trimethylammonium Chloride Modified Silica Nanoparticles and

New Brunswick, Canada E3B 5A3. This work investigated the synthesis, characterization, and evaluation of surface-modified cationic silica nanoparticle...
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Ind. Eng. Chem. Res. 2005, 44, 539-545

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Novel Retention System Based on (2,3-Epoxypropyl)trimethylammonium Chloride Modified Silica Nanoparticles and Anionic Polymer Norlito Cezar and Huining Xiao* Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3

This work investigated the synthesis, characterization, and evaluation of surface-modified cationic silica nanoparticles as part of retention or flocculation systems. Quaternary amine groups were introduced onto the fumed silica surface using the cationic reagent (2,3-epoxypropyl)trimethylammonium chloride under various reaction conditions. To characterize the surface properties of the modified silica particles, the ζ potential and charge density were determined. Dynamic flocculation experiments showed that, when used alone, the cationic silica nanoparticles contributed little to inducing clay flocculation. However, when the nanoparticles were used in conjunction with an anionic polymer of high Mw and low charge density, significant improvement in the flocculation of fine clay particles was achieved. The interaction between the cationic silica nanoparticles and anionic polymer, as well as the silica adsorption behavior on clay surfaces, was explored. The mechanism of flocculation induced by the dual-component system consisting of the cationic silica nanoparticle and anionic polymer was discussed. Introduction Retention aids, coagulants, or flocculants are chemicals used in wastewater treatment and papermaking processes. These chemicals are capable of altering the surface properties of colloidal substances in water and have become increasingly important for more efficient treatment or separation. The use of retention aids in papermaking involves the flocculation and attachment of fillers and other additives into a network of cellulose fibers. The separation of water from the fiber stock is a primary concern in the wet end section, with the sheet formation controlled to achieve optimum strength and properties of the paper product. The efficiency of retention aids determines not only the drainage and separation of water but also the sheet formation, which in turn affects the physical properties of paper.1 The most common retention aids include inorganic salts such as poly(aluminum chloride) and natural or synthetic polymers such as cationic starch and polyacrylamide. Polymers, especially with high molecular weight, are often used.2-4 The molecular weight and charge density of the polymers provide an extensive network of adsorbed cellulose fibers. Recently, it has been found that the bridging effects of the polymer are significantly enhanced in the presence of nano- or microparticles. Several microparticulate systems involved in flocculation include inorganic nanoparticles and microparticles that are often electrically charged.5 Because of synergistic effects, dual-component systems are increasingly becoming important, as they reduce the amount of the polymeric flocculant used.6,7 Microparticles provide the required surface area for the polymers to form a complex that bridges the filler particles and fibers. Consequently, the particle size of the microparticle has become increasingly important in the retention aid systems.8 * To whom correspondence should be addressed. Tel: 1-506453-3532. Fax: 506-453-3591. E-mail: [email protected].

The conventional retention aid systems consist of anionic nano- or microparticles and cationic polymers. Anionic silica particles are some of the inorganic particles widely used as one of the components in retention aid systems in conjunction with cationic polymers such as cationic starch (i.e., the Compozil system). Silica nanoparticles have become widely accepted templates for incorporating the desired functional group by surface modification.9,10 A number of surface-modified silica particles using various reagents with different functional groups have been reported.11,12 The modification was achieved by the reaction of the silanol groups on the surface of the silica particles and the reactive group of the reagent, usually the alkoxy groups of a silane coupling agent.13-15 In comparison to conventional retention aid systems, cationic nano- or microparticle retention aid systems showed fewer problems with a closed white water system, due to effective bonding of the cationic microparticle to fillers and fibers. These cationic systems also provide enhanced drainage and, thus, better sheet formation when used in conjunction with high-molecular-weight anionic polymers instead of cationic polymers.16 Several retention aid systems consisting of cationic inorganic or polymeric nano- or microparticles have been reported. The synergy of cationic polymeric microparticles (CPMs) with anionic polymers in inducing filler flocculation has been presented.6,8 An improved retention has also been demonstrated when the CPM particle size was reduced. However, particles of sizes less than 100 nm have been difficult to synthesize.17 The addition of nano- or microparticles has provided substantial advantages over conventional retention systems, which include accelerated drainage and better web formation. Fibrous-shaped cationic Al2O3 nanoparticles have demonstrated a synergistic effect with highmolecular-weight anionic polymers.7 Cationic silica sol modified with polyvalent metals has been used in conjunction with cationic or anionic polyacrylamide.14,18

