pubs.acs.org/Langmuir © 2009 American Chemical Society
Effects of Perfluorosulfonic Acid Adsorption on the Stability of Carbon Black Suspensions Hiroki Mizukawa and Masami Kawaguchi* Division of Chemistry for Materials, Graduate School of Engineering, Mie University 1577 Kurimamachiya, Tsu, Mie, 514-8507, Japan Received July 13, 2009 Carbon black particles were stabilized by the adsorption of perfluorosulfonic acid (PFSA) onto their surfaces in a mixture of water and isopropyl alcohol. The resulting carbon black suspensions were characterized by measurements of the adsorbed amounts of PFSA, the hydrodynamic diameter of carbon black particles with adsorbed PFSA, and by steady state shear viscosity measurements as functions of the PFSA and carbon black particle concentrations. An equilibrium adsorption of PFSA on carbon black particles was established within 1 day, and the resulting adsorption isotherm was dependent on the carbon black particle concentration; the amounts of adsorbed PFSA decreased with increased carbon black particle concentration. The hydrodynamic diameter of the carbon black particles in suspension increased with the amount of adsorbed PFSA, resulting in stabilization of the carbon black suspensions. Thus, the apparent steady-state shear viscosities of the carbon black suspensions at the first and second shear thinning regions decreased with an increase in the amount of adsorbed PFSA.
Introduction Primary particles of carbon black generally tend to form primary aggregate “structures”, which then form secondary aggregates, namely an agglomerate, with an increase in the carbon black concentration.1,2 Primary aggregates of carbon black particles in liquids are widely used in many applications, such as paints, coating, printing, inks, and batteries, because aggregation of these primary particles determines the ultimate morphology of the carbon black aggregates.1,2 In order to understand the stability and structure of the primary aggregates of carbon black particles in a dispersion media, the suspension of carbon black is one of the most important issues. When polymer chains are present in the dispersion media of a carbon black suspension, they can be expected to form a thick adsorption layer on the surface of the carbon black primary aggregates.3-5 Differences in stabilization between carbon black primary aggregates, in both the presence and absence of adsorbed polymer chains, can be clearly distinguished by investigation using scattering techniques and rheological methods.4,5 Light scattering techniques can be used not only to estimate the polymer layer thickness adsorbed on primary aggregates of carbon black particles, but also the structures of the primary aggregates. Rheological methods also provide information on the structures of the primary aggregates from the various rheological properties: (1) shear thinning, shear thickening, and yield stress for shear flow experiments accompanied by the deformation or partial breaking of the aggregated structures, and (2) viscoelastic behavior for oscillatory (dynamic) experiments with linear responses, where no *Corresponding author. E-mail address:
[email protected]. Phone: þ81-59-231-9432. Fax: þ81-59-231-9433.
(1) Donnet, J. B.; Voet, A. Carbon Black; Marcel Dekker: New York, 1976. (2) Dannenberg, E. M. Vanderbilt Rubber Handbook, 12th ed.; R. T. Vanderbilt Co.: New York, 1977. (3) Lin, Y.; Smith, T. W.; Alexandridis, P. Langmuir 2002, 18, 6147. (4) Barrie, C. L.; Griffiths, P. C.; Abbott, R. J.; Grillo, I.; Kudryashov, E.; Smyth, C. J. Colloid Interface Sci. 2004, 272, 210. (5) Ma, S.; Chen, Q.; Jøgensen, F. H.; Stein, P. C.; Skou, E. M. Solid State Ionics 2007, 178, 1568.
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deformation of the primary aggregated structures occur.6-10 Therefore, a combination of scattering and rheological measurements should lead to a deeper understanding of the stability of carbon black suspensions in the presence of polymer chains in terms of the changes in the aggregated structures of carbon black particles by polymer adsorption, i.e., suppression of primary aggregate agglomeration. In order to characterize the effects of polymer adsorption on the stability of primary aggregates of carbon black particles, we report transient shear stress and steady-state shear viscosity measurements of aggregated carbon black particles suspended in a mixture of water and isopropyl alcohol and in corresponding solutions of perfluorosulfonic acid (PFSA), together with dynamic light scattering measurements as a function of the carbon black particle and PFSA concentrations. PFSA should be effectively adsorbed on carbon black particles, and the adsorption characteristics are verified.
