The adsorption of starch derivatives onto kaolin

ELSEVIER

Colloids and Surfaces A: Physicochemical and Engineering Aspects 131 ( 1998 ) 145 159

COLLOIDS AND SURFACES

A

The adsorption of starch derivatives onto kaolin J.C. Husband E. C. C. International Central Research, c/o John Keay House, St. Austell, Cornwall PL25 4DJ, UK

Received 15 July 1996

Abstract

Modified starches are widely used as binders and thickening agents in paper coating formulations. Frequently, kaolin is used as the coating pigment. In this work, the adsorption of modified starches onto kaolin has been studied. Adsorption behaviour was found to depend both upon the type of chemical modification and botanical origin. With a phosphate ester of potato starch, adsorption was reduced in the presence of sodium polyacrylate used as a deflocculant on the clay. No fractionation of the starch components was observed during adsorption. These results imply an electrostatic mechanism of interaction with the edge aluminium sites. In contrast, a hydroxyethyl ether of corn (maize) starch showed preferential adsorption of amylose, which is shown to be a result of preferential etherification of this component with ethylene oxide. With this non-ionic derivative, adsorption was not reduced by the presence of polyacrylate. A mechanism of adsorption involving hydrogen bonding of hydroxyethyl groups to silanol or aluminol groups on the kaolin surface is proposed in this case. A hydroxyethyl ether of potato starch showed preferential adsorption of amylopectin, together with some sensitivity to polyacrylate. With potato starch, the adsorption behaviour is dominated by the presence of naturally occurring phosphate ester groups, which are associated with the amylopectin. © 1998 Elsevier Science B.V. Kevwords: Adsorption; Amylopectin; Amylose: Kaolin; Starch derivatives

1. Introduction

Kaolin (china clay) is widely used as a white pigment in the coating of paper. In order to achieve a high-solids suspension for coating, the clay is deflocculated using a polymeric dispersant such as a low molecular mass grade of sodium polyacrylate (NaPA). A binder must also be present to provide adhesion between the individual clay particles and with the paper substrate. Synthetic binders such as film-forming polymer latexes are often used, frequently in conjunction with a natural binder such as modified starch, which also increases the low-shear viscosity of the mix. A concise introduction to the principles of paper coating is given by Priest [ 1]. 0927-7757/98/$19.00 <¢~1998 Elsevier Science B.V. All rights reserved. PI1 S 0 9 2 7 - 7 7 5 7 ( 9 6 ) 0 3 8 8 1 - 2

A range of different starch types may be used in coating. Usually, these are based on potato or corn (maize) starch. These differ primarily in the ratio of branched (amylopectin) to straight-chain (amylose) component present. Derivatisation is carried out to modify the flow properties of the solution and reduce retrogradation. Either ionic or non-ionic functional groups may be introduced depending on the balance of properties required [2]. In this paper, adsorption studies of three types of starch derivative onto kaolin are reported in the absence and presence of NaPA. Adsorptive interactions are likely to influence the rheological properties of the coating suspension and the properties of the coating layer. The adsorption of paper

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J.C. Husband/Colloids Surfaces A: Physicochem. Eng. Aspects 131 (1998) 145 159

coating starches onto mineral surfaces has not been extensively studied, except by J~irnstr6m and co-workers [3-5] who have examined the effects of oxidised, hydrophobic and cationic derivatives on kaolin suspensions. Studies of starch adsorption onto calcite have also been published [6,7].

2. Materials

A standard grade of English coating clay, SPS ®, from ECC International (St. Austell, UK), was used in this study. As measured by sedimentation, 79 wt% of particles had an equivalent spherical diameter finer than 2 ~tm. X-ray diffraction gave the mineralogical composition as kaolinite (91%), mica (6%), feldspar (2%) and montmorillonite (1%). The surface area by nitrogen adsorption (BET method) was 12.1 mZg -1. The cation exchange capacity measured by ammonium acetate exchange [8] was 6.5 meq 100g -1 (65 gmol g 1). The major contaminating ions found in the clay supernatant were Na + and SO ] . with only ppm levels of Ca 2+ and Mg 2+. Only trace levels of sodium polyacrylate are present in this grade of kaolin. Dispex N40 ®, a sodium polyacrylate dispersant having a weight average molecular mass of 4600 (as determined by gel permeation chromatography/low-angle laser-light scattering (GPC/LALLS)) was supplied by Allied Colloids (Bradford, UK). The paper coating starch derivatives, designated D1-D3, are listed below. DI is a phosphate ester of potato starch, Nylgum A85 ® (Avebe, The Netherlands), prepared by reaction of raw starch with phosphate salts and urea [9]. The degree of substitution (DS) of the sample was 0.042 phosphate ester groups and 0.12 carbamate groups per anhydroglucose unit. D2 is a hydroxyethyl ether of yellow dent maize (corn) starch, Penford Gum 280 ® (Penford Products, Cedar Rapids, IA, USA). The sample had a DS of 0.057 hydroxyethyl (HE) groups per anhydroglucose unit. D3 is a hydroxyethyl ether of potato starch, Polaris Gum LV ® (Penford Products, USA). The DS was 0.081 HE groups per anhydroglucose unit. In addition this starch contained native phosphate ester groups equivalent

