Multi-salt coagulation of soft pitch colloids

Colloids and Surfaces A: Physicochem. Eng. Aspects 409 (2012) 74–80

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Multi-salt coagulation of soft pitch colloids Roland Lee a , Karen Stack a,∗ , Desmond Richardson b , Trevor Lewis a , Gil Garnier c,∗∗ a

School of Chemistry, University of Tasmania, Hobart, Tas, Australia Norske-Skog, Boyer, Tas, Australia c Department of Chemical Engineering, Monash University, Clayton, Vic 3800, Australia b

 Effects of single and multiple salts on stability of wood resin colloids studied.  Addition of second salt decreases first salt CCC and causes restabilsation of colloid.  Zeta potential measurements show charge reversal with addition of aluminium ions.  Experimental CCC values lower than predicted from theory.  Differences suggest specific ion adsorption occurs with calcium and aluminium ions.

a r t i c l e

i n f o

Article history: Received 14 April 2012 Received in revised form 17 May 2012 Accepted 9 June 2012 Available online 17 June 2012 Keywords: Soft colloid Pitch Multi-salt coagulation DLVO theory Critical coagulation concentration (CCC) Adsorption

g r a p h i c a l

a b s t r a c t

1.6 1.4 1.2 1

Log W

h i g h l i g h t s

0 mM Na

0.8

50 mM Na

0.6 0.4 0.2 0 -0.2 -3

-2

-1

Log [Al]

a b s t r a c t The dynamic coagulation of colloidal pitch was quantified under orthokinetic conditions. The effects of single salt, multiple salt and cation valency on the stability of the colloidal pitch were investigated as a function of salt concentration. Critical coagulation concentrations (CCCs) were determined for a range of individual cations. The CCC in the presence of a number of divalent or trivalent cations was investigated to gain an understanding of the effect of multiple salts normally found in industrial systems. Electrostatic destabilisation of wood resin colloids by a single salt is strongly influenced by salt valency (z) and mostly independent of the individual cation (at constant z). Addition of a second cation to solution resulted in a decrease in the CCC for both calcium and aluminium ions in the presence of sodium ions. The decrease in the CCC for the wood resin colloids was non-linear and showed restabilisation of the colloids above the CCC, unlike the effects observed for a single salt. Comparison of the experimental CCC results with the DVLO theory indicates that a higher Hamaker constant than reported for the interaction of abietic acid with talc in water (used here as a model interaction) is needed. This suggests that the aggregation process of the wood resin colloids involves a stronger interaction than abietic acid with talc. Experimental CCC values for divalent and trivalent ions were also lower than those predicted theoretically. To obtain agreement between theory and experimental results, reduced Stern potentials and higher Hamaker constants were required, suggesting specific ion adsorption of the multivalent cations is occurring to reduce surface charge of the colloid. © 2012 Elsevier B.V. All rights reserved.

1. Introduction ∗ 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: [email protected] (K. Stack), [email protected] (G. Garnier). 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2012.06.005

The stability and aggregation of colloidal particles are important to many industrial processes such as paint, food, petrochemical, pharmaceutical, mineral processing, and papermaking [1–3]. Industrial colloidal systems can be quite complex and a sound

