Cs

Ka

b

c

h

•

•

•

•

•

a

ARRAA

KCWPHA

0h

Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 101– 108

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

jo ur nal ho me page: www.elsev ier .com/ locate /co lsur fa

omplex formation and stability of colloidal wood resin pitchuspensions with hemicellulose polymers

aren Stacka,∗, Roland Leea, Desmond Richardsonb, Trevor Lewisa, Gil Garnierc,∗∗

School of Chemistry, University of Tasmania, Hobart, Tasmania 7001, AustraliaNorske-Skog Paper Mills (Australia) Ltd, Boyer, Tasmania 7045, AustraliaDepartment of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia

i g h l i g h t s

Effect of wood polymers and Ca saltson stability of wood resin colloidsstudied.Wood polymers caused both stabili-sation and destabilisation of colloids.Wood polymers unable to stabilisecolloids at high Ca levels.Low affinity Langmuir type adsorp-tion occurred in the absence of salt.With 10 mM Ca Freundlich typeadsorption isotherm obtained.

g r a p h i c a l a b s t r a c t

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

1 10 100 100 0 10000

Log

(W)

wood pol ymers mg/g pitch

r t i c l e i n f o

rticle history:eceived 31 May 2013eceived in revised form 18 August 2013ccepted 27 August 2013vailable online 4 September 2013

eywords:olloid stabilityood resin

itchemicellulosesdsorption isotherm

a b s t r a c t

This paper explores the impact of the wood polymers released by Pinus radiata on the stability of woodresin colloids under varying ionic strength conditions with the addition of calcium chloride. A photomet-ric dispersion analyser was used to study the kinetics of aggregation under shear conditions. Additionof the wood polymers to an aqueous dispersion of wood resins was found to cause destabilisation ofthe wood extractive colloids in two stages, separated by an apparent stabilisation region. The behaviourobserved is typical of aggregation due to polymer bridging at low polymer addition, followed by steric sta-bilisation of the colloids at medium polymer concentration and depletion flocculation at higher polymeradditions and then finally depletion stabilisation. At 10 mM calcium ion concentration, destabilisationof the colloids occurred at low wood polymer addition, while at wood polymer concentrations above50 mg/L restabilisation of the colloids occurred at this level of calcium. The wood polymers were foundto be unable to stabilise the colloids at high calcium levels of 50 mM. At high calcium levels the woodpolymers themselves were found to undergo aggregation. The stabilisation of the colloids was found tobe affected by the way in which the colloids and wood polymer complexes are formed and allowed toreorganise with time.

Adsorption isotherms for the adsorption of the wood polymers onto wood resin coated surfaces weredetermined. A low affinity Langmuir isotherm was obtained in the absence of electrolyte indicating that

interaction between the wood polymers and the wood resin arises due to hydrogen bonding between the carboxylic acid groups despiteshowed that the wood polymethe negative charge of the wooto be modelled by a Freundlichfolding of the wood polymer a

∗ Corresponding author. Tel.: +61 3 6226 2169; fax: +61 3 6226 2858.∗∗ Corresponding author. Tel.: +61 3 9905 3456; fax: +61 3 9905 3413.

E-mail addresses: Karen.Stack@utas.edu.au (K. Stack), Gil.garnier@monash.edu.au (G.

927-7757/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rittp://dx.doi.org/10.1016/j.colsurfa.2013.08.057

the negative charge on both. The higher than expected loading capacityrs adsorb as aggregates which extend from the surface and act to screend resin colloids. In the presence of 10 mM CaCl2 the isotherm was found

isotherm with a greater amount of wood polymer being adsorbed due tond reduction in its volume.

Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

Garnier).

ghts reserved.

1 ysico

1

ipbcsscicvidfduT(sctrt

wtliTpsas[tghaiodsrabcsp

psoadsac

2

2

2

f

02 K. Stack et al. / Colloids and Surfaces A: Ph

. Introduction

An understanding of colloidal stability and the variables affect-ng it is important in many industrial applications. Colloidalarticles are formed in papermaking from the release of hydropho-ic wood resins from the wood fibres into the process water. Theolloidal particles formed are negatively charged with a particleize of typically 0.2–2 �m. Instability of these colloids results inticky deposits, known as pitch, that are detrimental to the pro-ess efficiency and product quality [1,2]. Many factors have beendentified as contributing to destabilisation of the wood resin pitcholloids, including pH, temperature, salt concentration and saltalency [3–8]. While much of the research has been undertakennvestigating wood resins from Norwegian spruce, recent worketails the investigation of the colloidal stability of wood resinsrom Pinus radiata. The effect of shear’, temperature and pH on theynamics of salt induced coagulation [9] along with multisalt coag-lation of P. radiata wood resin colloids have been reported [10].hese studies showed that the critical coagulation concentrationCCC) of the wood resin pitch colloids is significantly influenced byhear, temperature and the presence of other salts. Also under typi-al industrial conditions of imposed shear and the presence of morehan one type of salt, deviations from classical DLVO theory waseported. The results indicated that specific ion adsorption of mul-ivalent cations is occurring to reduce colloid surface charge [10].

Hydrophilic polysaccharide substances also released from theood during the mechanical pulping process have been shown

o interact with the wood resins. This significantly affects col-oidal stability by helping to stabilise the colloids against electrolytenduced aggregation and reduces deposition tendency [5,11–14].hese hydrophilic substances consist mainly of hemicelluloses, inarticular glucomannan, and galactoglucomannan, as well as amall amount of lignin and lignan [15]. The hemicelluloses form

“protective hydrogel layer” around the wood resin colloids thatterically stabilise them and prevent aggregation and deposition6,7,16]. The molar mass of the hemicelluloses was found to affecthe stabilising ability with increasing molar mass resulting inreater stabilisation [16]. Changes in the chemical structure of theemicelluloses, that occur during bleaching and alkali treatment,lso affect the stabilisation properties of polysaccharides releasedn pulping [11,16–19]. Peroxide bleaching and alkaline treatmentf the thermomechanical pulp (TMP) pulp were found to causeeacetylation of the hemicelluloses and demethylation and dis-olution of pectic acids [19]. The deacetylation of hemicellulosesesult in their removal from the process water due to being stronglydsorbed onto pulp fibres and thus reduce their ability to sta-ilise wood resin colloids [16,17]. Pectic acids are highly negativelyharged and adsorb onto the wood resin colloids increasing theirurface charge and increasing colloidal stability, however in theresence of CaCl2, aggregation occurs [17,18].

