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Floc Strength Characterization Technique. An Insight into Silica Aggregation Mandalena Hermawan,† Graeme C. Bushell,*,† Vincent S. J. Craig,‡ Wey Yang Teoh,† and Rose Amal† School of Chemical Engineering and Industrial Chemistry, University of New South Wales, Sydney, NSW 2052, Australia, and Department of Applied Mathematics, Research School of Physical Science and Engineering, The Australian National University, Canberra, ACT, Australia Received February 10, 2004. In Final Form: May 5, 2004 This paper tests an approach to the estimation of relative particle bond strength based on the nondimensional floc and aggregation factors. The strength of flocs formed by aggregating nanosized silica particles with the addition of potassium chloride (KCl) or cationic surfactants, alkyltrimethylammonium bromide (mixture of CTAB, DTAB, and MTAB) was analyzed. The bonding force of the flocs formed in surfactant compared to that formed in the KCl system was estimated using the new dimensional analysis approach. This force ratio was then compared to that obtained by atomic force microscopy.
Introduction It is well understood that the efficiency of solid-liquid separation processes such as settling, filtration, and flotation depends greatly on the flocculation performance. During the flocculation process, aggregates of different properties (size, structure, and strength) are formed depending on the flocculation conditions (nature and concentration of electrolyte, pH, type and duration of shear, the presence of other species such as surfactant, polymer, and polyelectrolytes).1 With this knowledge, the flocculation is then tailored to form aggregates/flocs which are suitable for different solid-liquid separation methods, for example, strong, high-density flocs are favorable in filtration, but not in sedimentation processes. It is therefore essential to characterize the aggregates formed to optimize the process. Despite all that, aggregate properties characterization, in particular floc strength characterization, remains a challenge. Floc strength is not an independent property which can be measured trivially like size or structure and, hence, requires more sophisticated measurement and analysis. It has been well-known that aggregates formed during the flocculation process exhibit a fractal characteristic, implying that they are self-similar and scale invariant. So, for a fractal aggregate, the relationship between mass (M) and size (R) is simply
M ∝ RDf
(1)
where Df is the mass fractal dimension and can have a value between 1 and 3 in three-dimensional space. A lower Df indicates that the aggregate is open and more tenuous in structure. Reviews of the fractal dimension and ways to measure it have been published2,3 and will not be explained further in this work. * Corresponding author. E-mail:
[email protected]. † University of New South Wales. ‡ The Australian National University. (1) Scales, P. J.; Johnson, S. B.; Franks, G. V.; Boger, D. V.; Healy, T. W. Int. J. Miner. Process. 2000, 58, 267-304. (2) Hermawan, M.; Bushell, G.; Bickert, G.; Amal, R. Part. Systems Charact. 2003, 20, 1-8. (3) Bushell, G. C.; Woodfield, D.; Amal, R.; Yan, Y. D.; Raper, J. Adv. Colloid Interface Sci. 2002, 95, -50.