R. Lee et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 409 (2012) 74–80

understanding of the factors affecting colloidal stability is necessary in order for good process control to be achieved. In papermaking the naturally occurring lipophilic wood extractives (also called wood resins) released during wood pulping, form colloidal pitch particles that can aggregate and form sticky deposits that cause significant process problems [4–6]. These problems are aggravated as most pulp and paper mills actively reduce water usage through intense process water recycling. Water recycling increases the concentration of the lipophilic extractives and other substances, such as dissolved organic compounds and inorganic salts that affect colloidal stability of the wood extractives in the process water [6–8]. The aggregation process can be understood in terms of the attractive and repulsive forces acting on the particles [9,10]. The DLVO theory defines the coagulation forces in terms of the attractive van der Waals forces and the repulsive electrostatic forces under perikinetic (Brownian motion) controlled coagulation [9,10]. However, in most industrial systems orthokinetic (fluid shear) controlled coagulation predominates. This will influence the rate of particle–particle interaction and inter/intra particle forces resulting in discrepancies on comparison to DLVO predictions [1,11,12]. Experiments with systems of rigid, weakly charged particles at relatively low ionic strength conditions (<5 × 10−2 M) with monovalent electrolytes have shown agreement with traditional DLVO theory [13–15]. The addition of multivalent salts has been found to result in deviations from predicted values in some cases [16]. This is attributed to the multivalent salts not behaving as indifferent electrolytes but adsorbing and interacting with the surface to neutralise the surface charge [16] and the side reactions that they can undergo in solution [17]. High ionic strength (>0.1 M) and highly charged particles [13,14] also result in discrepancies. At high ionic strengths, the charge on the particle is screened and the barrier to coagulation lies at separations of less than 2 nm. At these short distances, it is believed that other short-range non-DLVO forces come into play [14,15]. Wood resins in solution form a soft colloidal system capable of reconformation [16] and coalescence of particles under orthokinetic conditions [17]. This paper aims to quantify the effect of various cations of different valency on the colloidal stability of a soft pitch colloid under orthokinetic conditions. The effect of multiple cations on the coagulation of the wood resin colloidal dispersions is also studied to determine if the addition of a second destabilising salt to the colloidal dispersion results in a competitive or additive effect on the destablisation of the colloids. 2. Experimental NaCl, KCl, CaCl2 and KNO3 (all 99.8%) and AR HNO3 were purchased from BDH. MgCl2 , Al2 (SO4 )3 and FeCl3 (99.8%) were obtained from Merck. All electrolytes were dissolved in deionised water as stock solutions. Thermomechanical pulp (TMP) made from Pinus radiata was collected from the primary refiners at Norske Skog, Boyer, Tasmania. The pulp was freeze-dried and soxhlet extracted for 8 h with hexane. The hexane was evaporated to obtain the lipophilic wood resins that were stored at −20 ◦ C until needed. 2.1. Colloidal wood resin dispersion preparation Aqueous wood resin dispersions of 100 mg/L concentration were prepared by dissolving the soxhlet-extracted wood resins in acetone (99.5%, Chem-Supply) and then adding the acetone solution to distilled water containing 1 mM KNO3 , and pH adjusted to 5.5 with 0.5 M HNO3 . The dispersions were stirred for 10 min using a magnetic stirrer at 500 rpm. Dialysis of the dispersion was

75

Table 1 GC operating conditions. Parameters

Description

Injector temperature

1.5 min at 90 ◦ C, then increase to 325 ◦ C at 180 ◦ C/min 2.0 min at 90 ◦ C then increase to 320 ◦ C at 15 ◦ C/min 350 ◦ C Ultra high purity helium 3.0 psi 54.8 cm/s

Oven/column temperature FID temperature Carrier gas Constant column pressure Corresponding linear velocity

performed using cellulose membrane tubing with a molecular mass cut off of 12,000 amu (Sigma–Aldrich D9402-100FT), to remove acetone. The wash water, containing 1 mM KNO3 at pH 5.5, was changed every hour for the first 5 h and after 24 h.

2.2. Wood resin analysis – GC Analysis and quantification of the wood resins was carried out through extraction from the aqueous colloidal dispersions using tertiary-butylmethylether (t-BME, CG, Sigma–Aldrich). An internal standard containing heptadecanoic acid (C17), pentadecanoic acid (C15), cholesteryl stearate (CS) and 1,3-dipalmitoyl-2-oleoylglycerol (DOG) was added to each sample for quantification purposes. Silylation of the samples was undertaken by adding 100 ␮L of pyridine (99% purity, Sigma–Aldrich) and 100 ␮L of N,Obis(trimethylsilyl)-acetamide (BSA, 98% purity, Sigma–Aldrich) to dry sample extracts and heating at 60 ◦ C for 20 min, according to the method described by McLean et al. [18]. The silylated samples were analysed using a Varian 3800 GC-FID fitted with a Varian 8400 auto sampler, FID detector and a Varian temperature programmable 1079 injector. Separation of wood resin components was achieved with a 15 m Phenomenex® 100% polydimethylsiloxane (ZB-1, 15 m × 0.53 mm ID × 0.15 ␮m FT) ZebronTM capillary GC column. Table 1 presents the GC operating conditions employed. Varian Star 5.5 software was used to analyse the GC chromatograms.

