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Starch derivatives of high degree of functionalization
Colloids and Surfaces A: Physicochem. Eng. Aspects 254 (2005) 75–80
Starch derivatives of high degree of functionalization 10. Flocculation of kaolin dispersions Svetlana Bratskayaa,b,∗ , Simona Schwarzb , Tim Liebertc , Thomas Heinzec a
Institute of Chemistry, Far East Department of Russian Academy of Sciences, 159, Prosp. 100-letiya Vladivostoka, Vladivostok 690022, Russia b Institute of Polymer Research Dresden, Hohe Strasse 6, D-01069 Dresden, Germany c Center of Excellence for Polysaccharide Research, Friedrich Schiller University of Jena, Humboldtstrasse 10, D-07743 Jena, Germany Received 17 June 2004; accepted 26 November 2004 Available online 7 January 2005
Abstract Flocculation properties of cationic potato starch derivatives (2-hydroxy-3-trimethylammonium-propyl starch chloride) with degree of substitution (DS) up to 1.54 were investigated in kaolin dispersions at various solid/liquid ratios and ionic strengths by means of turbidity measurements and colloid titration. It was found that flocculation has occurred long before the kaolin surface charge neutralization was reached. Flocculant amount required for a complete phase separation decreased with the increase of DS, and dispersion restabilization was observed at polymer overdose for all the studied starch derivatives and solid/liquid ratios. The mechanism of the kaolin dispersion flocculation with cationic starch derivatives was suggested to be a combination of bridging and “charge patch” processes. Addition of electrolyte proved to enhance the dispersion flocculation through lowering the required polymer dose and broadening the flocculation window, although no substantial influence of ionic strength on adsorption of the starch derivatives was found. © 2004 Elsevier B.V. All rights reserved. Keywords: Polyelectrolytes; Flocculation; Adsorption; Cationic starch derivatives; Kaolin
1. Introduction Natural starch has a capacity to acquire properties significantly extending its possible applications in food and nonfood industrial sectors as a result of physical and chemical modification. This is the main reason for a worldwide sustained growth of demand for starch products at approximate annual rate of 4%. Although a substantial amount of nonfood starch derivatives is used in mineral processing industry [1–4] and sludge dewatering [5], around 20% of overall natural and modified starches amount is consumed by the paper industry, where non-ionic, anionic, and cationic derivatives are used as binding, sizing, and wet strength agents. The efficiency of starch performance in the paper-making process is closely related to its ability to bind coating fillers and pigments as well as to retain on the cellulose fibers. Dif∗
Corresponding author. Tel.: +7 4232 313 583; fax: +7 4232 313 583. E-mail address: [email protected] (S. Bratskaya).
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.11.030
ferent types of starch have found application in paper coatings developed to improve smoothness, light-scattering ability, print gloss, and ink receptivity of paper. Several studies were focused on investigation of interaction between one of the most industrially important paper filler—kaolin, and various starch derivatives [6–9]. The effect of agglomerated clay particles bulk structure on the properties of the coatings formed had been also addressed [6]. It was concluded that cationic and amphoteric starches form more favourable bulk structure of coating dispersion [6] which, in combination with their high retention on cellulose fibers [10], determines superior properties of these derivatives in comparison with oxidized [6,11] and non-ionic [12] starches. However, most of the works cited above were carried out for cationic starch with very low degree of substitution (DS < 0.2), which is typical for commercially available derivatives [13,14]. Among starches of different botanic origin, a special attention was given in literature to the performance of potato starch [15], since it proved to have supe-
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rior properties over other starches as a wet-end additive for papermaking. Most likely, it is due to the presence of phosphate anionic sites, which can enhance retention through the complexes formation with polyvalent inorganic cations also used in the papermaking process [16]. Nevertheless, cationization of potato starch at very low DS cannot overcompensate net-negative charge of macromolecules at pH > 8 caused by contribution from ionized phosphate groups. Thus, the detailed investigation of cationic starch flocculation properties in kaolin dispersions [7] corresponds to the case when substrate and polymer are both negatively charged, and only insignificant attraction can occur. Aside from the coating process in papermaking, cationic starches, especially those with high DS, have a high potential for application in closed white water systems due to the efficient flocculation of anionic trash and its fixation to larger fibers [17]. In this paper we discuss the effect of DS, solid/liquid ratio, and salt addition on adsorption and flocculation in the system kaolin/cationic starch derivatives with DS values in the range from 0.25 to 1.54. 2. Experimental 2.1. Materials 2.1.1. Kaolin Kaolin (USP-type, Sigma) was used in all experiments as supplied. Kaolin dispersions in water (with and without KCl additions) and in Tris–HCl buffer solution (pH = 8 ± 0.1) were prepared with ultrasonic treatment for 15 min followed by vigorous stirring during 1 h. Solid content in the dispersions varied from 0.01% to 0.5% (w/v). Average size of kaolin particles in dispersions (Tris–buffer, pH = 8) was 310 nm as determined by means of the photon correlation spectroscopy on the device specially designed for measurements in concentrated systems (Microtrac UPA 150). 2.1.2. Starch Cationic starch derivatives (2-hydroxy-3-trimethylammonium-propyl starch chloride) with DS from 0.25 to 1.54, corresponding to 25 and 154 quaternary ammonium groups per 100 glucose units, respectively, were synthesized according to the route described elsewhere [18]. Potato starch with amylose content of 28% and molecular weight of 40 × 106 g mol−1 (Emsland St¨arke GmbH, Emlichheim, Germany) was used as a raw material. Substitution degree of derivatives was determined by nitrogen and chlorine elemental analysis as described in [18]. Starch stock solutions with concentration 1 and 5 g l−1 were prepared by dissolving an appropriate amount of starch with stirring for 1 h at 60 ◦ C and then for 24 h at room temperature. 2.1.3. Other chemicals Other chemicals were of analytical grade. Distilled water with conductivity <1 S cm−1 was used in all experiments.