Experimental Section Samples. PFSA dispersed in a mixture of water, isopropyl alcohol, and ethyl alcohol was purified by precipitation in 1,4dioxane, and the precipitated PFSA was dispersed in a mixture of water and isopropyl alcohol. The resulting PFSA dispersions were freeze-dried, and the purified PFSA was obtained in the solid state. Carbon black powders with a surface area of 165.8 m2/g were dried under vacuum before use. Water was purified by a Milli-Q Academic A10 ultrapure water system. Guaranteed grade 1,4-dioxane and isopropyl alcohol were used without further purification. Preparation of Carbon Black Suspensions. To prepare a carbon black suspension, a given weight of carbon black powder was added to 24 mL of water in a glass bottle to which 6 mL of (6) Boonstra, B. B. Polymer 1979, 20, 691. (7) Rigbi, Z. Adv. Polym. Sci. 1980, 36, 21. (8) Amari, T.; Watanabe, K. J. Rheol. 1990, 34, 207. (9) Amari, T.; Uesugi, K.; Suzuki, H. Prog. Org. Coat. 1997, 31, 171. (10) Kawaguchi, M.; Okuno, M.; Kato, T. Langmuir 2001, 17, 6041.
Published on Web 09/21/2009
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isopropyl alcohol or 6 mL of a PFSA isopropyl alcohol solution with a given concentration was then added. The carbon black concentrations of the suspensions were fixed at 5.0 and 8.0 wt %. The dosing concentrations of PFSA were changed from 0.01 to 0.4 g/100 mL. The resulting carbon black suspensions were agitated at 1500 rpm for 1 day at 25 °C in a mixer (Tokyo-Rikaki CM1000) with 20 g of 1 mm zirconia beads. The carbon black suspensions including PFSA were sedimented using a Kubota 6500 centrifuge at 20000 rpm. The separated carbon black suspensions were rinsed three times with a 4/1 vol% mixture of water and isopropyl alcohol (W/IPA mixture), and the resulting separated carbon black suspensions were then redispersed in the W/IPA mixture to maintain the same carbon black concentration, hereafter referred to as redispersed carbon black suspensions. In order to determine whether adsorption of PFSA on the zirconia mixing beads occurs, mixtures of 24 mL of water and 6 mL of PFSA isopropyl alcohol solution with a given concentration were agitated together in a glass bottle with the corresponding zirconia beads for 1 day under the same conditions as those used for the adsorption experiments of PFSA on the carbon black suspensions. Measurements of Adsorbed Amounts of PFSA. To quantitatively determine the amounts of adsorbed PFSA, 10 mL of the supernatant solution remaining after sedimentation was extracted, the W/IPA mixture was evaporated by heating, and the residue was weighed after drying in vacuum. This gravimetric analysis provides the concentrations of PFSA remaining in the supernatant solution. In order to determine the actual amount of adsorbed PFSA, the calculated amount of PFSA were subtracted from the initially added amount of PFSA. The gravimetric analysis for the amount of adsorbed PFSA was performed at least twice, and the experimental error was less than 5%. This method allowed determination of concentration down to 2 10-6 g/10 mL, according to the sensitivity of the electronic balance (Mettler AT250) used. Dynamic Light Scattering Measurements. Dynamic light scattering measurements of carbon black suspensions with concentrations from 0.02 to 0.16 wt %, obtained by dilution of the 5.0 and 8.0 wt % redispersed carbon black suspensions in the W/IPA mixture, were performed at 25 °C using a fiber-optics particle analyzer (Otsuka Electronics FPAR-1000). For a comparison, dynamic light scattering measurements of the carbon black suspensions in the absence of PFSA were also investigated at dilute carbon black concentrations of 0.02 and 0.1 wt %. The results were analyzed by the photon correlation spectroscopy technique, and an average diameter of the carbon black particles was calculated using the Marquardt method. Rheological Measurements. Rheological experiments of the carbon black suspensions and redispersed carbon black suspensions were carried out using a rheometer (Anton Paar Physica MCR-300) with a cone-plate fixture with a cone angle of 1° and a cone diameter of 50 mm (CP50-1). Experiments were conducted under steady shear flow at 25 °C. For each steady shear flow experiment, the corresponding carbon black suspensions were preconditioned at a shear rate of 2000 s-1 for 5 min to attain a constant shear stress, a 1 min rest time was applied, and then shear rates ranging from 1 to 1000 s-1 were applied to achieve steadystate as a function of time. Respective measurements were repeated at least twice, and the experimental errors were within 10%.