10.1021/ie030830w CCC: $30.25 © 2005 American Chemical Society Published on Web 01/06/2005

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Recently, we have reported the surface modification of silica nanoparticles using cationic silane and its application in inducing clay flocculation in conjunction with water-soluble anionic polymer.19 In this work, we have investigated the surface modification of nonporous silica nanoparticles (particle size 14 nm) using the cationic reagent (2,3-epoxypropyl)trimethylammonium chloride (ETMAC). ζ potential values have been used to evaluate the introduction of the quaternized group on the surface of the silica nanoparticles. The flocculation of fine clay fillers using the ETMAC-modified silica nanoparticles and anionic polymer has been performed. Furthermore, the structure-property relationship of the cationic modified silica nanoparticles has also been established. Experimental Section Materials. Fumed silica from Sigma-Aldrich has an average particle size of 14 nm in diameter and 200 m2/g surface area and was used as received. The silica nanoparticle has an estimated 3.5-4.5 hydroxyl groups/ nm2. (2,3-Epoxypropyl)trimethylammonium chloride (ETMAC, 71 wt %, water 25 wt %) from Sigma-Aldrich was also used as received. Standard solutions of NaOH and HCl were used for adjusting pH. The dialysis tube was obtained from Fisher Scientific and has an Mw cutoff of 6000 and 18 Å pore size. The dialysis tube was pretreated by soaking the required tube length in distilled water for 30 min. The filler clay was obtained from Irving Paper in New Brunswick, Canada, and was diluted to about 7 wt % prior to the dynamic flocculation experiment. The average size of the clay is about 1.9 µm in equivalent diameter, measured using an Analysette 22 laser particle sizer (Fritsch GmbH, Germany). Blotting paper (4.2 wt % moisture) was obtained from Labtech Instruments, Inc. (Quebec, Canada). A disintegrator (Labtech, Canada) was used to obtain wood fiber. The anionic polymers Percol 173 and Percol E24 were supplied by CIBA Chemicals U.K. The molecular weights of Percol 173 and Percol E24 are 12,000,000 and 14,000,000, respectively, and the degrees of substitution are 5 and 15%, respectively.20 Quaternization of Silica Nanoparticles. The preparation of cationic silica nanoparticles was performed according to the following procedures. In a 250 mL three-necked flask equipped with continuous reflux, 1.3 g of fumed silica particles was dispersed in 50 mL of deionized and distilled water, and the pH was adjusted to about 4 by adding drops of 1 mM HCl. A 4.0 mL portion of ETMAC (71 wt %) was then added, and the mixture was continuously stirred in a water bath for 2 h. The silica sample dispersions were then dialyzed in deionized water until the conductivity reached that of the deionized water. Characterization of Cationic Silica Nanoparticles. A Coulter DELSA 440SX instrument was used to determine the average ζ potential. In a 50 mL beaker, 1 mL of the dialyzed silica dispersion was added along with 20 mL of 1 mM NaCl. Unless otherwise indicated, the pH was measured and adjusted to about 4.5 by adding 0.05% vol HCl. Samples were then injected inside the cell chamber of the equipment, and the cell temperature was allowed to reach 25 °C. The ζ potential was determined by following the standard procedure of the instrument operation.