Little is understood about the stability of wood resins in theresence of extracted wood polysaccharides under the high ionictrength conditions typical of system closure, especially for speciesriginating from southern hemisphere wood types, such as P. radi-ta. This paper aims to investigate the effect of electrolytes andissolved wood polysaccharides from P. radiata on the colloidaltability of wood resins from P. radiata. A photometric dispersionnalyzer was used to study the kinetics of aggregation under shearonditions.

. Experimental

.1. Materials

.1.1. PulpThermo-mechanical pulp (TMP) from P. radiata was collected

rom the primary refiners at Norske Skog’s Boyer mill in Tasmania.

chem. Eng. Aspects 441 (2014) 101– 108

The pulp was freeze dried then soxhlet extracted with hexane toremove any hydrophobic wood resin pitch.

2.1.2. Wood resin pitch colloidal suspensionsThe hydrophobic wood resin fraction was extracted from

thermo-mechanical pulp by soxhlet extraction with hexane for 8 h.After removal of the hexane by rotary evaporation the oily woodresin was stored at −4 ◦C until required. Aqueous dispersions of thewood resin were prepared by dissolving the wood resin in acetone(99.5% purity) and adding a small amount of the acetone solutionto distilled water containing 1 mM KNO3, with pH adjusted to 5.5.The dispersion was stirred for 10 minutes using a magnetic stirrerat 500 rev/min. After stirring, the dispersion was dialysed for 24 hto remove the acetone using cellulose membrane tubing (SigmaD-9402, 76 mm wide, >12,000 MW).

2.1.3. Preparation of extracted wood polymersExtracted wood polymers were obtained using soxhlet

extracted pulp from P. radiata TMP following the procedure of Orsa[20]. The pulp was disintegrated in water to 2% consistency at 60 ◦Cfor 3 h and then filtered. The filtrate was used to dilute more pulpand repeat the extraction process. This procedure was repeated upto 5 times in order to concentrate the filtrate. The filtrate was thenfiltered through 0.45 �m filter paper to obtain the dissolved organicwood polymers. This dispersion was dialysed for 24 h in 1 mM KClwith pH adjusted to 5.5 to remove impurities using a cellulosemembrane tubing (Sigma D-9402, 76 mm wide, >12,000 MW).

2.2. Methods

2.2.1. NMR of extracted wood polymersAll NMR experiments were carried out using a VARIAN 400 MHz

Nuclear Magnetic Spectrometer. Pre-saturation of the water peakwas carried out prior to each NMR analysis. Studies were done inaqueous media with D2O as the internal standard.

2.2.2. Lignin analysis of wood polymersFor lignin analysis, the sample was prepared by performing a

1 in 10 dilution of the wood polymers solution. UV absorbance at205 nm was measured using a Shimadzu UV-160 spectrophotome-ter with 1 cm quartz sample cell and an extinction coefficient ofε = 110.0 L g−1 cm−1 [21]. The lignin concentration was determinedusing the Beer–Lambert equation.

2.2.3. Total carbohydrates of wood polymersTotal carbohydrate concentration was determined using the

Orcinol method [22]. Orcinol reagent was prepared by dissolving0.2 g of orcinol in 100 mL of concentrated sulphuric acid. The sam-ples were prepared by performing a 1 in 25 dilution of the woodpolymer sample with distilled water. Calibration curves were pre-pared using glucose solutions in distilled water at concentrationsof 0, 10, 30, 60 and 100 mg/L.

Wood polymer samples (1.00 mL) were pipetted into Teflontest tubes and made up to a final volume of 3.0 mL with orci-nol reagent. During the orcinol reagent addition, solutions werecooled with cold water. The solution was then placed in an 80 ◦Cwater bath for 15 min, then allowed to cool to room temperatureSample absorbance was measured with a Shimadzu UV-Vis 160at 540 nm immediately after cooling. Glucose standards and blankwere treated in the same manner as the samples.

2.2.4. Methanolysis and gas chromatography (GC) of wood

polymers

Analysis of the total wood polymer (hemicellulose) content andits composition was undertaken using the method of Sundberget al. [23]. Acid methanolysis reagent was prepared by adding 14 mL

ysicoc

ai

opttio

3fbroad

maaoIpttappop

2

tMn

2

Aatsdta

2

ot–upcoda

ddS

K. Stack et al. / Colloids and Surfaces A: Ph

cetyl chloride to 86 mL of dried methanol in an ice bath. The result-ng acid methanolysis reagent was stored at −24 ◦C.

Wood polymer samples (2.0 mL) were weighed and freeze-driedvernight. Methanolysis reagent (3.0 mL) was added to the dry sam-les and solutions were stirred for 5 h at 70 ◦C. The samples wherehen cooled to room temperature and 100 �L of pyridine was addedo neutralise the solution. Dry methanol solution (4.0 mL) contain-ng 0.1 g/L sorbital was then added to each solution. A 1.0 mL aliquotf the resulting solution was removed and blown to dryness.

The dried samples were silylated with 100 �L of pyridine and00 �L of bis-trimethylsilyl acetamide (BSA) and heated at 60 ◦Cor 20 min. All samples were made up to 1.0 mL with toluene. Sor-itol was used as the internal standard for quantification. Sugaretention times and peak assignments were found from standardsf mannose, glucose, galactose, glucouronic acid and galactouroniccid. Standards where methylated and silylated in the same methodescribed for the samples.