added PFSA concentrations for the respective carbon black suspensions, which indicates that adsorption of PFSA occurs on the surfaces of the carbon black particles. An increase in adsorption time leads to an increase in the amount of adsorbed PFSA, and an equilibrium adsorbed amount of PFSA is attained at adsorption times of less than 24 h. The resulting equilibrium adsorption time is usual for polymer adsorption behavior on various solid surfaces from polymer solutions.11 Adsorption interaction of PFSA is mainly due to hydrophobic interaction, because PFSA consists of a perfluorinated polymer backbone. However, since PFSA contains ether side chains terminated with sulfonate groups, hydrophilic adsorption interaction could occur for the adsorption of PFSA on carbon black particles, due to the presence of some functional groups on their surfaces. Adsorption isotherms of PFSA for the 5.0 and 8.0 wt % carbon black suspensions are displayed in Figure 1. The amounts of adsorbed PFSA increase with an increase in the amount of PFSA added, and they attain a plateau value, irrespective of the carbon black concentration. The magnitude of the amount of adsorbed PFSA for the 5.0 wt % carbon black suspension is larger than that for the 8.0 wt % suspension. This means that the effective adsorption surface area becomes smaller with increased carbon black concentration, due to the formation of highly aggregated structures in the carbon black suspensions. Similar polymer adsorption behavior has been observed for colloidal silica12 and fumed titania particles.13 The amounts of adsorbed PFSA seem to be 1 order of magnitude less than those for the adsorption of various neutral polymers;11 however, the magnitude is almost the same as that for adsorption of PFSA on carbon as well as on noble metal catalyst in water.5 The smaller adsorption amount may be attributed to the electrostatic repulsions between the sulfonate groups in PFSA. In addition, since any desorption of PFSA from the PFSA chains adsorbed on carbon black particles was not detected within the experimental error, it can be regarded that the amount of adsorbed PFSA is not changed by rinsing with the W/ IPA mixture. Similar results have been reported for the other systems.14-16 Hydrodynamic Diameters. The dynamic light scattering technique is useful to determine the hydrodynamic diameter of a particle and a polymer chain suspended in a fluid by the Stokes-Einstein relation using the diffusion coefficient measured from the corresponding technique. The redispersed 5.0 wt % carbon black suspensions are black in color, so that it is very difficult to detect light scattering from such carbon black suspensions without dilution using the W/IPA mixture. Thus, the hydrodynamic diameter of carbon black in dilute carbon black suspensions with carbon black concentrations ranging from 0.02 to 0.10 wt % were 300 ( 10 nm, irrespective of the carbon black concentration. The resulting hydrodynamic diameters could correspond to the sizes of the primary aggregated carbon black particles in carbon black suspensions without dilution. When PFSA is adsorbed on the primary aggregated carbon black particles without breaking of their aggregated structures, the hydrodynamic diameters of the primary aggregated carbon black particles with adsorbed PFSA can be expected to increase
Results and Discussion
(11) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993. (12) Kawaguchi, M.; Naka, R.; Imai, M.; Kato, T. Langmuir 1995, 11, 4323. (13) Doi, M.; Kawaguchi, M.; Kato, T. Colloids Surf., A: Physicochem. Eng. Aspects 2003, 211, 223. (14) Kawaguchi, M.; Hayashi, I.; Takahashi, A. Polymer J. 1981, 13, 783. (15) Shimono, N.; Koyama, N.; Kawaguchi, M. Jpn. J. Appl. Phys. 2006, 45, 4196. (16) Nishiguchi, N.; Ueno, T.; Kawaguchi, M. J. Dispersion Sci. Technol., in press.