The surface charge density of silica particles was determined using a Mutek Charge Titrator PCD03 (Mutek, Germany). A 5.0 mL portion of the silica sample mixture was added to the titration cell containing 5 mL of deionized and distilled water and was titrated with standardized solutions of poly(diallyldimethylammonium chloride) (poly-DADMAC) or potassium poly(vinyl sulfate) (PVSK). The particle charge density was estimated from the volume of the poly-DADMAC or PVSK solution (1.0 mN) required to reach the end point during titration. Characterization of Clay Flocculation. The modified silica particles were used to induce clay flocculation using a dynamic drainage jar (DDJ) setup and a Photometric Dispersion Analyzer (PDA 2000, Rank Brother, Inc., U.K.). The flocculation was conducted as described elsewhere.7 The pH of the clay suspension was adjusted to about 5.5, unless otherwise indicated. The clay suspension was circulated through the PDA using a peristaltic pump at a rate of 65 mL/min. The propeller speed in the DDJ was maintained at 500 rpm for all experiments unless otherwise indicated. There was no electrolyte added to the suspension. The cationic silica particles were added prior to the anionic polymer. The flocculation of clay in the presence of wood fibers followed the same procedures as above, except for the fibers being added to the jar prior to clay particles. Analysis of Clay Floc Particle Size. The clay flocs were collected and analyzed for particle size distribution using laser light diffraction. The clay suspension was collected after dynamic clay flocculation. The flocs were then separated, and the particle size distribution was analyzed using the same instrument noted previously (i.e., Analysette 22 laser particle sizer). The samples were sonicated over the entire duration of analysis. Interaction between Anionic Polymers and Cationic Silica Nanoparticles. The interaction between the cationic silica and anionic polymer was explored without adding electrolyte to the system. A range of 0.01-5 mL of 0.1 wt % Percol 173 or 0.1 wt % Percol E24 solution was added to a test tube containing 5 mL of 0.25 wt % cationic silica. The mixture was then further diluted with deionized and distilled water to a total volume of 10 mL and the pH adjusted to about 4.5. After thorough mixing, the test tubes stood overnight at room temperature to allow the nanoparticle/ polymer complex to settle. The precipitates were then centrifuged at 5000 rpm for 30 min. The supernatant was separated and analyzed. The amount of free anionic polymer was determined by colloidal titration using Mutek PCD03. The amount of bonded anionic polymer (i.e., in complex) was estimated by comparing the titration results with the control samples. Interaction Between Cationic Silica and Clay Particles. To determine the interaction between the cationic silica nanoparticles and clay particles, different amounts of 0.25 wt % cationic silica were added to a test tube containing a mixture of 5 mL of 0.04 wt % clay and 5 mL of 1 mM NaCl. The mixture was then mixed thoroughly. The pH was then adjusted to about 4.5 prior to analysis of the ζ potential of the clay suspension using DELSA 440SX. Results and Discussion Surface Modification of Silica Nanoparticles. The silica particles showed cationic properties after surface modification with ETMAC. As shown in Table

Ind. Eng. Chem. Res., Vol. 44, No. 3, 2005 541 Table 1. Temperature and pH Effects on the ζ Potential of ETMAC-Modified Silica Products after t ) 2 ha silica type

pH

temp (°C)

ζ potential (mV)

CESi-1 CESi-2 CESi-3 CESi-4 CESi-5 CESi-6

3.8 7.1 8.6 9.8 3.9 9.8

20 20 20 20 70 70

23.5 17.1 21.1 21.9 18.9 19.3

a

ζ potential analysis at pH 4.5.

Figure 2. Effect of different reaction conditions of silica particles on the relative turbidity at varying amounts of cationic silica added ([Percol 173] ) 0.1% w/w): (0) CESi-1; (4) CESi-3; ([) CESi-4.

Figure 1. Effect of pH on the ζ potential of ETMAC-modified silica nanoparticles: (]) CESi-3; (9) unmodified silica.

1, the introduction of the quaternary group on the surface of silica rendered the silica nanoparticles cationic. The modified silica nanoparticles have ζ potential values that range from +17.1 to +23.5 mV. Modification reactions at different reaction pH values and temperatures led to a range of modified silica nanoparticles with different zeta potentials. The influence of pH on the surface modification of silica particles is attributed to the reaction of the epoxide ring of ETMAC. At the same temperature, the increase of the ζ potential of the silica product confirms the requirement for basecatalyzed (whereby the epoxide group underwent nucleophilic attack) or acid-catalyzed (where the epoxide was protonated prior to being attacked by a nucleophile) epoxy ring cleavage to silanol groups.21 Both low and high pH values favor stable colloid dispersions of the silica particles with high ζ potential. The lowest ζ potential occurred at +17.1 mV when the reaction pH is 7.1. As the pH was increased, the ζ potential increased slightly. Reaction temperature provided a different effect on achieving a high ζ potential. Unlike cationic starch, where preparation is usually done at a high temperature to achieve better cationicity, moderate temperature resulted in higher ζ potential for cationic silica preparation. At the same pH, the ζ potential decreased when the temperature was increased. The decrease in ζ potential would mean less surface modification and inefficient reaction at high temperatures. The highest ζ potential of 23.5 mV was achieved for silica particles reacted at room temperature and at pH 3.80. The effect of solution pH on the ζ potential of the cationic silica nanoparticles is shown in Figure 1. The isoelectric point (IEP) of the silica sample CESi-3 is achieved at pH 7.2. This implies that silica products from surface modification tend to form stable colloidal dispersions for solutions whose pH is below 7.2. Apparently, the ζ potential of the cationic silica nanoparticles increases as the pH is decreased. This is expected because the addition of hydronium ions (H3O+) or protons alters the ion equilibria that surround the silica colloidal particles, leading to more cationic charges in

