Stabilities and Rheological Properties of Coagulated Silica Sols

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Stabilities and Rheological Properties of Coagulated Silica Sols Formed from Fumed Silica Suspensions in the Presence of Hexadecyltrimethylammonium Chloride Ryotaro Komada and Masami Kawaguchi* Division of Chemistry for Materials, Graduate School of Engineering, Mie University 1577 Kurimamachiya, Tsu, Mie 514-8507 Japan Received January 27, 2010. Revised Manuscript Received February 16, 2010 The stability and rheological response of coagulated silica sols formed from fumed silica suspensions were investigated in aqueous KOH solution at pH 11 upon the addition of hexadecyltrimethylammonium chloride (C16TAC) as functions of silica and surfactant concentration. The coagulated silica sols with negative charges are stable at given concentration ranges of silica and C16TAC, and the C16TAC molecules are completely adsorbed on the silica surface in these ranges. The average hydrodynamic diameters of the coagulated silica colloidal sols at C16TAC concentrations from 1.0
10-4 to 5.0
10-4 M are almost twice the diameter observed in the absence of C16TAC, independent of silica concentration. At C16TAC concentrations below 4.0
10-4 M, the resulting coagulated colloidal sols showed Newtonian responses under hysteresis loop measurements, whereas at above 5.0
10-4 M, the flow curve changes from a positive hysteresis loop to a crossover hysteresis loop with increasing concentration. This change under shear flow is due to a partial breakdown of the coagulated structure of the fumed silica suspensions as a result of electrical neutralization. Finally, the coagulated colloidal sols which gave a crossover hysteresis loop present a shearthinning behavior and their transit shear stresses exhibit overshoots whatever the shear rate used. Plots of the resulting steady-state viscosities against shear rate indicate shear-thinning behavior.

Introduction Aqueous suspensions of fumed silica particles have been widely employed in academic research and in applications.1-3 Aqueous suspensions of fumed silica particles have been used to polish the surfaces of silicone oxides in chemical and mechanical polishing (CMP) due to their high purity.4 In the CMP process, the presence of coagulates formed from aggregated fumed silica particles can lead to scratches on the corresponding surfaces. In order to understand the formation mechanism and physical properties of coagulates or gels formed from fumed silica aggregates, we have investigated the effects of adding KCl5 and cationic surfactants6 such as dodecyltrimethylammonium chloride (C12TAC) and hexadecyltrimethylammonium chloride (C16TAC) on gel formation. At pH 11, the aggregates are electrostatically stabilized by negative charges,5,6 such that the Kþ and trimethylammonium ions are preferentially adsorbed at the negative silica surfaces. In previous papers,5,6 we reported that colloidal gels formed from aggregates of the fumed silica particles could be regarded as weaklink gels based on a comparison of the silica volume fraction *E-mail address: [email protected]. Phone: þ81-59-231-9432; FAX: þ81-59-231-9433.

(1) Iler, R. K. The Chemistry of Silica; Wiley Interscience: New York, 1979. (2) Bergna, H. E., Ed. The Colloid Chemistry of Silica; American Chemical Society: Washington, DC, 1994. (3) Kawaguchi, M.; Kimura, Y.; Tanahashi, T.; Takeoka, J.; Kato, T.; Suzuki, J.; Funahashi, S. Langmuir 1995, 11, 563. (4) Cook, L. M. J. Non-Cryst. Solids 1990, 120, 152. (5) Yokoyama, K.; Koike, Y.; Masuda, A.; Kawaguchi, M. Jpn. J. Appl. Phys. 2007, 46, 328. (6) Asai, H.; Masuda, A.; Kawaguchi, M. J. Colloid Interface Sci. 2008, 328, 180. (7) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (8) Dong, D. C.; Winnik, M. A. Photochem. Photobiol. 1982, 35, 17. (9) Zana, R. In Surfactant Solutions. New Methods of Investigation, Zana, R. Ed.; Dekker: New York, 1987; Chapter 5. (10) Stechemesser, H.; Sonntag, H. In Coagulation and Flocculation, 2nd ed.; Stechemesser, H., Dobias, B., Eds.; CRC Press: New York, 2005; Chapter 2.