The concentration of the silylated monomeric sugars was deter-ined using a Varian 3800 GC equipped with a Varian 8400

uto sampler, programmed temperature on-column injector and flame ionisation detector using. Samples (1 �L) were injectednto a Phenomenex® 100% polydimethylsiloxane (15 m × 0.53 mm.D. × 0.15 �m FT) ZebronTM capillary GC column. The injector tem-erature was held at 90 ◦C for the first 0.5 min after injection andhen increased to 260 ◦C at a rate of 200 ◦C/min. The column ovenemperature was held at 90 ◦C for the first 1.5 min after injectionnd then increased to 130 ◦C at a rate of 20 ◦C/min. The FID tem-erature was held at 290 ◦C for approximately 24 min. Ultra highurity helium was used as the carrier gas and a constant flow ratef 3.0 ml/min. GC data was analysed using the Varian Star softwareackage.

.2.5. Cationic demandWood polymer charge density was determined at a pH of 5.5 by

itration of 10 mL of the sample with 0.001 N polyDADMAC using autek PCD-02 (Particle Charge Detector) to determine the charge

eutralisation endpoint.

.2.6. Zeta potentialZeta potential was measured using a Malvern Zetasizer Nano ZS.

100 mg/L aqueous wood resin pitch dispersion was prepared bydding hexane extracted wood resin dissolved in acetone to dis-illed water containing 1 mM KNO3 with pH adjusted to 5.5 andtirring for 10 min at 500 rpm. Subsamples of this dispersion wereiluted to 50 mg/L at varying extracted wood polymer concentra-ions. Measurements on the Zetasizer were performed immediatelyfter each addition of wood polymer.

.2.7. Wood polymers molecular weight distributionHigh performance size-exclusion chromatography was carried

ut on a Waters – 2695 separation unit using a Waters differen-ial refractometer detector (Waters Assoc.) and an Aqua Pore OH

100 �m lot 1817 column (USA Brownlee labs) at 50 ◦C. This col-mn was found to be suited to the size range of the extracted woodolymers in the crude sample. The standards were made to a con-entration of 500 mg/L and the sample at 800 mg/L. The volumef the injected sample was about 50 �L. The eluent was de-ionisedistilled water at a flow rate of 0.8 mL/min. The signal was recordednd processed by Empower Pro software (Waters Assoc.).

Molecular weight calibration of this system was achieved using (+) galactose (BDH Chemicals Ltd.) and a set of fractionatedextran standards (5.22 kDa, 11.6 kDa, 23.8 kDa, and 48.6 kDa,igma–Aldrich).

hem. Eng. Aspects 441 (2014) 101– 108 103

2.2.8. PDA aggregation analysisA Photometric Dispersion Analyzer (PDA 2000, Rank Brothers,

UK) was used to monitor changes in aggregation of colloidal woodresin pitch dispersions and also the wood polymers. Samples werestirred continuously using a 3 cm radius flat blade impeller at500 rpm. The samples were pumped to the PDA by a Cole PalmerMasterflex L/S peristaltic pump at a flow rate of 70 mL/min. Thesamples were recirculated back to the sample vessel using 3 mmtubing. The instrument was initially calibrated with distilled waterand the DC gain control was adjusted to give a DC value of 10 V assuggested in the operating manual and Hopkins and Ducoste [24].

The stability ratio, W, was determined as the change in slopefor the initial growth of colloidal particles [25,26]. The slope isconverted to W with the use of Eq. (1):

W = K∗K1

(1)

where K* is the fastest rate of coagulation and K1 is the rate ofcoagulation of interest.

2.2.9. Viscosity measurementsViscosity measurements were performed on 5.0 g/L extracted

wood polymer solutions using an Ostwald viscometer. A measure-ment of the time for a wood polymer solution of known CaCl2concentration in relation to the time for water to pass the top andbottom timing lines can be used to calculate the relative viscosity(�r) of each solution.

�r = Tpolymer

Twater(2)

where �r is the relative viscosity, Tpolymer is the time required forwood polymer solutions to pass through the Ostwald viscometerand Twater is the time required for water to pass through the Ostwaldviscometer. Each measurement was done in triplicate and averagevalues are reported.

2.2.10. Adsorption of wood polymers onto wood resin coatedslides

Hydrophobised glass slides (Chase Scientific Glass MicroscopeCover Glass, 22 mm × 22 mm) were prepared by immersing theglass slides in a 50/50 solution of trimethylchlorosilane (TMCS)(≥99% Sigma–Aldrich) and pyridine (≥99% Sigma–Aldrich) for 12 h.The slides were then cleaned with hexane (99.8% Sigma–Aldrich)and air dried.

Colloidal wood resins were adsorbed onto the surface ofhydrophobised glass slides by immersion of the slides into an aque-ous wood resin pitch dispersion for 24 h. The adsorbed mass ofwood resins was determined via the change in the solution con-centration with the use of the established GC method. The resultingsurfaces were placed into dissolved wood polymer solutions atvarious concentrations (wood polymer concentrations were deter-mined with the orcinol method) and allowed to stand for 24 h. Thedifference in the concentration of wood polymers was determinedto be the mass of wood polymers adsorbed to the wood resinssurface. It was assumed that complete coverage of the surfacesoccurred and the surface area for the wood resins was determinedto be the surface area of the glass slides (4.84 × 10−4 m2).

3. Results

3.1. Characterisation of the system

3.1.1. PitchThe chemical composition (determined by gas chromatogra-

phy), size and charge of the colloidal particles formed by theaqueous P. radiata wood resin pitch dispersion are given in Table 1.

104 K. Stack et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 101– 108

Table 1Characterisation of P. radiata wood resin pitch colloids.

Chemical composition•Fatty acid 1.44 mg/g dry fibre•Resin acid 6.17 mg/g dry fibre•Triglycerides 5.89 mg/g dry fibre

Av. particle diametera 800 nmZeta potentiala −50 mV

a 50 mg/L wood resin pitch dispersion, pH 5.5 and 1 mM KNO3.

Table 2Composition of wood polymers extracted from P. radiata TMP.