the system. Likewise, a shift in ion equilibria will exist if the pH is increased by the addition of a sodium hydroxide. The negative charge-carrying OH- groups from the base neutralize the charge characteristics around the silica nanoparticles, decreasing the ζ potential until it is rendered anionic. Also presented in Figure 1 is the isoelectric point for the unmodified silica, which was found in this work to be at about pH 1.9. This result is in close agreement with that in the literature whereby the IEP of unmodified silica particles was reported to be between pH 1 and 2.22 In one study, the isoelectric point of the unmodified silica was at about pH 2.8.23 The IEP obtained for ETMAC-modified silica nanoparticles is evidently much higher than that of the unmodified silica particles. The importance of the isoelectric point will be investigated further in the following sections. Clay Flocculation Induced by the Silica Nanoparticles in Conjunction with Anionic Polymers. Prior to performing the clay flocculation using cationic nanoparticles in conjunction with anionic polymer, we evaluated silica particle systems alone. The results indicated that the cationic-modified silica nanoparticles contributed very little to inducing clay flocculation when used alone. For cationic silica dosages between 0.1% and 2.0% w/w clay, the relative turbidity of the clay suspension was not reduced. However, the addition of an anionic polymer of high molecular weight to cationic silica particle systems significantly improved the clay flocculation. The influence of varying cationic silica nanoparticle dosage at a constant anionic polymer dosage (0.1% w/w clay) is presented in Figure 2. A sample addition order was arranged in which the cationic silica particles were always added first, followed by anionic polymer. A lower value of the relative turbidity simply means better flocculation. As can be seen, three types of cationic silica samples performed similarly, particularly at a dosage below 1.5% (w/w on clay). Effective clay flocculation was achieved even at a low dosage (0.02%) for all three silica samples. At a high dosage (i.e., 2.0%), the silica sample CESi-1 appeared to be the best among the three samples, leading to a relative turbidity close to 0.4 (i.e., more than 60% of single clay particles being flocculated). The sample CESi-1 also has the highest ζ potential of +23.5 mV (see Table 1), which implies the correlation between the flocculation efficiency and the surface charge characteristics of cationic silica particles. Rendering the silica sample to be highly cationic tends to improve the clay flocculation in the presence of an anionic polymer as a flocculant.

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Figure 3. Effect of Percol 173 dosage on the relative turbidity with cationic silane modified silica fixed at 0.2% w/w clay: (0) CESi-1; (]) CESi-4; (2) Percol 173 alone.

Figure 4. Effect of pH on clay flocculation, with cationic silica (1% w/w) added prior to the anionic polymer Percol 173 (0.1% w/w): ([) CESi-3/PERCOL 173; (0) Percol 173 alone.

Figure 3 shows the effect of varying the anionic polymer dosage on clay flocculation. The addition order was the same as addressed above (i.e., the cationic silica nanoparticles were added to the clay suspension prior to the polymer). At a fixed silica concentration of 0.2% w/w clay, the best flocculation achieved by CESi-1 occurred at a polymer dosage of 0.1% w/w clay. However, synergy was not significant when polymer was added at dosages lower than 0.1%, which produced relatively poor flocculation. Increasing the amount of polymer beyond 0.1% also resulted in a higher relative turbidity of the clay suspension, probably due to the presence of excess negative charges (occurrence of electrostatic repulsion) and steric effects from polymer chains. A relative turbidity close to 1.0 without polymer addition again reflected the ineffectiveness of the cationic silica system alone. For the system involving the silica sample CESi-4, the reduction in relative turbidity proceeded as the polymer dosage was increased. The effect of adding ETMAC-modified cationic silica nanoparticles as part of the dual system was most noticeable when the polymer dosage was above 0.05% w/w clay. Synergy was observed at polymer dosages above this value. These results imply that improved bridging flocculation might have taken place in the presence of the cationic nanoparticles. Figures 2 and 3 show that the best cationic silica-to-polymer ratio for flocculation is between 2:1 and 5:1. Effect of pH on Flocculation. The results of varying the pH during clay flocculation are shown in Figure 4 for the Percol 173 system alone and the dualcomponent system. The anionic polymer Percol 173 alone was very sensitive to pH, with flocculation reduced as the pH was increased. For the dual-component system using CESi-1 and the anionic polymer Percol 173, the pH dependency is likewise evident, probably due to the polymer. Effective

Figure 5. Relative turbidity of clay suspension as a function of polymer charge density and cationic silica CESi-3 dosage ([polymer] ) 0.1% w/w clay): ([) Percol 173; (2) PE24.