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dependence of the critical strain and the storage modulus with the power-law predicted by a fractal gel model.9-13 In such a weaklink gel, the bonding between the silica particles in the aggregates, namely, intrafloc links, is stronger than that between the fumed silica aggregates (interfloc links), and the gel network forms by electrical neutralization. However, gelation in the presence of C12TAC and C16TAC occurred at concentrations 3 orders of magnitude less than that observed for KCl at the same silica concentrations, and moreover, the added C16TAC were completely adsorbed on the silica surfaces. This means that C16TAC shows a higher adsorption affinity onto the silica surface than that of C12TAC. On the other hand, there also exist sol-state regimes in the phase diagram of the silica suspensions in the presence of C16TAC6 and the aggregated fumed silica particles in the corresponding regimes are coagulated, although not precipitated. Since characterization of such coagulates of aggregated silica particles was limited in a previous paper,6 we performed more detailed characterization of the coagulates of the aggregated silica particles in the present study. To more deeply understand the effects of C16TAC adsorption on aggregated silica particles, we measured the particles using dynamic light scattering, hysteresis loop, transient shear stress, and steady state viscosity measurements as functions of silica and C16TAC concentration.

Experimental Section Samples. The aqueous fumed silica suspension (original silica suspension) was kindly supplied by Cabot Microelectronics Co. (11) Cheremisinoff, N. P. In Encyclopedia of Fluid Mechanics, vol. 7, Rheology and Non-Newtonian Flows, Cheremisinoff, N. P., Ed.; Gulf Publication Company: Houston, 1986; Chapter 26. (12) Lapasin, R.; Pricl, S. In Rheology of Industrial Polysaccharides: Theory and Applications; Blackie Academic & Professional: London, 1995; Chapter 3. (13) Chhabra, R. P. In Bubbles, Drops, and Particles in Non-Newtonian Fluids, 2nd ed.; Taylor & Francis: Boca Raton, 2007; Chapter 2.

Published on Web 02/23/2010

DOI: 10.1021/la100391c

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(Tsu, Japan). The original silica suspension contains 25 wt % solid contents, has a pH maintained at 11, and a specific density of 1.16. The average diameter and zeta potential of the aggregated silica particles in the original silica suspension was determined to be 170 nm and -50 ( 5 mV, respectively.5,6 The dispersion medium used to prepare the original silica suspensions was also kindly supplied by Cabot Microelectronics Co. The dispersion medium was also used as a solvent for C16TAC.6 C16TAC purchased from Tokyo Chemical Industry Co. (Tokyo, Japan) was used after purification by the same method as given in a previous paper.6 For aqueous solutions of the purified C16TAC, the critical micelle concentrations (CMC) were determined to be 0.1 mM6 by a fluorescence spectroscopy technique using pyrene as a probe.7-9

Preparation of Silica Suspensions in the Presence of C16TAC. To prepare a silica suspension with the desired silica