Average wood polymer concentration 873 mg/LCarbohydrate 95 ± 2%Lignin 7.6 ± 0.2%Total sugars 72 ± 10%Cationic demand mmol/g equiv charge [16] 0.42Galactose:glucose:mannose:uronic acid ratio 0.4:1:2.4:0.6Average molecular weight 16.3 kDa

3

(tldi7poTttfrote

mTTctwi2

FG

pitch. After polymer addition to the wood resin pitch suspension(at 450 s), three distinct regions were obvious: a region of linearincrease of the signal to a maximum, a region of exponential decay,

.1.2. Water soluble wood polymersThe water-soluble wood polymer fraction was characterised

Table 2). The polysaccharide fraction represents around 95% ofhe mass with 12% being uronic acids and 88% being hemicellu-oses. A small amount of lignin oligomers of approximately 8% wasetected. The total yield of sugar monomers was only 72%. This

s similar to literature values using the same technique in which7–79% total yield of sugar monomers was obtained [27]. It wasroposed that the reasons for this are due to incomplete cleavagef the hemicellulose bonds and/or degradation of the monomers.he saccharide ratio of galactose: glucose: mannose was foundo be 0.4:1:2.4 using gas chromatography. These values are lowerhan literature values for dissolved and colloidal substances (DCS)rom TMP Spruce of 3–4:1:0.5–1 and also the values previouslyeported for the water soluble galactoglucomannans from P. radiataf 4.5:1.0:1.3 [28]. The wood polymers also bear a slight nega-ive charge as demonstrated by the cationic demand of 0.42 mmolquivalent/g polymer.

The extracted wood polymer was found to have a polydispersedolecular weight distribution ranging from 1.0 to 500 kDa (Fig. 1).

he majority of the hemicellulose is between 5 kDa and 50 kDa.he average molar mass for the extracted wood polymers is cal-ulated to be 16.3 kDa. This equates to about 92 sugar units inhe polysaccharide chain. Dialysis of the extracted wood polymersould remove the low molecular weight fraction up to 12 kDa thus

ncreasing the average molecular weight of the wood polymers to5–30 kDa.

ig. 1. Water soluble wood polymer molecular weight distribution measured byPC.

Fig. 2. Effect of CaCl2 on the dynamics of wood polymers destabilisation andrearrangement (wood polymer concentration = 360 mg/L).

3.2. Wood polymers and pitch complex

3.2.1. Effect of ionic strength on wood polymersA PDA was employed to investigate the aggregation behaviour

and colloidal properties of the extracted wood polymers at pH 5.5and room temperature (Fig. 2). In the absence of CaCl2, the PDAsignal is relatively flat, indicating that no aggregation is occurringand that the extracted wood polymers are stable under the shearconditions imposed. Upon addition of 10 mM CaCl2, a small increasein the PDA signal occurs, though the suspension remained clear.This response represents an aggregation or reorganisation of thewood polymers. The plateau in the signal following the aggregationof the wood polymers indicates that the aggregated polymers arestable to deposition, disintegration and further coagulation. Theaddition of 20 mM CaCl2 resulted in a significant increase in thePDA signal, with a clear peak evident around 600 s.

The effect of calcium chloride concentration on the wood poly-mer relative viscosity was quantified (Fig. 3). The wood polymerrelative viscosity decreased with increasing CaCl2 addition until asignificant reproducible peak was observed at 20 mM CaCl2 whichwas followed by a gradual increase with continued CaCl2 addition.

3.3. Effect of wood polymers concentration on wood resin pitchstability

The effect of wood polymers concentration on the wood resinpitch colloidal stability was measured by PDA as a function oftime (Fig. 4). The wood polymers concentration ranged over nearlythree orders of magnitude, from 0 to 820 mg/L or 0 to 8.2 mg/mg

and a plateau. The slope, the maximum reached and the plateau

Fig. 3. Effect of salt concentration on the relative viscosity of wood polymers.

K. Stack et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 441 (2014) 101– 108 105

Fc

art

cdmplavadflppmprpi

3r

fT−

3

wa

Fig. 6. Effect of wood polymer on the zeta potential of wood resin pitch colloids.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

8006004002000

Woo

dpo

lymersa

dsorbe

d(g/m

2)

Equil Conc. wood polymers (mg/L)

10 mM Ca

0 mM Ca

ig. 4. Effect of wood polymers concentration on the dynamics of colloidal pitchoagulation, pitch concentration = 100 mg/L. The polymer was added at 450 s.

re all function of polymer concentration; the highest values wereeached at a polymer concentration of 410 mg/L; that correspondso a wood polymer–pitch ratio of 4.1 mg/mg.

To quantify the effect of wood polymers concentration on theolloidal wood resin pitch stability the stability factor, W, wasetermined from the initial slopes in Fig. 4 for each wood poly-er concentration. Fig. 5 plots log W as a function of the wood

olymer concentration plotted on a logarithmic scale. In this figure,og W = 0 indicates the highest flocculation rate (ki = kfast), pitch is

stable colloid at the high log W values, and unstable at the lowalues. The original pitch suspension is stable. Four distinct regionsre observed in Fig. 5. In the first region, pitch stability steadilyecreases with wood polymer concentration to reach maximumocculation at a polymer concentration of 100 mg wood polymerer g pitch. In the second region, ranging from 100 to 900 mg/g,itch stability increases to reach again full stability at 900 mg poly-er per g pitch. In the third region, extending from 900 mg per g

itch to 7000 mg polymer per g pitch, pitch stability decreases toeach a second minima indicating full flocculation at 7 g polymerer g pitch. At higher polymer concentration, >7 g/g, pitch stability

ncreases, representing the fourth region.

.3.1. Effect of wood polymers on charge (zeta potential) of woodesin pitch colloidal particles

The addition of wood polymers to wood resin pitch colloids wasound to reduce the charge on the wood resin pitch colloids (Fig. 6).he charge was found to reach a constant value of approximately10 mV after the addition of 2 g wood polymer per g pitch.