While size, density, and structure are the aggregate properties that are most commonly investigated in solidliquid separation, there is one other very important property that needs to be focused on: floc strength. However, floc strength is not an independent property; both the floc structure and particle bond strength are interrelated with floc strength. For example, aggregates with compact structure tend to be stronger and more resistant to break up as there are usually more interparticle bonds within the aggregates compared to those of aggregates of looser structure. However, in some cases, strong bonds between the individual particles will enable the flocs to grow large with an open structure. While techniques to characterize floc size and structure using scattering, settling, and imaging are readily available and have been used widely in research and industry,4-10 floc strength is harder to characterize and there is, as yet, no straightforward technique to characterize the floc strength experimentally and there remain significant challenges to describing the floc strength adequately by theoretical means. Earlier research11 determined the strength of aggregates by combining the strength of adhesion of particles in coagulating or flocculating media and the floc size distribution of the suspension after it had been subjected to certain shear rates. A correlation was found between floc size and the strength of particle adhesion (for a given rate of shear and the resultant particle size of suspension). In later research,12,13 the floc strength was defined from (4) Cametti, C.; Codastefano, P.; Tartaglia, P. J. Colloid Interface Sci. 1989, 131(2), 409-423. (5) Schaefer, D. W.; Beaucage, G. J. Non-Cryst. Solids 1994, 172174, 797-805. (6) Axford, S. D. T.; Herrington, T. M. J. Chem. Soc., Faraday Trans. 1994, 90 (no. 14), 2085-2093. (7) Matsumoto, K.; Mori, Y. J. Chem. Eng. Jpn. 1975, 8 (2), 143-147. (8) Uribe-Salas, A.; Vermet, F.; Finch, J. A. Chem. Eng. Sci. 1992, 48 (4), 815-819. (9) Turian, R. M.; Ma, T. W.; Hsu, F. L. G.; Sung, D. J. Powder Technol. 1997, 93, 219-233. (10) Farias, T. L.; Brasil, A. M.; Carvalho, M. G. J. Aerosol Sci. 1999, 30 (10), 1379-1389. (11) Smith, D. K. W.; Kitchener, J. A. Chem. Eng. Sci. 1978, 33, 1637-1643. (12) Glasgow, L.; Hsu, P. J. Am. Ind. Chem. Eng. J. 1982, 28, 779285. (13) Tambo, N.; Hozumi, H. Water Res. 1979, 13, 421-427.
10.1021/la049651p CCC: $27.50 © 2004 American Chemical Society Published on Web 06/22/2004
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the relationship between the size of the surviving aggregate to the applied shear or normal stress that flocs were subjected to during turbulent flow. Other researchers attempted to evaluate floc strength by relating not only the size of the floc but also the porosity of the floc, to the applied stress.14,15 Pelton and Yeung16 introduced a micromechanical approach where a single floc particle was pulled apart in order to study the strength and break up of polymeric flocs. The rupture force was taken as the floc binding strength. They found that the average floc strength was dependent on the shear rate at which the flocs were formed and there was no scaling relationship found between floc strength and floc size. This is expected as the strength of the aggregates is not dependent on the size but is dependent on the floc structure, which is a governing factor in the floc break up mechanism. For example, an open structured floc will split more easily than a compact structured floc as there are more bonds between particles for the latter. Floc size, however, is merely the result of floc strength when it is subjected to a certain shear. Alternatively, other researchers have tried to determine the floc strength from the rheological behavior of the aggregate suspension under different flocculation conditions (pH, electrolyte, and additives).1,17 While aggregate strength is related to the way in which the particles are arranged within the structure (floc structure), the mechanical strength of aggregates is also dependent on the interparticle forces and the packing of the primary particles that make up the aggregates.18 Floc structure can generally be characterized by light scattering or image analysis3,19-21 and expressed in terms of the fractal dimension, which provides an insight into the arrangement of particles within the aggregate as well as the degree of compactness of aggregates. It is an important measure since increasing compactness often means greater interparticle bonding which implies that the floc is stronger. Through development of novel parameters via dimensional analysis, Selomulya et al.22 proposed a correlation between the properties of the particles flocculated under certain conditions and the characteristic of flocs produced. The analysis considers both aggregate properties and the fluid properties. The aggregate properties are comprised of parameters such as the size of the primary particles (d0), size of the aggregate (d), bonding force (F), scattering exponent (SE) or fractal dimension (Df), and the initial number of primary particles (n0). Meanwhile the fluid properties involve the fluid density (F), viscosity (µ), and the average energy dissipation rate () from the applied shear. By means of dimensional analysis, the aggregate properties can be grouped together to what is termed a floc factor:
Πf )
() d d0
1/SE
(2)
The power law form between the size ratio of (d/do) and structure (SE) was required to account for the scaling of the overall floc properties with size.22 (14) Bache, D. H.; Al-Ani, S. H. Water Sci. Technol. 1989, 21, 29537. (15) Sonntag, R. C.; Russel, W. B. J. Colloid Interface Sci. 1987, 115 (2), 378. (16) Pelton, R.; Yeung, A. K. C. J. Colloid Interface Sci. 1996, 184, 579-585. (17) Quemada, D.; Berli, C. Adv. Colloid Interface Sci. 2002, 98 (1), 51-85. (18) Tang, S.; Ma, Y.; Shiu, C. Colloid Surf., A 2001, 180, 7-16. (19) Bottero, J. Y.; Thill, A.; Wagner, M. J. Colloid Interface Sci. 1999, 220, 465-467. (20) Adachi, Y.; Kobayashi, M.; Ooi, S. J. Colloid Interface Sci. 1998, 208, 353-355.