volume fraction, φ, in the presence of C16TAC, the original silica suspension was mixed with a preprepared aqueous C16TAC solution of a given concentration in a glass bottle of 20 mL. The resulting silica suspensions were subjected to mechanical shaking in an incubator at 25 °C for 1 day to form coagulates of the aggregated silica particles. Adsorption Characteristics of C16TAC. The adsorption characteristics of C16TAC onto fumed silica particles were determined by an Orange II method, which is an extracting and color development method in which chloroform is added to an aqueous solution containing Orange II and C16TAC. The Orange II method is a kind of a quantitative analysis to confirm the presence of C16TAC in the supernatant aqueous solution centrifuged from the silica suspension. If C16TAC were present in the supernatant solution, the chloroform layer after extraction should become orange in color, which can be detected by monitoring at 485 nm in the visible spectrum. Dynamic Light Scattering Measurements. Dynamic light scattering measurements of silica suspensions in the range in volume fraction, φ, from 0.014 to 0.072 at C16TAC concentrations of 0, 1.0
10-4, 3.0
10-4, and 5.0
10-4 M were performed at 25 °C using a fiberoptic particle analyzer (Otsuka Electronics FPAR- 1000). Results were analyzed based on the photon correlation spectroscopy technique, and an average diameter of the partial coagulates of the aggregated silica particles was calculated using the cumulant method. Rheological Measurements. Measurements of the hysteresis loop and transient steady-state shear stress of the coagulates of the aggregated silica particles at φ values of 0.043, 0.057, and 0.072 and at C16TAC concentration ranges from 1.0
10-4 to 7.0
10-4 M were performed using a rheometer (Anton Paar Physica MCR 300) with a cone-plate fixture with a cone diameter of 75 mm and a cone angle of 1° (CP75-1) at 25 °C. The hysteresis loop measurements were performed by increasing the shear rate from 1 to 1000 s-1 and subsequently decreasing the shear rate from 1000 to 1 s-1 over 5 min in each direction. The transient steady-state shear stress measurements were performed at a given shear rate to obtain a plateau shear stress. Respective measurements were repeated at least twice, and their experimental errors were within 3%.

Results and Discussion Adsorption of C16TAC. Addition of C16TAC to the fumed silica suspensions gives rise to changes in the phase state as shown in a previous paper.6 The coagulates of the aggregated silica particles at a C16TAC concentration from 1.0
10-4 to 7.0
10-4 M are stable in the sol state below a volume fraction, φ, of 0.08 and no precipitation of the corresponding coagulates occurs. From the Orange II method, we could quantitatively determine the adsorption of C16TAC on the aggregated silica particles as follows. At φ = 0.057, the silica suspensions were mixed with an aqueous solution of C16TAC at various concentrations from 0.01 9394 DOI: 10.1021/la100391c

Figure 1. Plots of hydrodynamic diameters of the aggregated silica particles in the presence of 0 (filled circle), 1.0
10-4 (open circle), 3.0
10-4 (open square), and 5.0
10-4 M (open triangle) C16TAC.

to 1.0 mM, and the resulting suspensions were subjected to centrifugation to separate the silica particles and the supernatant. The supernatant was subjected to the Orange II method and checked for any color development. For all of the silica suspensions prepared in this concentration range, no color was detected, indicating that all C16TAC molecules are adsorbed onto the silica surface. Similar results were obtained at φ = 0.074 for the silica suspensions, as described in a previous paper.6 Hydrodynamic Diameters. The dynamic light scattering technique is a useful method for determining the average hydrodynamic diameter of a particle or a coagulate, i.e., a floc suspended in a fluid. The method is based on the Stokes-Einstein relation, which relates the diffusion coefficient measured by light scattering with the hydrodynamic diameter of the particle in solution. Figure 1 shows the average hydrodynamic diameters of the aggregated silica particles measured without C16TAC plotted as a function of φ. As reported in a previous paper,5 the resulting average diameter slightly increases with an increase in the silica concentration. Adsorption of C16TAC onto the silica suspensions induces coagulation of the aggregated silica particles, and it can be expected that partial coagulates of the aggregated silica particles should give a larger average hydrodynamic diameter than that observed without C16TAC. These coagulates form by clustercluster aggregation, in which a cluster corresponds to an initially aggregated silica particle as reported previously.6 The average hydrodynamic diameters of the partial coagulates of aggregated silica particles at C16TAC concentrations of 1.0
10-4, 3.0
10-4, and 5.0
10-4 M are also shown in Figure 1 as a function of φ. The resulting average hydrodynamic diameters are somewhat scattered and tend to increase with an increase in C16TAC concentration. However, the sizes are notably larger than that observed without C16TAC and are almost independent of the silica concentration. This means that the coagulation rate constant of the aggregated silica particles in the corresponding partial coagulates is almost independent of the silica concentration, and that the coagulation between the aggregated silica particles and the C16TAC molecules by electrical neutralization seems to be representative of a rapid coagulation mechanism.10 Rheological Properties. For rheologically complex materials, such as the coagulates of aggregated silica particles in this study, it is convenient to study the transient behavior, such as measurements of the hysteresis loop and transient shear stress before examining the steady-state rheological responses. Hysteresis loop measurements are often performed to distinguish Langmuir 2010, 26(12), 9393–9396