to a DS of 0.004. Both D2 and D3 are prepared by reaction of granular starch with gaseous ethylene oxide [ 10, 11 ]. The DS of each starch was determined by an appropriate analytical technique, Phosphorus contents were determined by inductively coupled plasma atomic emission spectroscopy (ICP AES) after dissolution of the starch (washed to remove residual phosphate) in boiling nitric acid (BDH, Dorset, UK. Aristar). DI also contained carbamate groups which were determined using a Kjeldahl method, assuming that potato starch contains no other source of nitrogen. HE starches were analysed using a modified method of Morgan as described by Lortz [12], with detection of the evolved ethyl iodide using gas-liquid chromatography (GLC). An estimate of the molecular mass of D1 and D2 was obtained using an angular light scattering technique. Solutions of each starch were prepared in 0.2 M aqueous NaC1 (filtered through a 0.01 ~tm membrane before use) and dimethyl sulphoxide (Aldrich, Dorset, UK. HPLC grade) and filtered (0.45 ~tm) after diluting to appropriate concentrations. Scattering was measured as a function of angle using laser light of wavelength 633 nm in a Malvern K7025 spectrogoniophotometer (Malvern, Worcs., UK.). Values for the refractive index/concentration increment (dn/dc) of starch were taken as 0.15 cm3g-1 in water and 0.074cm3g -1 in dimethyl sulphoxide (DMSO) [13]. The results were graphed using a Zimm plot following the procedure of Matthews [14]. Extrapolation to zero angle and zero concentration gave molecular mass values for DI of 2.8 x 107 and 2.4x 107 for NaC1 and DMSO solvents, respectively. For D2, values of 4.1 x 107 (NaC1) and l a x 10~ (DMSO) were obtained. Because light scattering gives a weight average value, the result is mainly influenced by the largest molecules, which in native starches are likely to be amylopectin. Both derivatives have broadly similar molecular masses, which are about an order of magnitude lower than those quoted by Banks and Greenwood for native amylopectins from a variety of sources [15]. A reduction in molecular mass normally occurs during the manufacture of paper coating binders from native starch [2].

J.C. Husband/Colloids Surfaces A: Physicochem. Eng. Aspects 131 (1998) 145 159 100

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Fig. 1. Filtration of 100 mg dm -3 starch solutions using Nuclepore :~ membranes. Eluted starch determined by anthrone assay. O, Phosphate starch DI: "-,, HE-corn starch D2: A, HE-potato starch D3.

Further information about the range of molecular sizes present in each derivative was obtained in the following experiment. Dilute (100 mg d m - 3) solutions of starch in 0.2 M NaC1 were filtered through a series of polycarbonate membranes (Nuclepore ®, Pleasanton, CA, USA) of various pore diameters from 0.01 lam to 0.10tam (10 100 nm). These membranes are track-etched to give cylindrical pores within 9 0 + 10% of the stated diameter. A stirred diafiltration cell (Amicon ~, Watford, U K ) was used to prevent blocking of the membrane by retained starch molecules. Starch concentrations were determined on the filtrates using both iodine and anthrone assays as described in a later section. Fig. 1 suggests that the three derivatives contain at least 50wt% of molecules in the size range below 50 nm. Both the potato derivatives DI and D3 contain significantly more low molecular weight material than the corn starch ether D2. The composition of the filtrate was also calculated and is given in Table 1 in terms of amylose content. These are close to the expected levels for corn and potato starches [2]. The results indicate that considerable breakdown of the amylopectin has taken place during manufacture of these derivatives, because in native starch, the

Table 1 Amylose content (wt%) of starch derivatives after filtration at a controlled pore size Derivative

DI D2 D3

Pore size/nm <80

<50

<30

< 10

17.5_+0.5 30.1 _+0.7 19.9+_0.3

17.0_+0.8 28.5+-0.4 19.0±0.3

15.7±1.0 37.O±3.4 21.0+-0.3

7.9±2.7 51.3_+8.2 18.2±0.2

components occupy distinctly separate molecular size ranges [16]. An enrichment of amylose to about 51% was found in the fraction of D2 smaller than 10 nm; in contrast DI exhibited depletion of amylose in this fraction. D3 showed little significant change in composition with molecular size.

3. Adsorption experiments Clay suspensions were prepared at 50 wt% solids in water with or without NaPA. The pH was adjusted to 8.5 + 0.1 using NaOH. Starch solutions were prepared at 20 wt% and cooked at 98-100'~C for 20 rain with stirring. After cooling, they were

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J. C. Husband/Colloids Surfaces A: Physicochem. Eng. Aspeets 131 (1998) 145-159

diluted to 10 wt% and filtered through a 5 gm pore diameter polycarbonate membrane (Cyclopore®, Kent, UK) to remove undissolved material. The activity of the solution was accurately determined by drying duplicate weighed aliquots to constant mass at 100°C. Adsorption experiments were completed within 48 h of preparing the stock starch solution. Adsorption studies were carried out by the solution depletion method. Additions of starch were made by weight to 20 g aliquots of clay and the suspension was diluted with distilled water to a clay concentration of 35 wt%. It was estimated that the ionic strength of the aqueous phase was equivalent to 0.015 M Na/SO 4. The suspension was stirred for a further 5 rain and transferred to sealed polyethylene bottles in a water bath maintained at 25_+0.5°C overnight. The pH was 8.0_+0.2, typical of paper coating suspensions. After 24 h, a portion of the aqueous phase was separated by pressure filtration under nitrogen through a 0.45 tam pore diameter polyamide membrane (Sartolon ®, Sartorius, G6ttingen, Germany). Visibly clear filtrates were obtained, and experiments established that the membrane did not retain starch molecules. 0.5-1 g of filtrate was collected, with dilution by factors of up to 100 being made by weighing before assay. 3.1. Starch assay

