Complexation of Ferric Oxide Particles with Pectins of Different Charge

Jul 25, 2008 - The effect of polyelectrolyte charge density on the electrical properties and stability of suspensions of oppositely charged oxide part...
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Langmuir 2008, 24, 9495-9499

9495

Complexation of Ferric Oxide Particles with Pectins of Different Charge Density Viktoria Milkova, Kamelia Kamburova, Ivana Petkanchin, and Tsetska Radeva* Institute of Physical Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria ReceiVed April 22, 2008. ReVised Manuscript ReceiVed June 11, 2008 The effect of polyelectrolyte charge density on the electrical properties and stability of suspensions of oppositely charged oxide particles is followed by means of electro-optics and electrophoresis. Variations in the electro-optical effect and the electrophoretic mobility are examined at conditions where fully ionized pectins of different charge density adsorb onto particles with ionizable surfaces. The charge neutralization point coincides with the maximum of particle aggregation in all suspensions. We find that the concentration of polyelectrolyte, needed to neutralize the particle charge, decreases with increasing charge density of the pectin. The most highly charged pectin presents an exception to this order, which is explained with a reduction of the effective charge density of this pectin due to condensation of counterions. The presence of condensed counterions, remaining bound to the pectin during its adsorption on the particle surface, is proved by investigation of the frequency behavior of the electro-optical effect at charge reversal of the particle surface.

Introduction Polymer coatings are widely used to control the suspension stability in a number of biological and technological processes. In particular, the adsorption of polyelectrolytes onto oppositely charged particles is of interest for applications ranging from foods and cosmetics to water treatment, paper manufacturing, and so forth. At low polyelectrolyte concentration, the adsorption of the oppositely charged polyelectrolyte leads to a decrease of the particle charge. This reduces the repulsive electrostatic interactions between the approaching particles and, subsequently, leads to an increase in the particle aggregation. The rate of the aggregation process is enhanced and leads to the formation of the biggest aggregates when the particle charge is neutralized by the adsorbed polyelectrolyte (near the particle isoelectric point).1–9 Beyond the isoelectric point, the adsorption of polyelectrolyte continues. This causes a reversal of the particle charge and restabilization of the suspension by electrostatic repulsion.2,5–11 The overcompensation of the particle charge has been explained by additional surface ionization in the vicinity of the adsorbing polymer, which drives further adsorption.12,13 On the other hand, Shklovskii et al.14,15 consider the reversal of the particle charge as a result of strong lateral correlations, which reduce the * To whom correspondence should be addressed. Telephone: 359 2 979 39 22. Fax: 359 2 971 26 88. E-mail: [email protected]. (1) Gregory, J. J. Colloid Interface Sci. 1973, 42, 448. (2) Bouyer, F.; Robben, A.; Yu, W. L.; Borkovec, M. Langmuir 2001, 17, 5225. (3) Lin, W.; Galletto, P.; Borkovec, M. Langmuir 2004, 20, 7465. (4) Kleimann, J.; Gehin-Delval, C.; Auweter, H.; Borkovec, M. Langmuir 2005, 21, 3688. (5) Radeva, Ts.; Petkanchin, I. J. Colloid Interface Sci. 1999, 220, 112. (6) Buleva, M.; Petkanchin, I. Colloids Surf., A 1999, 151, 225. (7) Buleva, M.; Peikov, V.; Pefferkorn, E.; Petkanchin, I. Colloids Surf., A 2001, 186, 155. (8) Buleva, M.; Petkanchin, I. Colloids Surf., A 2002, 209, 289. (9) Milkova, V.; Radeva, Ts. J. Colloid Interface Sci. 2006, 298, 550. (10) Ashmore, M.; Hearn, J.; Karpowicz, F. Langmuir 2001, 17, 1069. (11) Radeva, Ts. Colloids Surf., A 2002, 209, 219. (12) Bonekamp, B. C.; Lyklema, J. J. Colloid Interface Sci. 1986, 113, 67. (13) Hoogeveen, N. G.; Cohen Stuart, M. A.; Fleer, G. J. J. Colloid Interface Sci. 1996, 182, 133. (14) Nguyen, T. T.; Shklovskii, B. I. Phys. ReV. Lett. 2002, 89, 018101-1. (15) Grosberg, A. Yu.; Nguyen, T. T.; Shklovskii, B. I. ReV. Mod. Phys. 2002, 74, 329.