Komada and Kawaguchi

Figure 2. Flow curves of the coagulates of the aggregated silica particles at φ = 0.057 in the presence of 1.0
10-4 M C16TAC under increasing (filled circle) and decreasing (open circle) shear rate.

Figure 3. Flow curves of the coagulates of the aggregated silica particles at φ = 0.057 in the presence of 5.0
10-4 M C16TAC under increasing (filled circle) and decreasing (open circle) shear rate.

between Newtonian flow and non-Newtonian flows, such as thixotropic behavior, in which a negative hysteresis loop is observed, and rheopexic behavior, in which a positive loop is observed. These responses are accompanied by a partial breakdown of the microstructures of the complex materials under shear flow. Figures 2-4 show the shear rate dependences of the shear stresses, namely, the flow curves of the coagulates of the aggregated silica particles, with φ = 0.057 at C16TAC concentrations of 1.0
10-4, 5.0
10-4, and 7.0
10-4 M, respectively, under increasing and decreasing shear rate. The resulting hysteresis loop for the flocculates of aggregated silica particles for the lowest C16TAC concentration (Figure 2) can be regarded as nearly Newtonian flow, since the forward flow curves can almost be superimposed on the backward curve. A similar hysteresis loop is observed for silica particles with φ = 0.057 in the absence of C16TAC. An increase in the C16TAC concentration causes a drastic change in the shape of the hysteresis loop. At a C16TAC concentration of 5.0
10-4 M, the coagulates of the aggregated silica particles show a somewhat positive hysteresis loop, i.e., rheopexic flow, in which the shear stress under decreasing shear rates is slightly larger than that under increasing shear rates for all shear rates examined. At a C16TAC concentration of 7.0
10-4 M, however, a crossover hysteresis loop is observed such that Langmuir 2010, 26(12), 9393–9396

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Figure 4. Flow curves of the coagulates of the aggregated silica particles at φ = 0.057 in the presence of 7.0
10-4 M C16TAC under increasing (filled circle) and decreasing (open circle) shear rate.

thixotropic flow occurs at shear rates above 250 s-1 and rheopexic flow occurs at shear rates below 250 s-1. Such non-Newtonian flows should be strongly related to changes in the microstructure of the corresponding coagulates. In fact, the positive hysteresis loop is associated with breakdown of the microstructure, whereas the crossover hysteresis loop is connected with breakdown of the microstructure at higher and lower shear rates, respectively. When such hysteresis loops were repeatedly performed on the same coagulates at C16TAC concentrations of 5.0
10-4 and 7.0
10-4 M, similar shear stress responses were observed, even though the difference between the forward and backward flow curves becomes narrower with an increase in the number of hysteresis loops, suggesting that Newtonian flow curves are not obtained under shear flow. Moreover, the hysteresis loop measurements for the φ = 0.072 silica suspensions at C16TAC concentrations ranging from 1.0
10-4 to 7.0
10-4 M show similar hysteresis behaviors. However, the φ = 0.057 silica suspensions show a greater difference between the forward and backward flow curves than that observed for the φ = 0.072 suspensions as a result of microstructural changes occurring more easily for the φ = 0.057 suspensions. Figures 5 and 6 show typical shear stress responses for the coagulates of aggregated silica particles with φ = 0.057 at C16TAC concentrations of 5.0
10-4 and 7.0
10-4 M, respectively, for various shear rates. In the presence of 5.0
10-4 M C16TAC, the shear stresses at shear rates above 500 s-1 exhibit a slight sigmoidal increase with an increase in time, that is, structural buildup is observed. For shear rates from 30 to 100 s-1, however, a weak stress overshoot is observed and the shear stress soon attains its plateau value, irrespective of the shear rate. For shear rates below 10 s-1, the shear stresses are somewhat scattered and also tend to attain their plateau values, irrespective of the shear rate (not shown in Figure 5). For the 7.0
10-4 M C16TAC coagulates, an obvious stress overshoot was apparent, in contrast to that observed for the 5.0
10-4 M C16TAC coagulates, irrespective of the applied shear rate for shear rates above 50 s-1. Here, the peak in shear stress is observed after a longer shearing time with a decrease in the shear rate. For the 7.0
10-4 M C16TAC coagulates, the time taken for the shear stresses to attain steady state is 1 order of magnitude longer than the 5.0
10-4 M C16TAC coagulates. This indicates a more rigid microstructure for the 7.0
10-4 M C16TAC coagulates than for the 5.0
10-4 M C16TAC coagulates, due to the effective adsorption of C16TAC. However, the resulting DOI: 10.1021/la100391c