2.2. Methods 2.2.1. Flocculation Flocculation was investigated as batch tests in a series of ten beakers, each containing 50 ml of kaolin dispersions. After addition of starch solution aliquot (100–1500 l) mixtures were stirred for 15 min and allowed to sediment for 20 min. After that 10 ml of the supernatant was taken from the beaker surface, and its optical density (D500 ) was measured at 500 nm using Lambda 800 UV-VIS Spectrometer (Perkin Elmer). Residual turbidity (RT) was calculated according to 500 500 the formula: RT = (Dsupernatant /Ddispersion ) × 100%. 2.2.2. Floc size measurements Dispersions for floc size characterization were prepared according to the above described procedure for flocculation tests at cationic starch doses corresponding to the complete solid/liquid separation. Floc sizes were measured at various rotation speeds (500–3500 rpm) by means of the laser diffraction particle size analyzer Malvern-Sizer equipped with impeller mixer. 2.2.3. Colloid titration Charge densities of cationic potato starches and kaolin dispersions were determined by colloid titration until zero charge point in PC-controlled system connecting particle charge detector (PCD-03, M¨utek, Germany) and 702 SM Titrino (Metrohm, Switzerland). Kaolin dispersions containing different amounts of starch were prepared according to the same procedure as for flocculation tests; aliquots for titration were taken under constant stirring. Solutions of sodium polyethylene sulfonate (PES-Na) or polydiallyldimethyl ammonium chloride (PDADMAC) were used as titrants for cationic and anionic systems, respectively. Charge density (CD) (mequiv. g−1 ) was calculated according to the formula: Ctitrant Vtitrant , Vm where Ctitrant is the titrant (PDADMAC or PES-Na) concentration (mequiv. l−1 ), V is the volume of titrated solution, Vtitrant is the equivalent titrant volume, m is the content of starch or kaolin in titrated solution (g l−1 ).
CD =
2.2.4. Adsorption isotherms Adsorption of cationic starch derivatives on kaolin was investigated at 22 ◦ C. Kaolin dispersions in Tris–buffer (pH = 8 ± 0.1) with solid content 1 and 5 g l−1 and initial starch concentration from 20 to 450 mg l−1 were gently agitated during 24 h. After centrifugation at 10,000 rpm for 10 min, starch content in supernatant was determined by two methods—colloid titration, which is sensitive only to charged polymers, and photometric phenol–sulfuric method [19], which gives total concentration of polysaccharide in solution. The amount of adsorbed starch was calculated using the difference between known initial starch concentration and
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concentration measured in supernatant. The results obtained by both methods were found to be in a good agreement.
3. Results and discussion 3.1. Apparent charge density of cationic starches Cationic starch derivatives studied were water soluble and showed high positive zeta potential over broad pH range due to the presence of quaternary ammonium groups. Molecular dispersion of cationic starch in the solution was ensured by high substitution degree, providing sufficient electrostatic stabilization. Colloid titration of starches at pH = 8 ± 0.02 showed that there is a linear correlation between substitution degree (DS) and apparent charge density (CD) of the cationic starches up to DS = 0.7 (Fig. 1). Nevertheless, at higher DS the apparent CD was lower than it could be expected from the correlation found for starches with DS < 0.7. We assume that the most probable explanation should be the superstructure of the starch. The main fracture of potato starch is represented by highly branched, “bushy” amylopectin molecules. It was shown [20] that amylopectin structure in the solution can be represented as a porous sphere. Thus, due to the steric hindrances, cationic sites differing in location (sphere surface or interior) will not be equally involved into polyelectrolyte complexes formation or interactions with surface anionic sites. Moreover, as was shown previously [18], around 10% of glucose units in starch derivatives were di-substituted at high substitution degree (DS = 0.92). Therefore, positive charges in amylopectin can be located in a closer proximity in comparison with linear polyelectrolytes, including those used for colloid titration. Such a mismatch in charged segment spacing together with branched structure of amylopectin can result in nonstoichiometric charge neutralization and lower consumption of polyanion.
Fig. 1. Apparent charge density of cationic potato starches (pH = 8) in dependence on degree of substitution (DS).
Fig. 2. Adsorption of cationic starches with different degrees of substitution (DS) on kaolin surface at pH = 8. Solid content 0.5%: () DS = 0.25, () DS = 0.37, (♦) DS = 0.44; solid content 0.1%: () DS = 0.25.
3.2. Adsorption and flocculation Flocculation experiments, as well as investigation of starch adsorption on kaolin surface, were carried out at pH = 8 ± 0.2, which is typical pH of the coating process in papermaking industry. At this pH both kaolin faces (basal and edge) are negatively charged [21] that determines high attraction between cationic starches and substrate surface. In the previous investigations [7,9] of cationic starch adsorption on kaolin surface, a round shape of adsorption isotherms was found, most likely, due to weak electrostatic attraction between particles surface and starch derivatives with low DS. In our investigations, adsorption isotherms for all cationic starches studied were of high affinity type (Fig. 2) indicating strong interaction between the substrate and the polymer. Fig. 2 shows that the value of adsorption decreases with increase of cationic starch CD until DS = 0.44, while further increase of functionalization degree did not influence the value of starch adsorption. Such a behavior can be explained by two reasons: increasing electrostatic repulsion between adsorbed macromolecules and macromolecules in the solution at high substitution degree; and detrimental effect of flocculation on polymer adsorption. For highly substituted cationic starches, flocculation occurs very rapidly at low polymer doses, thus excluding substantial part of surface area and preventing further adsorption of polymer. Another feature of adsorption behavior was a tendency to higher adsorption values at lower solid content in kaolin dispersions. Such an observation is also in accordance with the assumption that the flocculation process is detrimental for the polymer adsorption due to the reduction of the accessible surface area [22]. As a result of adsorption of cationic starch on kaolin surface, expressed as Cp /Cs (mass ratio of polymer to substrate), destabilization of the dispersion was observed in a certain concentration range. It was followed by the dispersion restabilization at the polymer overdosing. Fig. 3 shows that the efficiency of phase separation increases with increasing DS, i.e. the higher DS, the lower amount of starch derivative is required to reach complete separation of solid and liq-
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Fig. 3. Flocculation of kaolin dispersion (pH = 8, solid content 1 g l−1 ) by cationic starches with different degrees of substitution (DS): () DS = 1.54, () DS = 0.96, () DS = 0.67, (䊉) DS = 0.25.