Starch concentrations were determined using two different colorimetric methods. The total carbohydrate was estimated using anthrone (9,10-dihydro-9-ketoanthracene, BDH). 1 cm 3 of the diluted filtrate was added to 4 cm 3 of a solution containing 0.232 g anthrone in 100 cm 3 of 80 vol% H2SO4 (BDH, Analar) and heated to 98_+2°C for 12 min. The absorbance was measured at a wavelength of 630nm [17,18]. Polyacrylate has been shown not to interfere with the determination of sodium carboxymethyl cellulose with this reagent [19]. Another portion of filtrate (10cm 3) was treated with 0.5 cm 3 0.025 M iodine-KI solution (BDH, Convol ®) and the absorbance measured at 580 nm. Because the blue complex with iodine principally involves the amylose (straight chain) component of starch [15], it was reasoned that

any discrepancy between the results from the two methods must indicate that fractionation of amylose and amylopectin had taken place during adsorption, leading to a change in the composition of the non-adsorbed starch. In order to quantify the amounts of amylose and amylopectin adsorbed, a similar method to that devised by van de Steeg et al. [20,21] was used. This required that extinction coefficients e of the iodine complexes with the pure components (61 am and q ap) should be known. The extinction coefficients of anthrone with amylose and amylop e c t i n (6AN T am and CANTap) were assumed to be identical and equivalent to that of the whole starch. The concentrations of amylose and amylopectin (Cam and Cap) present in the filtrate were estimated from the absorbances of the iodine and anthrone complexes (A~ and AAN T respectively) by solving a pair of simultaneous equations E"1 amCam -4-El apCap = A I

(1)

6"ANT amCam -'[-CANT apCap : AAN T

(2)

In order to estimate El ap, waxy maize starch (99% amylopectin) was used as supplied by Sigma (St. Louis, MO, USA), and as in Ref. [20] it was assumed that this closely resembled amylopectin from both corn and potato starches. It was not possible to obtain a hydroxyethyl derivative of waxy maize starch, but a hydroxypropyl derivative having a DS of 0.18 was available. The extinction coefficients with iodine of both these materials were measured. The values obtained were 7.02 and 4.00 cm 3 mg- ~c m 1 respectively, at 580 nm. (It is likely that a HE-derivative with a DS of 0.05 would give a value somewhere between these results.) The lower value is close to that reported in Ref. [20] (3.0cm3mg-lcm-1), but there are differences in the method and wavelength used; in the present work, 580 nm is closer to the 2max of the iodine-amylopectin complex, 550-560 nm [22]. Hovenkamp-Hermelink et al. [23] report a value of 5.1 cm 3 mg-1 cm-1 for potato amylopectin at 550 nm, but again, the quantities of reagents employed differ. In view of the uncertainty with which el ,p can be known, adsorption results were calculated using both values and the results

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,LC Husband/Colloids Surfiwes A: Physicochem. Eng. Aspects 131 (1998) 145 159

Table 2 Extinction coefficients for starch derivatives Dl, D2 and D3 with iodine, at a wavelength of 580 nrn Derivative Amylose/% e(total starch) E~amylose) icm3mg-lcm -I) (cm3mg-lcm-ll~ DI D2 D3

20 27 20

10.85 11.44 11.83

32.25 27.50 37.[5

aAssuming e(amylopectin)= 5.50 cm3 mg ~crn- ~. averaged, leading to an effective value for q ,p of 5.5_+ 1.5 cm 3 r a g - 1 cm Values for c of the amylose components of D 1-3 with iodine (q ,m) were then calculated assuming that the measured value of e for the whole starch is a weighted sum of the two component values. This procedure was also adopted by H o v e n k a m p Hermelink et al. [23]. Values calculated for the amylose component of each derivative are listed in Table 2 assuming amylose contents of 27% and 20% for corn and potato starch, respectively [2]. For comparison, values taken from the literature are listed in Table 3. In our work, q a m values are higher for all three derivatives than in the published references, probably because of differences in technique. The higher value of e found for potato compared with corn amylose is in agreement with data published by Baldwin et al. [24].

4. Results and discussion 4.1. Adsorption q f phosphate starch and H E corn starch

Adsorption isotherms for the phosphate ester derivative (D1) onto kaolin are shown in Fig. 2.