Figure 6. Relative turbidity of clay suspension as a function of anionic polymer added ([cationic silica] ) 1.0% w/w): ([) CESi-3/Percol 173; (2) CESi-3/Percol E24; (0) Percol 173 alone; (O) Percol E24 alone.

flocculation was achieved at a low pH, with the relative turbidity lower than that achieved by the polymer alone. As the pH was increased, the performance of the dualcomponent system in lowering the relative turbidity was impaired. An increase in pH of the clay suspension affected the cationic properties of the silica nanoparticles, where the ζ potential of the silica was also reduced, as discussed previously. This observation was evident when the suspension pH was beyond 7, where the dual-component system achieved higher relative turbidity compared to the polymer alone. Apparently, the cationic properties of the silica nanoparticles were lost at high pH (see Figure 1). Above the isoelectric point, the modified silica particles would have negative ζ potential and very few interactions between likecharged particles, i.e., clay particles and polymer, would be expected. At a high pH, charge repulsion existed between silica particles, polymer chains, and clay particles. This repulsion led to poorer flocculation. Overall, at a pH lower than the IEP of the cationic silica particles, i.e., at a pH lower than 7.2, the synergistic effect was obvious. Effect of the Charge Density of Anionic Polymers. An anionic polymer with higher charge density, Percol E24, was also used to investigate the effect of the anionic polymer charge density on clay flocculation. The comparison between Percol E24 and Percol 173 in the dual-component system is presented in Figures 5 and 6. The results show that the flocculation achieved by the CESi-3/Percol E24 system is not as effective as that achieved by the CESi-3/Percol 173 system, regardless of whether the flocculation was performed under constant dosage of the polymer or cationic silica. Better flocculation was observed for an anionic polymer with low charge density. This is in agreement with our previous work on microparticle/polymer systems.4,6,7

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Figure 7. Effect of shear rate on the relative turbidity of clay suspension: (0) 1.0% w/w CESi-3 and 0.1% w/w Percol 173; (9) 0.1% w/w Percol 173 alone.

Apparently, an anionic polymer with relatively high charge density tends to flatten on the substrate (i.e., the clay particle preadsorbed with cationic silica nanoparticles), leading to less effective bridging or flocculation. The repulsion between highly charged polymer chains will also weaken the polymer bridging between the particles. As a result, the CESi-3/PE24 system imparted little improvement of flocculation. Figure 6 also shows the synergistic cooperation between the silica particles and the anionic polymers. The more evident cooperation exists with the low charge density polymer; at a constant cationic silica dosage of 1.0% w/w, the relative turbidity decreases as the anionic polymer dosage increases. Effect of Shear. To determine the performance of the dual-component system at a high shear rate, the speed of stirring in the DDJ was increased. Results are presented in Figure 7. As the stirring speed was increased, improved clay flocculation was observed for both the anionic polymer alone and the dual-component systems, implying that strong flocs formed or the agglomerated clay flocs were not redispersed at high turbulence. This result is in agreement with previous work7 whereby shear-resistant flocs were obtained when cationic microparticles were used. The increase in shear rate during flocculation increased the turbulence in the system, thereby allowing more collisions among the particles. Such an increased collision might improve the polymer bridging efficiency, facilitating the agglomeration of the clay particles pretreated with cationic silica particles. Interaction of Cationic Silica with Anionic Polymer and Filler Clay. In the absence of clay, the interaction of the anionic polymer with cationic silica was investigated by determining the amount of the anionic polymers bonded with cationic silica nanoparticles via colloidal titration. Agglomeration of silica particles was observed immediately after an anionic polymer was added to the silica suspensions, implying that the cationic silica nanoparticles formed complexes with the polymer via electrostatic interactions. The results on two anionic polymers are presented in Figure 8. As can be seen, the amount of Percol 173 bonded with CESi-1 increased as its concentration increased and reached saturation at 27 mg/g. This is greater than that achieved by Percol E24, which is about 22 mg/g. The results obtained from colloidal or polyelectrolyte titration indeed provided quantitative information on anionic polymers bonded with silica particles. However, the analysis did not tell us how the polymer/silica nanoparticles were bonded. This has stimulated future work on revealing the structure of anionic polymer/ cationic silica particle complexes. For primary or singly