electrostatic repulsion between the polymer chains and allow more chains to adsorb than is necessary to neutralize the particle charge. Although the interaction between a polyelectrolyte and an oppositely charged particle is dominated by electrostatics, stoichiometric 1:1 charge neutralization is found only rarely in colloid-polyelectrolyte systems.10,16 Recently, superstoichiometric charging ratios, corresponding to the ratio of the polymer charge and the particle charge, were reported for particles in the presence of highly charged polyelectrolytes.4 This means that several charges of the polyelectrolyte are necessary to neutralize a single particle charge. Borkovec et al.4 have demonstrated that the charging ratio increases with increasing charge density of the polyelectrolyte when the polymer and the particle are either strong electrolytes or completely ionized at conditions of the experiment. This is attributed to an increase in the counterions binding to the polyelectrolyte.3,4 The phenomenon is known as counterion condensation, leading to a decrease of the bare polyelectrolyte charge to an effective value.17 When the charge density of the particle changes upon adsorption of a strong (or fully ionized) polyelectrolyte, the charging ratio is expected to remain close to unity. The reason is that the ionizable systems regulate their charge density in order to achieve as close as possible distance between charges on the polyelectrolyte and the particle surface.4 The purpose of the present study is to examine the effect of charge density of fully ionized polyelectrolytes on the stability and electrical properties of a suspension containing oppositely charged particles with ionizable surfaces. We employ electric light scattering and electrophoresis to follow the behavior of a β-FeOOH suspension as a function of concentration of pectins with different degrees of esterification. In an earlier report, we presented data on the behavior of a β-FeOOH suspension as a function of the charge density of another anionic polyelectrolyte, sodium carboxymethyl cellulose.18 They revealed that almost same polyelectrolyte concentrations were necessary for full neutralization of the particle charge. In that case, however, the charge density of all carboxymethyl cellulose samples was high (16) Ashmore, M.; Hearn, J. Langmuir 2000, 16, 4906. (17) Manning, G. S. J. Chem. Phys. 1969, 51, 924. (18) Radeva, Ts.; Kamburova, K. J. Colloid Interface Sci. 2006, 293, 290.

10.1021/la8012602 CCC: $40.75  2008 American Chemical Society Published on Web 07/25/2008

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Table 1. Characteristics of the Pectins: Molecular Weight (Mm), Galacturonic Acid Content (Gal), Average Degree of Esterification (DE), Contour Length (L), and Charge Density Parameter (ζ) pectin

Mm [kDa]

% Gal

% DE

L [nm]

ζ

5A 5C 5E 5G

58 78 54 67

85 91 92 94

71 51 36 21

145 200 140 175

0.4 0.7 0.9 1.1

enough to cause counterion condensation that results in equal values of their effective charge density. The present study examines three samples of pectin with lower charge density than is necessary to cause counterion condensation and one sample with higher charge density. We use the advantages of electrooptics to provide information on the mobility of counterions of the adsorbed polymers and, thus, to discriminate between free and condensed counterions.5–9 In addition, the electro-optical measurements were employed to probe the existence of lateral correlations between the adsorbed polymer chains in the stabilized suspensions. The aim is to achieve a better understanding of the particle charge overcompensation, since this process plays important role in the optimization of the suspension’s stability.1–4