All the parameters in the fluid properties are grouped together to form the aggregation factor:
Πa )
( ) ( ) () µ 1 1 (F) (n0) 2 d F 0
(3)
Selomulya et al.22 found a linear relationship between the floc factor Πf and aggregation factor Πa on a log-log plot using experimental data of latex aggregates for a certain shear rate range. The model has also been compared with results available in the literature.23-25 It is also believed that the model holds for geometrically similar systems. The linear relationship between the floc factor and the aggregation factor enables the estimation of the relative bonding force (F) between two aggregation systems. For example, the relationship would enable the determination of the relative bonding force of aggregates formed in the presence of electrolyte, surfactant, or polymer. An understanding of the interparticle interactions can be used to predict the stability of colloidal systems. The DLVO theory26,27 of colloidal stability describes the interparticle interaction energy in terms of the attractive van der Waals forces and (for like-charged particles) the repulsive electrostatic forces. It assumes that the colloidal particles are lyophobic, that is, they do not have strong specific interactions with the solvent. Silica however is lyophilic, and the strong solvent-silica interactions lead to colloidal stability that is not described by the DLVO theory. For example silica colloids are stable at low pH, where DLVO theory predicts the low surface charge will result in coagulation. In order for two silica particles to adhere it is necessary that during a collision the strong short-range hydration force is overcome. Cations are known to specifically interact with the silica surface. In doing so they replace surface-bound hydronium ions (H3O+) and can be seen as “dehydrating” the surfaces. At sufficiently high bulk concentrations of electrolyte, sufficient numbers of ions may be adsorbed to the silica surface in order to render the surface lyophobic, thereby removing the repulsive hydration force.28 This will result in coagulation of particles and flocculation, as at these high electrolyte concentrations the electrostatic doublelayer is highly compressed. Alternatively, the particle adhesion can be attributed to the bridging of two surfaces through the hydration layer of numerous adsorbed cations. The former of these arguments is supported by the observation that pyrogenic silica, whose surface is not highly hydrated, is flocculated by low levels of electrolyte. In any case it is difficult to predict the interparticle bond strength between silica particles in electrolyte solutions from theoretical considerations alone. (21) Amal, R.; Raper, J.; Waite, T. D. J. Colloid Interface Sci. 1990, 140 (1), 158. (22) Selomulya, C.; Amal, R.; Bushell, G.; Waite, T. D. Relationship between Floc Properties and Flocculation Conditions in Different Shear Environment. In Proceedings of the World Congress of Particle Technology 4, 2002, Sydney. (23) Oles, V. J. Colloid Interface Sci. 1992, 154, 351. (24) Serra, T.; Casamitjana X. J. Colloid Interface Sci. 1998, 206, 505-511. (25) Selomulya, C.; Bushell, G.; Amal, R.; Waite T. D. Aggregate Properties in relation to factors influencing aggregation under various applied shear environments. In Proceedings of Solid-Liquid Separation Systems III, Davos, Switzerland: United Engineering Foundation, New York, 2001. (26) Derjaguin, B. V.; Landau, L. Acta Phys. Chim. URSS 1941, 14, 633. (27) Verwey, E. J. W.; Overbeek, J. T. G. Theory of the Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948. (28) Iler, R. K. The Chemistry of Silica: Solubility, Polymerisation, and Surface Properties and Biochemistry; Wiley: New York, 1979.
