputtering yield wlearly observed in the bright areas of the TI image, and

nterpretation regarding these areas should be done car34]. This artifact is intrinsic in the ToF-SIMS experimentsddition to differences in ionization probability of differeolecules and clusters, and effects of topography. Tophy effects are supposed to be removed in the experimue to the large angle of collection of the instrument uhe distributions of EXT, STL and FA were even for Bt the raster size investigated. Some agglomeration ofas observed in the EXT and FA images.The distribution of extractives and metals for the DIP

resented inFig. 7. A clear patchy distribution was observor all investigated metals, while an even distributionbserved for extractives and contaminants. Similar artis observed for the BKP could be observed in the TI imhe distributions of Al, Si and Mg showed that clay andere present and confirmed the previous observation byhe Ca image was due to calcium carbonate while the of Na is difficult to determine because it is present in maper chemicals as salts or counter-ions. The Fe imagicated that traces of toner pigments were probably preXT, FA, RA and STL were evenly distributed. Contamina

CT) such as siloxane and phthalates were also observhis investigation and showed an even distribution withlomeration in some spots. Similar agglomeration waserved in the FA and EXT images, but not in RA or S


Fig. 6. ToF-SIMS imaging of BKP-U. Images of total ions (TI), extractive ions (EXT), sterols (STL), and fatty acids (FA) were taken using raster size of200�m× 200�m and positive mode.

Fig. 7. ToF-SIMS imaging of DIP-U. Images of total ions (TI), metals (Al, Si, Mg, Na, Fe, Ca), extractive ions (EXT), fatty acids (FA), resin acids (RA),sterols(STL), and contaminants (CT) were taken using raster size of 200�m× 200�m and positive mode.


Fig. 8. ToF-SIMS images of BKP and DIP after extraction using the AcP method. Total ion (TI) and extractive ions (EXT) images were taken using raster sizeof 200�m× 200�m and positive mode.

Extractives and contaminants were distributed on both fibersand minerals, with no particular dominance.

The distribution of EXT after extraction using the AcPmethod for BKP and DIP are presented inFig. 8. A fewspots were observed in the EXT image of BKP, although noagglomeration was noted. In the case of DIP, the EXT wasevenly distributed although some agglomeration of spots wasobserved. Extractives were still located in both fibers andmineral particles, after extraction.

3.2.5. Nanostructure of fiber surfaces by AFMThe nanostructure and surface morphology were inves-

tigated with AFM. The AFM images showed that differ-ent morphological components were unevenly distributed onthe surface of the unextracted samples. The BKP-U surfaces(Fig. 9) clearly showed more fibrillar nanostructures with adiameter in the range of 20–50 nm than DIP-U ones (Fig. 10).This was in agreement with the XPS results (Table 2) wherethe amount of carbon C1, the most abundant group in extrac-tive molecules, was considerably higher for DIP-U than forBKP-U. Additionally, the DIP samples were found to con-tain various inorganic materials such as mineral particles. Inspite of that and the fact the surface morphology varied onthe micrometer scale, some common observations could bemade. In the images before any extraction, a large amount ofb rgeg de-

tected for BKP-U (Fig. 9) and DIP-U (Fig. 10). Additionally,a very thin layer (marked with VTL) and structures similar insize and shape as previously observed for stearic acid calciumsalt (marked with #) in cotton linters were detected, togetherwith some hydrophilic spots (marked with a rectangular) andsmooth surface regions (marked with **).

The particles of stearic acid (marked with #) were ob-served on BKP–Ac, BKP–AcH, DIP–Ac, and DIP–AcH sur-faces. However, after extraction with the phosphate methodthese structures were not observed for BKP–AcP (Fig. 9),indicating that this method could remove most of them asalso observed by ToF-SIMS imaging. In case of DIP–AcP(Fig. 10), some particles were still present. This observationwas also in agreement with the XPS and ToF-SIMS results.Artifacts due to the effects of local height gradient duringlateral scanning (temporarily loose contact) were clearly ob-served as dark areas (marked with⇒ in Fig. 9). Thus anyinterpretation regarding this region should be avoided.