The amount of starch adsorbed in the absence of NaPA reached a plateau of between 16 and 17 mg g - 1. The shape of the isotherm is typical of a polymer having a broad range of molecular sizes [25,26]. There is an initial rapid uptake, followed by a slow approach to a plateau or 'pseudoplateau'. Pretreatment of the clay with 3.0 mg NaPA g-1 (from which 1.31 mg NaPA g i clay were adsorbed at equilibrium) reduced the amount of starch adsorbed to 3.0 mg g ~. G o o d agreement was obtained between the different methods of assay and it was concluded that no fractionation of amylose and amylopectin had occurred: in other words the adsorbed starch contained the same ratio of amylose: amylopectin as potato starch. The adsorption of the HE corn starch (D2) onto NaPA-free kaolin is plotted in Fig. 3, and the results tabulated in Table 4. Data for amylose and amylopectin adsorption are separated into Figs. 3 (a) and (b), respectively, to avoid confusion. Less starch is adsorbed than from D1 (6 7 mg g - t ) and considerable differences were found in starch concentrations using the two assay methods. Compared with the anthrone technique, the iodine method found less starch in the aqueous phase, indicating depletion of amylose from solution and therefore preferential adsorption of that component. The dashed lines represent the expected adsorption of amylose and amylopectin, i.e. 27% and 73% of the total starch, respectively. No change in the adsorbed amounts either of total starch or amylose were found to occur between 1 and 24 h after mixing, suggesting that a pseudoequilibrium has been attained in this time. Fig. 4 shows the effect of NaPA preaddition, which appears to slightly enhance starch adsorption, although this may be an artefact arising from a more tortuous filter cake structure in the presence

Table 3 Literature values for extinction coefficientsof starch-iodine complexes Reference

Starch type

)~m~x/'nm

elamylose),, mg ~cm ~)

¿cm 3

[20],[21 ] [20],[21] [23]

Potato Waxy maize Potato

625 625 550 618

(amylopectin)/ (cm3mg ~cm i1

31.01-35.15 15.50 23.40

2.711 3.06 5. l l~ ~.5o

J. C Husband/Colloids Surfaces A: Physicochon. Eng. Aspects 131 (1998) 145-159

150

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Fig. 2. Adsorptionof phosphate potato starch D1 onto 35 wt% solidskaolinat pH 7.7+0.3 (25~C). ~, No added polyacrylate:~, with preadditionof 3.0 mg NaPA g- 1clay.Open points, iodineassay; shadedpoints, anthroneassay. of the deflocculant, leading to physical retention of starch molecules. This sensitivity of the phosphate derivative D1 to NaPA suggests either that competitive adsorption onto the same surface sites takes place, or that an increased electrostatic repulsion occurs between the clay surface carrying adsorbed polyacrylate and the anionic starch molecules. In this respect the adsorption behaviour is similar to sodium carboxymethyl cellulose onto clay [19,27]. It has been shown that polyphosphate ions, frequently used as deflocculants for kaolin, adsorb via positively charged aluminium sites on the edges of the particles [28,29]. According to Bidwell et al. [29], these are also likely to be the sites for the uptake of NaPA. The pH of these experiments (7.7+0.3) is close to the point of zero charge of the kaolin edges (pH 7.3 + 0.2) where equal numbers of positive and negative sites coexist [30,31]. By carrying out measurements across a range of pH (Fig. 5), it was shown that the plateau adsorption of D1 decreased sharply above about 8.5, when positive sites no longer exist due to the hydroxylation of A1 species. The shape of the curve is similar to that published by Flegmann et al. for C1- ion adsorption onto kaolin [31]. It is not clear from these experiments whether the carbamate groups contribute to the adsorption

of D 1. A mechanism involving hydrogen bonding of amide groups with edge hydroxyl groups has been suggested by Pefferkorn et al. [32] to explain the adsorption of uncharged polyacrylamides onto kaolin. If we accept that adsorbed NaPA blocks the edge A1 sites, then the reduced starch adsorption (3.0mgg -1) may be a result of hydrogenbonding interactions via the amide groups. The observation that the adsorption of the nonionic corn starch ether D2 is not reduced in the presence of NaPA (Fig. 4) suggests that the starch adsorbs either on top of the polyacrylate layer or onto different sites. In view of the non-ionic nature of this derivative, hydrogen bonding onto silanol or aluminol groups is the most likely mechanism. The adsorption of D2 was also found to decrease with increasing pH on NaPA-free kaolin (Fig. 5). The shape of the curve is unlike that for D1 in having no plateau region between pH 7 and 9. It is likely that despite being predominantly nonionic, molecules of D2 may in fact carry a small anionic charge, probably due to the presence of a small number of carboxyl groups as a result of the starch processing. (Whilst this may be enough to provide a degree of repulsion from the mineral surface, the mechanism of adsorption cannot involve the carboxyl groups because NaPA does not compete.) Both Tadros [33] and Rubio and

151

J.C. Husband,/' Colloids Surfaces A: Physicochem. Eng. Aspects 131 (1998)145 159

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(b) T

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equilibrium [starch] in aqueous phase, mg cm -3

Fig. 3. Adsorption of hydroxyethyl corn starch D2 onto kaolin at 35 wt% solids, no added polyacrylate, pH 8.0_+0.1 (25 C ): (a) amylose, (b) amylopectin adsorption. C, Total starch by anthrone; A, amylose: 7', amylopectin. Dashed lines represent expected adsorption.