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Figure 5. Evolution of transient shear stresses of the coagulates of the aggregated silica particles at φ = 0.057 in the presence of 5.0
10-4 M C16TAC with time for various shear rates of 30 (open diamond), 100 (open triangle), 500 (open square), and 1000 s-1 (open circle).

Komada and Kawaguchi

Figure 7. Double-logarithmic plots of the apparent shear viscosity for the coagulates of the aggregated silica suspensions at φ = 0.057 as a function of shear rate in the presence of 5.0
10-4 (filled circle) and 7.0
10-4 M (open circle) C16TAC.

independent of the shear rate above shear rates of 100 s-1, suggesting that the coagulate microstructure at this C16TAC concentration are in a dynamic equilibrium state under shear flow.

Conclusions

Figure 6. Evolution of transient shear stresses with time for the coagulates of the aggregated silica particles at φ = 0.057 in the presence of 7.0
10-4 M C16TAC for various shear rates of 70 (open diamond), 200 (open triangle), 500 (open square), and 1000 s-1 (open circle).

shear stresses at shear rates below 50 s-1 are rather scattered and do not attain steady state even after shearing for over 1 h (not shown in Figure 6). In Figure 7, the steady-state apparent viscosities of the coagulates of the aggregated silica suspensions with φ = 0.057 in the presence of 5.0
10-4 and 7.0
10-4 M C16TAC are plotted against the shear rate. Both silica suspensions show typical pseudoplastic-flow behavior and the steady-state apparent viscosities at a C16TAC concentration of 5.0
10-4 M are almost

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Stable and aggregated fumed silica suspensions were effectively induced to form coagulates in the sol state by the addition of C16TAC, in which the C16TAC concentration was higher than the CMC. The resulting sol is stabilized by electrical neutralization along with adsorption of the C16TAC, owing to its high adsorption affinity for the negative charged silica surfaces, as shown by the complete adsorption of added amounts of C16TAC to the silica surfaces. The resulting hydrodynamic diameters of the coagulates increase with increasing C16TAC concentration, however, their magnitudes are almost independent of the silica concentration. The hysteresis loop of the coagulates is strongly dependent on the C16TAC concentration, and an increase in the C16TAC concentration results in a change from a Newtonian flow curve to crossover behavior through an initially positive hysteresis. The transient shear stresses of the corresponding silica suspensions are well correlated with the hysteresis loop behavior. Acknowledgment. This work is supported in part by the Grant-in-Aid for Scientific Research on Priority Area “Soft Matter Physics” (2006-2010) from the Ministry of Education, Culture, Sports, and Technology of Japan.

Langmuir 2010, 26(12), 9393–9396