.3.2. Adsorption of wood polymers onto wood resinThe adsorption of the dissolved dialysed wood polymers onto

ood resin coated slides is shown in Fig. 7. An increase in themount of dissolved wood polymers adsorbed onto the wood resin

Fig. 5. Effect of wood polymers concentration on colloidal pitch stability.

Fig. 7. Adsorption of dialysed wood polymers onto wood resin coated slides withand without calcium salt.

coated slides was observed with the addition of 10 mM calciumions.

The Langmuir and Freundlich equations were applied to deter-mine the best model for the adsorption isotherms. Parameters forthe two models are shown in Table 3. The experiments undertakenin the absence of added calcium ions were found to follow a Lang-muir isotherm. The Langmuir adsorption isotherm equation is givenby Eq. (3):

C

Q= 1

QmKL+ 1

QmC (3)

where C is the concentration of water-soluble wood polymers atequilibrium (mg/L), Q is the amount of wood polymers adsorbed perunit mass of the wood resins (g/g), Qm is the maximum adsorptioncapacity (g/g) and KL is the Langmuir constant related to the affinityof the binding sites and the energy of adsorption.

The solutions containing 10 mM calcium ions were found to

follow the Freundlich model. The Freundlich equation is given byEq. (4)

Q = KF C1/n (4)

Table 3Langmuir and Freundlich isotherm parameters for fitting models to the experimen-tal adsorption isotherms of dissolved wood polymers onto wood resin coated glassslides.

Langmuir model Freundlich model

Qm (g/m2) KL (L/mg) R2 n KF (mg/m2) R2

0 mM Ca 0.8 0.0275 0.92 2.72 53 0.8110 mM Ca 2.2 0.018 0.58 1.21 6 0.91

106 K. Stack et al. / Colloids and Surfaces A: Physico

Ff(

wlci

3

omaspt

omaamc

mfo

wioo

Ffo

ig. 8. Effect of extracted wood polymers concentration on the stability of pre-ormed pitch/extracted wood polymers complexes upon addition of 10 mM CaCl2above pitch CCC).

here Q is the amount of adsorbed wood polymers, C is the equi-ibrium concentration of the wood polymers, KF is the Freundlichonstant which is related to the maximum loading capacity and ns a parameter related to the intensity of the adsorption with n > 1.

.4. Colloidal stability of pitch–wood polymer complex

Complexes were formed by mixing the required concentrationf wood polymer with a 100 mg/L wood resin pitch suspension,ixing for 15 min then dialysing the solution for 24 h to remove

cetone from the wood resin pitch dispersion. A concentratedalt solution, sufficient to reach 10 mM CaCl2, was added to thereformed pitch-polymer suspension, and their stability was quan-ified as a function of time with the PDA.

A stability curve (Fig. 8) was generated from the initial slopesf the PDA plots with various pitch additions. At low wood poly-er concentrations, the colloidal complexes are destabilised upon

ddition of electrolyte, as shown by the decrease in log W to 0 atpproximately 500 mg/g wood resin. As the extracted wood poly-ers concentration is increased above 4000 mg/g wood resin the

olloids restabilise.Fig. 9 compares the effect of pre-forming the pitch/wood poly-

er complex (complex colloids formed over 12 h) and newlyorming the pitch wood polymer complex (complex colloids formedver 15 min) at 50 mM CaCl2.

In the presence of 50 mM CaCl2, the addition of wood polymer

as found to cause only destabilisation of the colloids. The restabil-

sation of the colloid that was evident in the absence and presencef 10 mM CaCl2 did not occur at 50 mM CaCl2 addition. The meansf forming the wood polymer and wood resin pitch colloids and

ig. 9. Effect of extracted wood polymers concentration on the stability of pre-ormed and newly formed pitch/extracted wood polymers complexes upon additionf 50 mM CaCl2 (above pitch CCC).

chem. Eng. Aspects 441 (2014) 101– 108

the length of time allowed for the colloids to reorganise is shownto affect the destabilisation of the colloids. Comparison of the newlyformed pitch wood polymers with the preformed wood polymers,in which the wood polymers were allowed to reach their equi-librium conformation on the pitch colloids, reveals that it takesfour times more wood polymer to reach maximum flocculationwith newly formed pitch/wood polymer complexes than with agedcomplexes. At the very high electrolyte concentration the adsorbedcollapsed wood polymer chains seem to have a lower surface cov-erage at a given concentration when newly formed than when thecolloids are allowed to reorganise over a period of time.

4. Discussion

4.1. System characterisation

4.1.1. Soluble wood polymersThe 1H NMR spectra showed that the extracted wood poly-

mers from P. radiata are similar in composition to other northernand southern hemisphere wood types [29]. The water-solublewood polymer fraction mostly consists of hemicellulose com-ponents (95%), predominantly galactoglucomannans, and ligninfragments (5–8 wt%). The lignin fraction, made of propyl phenolmonomers, consists of very short oligomers with a Mw < 1000 Dato remain water soluble [30]. The sugar composition of the P. radi-ata wood polymers is slightly different from that reported forSpruce using similar extraction methods and temperatures. Thehemicelluloses extracted from TMP from P. radiata has a lowermannose to glucose ratio (2.4 in Table 2) compared to 4 to 1 forSpruce [20,31]. Galactoglucommanans consist of a backbone of�-d-(1→4)-glucopyranose and �-d-(1→4)-mannopyranose unitswith side-chains of �-d-galactopyranose. It was noted by Willföret al. that the ratio of galactose to glucose varies from 0.1 to 1.4and mannose to galactose 3.4–5.4 in Spruce [32] depending on thetemperature and time of extraction.

The wood polymers fraction is highly polydispersed with abroad range of molecular weights. The lower molecular weightfractions (Mw < 10 kDa) consists of smaller chain sugars and pecticacids as shown by the presence of uronic acid (Table 2). Pectic acidsisolated from Spruce TMP have an average MW of 5.9 kDa [33] anda polydispersity of 3.3. The average molecular weight of the P. radi-ata TMP wood polymers of 16.3 kDa is lower than those reportedwhich vary from 29 to 60 kDa [31–33]. The dialysed wood poly-mers have significantly higher average molecular weight, closer to25–30 kDa.