Figure 8. Interaction between anionic polymers and cationic silica particles ([CESi-1] ) 2.5 g/L): ([) Percol 173; (0) Percol E24.

Figure 9. Effect of cationic silica CESi-1 dosage on clay ζ potential.

cationic silica nanoparticles, the silica particles might be entrapped into polymer coils on being mixed with anionic polymer solution, possibly leading to multiple silica particles per polymer. If cationic silica nanoparticles form aggregates, which cannot be avoided completely during modification processes due to the charge reversal, a much larger silica aggregate will be formed (up to a few hundred nanometers in diameter). In this case, the interaction between silica and polymer tends to be adsorption-based. However, this situation is not important for the retention or flocculation processes, since silica and anionic polymer would never be premixed prior to being added to filler or clay suspension in practical applications. The results above virtually confirm the strong interaction between anionic polymer and cationic silica particles, which is essential for the bridging mechanism addressed later. In filler retention processes, cationic silica particles were first added to the filler or clay suspension. Preadsorption of the silica particles on the clay surface provides binding sites for polymer bridging to occur, which is crucial for the effective retention or flocculation processes. To ensure the adsorption of silica particles on clay surfaces, the interaction between cationic silica and clay particles was investigated in the absence of polymer. At pH 4.5, the ζ potential of the clay particles was measure at about -38.9 mV. When cationic silica particles were added to the clay particles, the ζ potential of the clay particles decreased in magnitude. Increasing the dosage of cationic silica nanoparticles further increased the ζ potential, as shown in Figure 9. The increase in ζ potential suggested that the cationic silica particles indeed interacted with the anionic surfaces of the clay particles (i.e., top or bottom surfaces of the platelets). Due to their rigid nature, the modified silica particles are not flattened on the clay surfaces, allowing the discrete cationic charges to be available for further interaction with other clay particles. Cationic silica particles neutralize the charges on the clay plate surfaces, reducing the charge repulsion among clay

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Figure 10. Effect of fiber on clay flocculation using ETMACmodified silica nanoparticles ([fiber consistency] ) 0.2%; [clay] ) 0.1%; [Percol 173] ) 0.1% w/w clay): (- - -]- - -) CESi-3/Percol 173/ fibers; (s9s) CESi-3/Percol 173 only. Table 2. Floc Size of Clay Particles at Various CESi-1 to Percol 173 Ratios CESi-1:Percol 173 (w/w)

mean floc size diameter (µm)

10:1 5:1 2:1

16.8 13.8 7.9

particles. Apparently, agglomeration of clay particles was observed when the clay particles reached ζ potentials near 0 mV, or when the cationic silica CESi-1 dosage was about 9.5% w/w on clay. With the ζ potential of CESi-1 at about 23.5 mV, a high silica dosage was required to completely neutralize clay particles. This also suggests that the cationic silica added during flocculation (from 0.1 to 2.0% w/w clay) was too low to affect charge neutralization of clay particles, and therefore, no decrease in relative turbidity was observed. The mechanisms being proposed in this work are electrostatic attraction due to the interaction between the clay particles preadsorbed with cationic-modified silica nanoparticles and anionic polymers and bridging built by anionic polymers with extremely high molecular weight. If excess cationic nanoparticles are present freely in clay suspension, which likely happens at a high silica dosage, the complex bridging mechanism proposed in our previous work could be adopted. In this process, the free silica will form complexes with anionic polymer prior to adsorbing on clay. If the size of the complex is beyond a double-layer thickness of clay particles, the