uid phases. Although it is well known that polymers with lower charge density usually provide flocculation in broader concentration range, nearly the same width of flocculation window was observed for all studied starch derivatives. This fact, most likely, also originates from peculiarities of highly branched amylopectin structure not allowing the formation of favourable for bridging loops and tails even at comparatively low DS. Amylopectin has a rather compact inner structure with flexible exterior arms [23], each with a length of 20–30 glucose units [10], and this structure does not undergo significant changes upon cationization [24]. Thus, conformation of the adsorbed amylopectin does not remarkably depend on its charge density, and the formation of loops and tails typical for low and moderately charged linear polymers is not possible. At the same time, starches with lower DS show somewhat weaker restabilization effect guaranteeing their application in a broader concentration range. Fig. 4 summarizes the results of flocculation effectiveness of cationic starches in kaolin dispersions with different solid content. Cp /Cs optimum ratio corresponds to the lowest poly-
mer dose sufficient for complete phase separation (D500 of the supernatant <0.01 a.u.). There is no obvious difference between the performance of starch derivatives in dispersions with solid content of 1 and 5 g l−1 , which is in agreement with previously reported fact [25] that the polymer dose required for the system destabilization is proportional to the solid content, i.e. Cp /Cs ratio should remain constant, thus, ensuring flocculation at the same surface coverage degree. Interesting findings in [22] show that at high solid content a uniform coverage of particles surface is usually not reached due to the comparable times of dispersion agitation and polymer adsorption. If the affinity of the polymer to the substrate is high, one step addition of the polymer, even under vigorous agitation, results in local polymer overdose and formation of particles fraction with high coverage degree at the polymer concentration corresponding theoretically to much lower surface coverage. At low solid content polymers are distributed much more uniformly between particles, therefore, ensuring nearly the same coverage degree for all particles in the dispersion. In accordance with [22], at low solid content the system under study is more sensitive to changes in flocculant properties, and more pronounced difference in flocculating doses of starch derivatives with different DS is observed (Fig. 4, solid content 0.1 g l−1 ). Good phase separation found at low solid content of kaolin allows concluding that highly substituted cationic starches are also promising for application in closed water cycles in papermaking where it is important to remove low content of dispersed fillers and pigments remained in the effluent after the coating process. Comparison of the flocculation experiments and adsorption results shows that the optimum cationic starch dose corresponds to 25–35% of the maximum adsorption value, so that flocculation occurs long before the surface saturation is reached. Fig. 5 depicts residual turbidity and surface charge calculated from the results of kaolin dispersions colloid titration versus added amount of cationic starches of different CD. The zero charge points are in agreement with the results obtained in adsorption experiments; besides, the initial slop of the curves is the same for all cationic starches indicating their comparable affinity to the kaolin surface. Fig. 5 also unambiguously illustrates that dispersion restabilization starts when approximately only a half of total surface charge is neutralized. 3.3. Salt effect
Fig. 4. Dependence of the flocculant optimum dose (Cp /Cs optimum) on its charge density in dispersions with different solid contents: () 0.1 g l−1 , () 1 g l−1 , (䊉) 5 g l−1 .
Strong electrolytes are always present in industrial solutions including those of papermaking process. Thus, it is important to know the influence of ionic strength on flocculation effectiveness of the polymers used. The effect of electrolyte concentration on kaolin dispersion stability in the presence of cationic starches was investigated at 1, 10, and 100 mM of KCl in water solutions without pH adjustment, so the resulting pH was around 6 ± 0.2. Although the polymer adsorption was nearly unaffected by salt concentration (1 and
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An interesting feature of the flocculation in the presence of salt is a considerable widening of the flocculation window. Such a behavior having electrostatic origin was qualitatively predicted by DLVO theory and experimentally proved in several charge-stabilized systems [26]. Addition of electrolyte leads to the surface charge screening and, consequently, to the decrease of electrostatic repulsion between particles, that in its turn allows attracting in the broader concentration range of flocculant. 3.4. On the mechanism of kaolin dispersion flocculation by cationic starches
10 mM), the flocculation effectiveness in comparison with water was significantly enhanced at all studied ionic strengths (Fig. 6). Addition of electrolyte results in compression of the electrical double layer and decrease of the gyration radius of starch in the solution, although the former parameter is affected to remarkably higher extent. The effective thickness of the electrical double layer is around 1000 nm in salt free solution, and it reduces to 100 and 10 nm at 10−5 and 10−3 mol l−1 solutions of 1:1 electrolytes, respectively, while the gyration radius of cationic starches changes from 400 to 200 nm when 10–100 mM of salt is added [26].
In most of the previous investigations of cationic starch flocculation properties, the bridging mechanism of the flocculation was suggested. The only exception, as far as we know from the literature search, concerns highly substituted cationic starch, so called fixative, where the possibility of the charge patch mechanism was discussed [17]. We assume that for highly charged cationic starch derivatives no explicit difference between the charge patch and bridging mechanisms can be outlined. Taking into account the fact that the length of the polymer should exceed the double layer thickness, one can see that possibilities for the bridging flocculation in aqueous dispersions without electrolyte addition are very limited, since the thickness of the diffuse layer (1000 nm) is more than twice of the amylopectin gyration radius (maximum 400 nm). But in the presence of electrolytes reducing the double layer thickness formation of bridges becomes theoretically possible. It is well known that the higher the charge density of polyelectrolyte, the flatter its conformation upon adsorption, and the lower the possibilities for bridging. Fig. 7 shows that largest flocs were formed with cationic starch of lowest functionalization degree (DS = 0.25) at low shear rate (500 rpm) that is, most likely, the result of more favourable bridging between particles in comparison with starches of higher functionalization degree. Under increasing shear rate,
Fig. 6. Effect of KCl addition on flocculation of kaolin dispersion by cationic starch (DS = 0.25): () without salt addition, () 1 mM KCl, () 10 mM KCl, () 100 mM KCl.