2.3. PDA coagulation analysis A photometric dispersion analyser (PDA 2000, Rank Brothers, Cambridge, UK) was used to monitor the changes in aggregation of the colloidal wood resin dispersions. A Cole Palmer Masterflex L/S peristaltic pump and 3 mm tubing were used to circulate the suspension. The instrument was initially calibrated [19,20] with distilled water and the DC gain control was adjusted to give a DC value of 10 V. The PDA measures turbidity fluctuations of a flowing suspension under controlled shear conditions. The PDA signal is the ratio of the root mean square (rms) of the AC voltage (related to fluctuations in transmitted light) to the DC voltage (average transmitted light). This ratio is related to particle concentration and particle size and is plotted as a function of time. Three replicates were measured for each condition. The PDA signal was smoothed using a 40-point moving average. Peristaltic pump flow rate was set at 70 mL/min and stirring of the colloidal wood resin dispersions was undertaken using a flat impeller (4 cm diameter) at 500 rpm. The PDA system was connected with 2 mm tubing to the cell tubing of 1 mm. Total sample volume was 200 mL, consisting of 175 mL of colloidal wood resin solutions at about 100 mg/L and 25 mL of dissolved salt at a concentration to make the desired final concentration in 200 mL.

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Table 2 Characterisation of Pinus radiata wood resin colloids. Chemical composition • Fatty acid • Resin acid • Triglycerides Av. particle diametera Zeta potentiala a

1.44 mg/g dry fibre 6.17 mg/g dry fibre 5.89 mg/g dry fibre 800 nm −50 mV

Aqueous wood resin dispersion 50 mg/L concentration pH 5.5 and 1 mM KNO3 .

2.3

Addition of CaCl2

2.1 1.9

Signal

1.7

Fig. 2. Effect of CaCl2 concentration on the stability factor (W) of pitch colloid suspension.

0 mM Ca

1.5

1.0 mM Ca

1.3

2

5 mM Ca

1.1

10 mM Ca

0.9 0.7

20 mM Ca

Aggregation region

1.5

0.5 5

10

NaCl

15

Time x 100 sec Fig. 1. Effect of CaCl2 concentration on the coagulation kinetics of colloidal pitch dispersions measured by PDA.

A Malvern Zetasizer Nano ZS was used to measure the zeta potential and particle size. A 100 mg/L aqueous dispersion was prepared by adding hexane extracted wood resin dissolved in acetone to distilled water containing 1 mM KNO3 and pH adjusted to 5.5 and stirring for 10 min at 500 rpm. Subsamples of this dispersion were diluted to 50 mg/L at varying salt concentrations and salt valency by adding appropriate amounts of different salts and distilled water. Measurements on the Zetasizer were performed immediately after each dilution and salt concentration. 3. Results Table 2 summarises the chemical composition, particle size and zeta potential of P. radiata wood resin. Salt induced coagulation experiments were performed with monovalent (NaCl and KCl), divalent (CaCl2 and MgCl2 ) and trivalent (FeCl3 and Al2 (SO4 )3 ) salts. Fig. 1 shows the aggregation behaviour observed by PDA, on the addition of various concentrations of CaCl2 to the colloidal wood resin dispersion. Similar curves were obtained for all the salts. The slope of the aggregation region represents the rate of coagulation of the colloid. As the concentration of electrolyte increased the rate of coagulation increased. The stability factor (W) was determined from the rate of change in the coagulation region of the PDA curves using the following relationship: ko

2

k2

CaCl2

1

MgCl2 FeCl3 Al2(SO4)3

0.5

2.4. Wood resin particle size and charge

W=

KCl

Log (W)

0

(1)

where k2o is the rate of fastest coagulation and is taken to be the slope of the curve with steepest slope corresponding to maximum coagulation. k2 is the rate of coagulation for the line of interest. In Fig. 2, the fastest slope was taken to be the slope for the 10 mM CaCl2 curve. A plot of log W against salt concentration defines the colloidal stability of the system. A typical inverse sigmoidal curve with three distinct regions is observed as shown in Fig. 2. The three regions are:

0 -2

-1

0

1 [Salt]

2

3

4

Fig. 3. Colloidal stability curves for pitch colloids under dynamic conditions for the addition of various electrolytes (pH 5.5, 23 ◦ C).