Experimental Section Materials. Four samples of citrus pectin, which are chemically de-esterified (PEC 5A, 5C, 5E, and 5G), are obtained from CP Kelco. The galacturonic acid (Gal) content in all pectins is with essentially random distribution of the free galacturonosyl residues along the polymer chain.19 Correspondingly, the charge density parameter of the pectins ζ varies from 0.4 to 1.1 (see Table 1), taking into account that the mean charge distance d for the polygalacturonic acid is 0.475 nm.20 (The charge density parameter ζ ) lB/d is defined as the ratio of the Bjerrum length lB ) e2/(εkT) to the charge spacing d, where e is the electronic charge, ε is the dielectric constant of the solution, and kT is the Boltzmann energy term.17 At ζ > 1, monovalent counterions condense on the polymer backbone to reduce its charge to an effective value, corresponding to ζ ) 1.) The molecular weight of the pectin samples and the contour length of their chains are also shown in Table 1. Pectin stock solutions of concentration 1 g dm-3 are prepared by dissolution in double-distilled water. β-Ferric hydrous oxide particles (β-FeOOH) are prepared by acid hydrolysis of 1.8 × 10-2 M FeCl3 · 6H2O solution containing 10-5 M HCl over a period of 3 weeks at room temperature.21 The extraneous Fe3+ ions are removed by centrifugation in double-distilled water. This procedure gives particles of uniform shape (ellipsoids) and narrow size distribution. By electron microscopy, the average dimensions of the major and minor axes of the particles are determined to be a ) 285 ( 50 nm and b ) 72 ( 10 nm (axial ratio 4). In water solution, the β-FeOOH particles are positively charged, with a surface charge density of less than 0.015 C m-2.22 In this experiment, the concentration of the particles is 8 × 10-3 g dm-3 (about 2 × 109 particles in 1 cm-3) in order to avoid the multiple light scattering effects. The conductivity of the suspension is 2 × 10-6 S cm-1, and the pH is 5.8. The colloid/polyelectrolyte complexes are formed by adding pectin at an appropriate concentration to the suspension of β-FeOOH particles and stirring for 20 min. The pH of the suspensions after addition of pectin is approximately pH 5.5, which allows full ionization of all pectin samples (the intrinsic pK value of the galacturonic acid of pectin is 2.9).23 Electric Light Scattering. In electro-optics, the intensity of scattered light by colloids is measured in the presence of an externally (19) Krzeminski, A.; Marudova, M.; Moffat, J.; Noel, T. R.; Parker, R.; Wellner, N.; Ring, S. G. Biomacromolecules 2006, 7, 498. (20) Walkinshaw, M. D.; Arnott, S. J. Mol. Biol. 1981, 153, 1075. (21) Zocher, H.; Heller, W. Z. Anorg. Allg. Chem. 1930, 186, 75. (22) Kanungo, S. B.; Mahapatra, D. M. Colloids Surf., A 1989, 42, 173. (23) Ralet, M.-C.; Dronnet, V.; Buchholt, H. C.; Thibault, J.-F. Carbohydr. Res. 2001, 336, 117.

Figure 1. Electrophoretic mobility Ue of β-FeOOH particles as a function of the concentration of pectins with different degrees of esterification: (9) PEC 5A; (b) PEC 5C; (0) PEC 5E; and (2) PEC 5G.

applied electric field. The interaction between the particle electric moments (permanent and induced) and the electric field leads to orientation of anisometric particles. As a consequence, the light scattered by the suspension is changed. The steady-state electrooptical effect R presents a quadratic function of the electric field strength when the energy of particle orientation is lower than the Boltzmann energy kT:

R)

[

]

E2 A(Ka, Kb) p′2 + (γa - γb) I0(Ka, Kb) kT 4kT

(1)