Fig. 7. Dependence of floc size on applied shear rate in kaolin dispersions treated with optimum doses of cationic starches: () DS = 1.54, () DS = 0.6, (䊉) DS = 0.25.
Fig. 5. Kaolin dispersion residual turbidity (a) and surface charge (b) determined by colloid titration in the presence of cationic starch derivatives with DS: () 1.54; () 0.44; () 0.25.
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bridges are destroyed, and the floc size for all three starches (DS = 0.25, 0.6 and 1.54) becomes comparable, although still with slightly larger size in the case of starch with the lowest DS. It is worth mentioning that floc destruction in the studied systems is a reversible process, which indicates that bridging corresponds to the equilibrium conditions. Taking into account high affinity of cationic starches to substrate and strong correlation between cationic starch CD and required flocculant dose, we can conclude that electrostatic interactions are the main driving force in flocculation of kaolin dispersions, although contribution from bridging, increasing with decreasing cationic starch DS, should not be neglected.
4. Conclusions 1. Apparent charge density of the cationic starch derivatives obtained was proportional to DS up to 0.7. Further increase of DS resulted in lower CD than it can be expected from the linear correlation found. Steric hindrances and possible charge spacing mismatch in complex formation between amylopectin and oppositely charged polyelectrolytes due to highly branched structure of the former were suggested as possible reasons for behavior deviations. 2. Adsorption of cationic starch derivatives on kaolin surface was found to be of high affinity type, with maximum adsorption value decreasing with increasing polymer substitution degree up to DS = 0.5, after which no significant changes in amounts of adsorbed starches were detected. Adsorption values were lower at higher solid content in dispersions due to the detrimental effect of flocculation on polymer adsorption. 3. All investigated starch derivatives showed high flocculation efficiency at optimum flocculant doses that correlated well with CD of derivatives: the lower CD, the higher amount of starch derivative was required for complete phase separation. Overdosing of the flocculants resulted in dispersion restabilization in all studied cases. 4. Addition of electrolyte significantly enhanced flocculation and widened flocculation window, however, it did not have a noticeable effect on adsorption of starches. 5. Simultaneous contributions from both bridging and charge patch mechanisms were suggested for flocculation of kaolin dispersion by cationic starch derivatives.
Acknowledgement Financial support from Arbeitsgemeinschaft Industrieller F¨ordervereinigungen e.V. (AiF) under the project 13558BG is gratefully acknowledged. References [1] A.I. Arol, I. Iwasaki, Separat. Sci. Technol. 38 (2003) 647. [2] R.J. Deshpande, K.A. Natarajan, S.G. Kittur, T.R.R. Rao, Trans. Indian Inst. Met. 50 (1997) 391. [3] S.S. Ibrahim, N.A. Abdelkhalek, Miner. Eng. 5 (1992) 907. [4] C.O. Nebo, L.O. Asuquo, C.K. Nworu, M.C. Fuerstenau, J. Dispers. Sci. Technol. 17 (1996) 23. [5] W.G. Hunt, R.J. Belz, US Patent 3,962,079 (1976). [6] X.Q. Wang, J. Gron, D. Eklund, J. Pulp Paper Sci. 22 (1996) 486. [7] L. J¨arnstr¨om, L. Lason, M. Rigdahl, Colloids Surf. A: Physicochem. Eng. Aspects 104 (1995) 191. [8] L. J¨arnstr¨om, L. Lason, M. Rigdahl, U. Eriksson, Colloids Surf. A: Physicochem. Eng. Aspects 104 (1995) 207. [9] J.C. Husband, Colloids Surf. A: Physicochem. Eng. Aspects 131 (1998) 145. [10] D. Eklund, T. Lindstr¨om, Paper Chemistry. An Introduction, DT Paper Publications, Grankulla, Finland, 1991. [11] H.L. Lee, J.Y. Shin, C.H. Koh, H. Ryu, D.J. Lee, C. Sohn, Tappi J. 1 (2002) 34. [12] J.C. Formento, M.G. Maximino, L.R. Mina, M.I. Srayh, M.J. Martinez, Appita J. 47 (1994) 305. [13] M.R. Kweon, F.W. Sosulski, P.R. Bhirud, Starch/Starke 49 (1997) 59. [14] M.R. Kweon, P.R. Bhirud, F.W. Sosulski, Starch/Starke 48 (1996) 214. [15] M. Bjorklund, L. Wagberg, Colloids Surf. A: Physicochem. Eng. Aspects 105 (1995) 199. [16] H.G.M. Vandesteeg, A. Dekeizer, M.A.C. Stuart, B.H. Bijsterbosch, Colloids Surf. A: Physicochem. Eng. Aspects 70 (1993) 91. [17] V. Bobacka, D. Eklund, Colloids Surf. A: Physicochem. Eng. Aspects 152 (1999) 285. [18] V. Haack, T. Heinze, G. Oelmeyer, W.M. Kulicke, Macromol. Mater. Eng. 287 (2002) 495. [19] M. Dubois, U.A. Giles, J.K. Hamilton, P.A. Rebers, F. Smith, Anal. Chem. 28 (1956) 350. [20] A. Larsson, M. Rasmusson, Carbohydrate Res. 304 (1997) 315. [21] M.B. Hocking, K.A. Klimchuk, S. Lowen, J. M. S. Rev. Macromol. Chem. Phys. C39 (1999) 177. [22] R. Hogg, Colloids Surf. A: Physicochem. Eng. Aspects 146 (1999) 253. [23] A. Larsson, Colloids Surf. B: Biointerf. 12 (1998) 23. [24] W. Burchard, M. Frank, E. Michel, Ber. Bunsenges. Phys. Chem. 100 (1996) 807. [25] S. Baran, D. Gregory, Colloid J. 58 (1996) 9. [26] S. Wall, P. Samuelsson, G. Degerman, P. Skolund, A. Samuelsson, J. Colloid Interf. Sci. 151 (1992) 178.