Kitchener [34] observed that the adsorption of non-ionic polymers such as polyvinyl alcohol and polyethylene oxide onto silica was reduced with increasing pH, which the authors attributed to the ionisation of silanol groups which reduced the available sites for hydrogen bonding. It has been argued by Ferris and Jepson [35] that the kaolin

surface is coated with a silicic acid gel, rich in silanol groups. Preferential adsorption of the amylose component, which in native starch is the smaller molecule, is not predicted thermodynamically unless the system has not attained equilibrium, or if the very different shapes of the two molecules lead to an

J.C. Husband/Colloids Surfaces A: Physicochem. Eng. Aspects 131 (1998) 145-159

152

Table 4 Adsorption results for sample D2, a hydroxyethyl ether of corn starch, onto kaolin, 35 wt% clay solids, no added sodium polyacrylate Total [starch] in soln./(mg cm -3)

[Amylose] in soln./ (mg cm-3)

Amylose: amylopectin in soln.

Total starch adsorbed/ (mg g- 1)

Amylose adsorbed/ (mg g- 1)

Amylose: amylopectin adsorbed

0.388 0.889 1.603 2.883 3.751 4.736 6.035 6.533 7.708

0.066 0.096 0.294 0.547 0.810 1.049 0.572 0.906 1.092

17.0:83.0 10.8:89.2 18.3:81.7 19.0:81.0 21.5:78.5 22.1:77.9 9.5:90.5 13.9:86.1 14.2:85.8

1.654 2.713 3.019 4.052 5.489 6.268 6.391 6.678 6.226

0.522 0.978 1.075 1.528 1.862 2.115 3.704 3.373 3.521

31.6:68.4 36.0:64.0 35.6:64.4 37.7:62.3 33.9:66.1 33.7:66.3 58.0:42.0 50.5:49.5 56.6:43.4

10

73% ~qr ' ~

T

1

~

27~

2

0

I

0

1

2

3

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4

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5

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6

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7

equilibrium [starch] in aqueous phase, mg cm -3

Fig. 4. Adsorption of hydroxyethyl corn starch D2 onto kaolin at 35 wt% solids, with preaddition of 3.0 mg NaPA g- 1, pH 8.0 + 0.1 (25°C): 0, total starch by anthrone; &, amylose; V, amylopectin. entropic preference for the linear and more flexible amylose molecule, which is unlikely. Van de Steeg et al. [20,21] found a similar result for cationic potato starch onto cellulose, which they attributed to diffusion of the smaller amylose molecules through pores in the outer wall of the cellulose fibres, which the very large amylopectin molecules could not penetrate. At the volume solids of the clay in these adsorption experiments (17%) it is possible that a similar size-exclusion mechanism may take place in the pores between particles.

4.2. Effect of clay concentration on adsorption of D2 Further adsorption experiments were conducted with D2 onto kaolin suspensions at a lower solids concentration in order to examine the effect of increasing the interparticle separation. The ionic strength was maintained by diluting with filtrate from a 35 wt% solids clay suspension. The isotherms are plotted in Figs. 6(a) and (b) and the results are set out in Table 5. At 10 wt%, a two-

J.C. Husband/Colloids Surfaces A. Physicochem. Eng. Aspects 131 (1998) 145 159

imposing the amount of total starch adsorbed from D2 at 10 and 35 wt% clay solids against tile weight of unadsorbed starch per g of clay. The filtration experiments in Fig. 1 and Table 1 suggest that fractionation of starch components by size is not possible because considerable homogenisation with respect to composition has taken place across the size range. The fractionation seen during the adsorption of D2 is therelbre unlikely to be the result of a size exclusion mechanism unless the channels between clay particles are as small as 10 nm in diameter. Mercury porosimetry experiments by Kettle [36] suggest that, in a dried matrix of this grade of kaolin, the pore diameters are at least one order of magnitude higher than this, between 100 and 150nm. In an aqueous suspension, the pores will be even larger.

22 20

% E

18 16

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D2 0

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pH

Fig. 5. Plateau adsorption of phosphate potato starch DI (open points) and hydroxyethyl corn starch D2 (shaded points) onto polyacrylate-free kaolin at 35 wt% solids as a function of suspension pH, 25 C (anthrone assay).

fold increase in the adsorbed amount of D2 to around 15 mg g- ~ clay was found compared with at 35 wt%. Preferential adsorption of amylose is still observed at the lower clay concentration. The adsorption of polydisperse homopolymers has been considered theoretically by Cohen Stuart et al. [25] and the predictions were confirmed experimentally in a later paper by Hlady et al. [26]. Because entropically (at equilibrium) the largest molecules adsorb preferentially, for similar solution polymer concentrations the proportion of large molecules adsorbed will depend upon the amount of clay surface available. At higher clay concentrations, there may not be enough large molecules available to satisfy all the available sites, and the proportion of smaller species adsorbed therefore increases, which tends to reduce the plateau adsorption. Hlady et al. [26] showed that adsorption isotherms determined for the same system at different adsorbent concentrations fell on the same curve if the amount of unadsorbed polymer was normalised to the surface area (or weight) of adsorbent. Fig. 7 confirms this by super-