1. an initial flat region at low salt concentration (a stability zone); 2. a region in which log W decreases rapidly (a transition zone of colloidal instability); and 3. a region in which log W is 0 at higher salt concentration (complete aggregation of the colloid). The point at which log W intercepts the x-axis defines the critical coagulation concentration (CCC) for that particular salt. Fig. 3 shows the stability curves for the addition of each individual electrolyte and Table 3 summarises the critical coagulation concentrations (CCCs) obtained from Fig. 3. It is clear from this figure that pitch stability is strongly dependent on the salt valency, and as the salt valency increased, the CCC decreased. While only slight differences in the CCCs for the different cations of the same valency were observed. The effect of two salts on the stability of the wood resin colloids was then investigated in the same manner. Either Ca2+ or Table 3 Critical coagulation concentrations (CCCs) under dynamic conditions for different cations at pH 5.5. Salt

z

CCC (mM) ± standard dev.

+

1 1 2 2 3 3

720 670 7.8 6.5 0.065 0.075

Na K+ Ca2+ Mg2+ Al3+ Fe3+

± ± ± ± ± ±

40 30 0.3 0.4 0.003 0.005

R. Lee et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 409 (2012) 74–80

Table 4 Critical coagulation concentrations for calcium and aluminium salts at different sodium ion concentrations.

2.2 2 1.8

0 mM Al

Addition of Al2(SO4)3 and NaCl

1.6

Ratio

77

0.01 mM Al

1.4 0.02 mM Al

1.2 0.2 mM Al

1

[Na] (mM)

CCC [Ca] (mM)

Cation ionic strength (mM)

CCC [Al] (mM)

Cation ionic strength (mM)

0 50 150 300 400 720

7.8 6.6 3.0 2.0 0.92 0

15.6 38.2 81 154 202 360

0.065 0.059 0.024 N/A 0.013 0

0.2925 25.27 75.11 N/A 200.1 360.0

0.8 Particle growth region

9

0.6 0

250

500

750

1000

1250

8

Time (sec)

CCC [Ca] (mM)

7

Fig. 4. PDA response to the simultaneous additions of various concentrations of Al3+ with 150 mM Na+ to a pitch suspension under dynamic conditions (pH 5.5, 23 ◦ C and 500 rpm).

6 5 4 3 2 1 0 0

200

400

600

800

[Na] (mM) Fig. 7. Effect of sodium ion concentration on the critical coagulation concentration of calcium ions in a pitch suspension.

salts or sodium and aluminium salts, respectively. The cation ionic strength was calculated as: Fig. 5. Plot of log W against log [Ca2+ ] for various concentrations of Na+ (pH 5.5, 23 ◦ C and 500 rpm).

1 2 ci zi 2 n

IS =

i

Al3+

Na+ ,

was added, along with a constant concentration of to the colloidal wood resin suspension. A similar coagulation response as for a single salt was found (Fig. 4). The effects of the addition of two salts on the colloidal pitch stability factor (log W) are shown in Figs. 5 and 6 for CaCl2 /NaCl and Al2 (SO4 )3 /NaCl. These curves show destabilisation of the colloid with increasing Ca2+ (or Al3+ ) concentration. They also illustrate that the minimum in the stability curve, corresponding to the CCC, shifted to the left as the concentration of the second salt (Na+ ) increased. Restabilisation of the colloids occurred as the concentration of the multivalent salt increased above the CCC. Table 4 presents the CCC and cation ionic strength at the CCC, for the colloidal wood resins with the addition of sodium and calcium

Fig. 6. Plot of log W against log [Al3+ ] for various concentrations of Na+ (pH 5.5, 23 ◦ C and 500 rpm).

where c is the concentration of the cation and z is the charge of the cation. The CCC was determined similarly as with single salts. The effect of sodium ion concentration on the CCC for Ca2+ and Al3+ is shown in Figs. 7 and 8. The CCCs for calcium and aluminium ions decreased in a non-linear way with sodium ion concentration. The effects of salt type and concentration on the wood resin colloid charge were also studied. The charge on the colloids, as measured by the zeta potential, was found to decrease from −50 mV with no salt addition to a constant value of −10 mV with increasing concentration of both NaCl and CaCl2 as shown in Fig. 9. The addition of aluminium salt was found to result in the charge decreasing

Fig. 8. Effect of sodium ion concentration on the critical coagulation concentration of aluminium ions in a pitch suspension.