Starch derivatives of high degree of functionalization 10. Flocculation of kaolin dispersions Svetlana Bratskayaa,b,∗ , Simona Schwarzb , Tim Liebertc , Thomas Heinzec a
Institute of Chemistry, Far East Department of Russian Academy of Sciences, 159, Prosp. 100-letiya Vladivostoka, Vladivostok 690022, Russia b Institute of Polymer Research Dresden, Hohe Strasse 6, D-01069 Dresden, Germany c Center of Excellence for Polysaccharide Research, Friedrich Schiller University of Jena, Humboldtstrasse 10, D-07743 Jena, Germany Received 17 June 2004; accepted 26 November 2004 Available online 7 January 2005
Abstract Flocculation properties of cationic potato starch derivatives (2-hydroxy-3-trimethylammonium-propyl starch chloride) with degree of substitution (DS) up to 1.54 were investigated in kaolin dispersions at various solid/liquid ratios and ionic strengths by means of turbidity measurements and colloid titration. It was found that flocculation has occurred long before the kaolin surface charge neutralization was reached. Flocculant amount required for a complete phase separation decreased with the increase of DS, and dispersion restabilization was observed at polymer overdose for all the studied starch derivatives and solid/liquid ratios. The mechanism of the kaolin dispersion flocculation with cationic starch derivatives was suggested to be a combination of bridging and “charge patch” processes. Addition of electrolyte proved to enhance the dispersion flocculation through lowering the required polymer dose and broadening the flocculation window, although no substantial influence of ionic strength on adsorption of the starch derivatives was found. © 2004 Elsevier B.V. All rights reserved. Keywords: Polyelectrolytes; Flocculation; Adsorption; Cationic starch derivatives; Kaolin
1. Introduction Natural starch has a capacity to acquire properties significantly extending its possible applications in food and nonfood industrial sectors as a result of physical and chemical modification. This is the main reason for a worldwide sustained growth of demand for starch products at approximate annual rate of 4%. Although a substantial amount of nonfood starch derivatives is used in mineral processing industry [1–4] and sludge dewatering [5], around 20% of overall natural and modified starches amount is consumed by the paper industry, where non-ionic, anionic, and cationic derivatives are used as binding, sizing, and wet strength agents. The efficiency of starch performance in the paper-making process is closely related to its ability to bind coating fillers and pigments as well as to retain on the cellulose fibers. Dif∗
Corresponding author. Tel.: +7 4232 313 583; fax: +7 4232 313 583. E-mail address: [email protected] (S. Bratskaya).
0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.11.030
ferent types of starch have found application in paper coatings developed to improve smoothness, light-scattering ability, print gloss, and ink receptivity of paper. Several studies were focused on investigation of interaction between one of the most industrially important paper filler—kaolin, and various starch derivatives [6–9]. The effect of agglomerated clay particles bulk structure on the properties of the coatings formed had been also addressed [6]. It was concluded that cationic and amphoteric starches form more favourable bulk structure of coating dispersion [6] which, in combination with their high retention on cellulose fibers [10], determines superior properties of these derivatives in comparison with oxidized [6,11] and non-ionic [12] starches. However, most of the works cited above were carried out for cationic starch with very low degree of substitution (DS < 0.2), which is typical for commercially available derivatives [13,14]. Among starches of different botanic origin, a special attention was given in literature to the performance of potato starch [15], since it proved to have supe-
76
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rior properties over other starches as a wet-end additive for papermaking. Most likely, it is due to the presence of phosphate anionic sites, which can enhance retention through the complexes formation with polyvalent inorganic cations also used in the papermaking process [16]. Nevertheless, cationization of potato starch at very low DS cannot overcompensate net-negative charge of macromolecules at pH > 8 caused by contribution from ionized phosphate groups. Thus, the detailed investigation of cationic starch flocculation properties in kaolin dispersions [7] corresponds to the case when substrate and polymer are both negatively charged, and only insignificant attraction can occur. Aside from the coating process in papermaking, cationic starches, especially those with high DS, have a high potential for application in closed white water systems due to the efficient flocculation of anionic trash and its fixation to larger fibers [17]. In this paper we discuss the effect of DS, solid/liquid ratio, and salt addition on adsorption and flocculation in the system kaolin/cationic starch derivatives with DS values in the range from 0.25 to 1.54. 2. Experimental 2.1. Materials 2.1.1. Kaolin Kaolin (USP-type, Sigma) was used in all experiments as supplied. Kaolin dispersions in water (with and without KCl additions) and in Tris–HCl buffer solution (pH = 8 ± 0.1) were prepared with ultrasonic treatment for 15 min followed by vigorous stirring during 1 h. Solid content in the dispersions varied from 0.01% to 0.5% (w/v). Average size of kaolin particles in dispersions (Tris–buffer, pH = 8) was 310 nm as determined by means of the photon correlation spectroscopy on the device specially designed for measurements in concentrated systems (Microtrac UPA 150). 2.1.2. Starch Cationic starch derivatives (2-hydroxy-3-trimethylammonium-propyl starch chloride) with DS from 0.25 to 1.54, corresponding to 25 and 154 quaternary ammonium groups per 100 glucose units, respectively, were synthesized according to the route described elsewhere [18]. Potato starch with amylose content of 28% and molecular weight of 40 × 106 g mol−1 (Emsland St¨arke GmbH, Emlichheim, Germany) was used as a raw material. Substitution degree of derivatives was determined by nitrogen and chlorine elemental analysis as described in [18]. Starch stock solutions with concentration 1 and 5 g l−1 were prepared by dissolving an appropriate amount of starch with stirring for 1 h at 60 ◦ C and then for 24 h at room temperature. 2.1.3. Other chemicals Other chemicals were of analytical grade. Distilled water with conductivity <1 S cm−1 was used in all experiments.