Here, A(Ka, Kb) and I0(Ka, Kb) are optical functions depending on the particle dimensions and K ) 2π/λ sin θ/2, where λ is the wavelength of the incident light, θ is the angle of observation, and p′, γa, and γb are the permanent dipole moment and the electrical polarizability with respect to the major and minor axes of the particle, respectively.24,25 The transient process of Brownian particle disorientation after switching the electric field off permits determination of the rotational diffusion coefficient Dr ) 1/6τ, relative to the particle dimensions. From the change in the particle dimensions due to the polyelectrolyte adsorption, one can calculate also the hydrodynamic thickness LH of the adsorbed layer. Two effects are generally observed in colloidal suspensions when a sinusoidal electric field is applied.24,25 The first one appears in the range of particle rotation (Hz region), and it is related to the “permanent” dipole moment of the particle. The second electrooptical effect (in the kHz range) is related to polarization of the diffuse part of the particle electrical double layers. The mobility of ions in the diffuse layer is accepted as equal to that of free ions in a salt solution. In a suspension stabilized by adsorption of highly charged polyelectrolyte, an additional (third) effect appears near the range of particle rotation. It is attributed to polarization of condensed counterions near the adsorbed polyion surface.5,26 The mobility of the condensed counterions is lower than the mobility of the free ions because of the strong attraction to the polyion surface.27 In this experiment, the electric light scattering is recorded at an angle of 90° with respect to the electric field, using white unpolarized light. The technical details are described in ref 25. Electrophoresis. The particle electrophoretic mobilities Ue are measured using a Rank Brothers Mark II apparatus with a flat quartz cell at 25 °C.

Results and Discussion Electrophoretic and Stability Measurements. Figure 1 shows variation in the electrophoretic mobility of β-FeOOH particles as a function of the concentration of pectins with different charge density. At low pectin concentration, the electrophoretic mobility (24) O’Konski, C. T.; Yoshioka, K.; Orttung, W. H. J. Phys. Chem. 1959, 63, 1558. (25) Stoylov, S. P. AdV. Colloid Interface Sci. 1971, 3, 45. (26) Radeva, Ts. J. Colloid Interface Sci. 1995, 174, 368. (27) Ookubo, N.; Hirai, Y.; Ito, K.; Hayakawa, R. Macromolecules 1989, 22, 1359.

Ferric Oxide Particle/Pectin Complexation

of the particles decreases as expected due to the partial neutralization of their charge. Under certain concentrations, the neutralization of the particle charge leads to the isoelectric point. For PEC 5E, the isoelectric point is located at a concentration of ∼5 × 10-5 g dm-3, while 10-4 and 2 × 10-4 g dm-3 PEC 5C and PEC 5A, respectively, are necessary to reach full neutralization of the particle charge. This indicates that the concentration of polyelectrolyte, necessary for full neutralization of the particle charge, increases with decreasing charge density of the adsorbed pectin. The exception to this order is the most highly charged PEC 5G. The isoelectric point in the presence of PEC 5G is shifted toward higher concentration as compared to the less charged PEC 5E and coincides with that of PEC 5C (10-4 g dm-3). Our explanation is that, due to partial condensation of counterions, the charge density of PEC 5G is reduced and only its effective charge contributes to neutralize the particle charge. The increase in concentration of PEC 5G, necessary for full neutralization of the particle charge, indicates that the condensed counterions of PEC 5G remain bound to the polyion during the process of the surface charge neutralization. One may expect, therefore, that the charging ratio at the isoelectric point will be higher than unity if one used the charge density of the PEC 5G backbone for the estimation. The explanation of the above presented result is based on the prediction of Sens and Joanny28 that a highly charged polyelectrolyte may retain its condensed counterions when it adsorbs onto a weakly charged substrate. Estimations of the fraction of condensed counterions and experimental evidence for their participation in the creation of the electro-optical effect from β-FeOOH particles, covered with highly charged polyelectrolyte, were already reported.9,18 Experimental evidence that a substantial fraction of condensed counterions remains bound after the adsorption of highly charged polyelectrolyte onto oppositely charged substrate has also been obtained by other techniques.29,30 For the less charged pectins, the charging ratio is expected to remain close to unity, in line with predictions of Borkovec et al. that the ionizable systems shift their ionization degree in order to match more closely the conditions of 1:1 stoichiometric charge neutralization.4 From the known value of the β-FeOOH charge density (ca. 0.015 C m-2 at ionic strength 10-4 M, corresponding to an average distance between its charged groups of 3 nm22) and the distance between the charged groups on the fully ionized PEC 5A, 5C, 5E, and 5G (1.9, 1.1, 0.8, and 0.6 nm, respectively, corresponding to the values of the charge density parameter shown in Table 1), we calculate charging ratios larger than unity for all samples (1.6, 2.7, 3.8, and 5.0 respectively). However, our calculation is approximate for at least two reasons. The first one is that we used values for the nearest-neighbor charge distances of pectins and oxide particles before mixing, which means that the additional surface ionization is not taken into account. (The additional ionization of the oxide particles would bring the charging ratio closer to unity.) The second reason is that such an estimation is reasonable only for flat adsorbed layers, where the polymer chains adsorb exclusively in a trainlike conformation and a surface charge can neutralize one charge on the polyelectrolyte.4 One interesting quantity to consider is the adsorbed amount, which depends on the polyelectrolyte charge density. It is reasonable to expect that the increase in the particle charge due to the adsorption of highly charged pectin will force more chains (28) Sens, P.; Joanny, J.-F. Phys. ReV. Lett. 2000, 84, 4862. (29) Sennato, S.; Bordi, F.; Cametti, C.; Diociaiuti, M.; Malaspina, P. Biochim. Biophys. Acta 2005, 1714, 11. (30) Shin, Y.; Roberts, J. E.; Santore, M. M. J. Colloid Interface Sci. 2002, 247, 220.