The surface nanostructure of both pulp samples was af-fected by all of the extraction methods which decreased theamount and size of large granules. In most of the images ofextracted samples they were not observed at all. Similarly, noevidence of the presence of very thin layers was observed forany extracted samples, indicating that they may be formedby some type of extractives, probably oleic acid. Acetone ex-t gionsa d a

oth the small (<150 nm, marked with an arrow) and laranules (>150 nm, marked with an oval) were regularly

raction considerably increased the smooth surface rend the hydrophilic surface structures (marked with ** an


Fig. 9. AFM phase contrast images of BKP before (U) and after extractions using the Ac, AcH, and AcP methods. Different surface structures, such as thosesimilar in size and shape to the ones observed for stearic acid on cotton linter onFig. 1b, small (→) and large granules (marked with oval), very thin layers (VTL),hydrophilic spots (marked with rectangular) and smooth surface regions (**) were observed in BKP-U. The occurrence of small granules decreased drasticallyafter acetone (BKP–Ac) and acetone–acetic acid (BKP–AcH) extractions, but a similar effect was not observed for the acetone–phosphate (BKP–AcP) extractedsample. Artifacts were also observed (⇒). The Image size is 3�m× 3�m and theZ-scales are given in each image.

Fig. 10. AFM phase contrast images of DIP before (U) and after extraction using the Ac, AcH, and AcP methods. Different surface structures, such as thosesimilar in size and shape to the ones observed for stearic acid on cotton linter onFig. 1b (#) were present in DIP-U. These structures were not removed by acetone(DIP–Ac), acetone–acetic acid (DIP–AcH), acetone–phosphate (DIP–AcP) extractions. Artifacts were also observed (⇒). The image size is 3�m× 3�m, but2.3�m× 2.3�m for DIP-Ub. TheZ-scales are given in each image.


rectangular, respectively) for BKP–Ac (Fig. 9) and DIP–Ac(Fig. 10). The amount of small granules decreased drasticallywith Ac or AcH extraction for both samples, while the AcPmethod did not seem to have any considerable effect on theamount, although some pure cellulose surfaces were observedfor DIP–AcP (Fig. 10) and especially for BKP–AcP (Fig. 9).Since ToF-SIMS and XPS indicated that the extractives wereextensively removed from the BKP–AcP surfaces, thereforethe hydrophobic granules were suggested to be from contami-nants such as hydrocarbons and the hydrophilic granules fromhemicelluloses.

4. Discussion

4.1. Morphological and chemical heterogeneities inpulp fibers: facts and consequences

Pulp samples are heterogeneous multiphase systems com-posed of different cells, mineral particles, crystalline andamorphous cellulose, hemicelluloses, different extractivesand contaminants. These components are arranged in diversedomains which can be exposed to different extents forming acertain interface, i.e. a definition of a surface for this systemis therefore not often clear and much depends on the probingm a-r thet redf pho-t areasa daryi ionsi ters,t bulka am ofp elec-t

d tot How-e ere isn u-t in thee ho-t facep tionf alcu-l lec-t bersa encei ricte tionsa efi aket ativea ti-m with

ToF-SIMS and AFM, and thus strengthen the value of thesetechniques. An interesting aspect about ToF-SIMS is the ca-pability to identify artifacts such as topographic effects, ionbeam shadowing and effects of impact angle in the images.

It was clear that for both pulps (BKP, DIP) the differentextractions increased the O/C ratio and reduced the C1 rela-tive area in XPS. This observation has been reported for otherpulps using different XPS instruments[14–18]. In parallel,the ToF-SIMS results illustrated a significant decrease in ex-tractive secondary ions after extractions, particularly usingthe AcP method for the BKP. Another interesting observa-tion regarding the contamination present after extraction inboth pulps could be noted by the C1 estimation in XPS and theidentification of phthalates and siloxane ions in ToF-SIMS.The AFM results indicating the increase in fibrillar structuresafter extractions agreed with the information obtained by XPSand ToF-SIMS. This was unexpected considering that XPShad an assumed sampling depth of 5–10 nm while ToF-SIMSand AFM probed less than 1 nm. This was a evidence that theextractives and contaminants form a surface layer of a fewnm. The role of these low-molar mass components in the fibersurface is discussed in the next section.

4.2. Role of extractives in fiber surfaces

t ins s offi ex-t entlyb rn onfi tiono fact,t bec le ob-s byT2 theE lom-e tionso e de-s sultsp ble toa delsw r saltsh gre-g d byA event n ofd ld bef