4.3. Pr~/~rential substitution oJ'amylose in D2 Assuming that the proposed hydrogen bonding of D2 onto kaolin occurs principally via the hydroxyethyl groups, an alternative explanation for the fractionation may be that the amylose has been preferentially derivatised during the reaction of the granular starch with ethylene oxide. There is evidence in the published literature that this occurs during the preparation of methyl [37], hydroxypropyl [38], and diethylaminoethyl [39] ethers from granular starch. The proposed mechanism l\)r this effect is that the amylose is located in the less crystalline regions of the granule and undergoes etherification more readily. An experiment was carried out to recover a sample of non-adsorbed starch D2 from the aqueous phase in equilibrium with kaolin. 5.4 g D2 were added as a solution to a suspension of clay containing 420 g (a dose of 12.9 mg g I) and the clay solids adjusted to 35 wt%. The pH was adjusted to 8.0. After 24 h, the suspension was centrifuged at 8000revmin 1 f o r 5 0 min. The supernatant was carefully decanted and filtered through a polyamide membrane of pore diameter 0.20 gm to remove ultrafine clay particles. The resulting starch solution was concentrated under reduced pressure in a rotary evaporator and dried at 98~C. The recovered starch (2 g) contained no more than 2 wt% of clay, as determined by analysis

154

J. C Husband / Colloids Surfaces A: Physicochem. Eng. Aspects 131 (1998) 145-159 18

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equilibrium [starch] in aqueous phase, mg cm -a

Fig. 6. Adsorptionof hydroxyethylcorn starch D2 onto kaolin at 10wt% solids, pH 7.9+0.1, 25~C:(a) amylose,(b) amylopectin adsorption. O, total starch; &, amylose, V, amylopectin.Dashed lines representexpectedadsorption.

of A1 and Si by ICP AES. Its amylose content was estimated as 15.5 + 3.2 wt%. The results of the HE group analysis (Table 6) show that the DS of the non-adsorbed starch is lower than that added, indicating that the sorbed material has been preferentially substituted. It was also possible to estimate the DS of the amylose and amylopectin fractions,

again by solving a pair of simultaneous equations 1.54 =0.27x +0.73y

(3)

1.19 =0.155x +0.845y

(4)

where x and y are the HE content (%) of the amylose and amylopectin fractions respectively.

J.C. Husband/Colloids Surfaces A. Physicochem. Et ,~ Aspects 131 (1998) 145 159

155

Table 5 Adsorption results for sample D2, a hydroxyethyl ether of HE-corn starch, onto kaolin, 10 wt% clay solids Total [starch] in soln., (mg cm- 3)

[Amylose] in soln./ (mg cm 3)

0.072 0.324 0.560 1.242 2.212 3.146 4.252 5.079 6.099 7.058

Amylose: amylopectin insoln.

Total starch adsorbed (mgg 1)

Amylose adsorbed,, ( m g g ~)

Amylose: mylopectin adsorbed

29.9:70.1 21.0:79.0 21.3:78.7 20.1:79.9 21.4:78.6 18.8:81.2 20.2:79.8 20.4:79.6 20.6:79.4 21.3:78.7

2.143 3.611 4.698 6.990 9.838 10.30 10.82 13.30 13.88 14.92

0.560 I. 156 1.554 2.649 3.779 5.114 5.473 6.6113 7.213 7.635

26.1:73.9 32./):68.0 33.1:66.9 37.9:62.1 38.4:61 .(-, 49.7:50.3 5t).6:49.4 49.6:50.4 52.o:48.0 51.2:48.8

0.022 0.068 0.1 l 9 0.250 0.473 0.592 0.861 1.034 1.259 1.503

10

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non-adsorbed starch, mg g-1 Fig. 7. Adsorbed amount of hydroxyethyl corn starch D2 at 10 (open points) and 35 (shaded points) wt% clay concentrations, plotted against non-adsorbed amount normalised to constant mass of adsorbent.

Table 6 confirms that the HE-content of the amylose fraction of D2 is between five and eight times higher than the amylopectin fraction, and is almost certainly the reason for the observed preferential adsorption. This heterogeneity in DS will lead to analogous adsorption behaviour to that proposed by Hlady et al. [26] for a spread of molecular sizes. In the same way that there is an entropic preference for the largest molecules, there will also be an enthalpic preference for those molecules which have the

highest interaction energy with the surface [40]. This is described by the Flory-Huggins parameter Z, which in this case is influenced by the content of HE groups. In this model, the most highly substituted material is adsorbed preferentially at equilibrium. (Superimposed on this will be the entropic preference for larger molecules, which may limit the extent of the fractionation if the largest molecules have the lowest DS.) In support of this mechanism, Figs. 3 and 4 show that, as more starch is added, the amount of

156

J.C. Husband/Colloids Surfaces A: Phvsicochem. Eng. Aspects 131 (1998) 145 159

Table 6 Hydroxyethyl (HE) contents and degrees of substitution (DS) for non-adsorbed hydroxyethyl ether starch D2 Starch fraction

HE content/%

DS

Whole starch Non-adsorbed starch Amylose (estimated) Arnylopectin (estimated)