2.2. Methods 2.2.1. Flocculation Flocculation was investigated as batch tests in a series of ten beakers, each containing 50 ml of kaolin dispersions. After addition of starch solution aliquot (100–1500 l) mixtures were stirred for 15 min and allowed to sediment for 20 min. After that 10 ml of the supernatant was taken from the beaker surface, and its optical density (D500 ) was measured at 500 nm using Lambda 800 UV-VIS Spectrometer (Perkin Elmer). Residual turbidity (RT) was calculated according to 500 500 the formula: RT = (Dsupernatant /Ddispersion ) × 100%. 2.2.2. Floc size measurements Dispersions for floc size characterization were prepared according to the above described procedure for flocculation tests at cationic starch doses corresponding to the complete solid/liquid separation. Floc sizes were measured at various rotation speeds (500–3500 rpm) by means of the laser diffraction particle size analyzer Malvern-Sizer equipped with impeller mixer. 2.2.3. Colloid titration Charge densities of cationic potato starches and kaolin dispersions were determined by colloid titration until zero charge point in PC-controlled system connecting particle charge detector (PCD-03, M¨utek, Germany) and 702 SM Titrino (Metrohm, Switzerland). Kaolin dispersions containing different amounts of starch were prepared according to the same procedure as for flocculation tests; aliquots for titration were taken under constant stirring. Solutions of sodium polyethylene sulfonate (PES-Na) or polydiallyldimethyl ammonium chloride (PDADMAC) were used as titrants for cationic and anionic systems, respectively. Charge density (CD) (mequiv. g−1 ) was calculated according to the formula: Ctitrant Vtitrant , Vm where Ctitrant is the titrant (PDADMAC or PES-Na) concentration (mequiv. l−1 ), V is the volume of titrated solution, Vtitrant is the equivalent titrant volume, m is the content of starch or kaolin in titrated solution (g l−1 ).
CD =
2.2.4. Adsorption isotherms Adsorption of cationic starch derivatives on kaolin was investigated at 22 ◦ C. Kaolin dispersions in Tris–buffer (pH = 8 ± 0.1) with solid content 1 and 5 g l−1 and initial starch concentration from 20 to 450 mg l−1 were gently agitated during 24 h. After centrifugation at 10,000 rpm for 10 min, starch content in supernatant was determined by two methods—colloid titration, which is sensitive only to charged polymers, and photometric phenol–sulfuric method [19], which gives total concentration of polysaccharide in solution. The amount of adsorbed starch was calculated using the difference between known initial starch concentration and
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concentration measured in supernatant. The results obtained by both methods were found to be in a good agreement.
3. Results and discussion 3.1. Apparent charge density of cationic starches Cationic starch derivatives studied were water soluble and showed high positive zeta potential over broad pH range due to the presence of quaternary ammonium groups. Molecular dispersion of cationic starch in the solution was ensured by high substitution degree, providing sufficient electrostatic stabilization. Colloid titration of starches at pH = 8 ± 0.02 showed that there is a linear correlation between substitution degree (DS) and apparent charge density (CD) of the cationic starches up to DS = 0.7 (Fig. 1). Nevertheless, at higher DS the apparent CD was lower than it could be expected from the correlation found for starches with DS < 0.7. We assume that the most probable explanation should be the superstructure of the starch. The main fracture of potato starch is represented by highly branched, “bushy” amylopectin molecules. It was shown [20] that amylopectin structure in the solution can be represented as a porous sphere. Thus, due to the steric hindrances, cationic sites differing in location (sphere surface or interior) will not be equally involved into polyelectrolyte complexes formation or interactions with surface anionic sites. Moreover, as was shown previously [18], around 10% of glucose units in starch derivatives were di-substituted at high substitution degree (DS = 0.92). Therefore, positive charges in amylopectin can be located in a closer proximity in comparison with linear polyelectrolytes, including those used for colloid titration. Such a mismatch in charged segment spacing together with branched structure of amylopectin can result in nonstoichiometric charge neutralization and lower consumption of polyanion.
Fig. 1. Apparent charge density of cationic potato starches (pH = 8) in dependence on degree of substitution (DS).
Fig. 2. Adsorption of cationic starches with different degrees of substitution (DS) on kaolin surface at pH = 8. Solid content 0.5%: () DS = 0.25, () DS = 0.37, (♦) DS = 0.44; solid content 0.1%: () DS = 0.25.
3.2. Adsorption and flocculation Flocculation experiments, as well as investigation of starch adsorption on kaolin surface, were carried out at pH = 8 ± 0.2, which is typical pH of the coating process in papermaking industry. At this pH both kaolin faces (basal and edge) are negatively charged [21] that determines high attraction between cationic starches and substrate surface. In the previous investigations [7,9] of cationic starch adsorption on kaolin surface, a round shape of adsorption isotherms was found, most likely, due to weak electrostatic attraction between particles surface and starch derivatives with low DS. In our investigations, adsorption isotherms for all cationic starches studied were of high affinity type (Fig. 2) indicating strong interaction between the substrate and the polymer. Fig. 2 shows that the value of adsorption decreases with increase of cationic starch CD until DS = 0.44, while further increase of functionalization degree did not influence the value of starch adsorption. Such a behavior can be explained by two reasons: increasing electrostatic repulsion between adsorbed macromolecules and macromolecules in the solution at high substitution degree; and detrimental effect of flocculation on polymer adsorption. For highly substituted cationic starches, flocculation occurs very rapidly at low polymer doses, thus excluding substantial part of surface area and preventing further adsorption of polymer. Another feature of adsorption behavior was a tendency to higher adsorption values at lower solid content in kaolin dispersions. Such an observation is also in accordance with the assumption that the flocculation process is detrimental for the polymer adsorption due to the reduction of the accessible surface area [22]. As a result of adsorption of cationic starch on kaolin surface, expressed as Cp /Cs (mass ratio of polymer to substrate), destabilization of the dispersion was observed in a certain concentration range. It was followed by the dispersion restabilization at the polymer overdosing. Fig. 3 shows that the efficiency of phase separation increases with increasing DS, i.e. the higher DS, the lower amount of starch derivative is required to reach complete separation of solid and liq-
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Fig. 3. Flocculation of kaolin dispersion (pH = 8, solid content 1 g l−1 ) by cationic starches with different degrees of substitution (DS): () DS = 1.54, () DS = 0.96, () DS = 0.67, (䊉) DS = 0.25.