The wood polymers were found to undergo aggregation withthe addition of electrolyte. At higher electrolyte addition the PDAsignal decreases after the initial aggregation (Fig. 2). This decreasecan be attributed to a number of causes. Under the shear condi-tions in the PDA, break-up of the flocs may be occurring due to theweak bonding holding together flocs [24]. Collapse of the polymerchains occurs with the addition of electrolyte which also influencesthe PDA signal. The viscosity measurements (Fig. 3) support thedecrease in polymer size due to shrinking of the polymer chainsand decrease in the effective volume fraction on the addition ofsalt. This behaviour has been reported for guar gum (a polysaccha-ride very similar to the wood polymers) [34] and other polymers[35–39].

Wood polymer solution viscosity slightly increases at 20 mMCaCl2 addition (Fig. 3). This increase corresponds to an increase in

the hydrodynamic radius of the polymer and indicates a change inthe intermolecular structure and network of the wood polymers.The increase may indicate the formation of a network of coagu-lated material due to screening of the weak charge on the woodpolymers.

ysicoc

4

whtp[hsPfiwrataFaAsapotlh

wctBsgfb

woTettag(lrhwm

cttcltm

4

it

K. Stack et al. / Colloids and Surfaces A: Ph

.2. Wood polymer and pitch complex

Research into the stabilisation and destabilisation of lipophilicood resin pitch colloids from unbleached Norwegian Spruce TMPas shown that the colloids are relatively stable against elec-rostatic induced aggregation because the water soluble woodolymers in the pulp stabilise the wood resin pitch particles2,12–15,17,40,5]. Ratios of 0.5:2 wood polymers to wood resinave been found to stabilise the wood resin pitch colloids againstalt induced aggregation. In the current work with wood resin from. radiata, a ratio of 0.9 (900 mg/g wood resin) (Fig. 5) was alsoound to stabilise the colloids. The important observation from thiss that aggregation occurs at low wood polymer addition (10 mg/g

ood resin) and at higher wood polymer addition of 7 g/g woodesin followed by a restabilisation at even higher wood polymerdditions. This behaviour is typical of polymer bridging floccula-ion at low additions, followed by steric stabilisation with polymersdsorbing onto the surface of colloids and covering them [41,42].urther addition results in depletion flocculation arising from non-dsorbed polymers and an osmotic pressure effect as proposed bysakura and Oosawa [43]. At even higher concentrations stabili-ation occurs again. This effect known as “depletion stabilisation”rises from the increase in free volume of the un-adsorbed polymerreventing colloids approaching each other. Direct measurementsf the depletion forces have been made using AFM [44]. We believehis is the first report of full stability-aggregation behaviour of col-oidal wood resin pitch occurring over four orders of magnitude ofemicellulose polymer concentration.

The adsorption isotherm for wood polymers adsorbing ontoood resin coated slides in the absence of salt in Fig. 7 can be

lassified as low affinity Langmuir isotherm [45], indicating thathe wood polymers are weakly adsorbed onto the wood resins.oth the wood polymers and wood resin are negatively chargedo the adsorption is driven by weak attractive forces due to hydro-en bonding between the carboxylic acid groups and van der Waalsorces. At the pH of the experiments (pH 5.5) both polymers woulde partially deprotonated.

The maximum loading capacity of the wood polymers on theood resin coated slides was found to be 0.8 g/m2 in the absence

f calcium salt and 2.2 g/m2 in the presence of 10 mM calcium salt.he loading capacity is significantly higher than the 0.1–1 mg/m2

xpected for molecular monolayer coverage [42]. This suggests thathe wood polymers are not forming a molecular monolayer onhe surface but are adsorbing as aggregates. The assumed surfacerea is very likely significantly higher than the surface area of thelass plate. The amount of wood resin adsorbed onto the slides1.3 g/m2) is also significantly higher than the expected molecu-ar monolayer coverage indicating a relatively thick layer of woodesin on the glass slide. Surface roughness and heterogeneity mayave increased the surface area. Further investigation is required,hich might involve measuring pitch adsorption by quartz crystalicrobalance (QCM-D) with a pitch-coated sensor.The adsorption of the wood polymers onto the wood resin pitch

olloids resulted in a shifting of the plane of shear and reduction inhe zeta potential as shown in Fig. 6. The zeta potential was foundo decrease to −10 mV at additions of 2 mg/mg wood resin. At theseharge measurements the thickness of the wood polymer adsorbedayer is sufficient to overcome the repulsive force between the par-icles and allow aggregation to occur through a polymer bridging

echanism.

.3. Colloidal stability of pitch wood polymer colloids

Stabilisation of the colloidal wood resins by the wood polymerss dependent on a number of different variables. For example, theime allowed for the stabilised colloids to form and the manner

hem. Eng. Aspects 441 (2014) 101– 108 107

in which they are formed (with or without shear) influences theeffect of salt. Furthermore, the concentration of electrolyte addedto solution affects the stability of the stabilised colloids. Calcium(Ca2+) was investigated in this study; adding calcium ions canaffect pitch stability in two ways. First, by increasing ionic strength,which decreases the thickness of the electrical double layer; second,through chemical reaction with the carboxylic groups of pitch.

In the presence of 10 mM calcium salt the adsorption no longerfollowed a Langmuir isotherm but instead fitted a Freundlichisotherm, indicating that the available sites for adsorption wereheterogeneous and that some interaction between adsorbing par-ticles with nearby occupied sites was occurring. Significantly morewood polymer was adsorbed onto the wood resin coated slidesunder these conditions. Screening of the wood resin charge by thesalt would have occurred allowing for a greater interaction anddeposition of wood polymers. The collapse and decrease in volumeof the wood polymers molecules by salt would contribute to morepolymer being adsorbed onto the surface.