1.54, 1.54 1.20, 1. l 8 3.95 _+0.65 0.65 _+0.25

0.056, 0.056 0.043, 0.042 0.148 _+0.024 0.024 + 0.009

4.4. Adsorption of hydroxyethyl potato starch Adsorption experiments with HE potato starch (D3) were conducted at 35 wt% kaolin solids and gave the results shown in Figs. 8 and 9. The results for the NaPA-free system are also summarised in Table7, and show a plateau adsorption of 6.0-6.5 mg g 1, similar to the HE corn starch D2. The iodine method indicated more starch in the aqueous phase than the anthrone assay, and analysis of the data shows that preferential adsorption of amylopectin takes place with this derivative (Fig. 8). (Again, the dashed lines indicate the expected adsorption if the components adsorb in the ratio which they occur in potato starch.) In the presence of NaPA (Fig. 9), less total starch was adsorbed (2.0mgg-1), and the ratio of the adsorbed species was the same as occurred in the added starch. Hence, preferential adsorption of amylopectin was eliminated in the presence of NaPA. The observation that adsorption of D3 is reduced by NaPA addition suggests an ionic mechanism. This is easily understood because it is well documented that potato starch amylopectin differs

amylose adsorbed tends to increase at the expense of amylopectin; indeed, the adsorbed amount of the latter is seen to decrease when the plateau is approached. At the limit of starch addition in these experiments (around 2.0 wt% on clay) the proportion of adsorbed amylose reaches about 50 wt%, and it is possible that at the higher levels of addition such as are used in paper coating (4-8 wt%) amylose may adsorb almost exclusively. This was difficult to investigate experimentally because the very high aqueous phase viscosity at these extreme starch doses led to problems in filtering the suspensions. Centrifugation at these levels also gave anomalous results.

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equilibrium [starch] in aqueous phase, mg cm -3 Fig. 8. Adsorption of hydroxyethyl potato starch D3 onto 35 wt% solids kaolin at pH 7,7,+0.1 (25°C). No added polyacrylate. L), Total starch; ~ , amylose; V, amylopectin. Dashed lines represent expected adsorption.

157

J.C. Husband/Colloids Surfaces A: Physicochem. Eng. Aspects 131 (1998) 145 15u 3

T O

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o 80%

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equilibrium [starch] in aqueous phase, mg cm -3

Fig. 9. Adsorption of hydroxyethylpotato starch D3 onto 35 wt% solids kaolin at pH 7.7 ± 0.1 ( 25 C ). With preaddition of 3.0 mg NaPA g-~ clay. Symbolsas in Fig. 8. Dashed lines represent expectedadsorption. Table 7 Adsorption results for hydroxyethyl ether potato starch D3 onto kaolin, 35 wt% clay solids, no sodium polyacrylatc Total [starch] in soln.: (rag cm-3)

[Amylose] in soln./ (rag cm 3)

Amylose: amylopectin in soln.

Total starch adsorbed,,' (rag g 1)

Amylose a d s o r b e d (mg g 1)

Amylose: amylopectin adsorbed

0.094 0.387 0.915 1.796 2.445 3.697 4.797 6.435 8.729

0.024 0.116 0.245 0.773 0.788 1.098 1.430 1.852 1.885

25.5:74.5 30.0:70.0 26.8:73.2 43.0:57.0 32.2:67.8 29.7:70.3 29.8:70.2 28.8:71.2 21.6:78.4

0.712 1.545 2.234 3.395 3.908 4.869 5.446 6.058 6.324

0.133 0.238 0.331 0.365 0.231 0.311 0.220 0.165 1.004

18.7:81.3 15.4:84.6 14.8:85.2 10.8:89.2 5.9:94.1 6.4:93.6 4.0:96.0 2.7:97.3 15.9:84. I

from that of corn in that it contains a small number of native phosphate ester groups [41,42]. Bourne et al. [43] suggested that adsorption of amylopectin onto aluminium hydroxide might be used to fractionate potato starch. The phosphorus analysis (Section 2) suggests that, for the amylopectin fraction of D3, there are about five phosphate groups per 1000 anhydroglucose units, assuming that potato starch contains 80% amylopectin. This small substitution is sufficient to confer polyelectrolyte character upon the molecule [42], and the adsorption behaviour follows that of the

phosphate ester D1. This again leads to heterogeneity in Zs and in this instance the adsorption of amylopectin is favoured. This suggests that the ionic mechanism of adsorption is enthalpically favoured over hydrogen bonding, which seems intuitively reasonable. Khosla et al. [44] found that the adsorption of phosphorylated starch onto ferric oxide was markedly exothermic. As the level of adsorbed starch approaches the plateau, the proportion of adsorbed amylose falls to less than 5% of the total adsorbed starch. When the ionic interaction is blocked by the presence of polyacry-

158

E.C. Husband/Colloids SurJaces A: Physieochem. Eng. Aspects 131 (1998) 145-159

late, the adsorbed species has the same composition as the parent starch.

acetylated derivatives, for example. Work is also needed to characterise these industrial starch derivatives more fully.