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Figure 2. Time for disorientation after switching the electric field off τ as a function of the concentration of pectins with different degrees of esterification: (9) PEC 5A; (b) PEC 5C; (0) PEC 5E; and (2) PEC 5G.

Figure 3. Dependence of the electro-optical effect R on the electric field strength E2 (at frequency 1 kHz) for particles coated with pectins of different charge density: (9) PEC 5A; (b) PEC 5C; (0) PEC 5E; and (2) PEC 5G. The concentration of pectin is 10-2 g dm-3. Inset: electrooptical effect at high field strengths.

to adsorb here than on a surface of the same initial constant charge. The adsorption of less charged pectin will cause a smaller increase in the particle charge, thus leading to a less adsorbed amount than that in the first case. On the other hand, more polymer chains of the less charged pectin are necessary to neutralize the particle charge in comparison to the more highly charged one. For these reasons, we expect insignificant dependence of the layer thickness on the pectin charge density in our systems. This expectation is tested by comparing thicknesses of the adsorbed layers in stabilized suspensions, where pectin chains adsorb onto single oxide particles. Figure 2, which shows variation in the rotational relaxation times of the particles τ as a function of the polyelectrolyte concentration, strongly confirms our suggestion: almost same hydrodynamic thicknesses are calculated for the adsorbed layers of all pectins at concentrations g 5 × 10-3 g dm-3 (LH ∼ 6 nm). Figure 2 shows a maximum of particle aggregation at the isoelectric point and stabilization of the suspension away from it. (The electro-optical effect at the isoelectric point is not high enough to allow determination of τ with accuracy. However, values of τ > 3 ms are measured in all suspensions, which indicates a steep rise in the particle size.) Coincidence between the isoelectric point and the maximum of aggregation was reported for various systems, which indicates that the process is chargedriven and that the charge reversal governs the stability of these systems.1–9,18 Effect of Charge Density on Behavior of Stabilized Suspension. The electro-optical effect from stabilized suspensions can provide important information on the electrical properties of the single polymer-coated particle. An increase in the electrooptical effect value with increasing charge density of the adsorbed pectin is shown in Figure 3 for the stabilized suspensions (in the presence of pectins with a concentration of 10-2 g dm-3). At low field strengths, the linear increase of the electro-optical effect

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Figure 4. Frequency dependence of the electro-optical effect for bare oxide particles (/) and for particles in the presence of PEC 5G with different concentrations: (b) 10-5; ([) 2 × 10-4; (]) 10-3; (∆) 5 × 10-3; (2) 10-2; and (O) 2 × 10-2 g dm-3. Field strength E ) 2.3 × 104 V m-1. Inset: electro-optical effect at 5 × 10-5 g dm-3 PEC 5G (9).