uid phases. Although it is well known that polymers with lower charge density usually provide flocculation in broader concentration range, nearly the same width of flocculation window was observed for all studied starch derivatives. This fact, most likely, also originates from peculiarities of highly branched amylopectin structure not allowing the formation of favourable for bridging loops and tails even at comparatively low DS. Amylopectin has a rather compact inner structure with flexible exterior arms [23], each with a length of 20–30 glucose units [10], and this structure does not undergo significant changes upon cationization [24]. Thus, conformation of the adsorbed amylopectin does not remarkably depend on its charge density, and the formation of loops and tails typical for low and moderately charged linear polymers is not possible. At the same time, starches with lower DS show somewhat weaker restabilization effect guaranteeing their application in a broader concentration range. Fig. 4 summarizes the results of flocculation effectiveness of cationic starches in kaolin dispersions with different solid content. Cp /Cs optimum ratio corresponds to the lowest poly-
mer dose sufficient for complete phase separation (D500 of the supernatant <0.01 a.u.). There is no obvious difference between the performance of starch derivatives in dispersions with solid content of 1 and 5 g l−1 , which is in agreement with previously reported fact [25] that the polymer dose required for the system destabilization is proportional to the solid content, i.e. Cp /Cs ratio should remain constant, thus, ensuring flocculation at the same surface coverage degree. Interesting findings in [22] show that at high solid content a uniform coverage of particles surface is usually not reached due to the comparable times of dispersion agitation and polymer adsorption. If the affinity of the polymer to the substrate is high, one step addition of the polymer, even under vigorous agitation, results in local polymer overdose and formation of particles fraction with high coverage degree at the polymer concentration corresponding theoretically to much lower surface coverage. At low solid content polymers are distributed much more uniformly between particles, therefore, ensuring nearly the same coverage degree for all particles in the dispersion. In accordance with [22], at low solid content the system under study is more sensitive to changes in flocculant properties, and more pronounced difference in flocculating doses of starch derivatives with different DS is observed (Fig. 4, solid content 0.1 g l−1 ). Good phase separation found at low solid content of kaolin allows concluding that highly substituted cationic starches are also promising for application in closed water cycles in papermaking where it is important to remove low content of dispersed fillers and pigments remained in the effluent after the coating process. Comparison of the flocculation experiments and adsorption results shows that the optimum cationic starch dose corresponds to 25–35% of the maximum adsorption value, so that flocculation occurs long before the surface saturation is reached. Fig. 5 depicts residual turbidity and surface charge calculated from the results of kaolin dispersions colloid titration versus added amount of cationic starches of different CD. The zero charge points are in agreement with the results obtained in adsorption experiments; besides, the initial slop of the curves is the same for all cationic starches indicating their comparable affinity to the kaolin surface. Fig. 5 also unambiguously illustrates that dispersion restabilization starts when approximately only a half of total surface charge is neutralized. 3.3. Salt effect
Fig. 4. Dependence of the flocculant optimum dose (Cp /Cs optimum) on its charge density in dispersions with different solid contents: () 0.1 g l−1 , () 1 g l−1 , (䊉) 5 g l−1 .
Strong electrolytes are always present in industrial solutions including those of papermaking process. Thus, it is important to know the influence of ionic strength on flocculation effectiveness of the polymers used. The effect of electrolyte concentration on kaolin dispersion stability in the presence of cationic starches was investigated at 1, 10, and 100 mM of KCl in water solutions without pH adjustment, so the resulting pH was around 6 ± 0.2. Although the polymer adsorption was nearly unaffected by salt concentration (1 and
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An interesting feature of the flocculation in the presence of salt is a considerable widening of the flocculation window. Such a behavior having electrostatic origin was qualitatively predicted by DLVO theory and experimentally proved in several charge-stabilized systems [26]. Addition of electrolyte leads to the surface charge screening and, consequently, to the decrease of electrostatic repulsion between particles, that in its turn allows attracting in the broader concentration range of flocculant. 3.4. On the mechanism of kaolin dispersion flocculation by cationic starches
10 mM), the flocculation effectiveness in comparison with water was significantly enhanced at all studied ionic strengths (Fig. 6). Addition of electrolyte results in compression of the electrical double layer and decrease of the gyration radius of starch in the solution, although the former parameter is affected to remarkably higher extent. The effective thickness of the electrical double layer is around 1000 nm in salt free solution, and it reduces to 100 and 10 nm at 10−5 and 10−3 mol l−1 solutions of 1:1 electrolytes, respectively, while the gyration radius of cationic starches changes from 400 to 200 nm when 10–100 mM of salt is added [26].
In most of the previous investigations of cationic starch flocculation properties, the bridging mechanism of the flocculation was suggested. The only exception, as far as we know from the literature search, concerns highly substituted cationic starch, so called fixative, where the possibility of the charge patch mechanism was discussed [17]. We assume that for highly charged cationic starch derivatives no explicit difference between the charge patch and bridging mechanisms can be outlined. Taking into account the fact that the length of the polymer should exceed the double layer thickness, one can see that possibilities for the bridging flocculation in aqueous dispersions without electrolyte addition are very limited, since the thickness of the diffuse layer (1000 nm) is more than twice of the amylopectin gyration radius (maximum 400 nm). But in the presence of electrolytes reducing the double layer thickness formation of bridges becomes theoretically possible. It is well known that the higher the charge density of polyelectrolyte, the flatter its conformation upon adsorption, and the lower the possibilities for bridging. Fig. 7 shows that largest flocs were formed with cationic starch of lowest functionalization degree (DS = 0.25) at low shear rate (500 rpm) that is, most likely, the result of more favourable bridging between particles in comparison with starches of higher functionalization degree. Under increasing shear rate,
Fig. 6. Effect of KCl addition on flocculation of kaolin dispersion by cationic starch (DS = 0.25): () without salt addition, () 1 mM KCl, () 10 mM KCl, () 100 mM KCl.