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R. Lee et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 409 (2012) 74–80

Fig. 9. Effect of salt type and concentration on zeta potential of wood resin colloids.

Fig. 10. Effect of a calcium and aluminium salts at various sodium salt levels on zeta potential of wood resin colloids.

to zero then reversing to a small positive charge at aluminium ion concentrations above 0.5 mM. The effect of adding a combination of two salts (sodium/calcium and sodium/aluminium) is shown in Fig. 10. The addition of calcium salt to a colloidal suspension in a solution of sodium ions was found to have little effect on the zeta potential of the colloids over the salt concentration ranges investigated. Addition of aluminium salts, on the other hand, was found to decrease the zeta potential and cause charge reversal of the particle charge. 4. Discussion The Na+ and Ca2+ CCCs measured for a pitch colloidal suspension (Table 3) are higher than those previously reported [7,21,22]. The published Na+ CCC for pitch are 100 mM [22], 150 mM [7] and 200 mM [21]. These differences in CCC can be explained in two ways: differences in pitch composition and variations in analytical method and level of shear during the measurement. P. radiata wood resins are higher in resin acid content than spruce and so have a very different extractives composition (Table 5). Sihvonen Table 5 Comparison of P. radiata and spruce wood extractives composition.

Spruce [23] P. radiata

Resin acids (kg/t)

Fatty acids (kg/t)

Triglycerides (kg/t)

2.0 6.17

0.40 1.44

4.55 5.89

et al. [7] reported pine heartwood pitch, containing high concentrations of free fatty acids and resin acids, to have the highest stability against aggregation with NaCl compared to other wood resins of lower resin acid and fatty acid content. Differences in CCC values can also be related to differences in methods used to determine the CCC, especially with regard to the level of shear during measurement. The absolute CCC value is function of the experimental technique; most previous studies relied on a static technique while the high shear/dynamic PDA method was employed in the current work. Similar analysis [7,21] applies for pitch destabilisation by Ca2+ . Pitch CCCs with trivalent metal ions (Table 3) are also slightly lower than the 0.02 mM (AlCl3 ) value reported by Sundberg et al. [21] in their study on spruce extracts. This may be due to the higher resin acid content of the southern hemisphere species forming stable aluminium soaps that readily precipitate. The trivalent salt results are complicated by the formation of a variety of hydrated monomers, dimers, trimers and higher order species of varying ionic charge in aqueous solution [9,24–27]. The “free” trivalent ions are only found at low pH [9]. At pH 5.5, aluminium exists mostly as Al(OH)2+ and Al(OH)2 + , with small amounts of Al(OH)3 and Al(OH)4 − and also as polynuclear species such as Al2 (OH)5 + , Al4 (OH)10 2+ , Al6 (OH)15 3+ and Al4 (OH)20 4+ along with charged hydrated dimers of Al2 (OH)(H2 O)8 4+ and tridecameric species [AlO4 Al12 (OH)24 (H2 O)20 ]7+ . Our experimental CCC values (Table 3) indicate that iron and aluminium behave as trivalent or tetravalent ions. The addition of a second salt to pitch colloids significantly decreased the CCC of the first salt. The second salt has a greater effect than the simple additive effect expected from an increase in ionic strength (Table 4). This suggests that the salts are not acting as indifferent electrolytes; some may specifically adsorb onto pitch with one of the salts aiding the adsorption of the second salt. It is possible that the sodium ion may be acting as a “sensitiser” and causing thinning of the electrical double layer thus allowing for the specific adsorption of the multivalent ions. The specific adsorption of ions alters the chemistry of the surface by eliminating active charged sites so less positive charges are required. The charge reversal observed in the zeta potential measurements (Figs. 9 and 10) clearly indicates that specific ion adsorption of aluminium ions does occur. Restabilisation of the wood resin colloids occurred with multiple salt addition but not with single salt destabilisation (Figs. 5 and 6), over the concentration range studied. This restabilisation is not predicted by the DVLO theory but has been reported previously by Ruckenstein and Huang [24] and Dishon et al. [28]. The colloid restabilisation can be attributed to the adsorption of cations onto the negatively charged colloid surface, thus decreasing or even reversing charge at high salt concentration. Charge reversal was found to occur with aluminium salts and calcium salts were found to reduce the charge slightly more than the sodium salt. Charge reversal has been reported on silica surfaces with the addition of NaCl, KCl and CsCl [28]. The reversal was attributed to the condensation of positive ions onto the surface and driven by the entropy associated with the release of hydrating water molecules.