5. Conclusions The adsorption of phosphate ester starch derivatives onto kaolin is likely to occur via complexation of the anionic phosphate ligand with positivelycharged aluminium species on the kaolin edges. The adsorption was depressed in the presence of polyacrylate dispersants, which may be explained either by competition for surface sites, or through mutual electrostatic repulsion. With hydroxyethyl derivatives of corn starch, which are substantially non-ionic, hydrogen-bonding via silanol or aluminol groups is the suggested mechanism, and in this case no effect on adsorption was found when polyacrylate was added. Preferential adsorption of amylose was found to occur independently of clay concentration between l0 and 35 wt%, suggesting a higher affinity than amylopectin for the clay surface sites. The non-adsorbed starch was shown to have a lower content of HE groups than the original derivative, and calculations suggest that the DS of the amylose fraction is of the order of five times higher than the amylopectin in this particular corn starch sample. Preferential etherification of amylose has been well established in the literature for other starch ethers. Evidence also suggests that the amylopectin fraction in these derivatives appears to be heavily degraded, Hydroxyethyl derivatives of potato starch differ from corn starch, exhibiting ionic character due to the presence of phosphate ester groups. Because these occur almost exclusively on the branched amylopectin fraction, preferential adsorption of this component was found to take place on polyacrylate-free kaolin. In the presence of NaPA, where ionic interactions were blocked, no preferential adsorption of amylopectin was detected from the potato starch ether. This work has demonstrated that widely differing adsorption behaviour occurs onto kaolin, depending on the source of the starch and its derivatisation. Much of this appears to be new to the literature. Further studies are needed on other commonly used derivatives such as oxidised and

Acknowledgment The author wishes to thank Professor R.H. Ottewill, Dr. A. de Keizer, Dr. T. Cosgrove and Dr. Lars Wfigberg for valuable discussions, and the Board of E.C.C. plc for permission to publish this work. Thanks are also expressed to Dr. J.E. Keiser of Penford Products for carrying out the DS determinations on the hydroxyethyl starches, and to Dr. N.J. Elton for assistance in computer programming.

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J. ('. Husband/Colloids Surfaces A: Physicochem. Eng. Aspects 131 (19983 145 159 [15]W. Banks and C.T. Greenwood, Starch and its Components, Edinburgh University Press, 1975. [16] H.F. Zobel, Chapter 2 in Ref. [2]. [17] F.J. Viles, L. Silverman, Ind. Eng. Chem. Anal. Ed. 21 18) ( 19493 950. [18] H.D. Graham, G. Mitchell, J. Food Sci. 28 (19733 546. [19] L. J~irnstr6m, G. Strom, P. Stenius, Tappi J. 70 (9) (19873 101. [20] H.G.M. van de Steeg, A. de Keizer, M.A. Cohen Stuart, B.H, Bijsterbosch, Colloids Surf. A 70 ( 19933 91. [21] H.G.M. van de Steeg, PhD Thesis, Wageningen, 1992. [22] R.S. Higginbotham, Shirley Inst. Mere. 23 (19493 171. [23] J.H.M. Hovenkamp-Hermelink, J.N. De Vries, P. Adamse. E. Jacobsen, B. Witholt, W.J. Feenstra, Potato Res. 31 ( 19883 241. [24] R.R. Baktwin, R.S. Bear, R.E. Rundle, J. Am. Chem. Soc. 6611944) 111. [25] M.A. Cohen Stuart, J.M.H.M. Scheutjens, G.J. Fleer, J. Polym. Sci. Polym. Phys. Ed. 18 (19803 559. [26] V. Hlady, J. Lyklema, G.J. Fleer, J. Colloid Interface Sci. 87 (2) (19823 395. [27] P,M. McGenity, P.A.C. Gane, J.C. Husband and M.S. Engley, Proc. Tappi Coating Conf., Orlando, 1992, TAPPI Press, Atlanta, pp. 133 146. [28] A.S, Michaels, Ind. Eng. Chem. 50 (6) (19583 951~

159

[29] J.I. Bidwelk W.B. Jepson, G.L. Toms, Clav Miner. 8 ( 1970 ) 445. [30] B, Rand, I.E. Melton, J. Colloid Interface Sci. 60 (1977) 308. [31] A.W. Flegmann, J.W. Goodwin, R.H. Ottewill, Proc. Brit. Ceram. Soc. 13 (19693 31. [32] E. Pefferkorn, L. Nabzar, A. Carroy, J. Colloid Interlace Sci. 106 { 13 119933 94. [33] Th.F. Tadros, J. Colloid Interface Sci. 46 13~ ( 1974} 528. [34] J. Rubio, J.A. Kitchener, J. Colloid Interface Sci. 57 ~1 ) (19763 132. [35] A.P. Ferris. W.B. Jepson, J. Colloid lnlerface Sci. 51 ( 19751 245. [36] J.P. Kettle, PhD Thesis, University of Plymouth, 1996 [37] P.A.M. Steeneken, E. Smilh, Carbohydrate Res. 209 (1991) 239. [38] B.M.N.Mohd. Azemi, M. Wooton, St~irkc 46 ( 1994~ 440. [39] P.A.M. Steeneken. St~irkc 36 (19843 13. [40] T. Cosgrove, personnal communication. 6 December 1995. [41] M.W. Radomski, M.D. Smith, Cereal Chem. 40 ( 1%33 31. [42] W. Banks and C.T. Greenwood, p. 50 in Ref. [10]. [43] E.J. Bourne. G.H. Donnison. S. Peal and WA. Whclan, J. Chem. Sot., 119493 1. [44] N.K. Khosla, R.P. Bagat, K.S. Gandhi, 4.K. Biswas, Colloids Surl'. ,~ ( 19843 321.