Adsorbed Amounts of PFSA. Any changes in the measured concentrations of the supernatant solutions without carbon black particles were not detectable within the experimental error, so that it was considered that adsorption of PFSA on the zirconia beads does not occur. However, the measured concentrations of PFSA in the supernatant solutions were lower than the corresponding Langmuir 2009, 25(20), 11984–11987
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Figure 1. Adsorption isotherms of PFSA for the 5.0 (b) and 8.0 (9) wt % carbon black suspensions.
with the increase in the amount of adsorbed PFSA. Moreover, the dilute redispersed carbon black suspensions are much less easily sedimented into two phases than the dilute carbon black suspensions without PFSA according to visual observation; therefore, PFSA adsorbed on carbon black particles plays a role in stabilization of the corresponding carbon black particles. The hydrodynamic diameters of the dilute (1/250 with the W/IPA mixture) redispersed carbon black particles are plotted as a function of the equilibrium PFSA concentration at carbon black dosing concentrations of 5.0 and 8.0 wt % are given in Figure 2. The resulting hydrodynamic diameters of the redispersed carbon black suspensions increased with the increase in the equilibrium concentration of PFSA, irrespective of the carbon black concentration. The hydrodynamic diameter also correlates well with the amount of PFSA adsorbed on the carbon black particles. At the highest equilibrium PFSA concentration, the layer thickness of adsorbed PFSA on the carbon black particle was determined to be 30 ( 3 nm from the difference in the diameters of aggregated carbon black particles with and without PFSA. However, it is noted that the resulting layer thickness of 30 nm includes also the shell of solvent and it is an overestimation of the polymer layer. Rheological properties. For rheologically complex substances, such as carbon black suspensions, it is convenient to study the transient behavior before examining the steady-state behavior. The shear stress at shear rates larger than 100 s-1 are almost independent of time, whereas at shear rates lower than 50 s-1 they are somewhat scattered and slightly decreased with increased time, and attain respective plateau values, i.e., structural breakdown is observed. The apparent steady-state shear viscosities of the corresponding carbon black suspensions are plotted as a function of shear rate in Figure 3. The resulting plot can be classified into weak shear thickening, and the small bump due to shear thickening is appreciable at a shear rate of 50 s-1. The addition of PFSA to the carbon black suspensions causes stabilization of the suspension by the steric repulsive forces between the adsorbed PFSA layers; therefore, changes in the shear flow behavior should occur. The development of shear stress for the 5.0 wt % redispersed carbon black suspension at various shear rates is not clearly different from those in the absence of PFSA, irrespective of the amount of adsorbed PFSA. However, the differences in the apparent steady-state shear viscosities as a function of shear rate between the 5.0 wt % carbon black suspension and the 5.0 wt % redispersed carbon black suspension are clearly observed, as shown in Figure 3. The first shear thinning behavior at shear rates of less than 10 s-1 in the presence of PFSA is somewhat weaker than that in the absence of PFSA. The shear thickening behavior is almost suppressed by the adsorption of PFSA on the carbon black particles for shear rates ranging from 10 to 50 s-1, and the apparent steady-state shear viscosities at shear rates higher than 50 s-1 in the presence of 11986 DOI: 10.1021/la9025188
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Figure 2. Plots of hydrodynamic diameters Dp of carbon black particles for the diluted redispersed carbon black suspensions as a function of the equilibrium PFSA concentration Cp. Symbols are the same as in Figure 1.
Figure 3. Double-logarithmic plots of apparent steady-state shear viscosities of the 5.0 wt % redispersed carbon black suspensions at the added PFSA concentrations of 0 (O), 0.01 (0), 0.05 (4), 0.1 (3), 0.2 (b), and 0.4 (9) g/100 mL as a function of shear rate.