KPa ctivec ence.A t thes identw . The

ethod[38]. The morphological complexity is another pameter that affects the surface definition. Accordingly,erm “surface composition” needs to be critically consideor pulp fibers, because the surface probed by ions andons, as investigated here, is an “average” of the exposedt certain conditions that yield higher intensities of secon

ons (ToF-SIMS) or photoelectrons (XPS). These conditnclude, among many physical and instrumental paramehe surface morphology and type of interaction betweennd adsorbents, the nature and geometry of incident behotons or ions, and the depth and yield of ejected photo

rons and secondary ions[39].In XPS, the take-off angle of photoelectrons is relate

he analytical depth of flat and homogeneous surfaces.ver, fibers are rough and heterogeneous. As a result thot a defined take-off angle[30], but a large angular distrib

ion indicating that at least three artifacts can be presentxperiment: (1) the ejection depth and the intensity of poelectrons vary with roughness and orientation of surlateaus; (2) shadowing of photoelectrons omits informa

rom valleys and depression areas; (3) changes in the cation of atomic concentration and variations in photoeron peak ratios. Besides the morphological factors, fire insulators, i.e. there is surface charging and differ

n work functions of different surface components. A stxperimental set-up was needed to handle these limitand obtain repeatable data[15,17]. The complexity of thber surface and the nature of the XPS experiments mhis characterization more an estimation than a quantitpproach for surface components[30]. Nevertheless, the esations of XPS obtained in this study were in agreement

Extractable material with low-molar mass is presenpecific cells or tissues in wood. The different processeber liberation affect this distribution in a way such thatractives are spread to other pulp components. It is currelieved that extractives have a patchy distribution patteber surfaces[14] and both models used here for estimaf surface coverage were based on this assumption. In

erms like “patchy” and “even” are empiric and shouldarefully considered because they depend on the scaerved. A good illustration is the EXT images obtainedoF-SIMS for BKP-U (Fig. 6) and DIP-U (Fig. 7): at the00�m× 200�m raster size, i.e. at micrometer scale,XT spots appeared evenly distributed even though aggration of spots could also be noted. These agglomerabserved at nanometer scale as achieved by AFM can bcribed as patchy. Because XPS, ToF-SIMS and AFM reroved the same tendencies for both pulps, it is reasonassume that the patchy distribution in XPS estimation moas correct. It seemed that saturated fatty acids and theiad a tendency for patchy distribution or a formation of agates while oleic acid formed a uniform layer as indicateFM. Considering this, one may expect that a patchy or

endency will depend on the composition and proportioifferent extractive components. This suggestion shou

urther investigated.The similarity in surface coverage of extractives for B

nd DIP samples, despite the difference in total extraontent, was unexpected and can be ascribed to a coincidctually, DIP showed a higher resistance to extraction aurface volume investigated than the BKP. This was evhen C1 areas of extracted samples were compared


reason for this resistance may be the presence of mineralssuch as CaCO3 and talc. These particles have a basic surfacecharacter[40,41] and adhesion of resin and fatty acids isconceivably favored, being a possible competitive attractionforce to the solvent extraction.

The fact that extractives are small molecules comparedto cellulose, lignin and hemicelluloses also needs to be con-sidered. It is assumed that carbohydrates are organized ina porous and high surface area fiber wall[42]. The surfaceenergy of this system is high[43] and the adhesion of com-pounds that lower the surface energy such as extractives andcontaminants are thermodynamically favored at the fiber–airinterface[44]. Extractives and contaminants can move freelythrough different regions in the fiber wall according to theinterface involved. After drying, and in air, it is reasonableto suppose that a fiber surface is promptly covered by theselow-molar mass compounds which migrate from differentfiber regions. After extraction, however, a residual layer ofextractives has a favored thermodynamic tendency to remainattached to the surface or be replaced by a contaminationlayer from the air. This can explain why C1 peaks were ob-served for XPS spectra of pure cellulose films in other inves-tigations[45]. It should also be stressed that ToF-SIMS andXPS operate at the fiber–vacuum interface and it is difficultto determine if any artifact was introduced by this feature.C ands , theg to bei

ofc trac-t chedk sug-g ables .

5

nos-t lps,i kedp anda ction.X ed.O tainsB s thee DIPs f thei nd pht s. Ext rdingt istri-b d andi oleica ical

heterogeneity of fiber surfaces allows XPS to only estimatethe surface components. This estimation agrees well withqualitative observations by ToF-SIMS and AFM imaging.The presence of extractives and contaminants is suggestedto be thermodynamically favored due to the reduction of thefiber surface energy in air. Our results proved that XPS, ToF-SIMS and AFM give valuable synergistic information andare adequate tools for fiber surface characterization.

Acknowledgements

This work is part of the activities at theAbo Akademi Pro-cess Chemistry Centre within the Finnish Centre of Excel-lence Programme (2000–2005) by the Academy of Finland.Johanna Gustafsson acknowledges the Graduate School ofMaterials Research for financial support. Paivi Kokkonen isacknowledged for her supply of TMP hand sheets.

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Extractives on fiber surfaces investigated by XPS, ToF-SIMS and AFM