MilkoVa et al.

not high enough to cause counterion condensation.) However, the polarization of condensed counterions of PEC 5G causes the observed decrease in the effect relaxation above the isoelectric point. These results are in line with simulation studies of Linse and Jonsson,33 who found an excess of counterions to the particle below the isoelectric point and an excess of counterions to the adsorbed polyelectrolyte at charge reversal. Our interpretation of the observed decrease in the effect relaxation in the presence of the most highly charged PEC 5G is that condensed counterions, having lower mobility in comparison to the diffuse ones, are accumulated on the surface of the polymer-coated particle. The relaxation frequency of the effect arising from polarization of condensed counterions along the PEC 5G chain νcr can be estimated as

νcr )

Figure 5. Frequency dependence of the effect for bare oxide particles (/) and for particles coated with pectins of different charge density: (9) PEC 5A; (b) PEC 5C; (0) PEC 5E; and (2) PEC 5G. The concentration of pectin is 10-2 g dm-3. Field strength E ) 2.3 × 104 V m-1. The relaxation frequency of the effect from particles coated with PEC 5G is indicated by an arrow.

with E2 corresponds well to the increase of the pectin charge density. However, the effect values shift toward saturation at lower fields in the presence of the most highly charged pectin. The inset in Figure 3 shows the same behavior at higher fields. Such an earlier shift toward saturation has already been observed for other systems in the presence of highly charged polyelectrolytes.11,31 It was related to polarization of condensed counterions, having lower mobility than the diffuse ones, and leading to a “permanent dipolelike” behavior at relatively low field strengths.32 Figure 4 illustrates the effect of concentration of PEC 5G on the frequency behavior of β-FeOOH particles. The kHz electrooptical effect, which is due to the polarization of the particle electrical double layers, decreases with increasing PEC 5G concentration below the isoelectric point and increases again after crossing the isoelectric point. At concentration 5 × 10-3 g dm-3, the effect value is already close to saturation (similarly to the behavior of the electrophoretic mobility; Figure 1) and remains constant with increasing concentration of PEC 5G up to 2 × 10-2 g dm-3. To avoid complications that occur when aggregation is available, we compare the frequency behavior of the kHz electro-optical effects for suspensions at low (10-5 g dm-3) and high (>5 × 10-3 g dm-3) concentrations of PEC 5G, that is, only for stable suspensions. As can be seen from Figure 4, the relaxation of the kHz effect from polymer-coated particles below the isoelectic point is similar to that of bare oxide particles. This indicates that diffuse ions from the electrical double layers of the pectin-coated particles are responsible for creation of their kHz electro-optical effect. (The particle charge density is also (31) Radeva, Ts.; Stoimenova, M. J. Colloid Interface Sci. 1993, 160, 475. (32) Yamaoka, K.; Fukudome, K. J. Phys. Chem. 1990, 94, 6896.

4DI πL2

(2)

where DI is the diffusion coefficient of the condensed counterion and L is the polyion contour length.34 With a diffusion coefficient of bound counterion 10-12 m2 s-1 (roughly 10 times smaller than the one of free counterions) and contour length of PEC 5G chain 175 nm, we obtained the value of νcr ) 4 kHz, corresponding well to the experimentally obtained frequency of the electrooptical effect (indicated by an arrow in Figure 5). Figure 5 compares the frequency behavior of suspensions at concentration 10-2 g dm-3 of all pectin samples, when an excess of counterions to the polyelectrolyte is mainly responsible for the creation of the electro-optical effect. It is seen that the relaxation frequency of the effect in the presence of weakly charged polymers (without condensation of counterions) increases with increasing polyelectrolyte charge density. The explanation that the relaxation frequency of the kHz electro-optical effect increases due to the deposition of more thinner layers from the less charged pectins (when increasing the charge density) is rejected because almost equal thicknesses of the adsorbed layers are obtained for all pectins. (The increased particle size due to the formation of layers with LH ∼ 6 nm might explain only the decrease in the relaxation frequency of the effect from the polymer-coated particles as compared to the bare oxide particles.) A more probable explanation is that the relaxation frequency of the electro-optical effect increases because of the increase in the adsorbed pectin charge density. Such an increase has already been found for β-FeOOH particles in the absence of polymer.35 Structure of Adsorbed PEC 5G Layer. Since the adsorption of PEC 5G does not cause release of the condensed counterions, we can estimate the number of adsorbed chains from the electrical polarizability of the pectin-coated particle in a stabilized suspension. The electrical polarizability of the pectin-coated particle is determined from the initial slope of the effect dependence on the electric field strength (Figure 3). Using eq 1, we obtained a value for γ ) 8 × 10-31 F m2. The polarizability of PEC 5G in a solution is calculated according to Manning’s theory for polarization of condensed counterions of a rodlike polyelectrolyte36