Fig. 7. Dependence of floc size on applied shear rate in kaolin dispersions treated with optimum doses of cationic starches: () DS = 1.54, () DS = 0.6, (䊉) DS = 0.25.
Fig. 5. Kaolin dispersion residual turbidity (a) and surface charge (b) determined by colloid titration in the presence of cationic starch derivatives with DS: () 1.54; () 0.44; () 0.25.
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bridges are destroyed, and the floc size for all three starches (DS = 0.25, 0.6 and 1.54) becomes comparable, although still with slightly larger size in the case of starch with the lowest DS. It is worth mentioning that floc destruction in the studied systems is a reversible process, which indicates that bridging corresponds to the equilibrium conditions. Taking into account high affinity of cationic starches to substrate and strong correlation between cationic starch CD and required flocculant dose, we can conclude that electrostatic interactions are the main driving force in flocculation of kaolin dispersions, although contribution from bridging, increasing with decreasing cationic starch DS, should not be neglected.
4. Conclusions 1. Apparent charge density of the cationic starch derivatives obtained was proportional to DS up to 0.7. Further increase of DS resulted in lower CD than it can be expected from the linear correlation found. Steric hindrances and possible charge spacing mismatch in complex formation between amylopectin and oppositely charged polyelectrolytes due to highly branched structure of the former were suggested as possible reasons for behavior deviations. 2. Adsorption of cationic starch derivatives on kaolin surface was found to be of high affinity type, with maximum adsorption value decreasing with increasing polymer substitution degree up to DS = 0.5, after which no significant changes in amounts of adsorbed starches were detected. Adsorption values were lower at higher solid content in dispersions due to the detrimental effect of flocculation on polymer adsorption. 3. All investigated starch derivatives showed high flocculation efficiency at optimum flocculant doses that correlated well with CD of derivatives: the lower CD, the higher amount of starch derivative was required for complete phase separation. Overdosing of the flocculants resulted in dispersion restabilization in all studied cases. 4. Addition of electrolyte significantly enhanced flocculation and widened flocculation window, however, it did not have a noticeable effect on adsorption of starches. 5. Simultaneous contributions from both bridging and charge patch mechanisms were suggested for flocculation of kaolin dispersion by cationic starch derivatives.
Acknowledgement Financial support from Arbeitsgemeinschaft Industrieller F¨ordervereinigungen e.V. (AiF) under the project 13558BG is gratefully acknowledged. References [1] A.I. Arol, I. Iwasaki, Separat. Sci. Technol. 38 (2003) 647. [2] R.J. Deshpande, K.A. Natarajan, S.G. Kittur, T.R.R. Rao, Trans. Indian Inst. Met. 50 (1997) 391. [3] S.S. Ibrahim, N.A. Abdelkhalek, Miner. Eng. 5 (1992) 907. [4] C.O. Nebo, L.O. Asuquo, C.K. Nworu, M.C. Fuerstenau, J. Dispers. Sci. Technol. 17 (1996) 23. [5] W.G. Hunt, R.J. Belz, US Patent 3,962,079 (1976). [6] X.Q. Wang, J. Gron, D. Eklund, J. Pulp Paper Sci. 22 (1996) 486. [7] L. J¨arnstr¨om, L. Lason, M. Rigdahl, Colloids Surf. A: Physicochem. Eng. Aspects 104 (1995) 191. [8] L. J¨arnstr¨om, L. Lason, M. Rigdahl, U. Eriksson, Colloids Surf. A: Physicochem. Eng. Aspects 104 (1995) 207. [9] J.C. Husband, Colloids Surf. A: Physicochem. Eng. Aspects 131 (1998) 145. [10] D. Eklund, T. Lindstr¨om, Paper Chemistry. An Introduction, DT Paper Publications, Grankulla, Finland, 1991. [11] H.L. Lee, J.Y. Shin, C.H. Koh, H. Ryu, D.J. Lee, C. Sohn, Tappi J. 1 (2002) 34. [12] J.C. Formento, M.G. Maximino, L.R. Mina, M.I. Srayh, M.J. Martinez, Appita J. 47 (1994) 305. [13] M.R. Kweon, F.W. Sosulski, P.R. Bhirud, Starch/Starke 49 (1997) 59. [14] M.R. Kweon, P.R. Bhirud, F.W. Sosulski, Starch/Starke 48 (1996) 214. [15] M. Bjorklund, L. Wagberg, Colloids Surf. A: Physicochem. Eng. Aspects 105 (1995) 199. [16] H.G.M. Vandesteeg, A. Dekeizer, M.A.C. Stuart, B.H. Bijsterbosch, Colloids Surf. A: Physicochem. Eng. Aspects 70 (1993) 91. [17] V. Bobacka, D. Eklund, Colloids Surf. A: Physicochem. Eng. Aspects 152 (1999) 285. [18] V. Haack, T. Heinze, G. Oelmeyer, W.M. Kulicke, Macromol. Mater. Eng. 287 (2002) 495. [19] M. Dubois, U.A. Giles, J.K. Hamilton, P.A. Rebers, F. Smith, Anal. Chem. 28 (1956) 350. [20] A. Larsson, M. Rasmusson, Carbohydrate Res. 304 (1997) 315. [21] M.B. Hocking, K.A. Klimchuk, S. Lowen, J. M. S. Rev. Macromol. Chem. Phys. C39 (1999) 177. [22] R. Hogg, Colloids Surf. A: Physicochem. Eng. Aspects 146 (1999) 253. [23] A. Larsson, Colloids Surf. B: Biointerf. 12 (1998) 23. [24] W. Burchard, M. Frank, E. Michel, Ber. Bunsenges. Phys. Chem. 100 (1996) 807. [25] S. Baran, D. Gregory, Colloid J. 58 (1996) 9. [26] S. Wall, P. Samuelsson, G. Degerman, P. Skolund, A. Samuelsson, J. Colloid Interf. Sci. 151 (1992) 178.