PFSA are much less than those in the absence of PFSA. Moreover, the corresponding plots are clearly dependent on the amounts of adsorbed PFSA, and an increase in adsorbed PFSA causes increased stabilization of the carbon black suspension, which leads to lower apparent steady-state shear viscosities at shear rates above 50 s-1. Moreover, the steady-state shear viscosities of the carbon black suspensions observed in this study are mainly attributed to the easily disintegrated aggregated structures of the carbon black suspensions under shear flow with increased amount of adsorbed PFSA. This was confirmed by a preliminary experiment using a rheometer-based shear flow apparatus for small angle neutron scattering (SANS-U instrument of the Institute for Solid State Physics, University of Tokyo at JAERI Tokai, Ibaraki, Japan).17-19 The scattering densities of carbon black, PFSA, water, and isopropyl alcohol are 6.5 1010, 2.0 1010, -0.56 1010, and -0.33 1010 cm-2, respectively. The neutron scattering intensities of the 5.0 wt % redispersed carbon black suspension with added PFSA at a concentration of 0.2 g/100 mL at various shear rates of 0, 10, 50, 100, 200, and 500 s-1 are displayed as a function of the scattering vector, q, in Figure 4, which shows that the neutron scattering intensities are clearly dependent on the shear rate. However, it is noted that the scattering patterns show two power-law regimes, irrespective of the shear rate. At low q, a (17) Imai, M.; Nakaya, K.; Kato, T.; Takahashi, Y.; Kanaya, T. J. Phys. Chem. Solids 1999, 60, 1313. (18) Takahashi, Y.; Noda, M.; Kitade, S.; Noda, I. J. Phys. Chem. Solids 1999, 60, 1343. (19) Takahashi, Y.; Noda, M.; Naruse, M.; Kanaya, T.; Watanabe, H.; Kato, T.; Imai, M.; Matsushita, Y. Nihon Reoroji Gakkaishi 2000, 28, 187.
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Figure 4. Neutron scattering intensities I(q) of the 5.0 wt % redispersed carbon black suspensions at the added PFSA concentration of 0.2 g/100 mL for various shear rates of 0 (O), 10 (b), 50 (9), 100 (0), 200 (4), and 500 (3) s-1 as a function of the scattering vector q.
weak slope of -2.3 corresponds to a mass fractal regime, whereas at high q, a slope of -3.6 is observed for the surface fractal of the primary particle. This means that the basic fractal structures of the carbon black suspensions are maintained even under shear flow. The slope of -2.3 is slightly greater than that for spherical particles, and the neutron scattering results indicate that the fractal carbon black particle has a more open structure. Lin et al.3 and Barrie et al.4 also reported similar power law results for small angle neutron scattering measurements of some carbon black suspensions without shear flow. A typical emphasized plot of neutron scattering intensities at q = 0.01 nm-1 is displayed against the shear rate in Figure 5. Changes in the neutron scattering intensities with increase in the shear rate are well correlated with the shear flow behavior of the corresponding carbon black suspension. Therefore, for the first shear thinning regions and the plateau shear viscosity regions at shear rates less than 50 s-1, the neutron scattering intensity rapidly decreases to a minimum, due to the partial breaking down of the carbon black aggregated structures in suspensions under
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Figure 5. Plots of neutron scattering intensities I(q) at q = 0.01 nm-1 of the 5.0 wt % redispersed carbon black suspensions at the added PFSA concentration of 0.2 g/100 mL as a function of shear rate.
shear flow. For the second shear thinning regions with an increase in the shear rate beyond 50 s-1, the corresponding neutron scattering intensity increases and approaches a constant value above a shear rate of 200 s-1, as a result of the formation of newly ordered structures of partially ruptured carbon black suspensions under higher shear flow. Therefore, the changes in the neutron scattering intensities seem to be well correlated with changes in the structures in the carbon black suspensions under shear flow.
Conclusions Adsorption of PFSA on carbon black in suspension causes an improvement in stability. Improved stability leads to an increase in the hydrodynamic size of the carbon black particles in diluted carbon black suspensions and a decrease in the steady-state shear viscosity with the increase in the amount of adsorbed PFSA. Acknowledgment. We thank Professor Yoshiaki Takahashi (Kyushu University) for performing the small angle neutron scattering measurements.
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