4.1. Comparison with the DLVO theory From DVLO theory [10], the critical coagulation concentration, defined as the salt concentration at which a colloid coagulates, occurs when the potential energy of the system V = 0 and dV/dt = 0:

CCC =

9.85 × 104 ε3 kB5 T 5  4 NA e6 A2 z 6

(2)

R. Lee et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 409 (2012) 74–80

79

Fig. 11. Effect of cation valency on the pitch critical coagulation concentration (CCC); pH 5.5 at 23 ◦ C and 500 RPM.

=

exp[zed /2kB T ] − 1 exp[zed /2kB T ] + 1

(3)

From this the CCC is predicted to be inversely proportional to the sixth power of the valency of the metal ion (Eq. (2)). This relationship, also known as the Shultz Hardy Rule, is valid for large values of the surface potential for which Eq. (3) limits to 1. At low potentials, however, a less sensitive dependence on the metal ion valence is predicted [9,10] as CCC is proportional to (d 4 /z2 ). In the current work, a plot of log (CCC) against log (cation valency) yielded a linear plot with a slope of −8.2 if the aluminium and iron salts were considered as forming trivalent metal ions and a slope of −7.1 if they were considered as tetravalent polynuclear hydroxyl species (Fig. 12). An improved correlation results if the iron and aluminium species are assumed to be tetravalent. Although it is well established, through 27 Al NMR experiments, that under the pH conditions in this experiment, the tridecameric Al species is dominate species in aluminium chloride and nitrate solutions, the presence of sulphate ions has been shown [27] to prevent the formation of the tridecamer species and promote the formation of dimer [Al2 (OH)(H2 O)8 ]4+ species. The results in Fig. 11 suggest that the change in CCC is not proportional to z−2 for low surface potentials or z−6 for high surface potentials as expected from Eq. (2). Our experimental results indicate a significantly greater dependence of CCC on cation valency. It can be argued that the higher than expected slope in Fig. 11 is due to the Stern potential (d ) being dependent on the z value. Lowering of the Stern potential by compression of the electrical double layer and specific ion adsorption are two possible mechanisms for the greater than expected dependence of the CCC on the metal ion valency. The effect of the Hamaker constant and Stern potential on pitch stability can be calculated from the DLVO theory (Eq. (2)). Fig. 12 shows the effect of the Stern Potential and Hamaker constant on the predicted CCC. A Hamaker constant of 1.56 × 10−20 J was reported by Wallqvist et al. [29] for abietic acid interacting with talc in water. Calculation of the CCC using this value for the Hamaker constant, when combined with the measured zeta potential of the wood resin colloids (as an estimate for the Stern potential d ), results in a predicted CCC value of 350 mM for a monovalent metal ion. This is much lower than the experimental values of approximately 700 mM presented in Table 3. This would imply that the value of the Hamaker constant used was too low. This is not unreasonable as the interaction between abietic acid and talc was used as a model interaction to give a starting value for the Hamaker constant. The

Fig. 12. Effect of Stern potential and Hamaker constant on CCC for a monovalent salt.