The addition of 10 mM calcium salt to wood resin colloids sta-bilised with wood polymers significantly affected the colloidalstability. Only two distinct stability regions can be observed inFig. 9 compared to the four regions observed in the absence ofadded calcium salt (Fig. 5). Destabilisation of the colloids occurredat higher wood polymer additions followed by stabilisation of thecolloids due to weak hydrogen bonding between the adsorbed poly-mer coated colloids. No depletion flocculation and stabilisation wasobserved. This may result from less free (un-adsorbed) polymer atthe same wood polymer additions combined with the effect salthas on the free volume of the wood polymers (both adsorbed andunadsorbed). At 50 mM calcium salt addition only destabilisation ofthe colloid occurred. This salt concentration is above the pitch crit-ical coagulation concentration (CCC). Above the CCC, it is expectedthat the rate of coagulation remains unaffected by changes to saltconcentration.

5. Conclusion

Dissolved wood polymers released from P. radiata TMP pulp,consist mostly of hemicelluloses which are large oligomers of galac-toglucomannans. Upon addition of CaCl2, these wood polymersprecipitate into colloids which can further coagulate; the woodpolymer chains can also fold in the same way that is noted for guarwith the addition of sodium salts.

Wood polymers can stabilise P. radiata colloidal pitch againstelectrolyte-induced aggregation. However, pitch stability is acomplex function which varies widely upon hemicellulose con-centration – with or without electrolytes. In the absence of addedelectrolyte, low concentrations of wood polymer actually desta-bilise wood resins, while at higher wood polymer concentration,the wood resin pitch colloids are completely stabilised. This indi-cates steric stabilisation through polymer adsorption. At evenhigher wood polymer concentration, the wood resin pitch colloidscoagulate again, very likely by depletion coagulation.

The effect of wood polymer adsorption on colloidal resin pitchis affected by both ionic strength and time. Wood polymers adsorband reorganise into more compact coils at high ionic strengthand lower rearrangement time. However, steric stabilisation wasachieved at high wood polymer concentration only for salt concen-trations of 10 mM CaCl2. At very high salt concentrations (50 mMCaCl2) the wood polymers were unable to stabilise the wood extrac-tive colloids.

In the absence of salt the adsorption of the wood polymers

onto wood resin coated surfaces follows a low affinity Langmuirisotherm, while in the presence of 10 mM CaCl2, the adsorptionisotherm follows a Frendlich isotherm with a greater amount ofwood polymer being adsorbed due to the collapse and reduced

1 ysico

vbcbo

A

PPSU

R

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[force microscope, J. Colloid Interface Sci. 170 (1995) 604–606.

08 K. Stack et al. / Colloids and Surfaces A: Ph

olume of the wood polymers and screening of the charges onoth the polymers and wood resin colloids. The addition of hemi-ellulose polymer to wood colloidal pitch represents a uniqueiological system where the full stabilisation–destabilisation isbserved over 4 orders of magnitude of polymer concentration.

cknowledgements

Financial support for this project was provided by Norske-Skogaper and an Australian Research Council Linkage Grant LP882355.rofessor Pavel Nesterenko is acknowledged for his help with theEC along with Dr. James Horne, of Central Science Laboratories atniversity of Tasmania, for his assistance with the NMR.

eferences

[1] L.H. Allen, Pitch in wood pulp, Pulp Pap. Canada 76 (1975) 70–77.[2] L.H. Allen, Characterization of colloidal wood resin in newsprint pulps, Colloids

Polym. Sci. 257 (1979) 533–538.[3] L.H. Allen, The importance of pH in controlling metal-soap deposition, Tappi J.

71 (1988) 61–64.[4] T. Hassler, Pitch deposition in papermaking and the function of pitch-control

agent, Tappi J. 71 (1988) 195–201.[5] K. Sundberg, J. Thornton, R. Ekman, B. Holmbom, Interactions between simple

electrolytes and dissolved and colloidal substances in mechanical pulp, NordicPulp Pap. Res. J. 9 (1994) 125–128.

[6] K. Sundberg, B. Holmbom, Destabilization of colloidal wood resin caused bycellulosic fibers in thermomechanical pulp suspensions, Paperi ja Puu 79 (1997)50–54.

[7] A.L. Sihvonen, K. Sundberg, A. Sundberg, B. Holmbom, Stability and depo-sition tendency of colloidal wood resin, Nordic Pulp Pap. Res. J. 13 (1998)64–67.

[8] D.S. McLean, K.R. Stack, D.E. Richardson, The effect of wood extractives com-position, pH and temperature on pitch deposition, Appita J. 58 (52–55) (2005)76.

[9] R. Lee, K. Stack, D. Richardson, T. Lewis, G. Garnier, Effect of shear, temperatureand pH on the dynamics of salt induced coagulation of wood resin colloids,Colloids Surf. A: Physiochem. Eng. Aspects 396 (2012) 106–114.

10] R. Lee, K. Stack, D. Richardson, T. Lewis, G. Garnier, Multi-salt coagulation of softpitch colloids, Colloids Surf. A: Physiochem. Eng. Aspects 409 (2012) 74–80.

11] D. Otero, K. Sundberg, A. Blanco, C. Negro, J. Tijero, B. Holmbom, Effects of woodpolysaccharides on pitch deposition, Nordic Pulp Pap. Res. J. 15 (2000) 607–613.

12] K. Sundberg, J. Thornton, C. Pettersson, B. Holmbom, R. Ekman, Calcium-induced aggregation of dissolved and colloidal substances in mechanical pulpsuspensions, J. Pulp Pap. Sci. 20 (1994) J317–J322.

13] I. Johnsen, M. Lenes, L. Magnusson, Stabilisation of colloidal wood resin bydissolved material from TMP and DIP, Nordic Pulp Pap. Res. J. 19 (2004) 22–28.

14] U. Welkener, T. Hassler, M. McDermott, The effect of furnish components ondepositability of pitch and stickies, Nordic Pulp Pap. Res. J. 8 (1993), 223–225,232.

15] J. Thornton, Dissolved and colloidal substances in the production of wood-containing paper, Department of Forest Products Chemistry, Åbo AkademiUniversity Åbo, Finland, 1993 (PhD Thesis).