flocculation of clay particles will be induced simultaneously as the complex is formed. This is a dynamic or transit process, since the complex tends to collapse and become inactive in flocculation once the size of the complex is reduced significantly. It should be noted that the complex bridging mechanism is unlikely to be dominant in the current systems, due to the fact that the dosage of cationic silica is relatively low. Apart from the relative turbidity measurement, the formation of clay flocs was further confirmed by floc size measurement using a laser particle sizer. Results are presented in Table 2. Obviously, the floc size is larger than that of primary clay particles (approximately 1.9 µm). The higher the silica/polymer ratios, the larger the flocs. This implies that the clay particles preadsorbed with more cationic silica particles facilitates the bridging induced by the anionic polymer, producing relatively large clay flocs. Clay Flocculation in the Presence of Chemical Pulp. Clay flocculation using modified silica nanoparticles and anionic polymer as flocculants was also conducted in the presence of wood fiber. Silica type CESi-3 was used for this analysis together with the low charge density anionic polymer Percol 173. Results are presented in Figure 10. Clearly, the presence of wood fiber has minor impact on clay flocculation, and the relative turbidity of the suspension showed slight improvement for the cationic silica dosages both below 0.5% and above 1.5% (w/w on clay). The slightly better flocculation may be attributed to the stabilization of clay flocs caused by fiber or fiber network. In terms of the flocculation mechanism addressed in the previous section, Figure 11 presents a schematic of the retention process. When filler clay particles were added to the fiber suspension, most of the clay particles remained in suspension with few clay particles adsorbed onto fiber by electrostatic interaction between the cationic edges of the clay particles and the negatively charged fiber surface (I). Addition of cationic silica nanoparticles to the system modified the surface properties of fiber and clay particles when the nanoparticles collided with the fiber and clay surfaces (II). The addition of the anionic polymer led to the bridging formation which improved flocculation. The anionic polymer mainly interacted with cationic silica nanopar-

Figure 11. Schematic of the flocculation process in the cationic silica nanoparticle/anionic polymer retention system.

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ticles that were mostly preadsorbed onto the surfaces of clay or fibers (III). Formation of more flocs resulted from interparticle bridging induced by the polymer anchored through electrostatic interaction with the cationic silica nanoparticles (IV) attached to either clay or fiber. Charge reduction of the anionic fiber surface by cationic silica nanoparticles allowed more polymers to be adsorbed onto the fibers. The electrostatic interaction induced by the cationic silica particles attached to the filler and fiber surfaces led polymer chains to collide more effectively with fibers and filler clay particles. This potentially led to the floc formation in various manners, which could be filler-filler (IVa), fiber-fiber (IVb), or fiber-filler particles (IVc), thereby lowering further the relative turbidity. On the other hand, the presence of wood fibers provided the opportunity for mechanical entrapping of the clay particles or flocs (IVb). Overall, the flocculation mechanisms being proposed in this work are electrostatic attraction induced by the cationic silica nanoparticles and interparticle bridging by the anionic polymer. Conclusions The conclusions achieved from this work are as follows. (i) The quaternization of silica nanoparticles was most effectively performed at a relatively high or low pH. High cationicity of the modified silica nanoparticle was achieved at relatively moderate reaction temperatures. (ii) As part of a dual-component system, the cationic silica particles improved the flocculation performance of low-charge-density, high-Mw anionic polymer. (iii) The observed synergy was effective in reducing the amount of anionic polymer required to lower the relative turbidity of the clay suspension. The most effective ETMAC-modified silica particle to anionic polymer ratio was found to be between 2:1 and 5:1. (iv) The rigid nature and cationic properties of the modified silica particles improved the bridging of fiber and clay particles by the anionic polymer via partial neutralization and formation of a stronger complex with the polymer. Charge interaction and bridging flocculation are the mechanisms likely occurring and being proposed in the current systems. Literature Cited (1) Roberts, J. C. Paper Chemistry; Chapman and Hall: New York, 1991. (2) Lapcˇik, L.; Alince, B.; Van de Ven, T. G. M. Effect of Poly(Ethylene Oxide) on the Stability and Flocculation of Clay Dispersions. J. Pulp Pap. Sci. 1995, 21(1), J19. (3) Wall, S.; Samuelsson, P.; Degerman, G.; Skoglund, P.; Samuelsson, A. The Kinetics of Heteroflocculation in the System Cationic Starch and Colloidal Anionic Silicic Acid. J. Colloid Interface Sci. 1992, 151(1), 178. (4) Xiao, H.; Pelton, R.; Hamielec, A. Retention Mechanisms for Two-Component Systems Based on Phenolic Resins and PEO

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Received for review November 10, 2003 Accepted November 5, 2004 IE030830W