experimental value of 700 mM for a monovalent salt measured in the current work would indicate that the Hamaker constant for the interaction between monovalent salt and wood resin colloids should be 2.25 × 10−20 J. Based on the newly determined Hamaker constant (A) of 2.25 × 10−20 J, the effect of cation charge and Stern potential (␺d ) on the theoretical CCC can be calculated (see Fig. 13). Clearly the predicted CCC is very sensitive to the cation charge. For monovalent cations (z = 1) small changes in the Stern potential result in significant changes in the CCC. As the valency of the cation increases this effect is reduced. To get agreement between experimental and predicted CCC values for divalent and trivalent metal ions, either a lower Stern potential than the value of 10 mV measured for the wood resins, or a higher Hamaker constant, or both, is needed in the calculation. The need for a lower Stern potential indicates that compression of the electrical double layer and/or a decrease in the surface charge is occurring due to specific adsorption of the divalent and trivalent ions onto the surface of the wood resin colloids. The simple form of the DLVO theory does not account for the slight variations in CCC, observed experimentally, between different salts of the same valency. Some of the differences observed between different metals of the same valency may be due to variations in the Hamaker constant, though it is unclear what effect the various metal ions have on the Hamaker constant. Alternatively, they may be due to slight differences in the surface charge (the Stern potential) caused by specific ion adsorption or to slight differences in the electrical double layers around the colloids in the presence

800 700 600

CCC (mM)

where A denotes the Hamaker constant, NA is Avogadro’s number, z is the charge of the counter-ion, T is the temperature in Kelvin, ε is the permeability of the dispersion medium, kB is the Boltzmann constant, e is the electron charge, and  is defined by:

500

z=1

400 300

z=2

200

z=3

100

z=4

0 0

0.005

0.01

Stern Potential (V) Fig. 13. Effect of Stern Potential and salt valency on CCC (Hamaker constant = 2.25 × 10−20 J).

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R. Lee et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 409 (2012) 74–80

of different ions. Different metal ions are also known to affect the tackiness of pitch particles and their attachment to substrates or each other following contact [30,31]. 5. Conclusion The mechanisms by which multiple salt addition and salt valency can affect colloid stability under orthokinetic conditions were studied. A wood resin (pitch) colloidal suspension was selected as model system due to the high industrial impact of colloid destabilisation (pitch deposition). The addition of multiple cations significantly reduces the critical salt coagulation concentration (CCC) of a wood resin colloid suspension compared to that expected from single salt addition. Restabilisation of the pitch colloids also occurred with multiple salt addition at salt concentrations above the CCC. The adsorption of multivalent cations that leads to a reduction of the surface charge and charge reversal of the colloid (at high concentrations) are the suggested mechanisms. The relationship between the salt concentration, salt valency and stability ratio deviates slightly from theoretical expectations. An inverse seventh power relationship was shown when iron and aluminium are assumed to be tetravalent polynuclear hydroxide species, instead of the expected inverse sixth power at high surface potentials or inverse two power at low surface potentials. We believe these deviations are due to the salts not acting as indifferent electrolytes and affecting Stern potentials through specific ion adsorption. Calculations of the CCC with the simple form of the DLVO theory requires a higher Hamaker constant than reported for abietic acid and talc indicating that the aggregation of the wood resins involves a stronger attractive force than the interaction between abietic acid and talc. In the case of divalent and trivalent ions, lower Stern potentials were required to model the experimental results also supporting the specific ion adsorption of the multivalent ions onto the wood resin surfaces. Whether the ability of multiple salt addition to destabilise and restabilise colloids is specific to pitch or is a general observation deserves further attention. Acknowledgements Financial support for this project was provided by Norske-Skog Paper and an Australian Research Council Linkage Grant LP882355. References [1] M. Bostrom, V. Deniz, G. Franks, B. Ninham, Extended DLVO theory: electrostatic and non-electrostatic force in oxide suspensions, Adv. Colloid Interface Sci. 123 (2006) 5–15. [2] L. Kang, J. Cleasby, Temperature effects on flocculation kinetics using Fe(III) coagulant, J. Environ. Eng. 121 (1995) 893–901. [3] L. Hanus, R. Hartzler, N. Wagner, Electrolyte-induced aggregation of acrylic latex. 1. Dilute particle concentrations, Langmuir 17 (2001) 3136–3147.

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