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Cationic surfactants adsorb to silica surfaces electrostatically at low concentrations and neutralize the surface charge. As the surfactant concentration increases, hydrophobic interactions between the surfactant tails also contribute to the adsorption process. At still higher concentrations hydrophobic interactions lead to the formation of bilayered aggregates at the interface (surfactant headgroups facing into solution) and result in a reversal of the surface charge. For a complete description of the surfactant adsorption mechanism, readers should refer to work by Atkin et al.29 The solution pH will alter the native silica surface charge and therefore influence the surface excess of surfactant. A higher pH will lead to a higher surface charge and an increase surfactant surface excess for a given bulk concentration. The complex surfactant adsorption behavior results in an equally complex series of surface interactions. At low concentrations the surface charge is neutralized and the surface excess of surfactant is low. Experimentally it has been observed that a strong long-ranged attraction, termed the hydrophobic attraction, exists at these levels of surfactant adsorption, though the precise origin of the interaction is not known and the magnitude of the attraction is variable and strongly dependent on the precise level of surfactant surface excess.30 The adhesion in these systems can be very large. At higher concentrations the net surface charge is reversed and electrostatic double layer repulsion is measured that increases with increasing surfactant levels, as the surface charge continues to increase. Here the longrange interactions are generally well described by DLVO theory, but at short range, repulsive interactions due to the adsorbed surfactant are observed and very little adhesion is observed. With sufficient compressive force and time in contact the surfactant layers yield. When this has occurred the adhesive force is much larger. Therefore while the silica-cationic surfactant system stability can be understood at least qualitatively in terms of the surface interactions, it is nearly impossible to make quantitative predictions of interparticle bond strength based on theory. Clearly at this stage information on bond strength between primary silica particles is best obtained experimentally. Several techniques are available for measuring the interaction forces on approach and the adhesion force upon separation. Perhaps the most suitable for colloidal silica particles is the colloid probe technique31,32 which utilizes the atomic force microscope. The technique enables the force as a function of surface separation to be measured between a silica sphere and a flat surface in solution. The details of this technique are reviewed elsewhere32 and will not be covered here. In this study the relative strength of silica aggregates formed by addition of cationic surfactants to that formed by addition of salt is studied. It was reported in earlier publications33-35 that silica can be made to aggregate and result in different floc structures depending on the aggregation conditions. In the absence of mixing, relatively (29) Atkin, R.; Craig, V. J.; Wanless, E. J.; Biggs, S. Adv. Colloid Interface Sci. 2003, 103 (3), 219-304. (30) Craig, V. S. J.; Ninham, B. W.; Pashley, R. M. Langmuir 1998, 14, 3326-3332. (31) Kappl, M.; Butt, H. J. Part. Part. Syst. Charact. 2002, 19, 129143. (32) Senden, T. J. Curr. Opin. Colloid Interface Sci. 2001, 6, 95-101. (33) Hermawan, M.; Bushell, G.; Bickert, G.; Amal, R. Characterisation of Short-Range Structure of Silica AggregatessImplication to Sediment Compaction, in Proceedings of Solid-Liquid Separations III; United Engineering Foundation, Inc.: Davos, Switzerland, 2001. (34) Hermawan, M.; Bushell, G.; Bickert, G.; Amal, R. Relationship between Floc Short-Range Structure and Sediment Compaction. In Proceedings of World Congress of Particle Technology 4; WCPT 4: Sydney, 2002; paper no 378.