γ)

1 - ξ-1 e2L3 12kTd 1 - 2(ξ - 1) ln κd

(3)

where κ-1 is the Debye-Huckel screening parameter. With contour length L ) 175 nm and κ-1 ) 100 nm, the calculated value of γ for PEC 5G is 2.4 × 10-31 F m2. When dividing the (33) Jonsson, M.; Linse, P. J. Chem. Phys. 2001, 115, 3406. (34) Schwarz, G. J. Phys. Chem. 1962, 66, 2636. (35) Miteva, S.; Stoimenova, M. J. Colloid Interface Sci. 2004, 273, 490. (36) Manning, G. S. Biophys. Chem. 1978, 9, 65.

Ferric Oxide Particle/Pectin Complexation

polarizability of the polymer-coated particle by the polarizability of one pectin chain (assuming that each chain polarizes independently), we obtained approximately 3.3 chains of PEC 5G adsorbed on the particle surface. This number is close to the one obtained from the particle area occupied by the pectin chains, assuming lateral correlation between the adsorbed polymer chains (ca. 3.0 PEC 5G chains). In the second estimation, the distance between the adsorbed PEC 5G chains is accepted as equal to their length. This is based on the theoretical prediction that the correlation between the adsorbed chains leads to the formation of nonoverlapping cells when the polymer adsorbs at low ionic strength onto a weakly charged substrate.37,38 Despite the approximate character of these estimations, they are indicative of the important role of the correlation effects as a reason for the reversal of the particle charge.

Conclusion By comparing the stability and electrophoretic mobility of oxide particles in the presence of oppositely charged pectins, we found that both characteristics depend on the polyelectrolyte charge density. With increasing pectin charge density, one observes a decrease of the polymer concentration that is necessary to reach the particle isoelectric point and, correspondingly, to (37) Dobrynin, A. V.; Deshkovski, A.; Rubinstein, M. Macromolecules 2001, 34, 3421. (38) Cheng, H.; Olvera de la Cruz, M. J. Chem. Phys. 2003, 119, 12635.

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cause strong aggregation in the suspension. The exception to this order is the pectin with the highest charge density, owing to the strong electrostatic attraction of its counterions to the pectin charged groups, which causes counterion condensation. As a consequence, the pectin charge is reduced to an effective value, which is smaller than the bare pectin charge density. This shifts the isoelectric point toward a higher concentration of the most highly charged pectin PEC 5G as compared to the less charged samples. The information on the counterion mobilities, obtained from the frequency dependences of the electro-optical effect at charge reversal (in stabilized suspensions), confirms our suggestion that the condensed counterions of the most highly charged pectin remain bound to the polyion after its adsorption on the particle surface. This indicates that the charging ratio, corresponding to the ratio of the pectin to the particle charge, exceeds unity in this case. The number of adsorbed PEC 5G chains, estimated from the electrical polarizability of the polymer-coated particles, closely matches that calculated from the particle area, occupied by the adsorbed polymer. Despite the approximate character of this estimation, it indicates that the correlation effects play important role for the reversal of the particle charge when polyelectrolytes adsorb onto oppositely charged substrates. LA8012602