16] T. Hannuksela, B. Holmbom, Stabilization of wood-resin emulsions by dis-solved galactoglucomannans and galactomannans, J. Pulp Pap. Sci. 30 (6) (2004)159–164.

17] K. Sundberg, J. Thornton, B. Holmbom, R. Ekman, Effects of wood polysac-

charides on the stability of colloidal wood resin, J. Pulp Pap. Sci. 22 (1996)226–230.

18] V. Saarimaa, L. Vähäsalo, A. Sundberg, A. Pranovich, B. Holmbom, M. Svedman, F.Orsa, Influence of pectic acids on aggregation and deposition of colloidal pitch,Nordic Pulp Pap. Res. J. 21 (2006) 613–619.

[

chem. Eng. Aspects 441 (2014) 101– 108

19] A.V. Pranovich, K.E. Sundberg, B.R. Holmbom, Chemical changes in thermome-chanical pulp at alkaline conditions, J. Wood Chem. Technol. 23 (2003) 87–110.

20] F. Orsa, B. Holmbom, J. Thornton, Dissolution and dispersion of spruce woodcomponents into hot water, Wood Sci. Technol. 31 (1997) 279–290.

21] TAPPI Useful Method UM 250, Acid-soluble lignin in wood and pulp, Tappi,1991.

22] Measurement of soluble carbohydrates – Orcinol method, Norske SkogResearch, 2005.

23] A. Sundberg, K. Sundberg, C. Lillandt, B. Holmbom, Determination of hemi-celluloses and pectins in wood and pulp fibres by acid methanolysis and gaschromatography, Nordic Pulp Pap. Res. J. 11 (1996) 216–220.

24] C. Hopkins, J. Ducoste, Characterising flocculation under heterogeneous turbu-lence, J. Colloid Interface Sci. 264 (2003) 184–194.

25] H. Reerink, J.T.G. Overbeek, The Rate of Coagulation as a Measure of the Stabilityof Silver Iodide Sols, Discussions of Faraday Society, 1954, pp. 74–84.

26] R.J. Hunter, Introduction to Modern Colloid Science, Oxford University Press,Oxford, 1993.

27] F. Bertaud, A. Sundberg, B. Holmbom, Evaluation of acid methanolysis for analy-sis of wood hemicelluloses and pectins, Carbohydr. Polym. 48 (2002) 319–324.

28] D. Brasch, The chemistry of Pinus radiata. VI. The water-soluble galactogluco-mannan, Aust. J. Chem. 36 (1983) 947.

29] T.C. Hannuksela, Herve du Penhoat NMR structural determination of dissolvedO-acetylated galactoglucomannan isolated from spruce thermomechanicalpulp, Carbohydr. Res. 339 (2003) 301–312.

30] A. Pranovich, M. Reunanen, Dissolved lignin and other aromatic substances inthermomechanical pulp waters, J. Wood Chem. Technol. 25 (2005) 109–132.

31] S. Willför, P. Rehn, A. Sundberg, K. Sundberg, B. Holmbom, Recovery of water-soluble acetylgalactoglucomannans from mechanical pulp of spruce, Tappi J. 2(2003) 27–32.

32] S. Willför, R. Sjöholm, C. Laine, M. Roslund, J. Hemming, B. Holmbom, Charac-terisation of water-soluble galactoglucomannans from Norway spruce woodand thermomechanical pulp, Carbohydr. Polym. 52 (2003) 175–187.

33] V. Saarimaa, A. Pranovich, A. Sundberg, B. Holmbom, Isolation of pectic acidsfrom bleached TMP water and aggregation of model and TMP pectic acids bycalcium, Bioresources 2 (2007) 638–651.

34] M.R. Gittings, L. Cipelletti, V. Trappe, D.A. Weitz, M. In, J. Lai, The effect of solventand ions on the structure and rheological properties of guar solutions, J. Phys.Chem. A 105 (2001) 9310–9315.

35] R. Sartori, L. Sepulveda, F. Quina, E. Lissi, E. Abuin, Binding of electrolytes topoly(ethylene oxide) in aqueous solutions, Macromolecules 23 (17) (1990)3878–3881.

36] L. Guo, K.C. Tam, R.D. Jenkins, Effects of salt on the intrinsic viscosity of modelalkali-soluble associative polymers, Macromol. Chem. Phys. 199 (6) (1998)1175–1184.

37] M.M. Nagasawa, A. Holtzer, The helix-coil transition in solutions of polyglu-tamic acid, J. Am. Chem. Soc. 86 (4) (1964) 538–543.

38] A. Güner, Properties of aqueous salt solutions of polyvinylpyrrolidone. I. Vis-cosity characteristics, J. Appl. Polym. Sci. 62 (5) (1998) 785–788.

39] W. Banks, C.T. Greenwood, Hydrodynamic properties and dimensions of linearpotato amylose molecules in dilute aqueous salt solution, Die Makromol. Chem.67 (1) (2003) 49–63.

40] A. Swerin, L. Odberg, L. Wagberg, Preparation and some properties of the col-loidal pitch fraction from a thermomechanical pulp, Nordic Pulp Pap. Res. J. 8(1993), 298–301, 337.

41] R. Hunter, Introduction to Modern Colloid Science, first ed., Oxford UniversityPress, Oxford, 1993.

42] R. Hunter, Foundations of Colloid Science, second ed., Oxford University Press,Oxford, 1993, pp. 635.

43] S. Asakura, F. Oosawa, Surface tension of high polymer solutions, J. Chem. Phys.22 (1954) 1255.

44] A. Millings, S. Biggs, Direct measurement of the depletion force using an atomic

45] C. Giles, T. MacEwan, S. Nakhwa, D. Smith, Studies in adsorption. Part XI. Asystem of classification of solution adsorption isotherms, and its use in diag-nosis of adsorption mechanisms and in measurement of specific surface areasof solids, J. Chem. Soc. 786 (1960) 3973–3993.