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Figure 1. Schematic diagram of experimental arrangement for type I aggregation.
open flocs are produced. With continuous mixing, compact flocs are formed. In this paper, studies on the relative floc strength between silica flocs formed by mixed alkyltrimethylammonium bromide and KCl are presented. The relative floc strength was estimated from the linear relationship between floc factor and aggregation factor. A more detailed description on how this correlation can be used to estimate the floc strength is given in the Experimental Section. The relative floc strength estimated from this new approach was compared to that measured by atomic force microscopy (AFM). Experimental Method Materials. Silica Ludox TM-40 particles of 22 nm in diameter were made to aggregate by addition of electrolyte or surfactant. The silica stock solution was prepared and stored at 10 000 mg/ L. KCl at a concentration of 1.5 M was the chosen electrolyte, and several different concentrations of mixtures of cationic alkyltrimethylammonium bromide surfactants (mixture of CTAB (C16TAB), MTAB (C14TAB), and DTAB (C12TAB)) supplied as mixed powder from Sigma Aldrich which predominantly contains C14TAB but also CTAB (C16TAB) and DTAB (C12TAB) homologues were used to induce the aggregation without further purification. Sodium tetraborate, Na2B4O7‚10H2O, at a concentration of 0.005 M was used as a buffer to maintain the suspension at pH 8, and when aggregation was to be carried out at pH 5, hydrochloric acid was used to adjust the pH of the solution. All solutions were prepared using distilled water that was further purified using a Millipore filtration system. Methods. Aggregation and Characterization. Two types of aggregation methods were employed in characterizing the silica aggregates. The first type of aggregation as shown in Figure 1 (type I) involved continuous shearing in a 1 L baffled beaker using an axial flow impeller (three-blade fluid foil Lightnin A310 impeller). The impeller was positioned one-third of the height of the suspension from the bottom. The stirring speed was adjustable, allowing for different average shear rates, G (s-1), which were estimated using the expression36
G ) (/ν)1/2
(4)
where ν (m2/s) is the kinematic viscosity of the suspension and (m2/s3) is the average turbulent energy dissipation rate per unit mass of the suspension. The energy dissipation rate is related to the suspension volume, V, the impeller number, Np (taken as 0.3), the impeller diameter, D (taken as 64 mm), and the impeller speed, N (s-1), by the equation
) NpN3D5/V
(5)
For all the aggregation experiments, an 800 mL suspension of silica particles at a concentration of 100 mg/L was used. For (35) Hermawan, M.; Teoh, W. Y.; Bushell, G.; Bickert, G.; Amal, R. Short Range and Long-Range Silica Restructuring and Its Implication on Sediment Compaction. In Proceedings of Chemeca 2002; APCCHE: Christchurch, NZ, 2002; paper no 578. (36) Camp, T. R.; Stein, P. C. J. Boston Soc. Civil Eng. 1943, 30, 219.
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Figure 2. Schematic diagram of experimental arrangement for type II aggregation. the first type of aggregation experiment (type I) a peristaltic pump was used to circulate the suspension in and out of the scattering cell for continuous measurement. The pump was placed at the outlet of the scattering cell to prevent shearing of the aggregates in the pinch portion of the pump prior to the measurement. Samples were withdrawn from midway between the impeller and the top of the suspension. The second aggregation method employed (type II) has a similar experimental arrangement as the type I aggregation except for the peristaltic pump being replaced by a syringe as shown in Figure 2. In this arrangement the solution was introduced into the scattering cell by suction using a syringe. This was to ensure that the shear rate imposed by the impeller was greater than that imposed by transferring the suspension to the scattering cell, even for the condition when the impeller was operating at a low speed. Small-angle laser light scattering was used to characterize the floc size and structure using a Malvern Mastersizer/S. The instrument consists of a 5 mW He-Ne laser source, an optical lens, and a series of photosensitive detectors. The expanded and collimated laser beam is scattered by the particles in the suspension. The scattered intensity as a function of scattering angle, is measured by photosensitive detectors and used to obtain information on floc structure as a function of length scale.37 Atomic Force Microscopy. Interaction Forces Characterization. The interaction forces between two silica surfaces were characterized using an atomic force microscope. The method is commonly used and fully described elsewhere,29,30,32,38-42 therefore only a brief description will be given here. The instrument employs the light lever technique to sense the deflection of a fine spring due to the action of surface forces. Microfabricated cantilever springs (Nanoscope, Digital Instruments) with a spring constant of 0.10 N/m were used. A borosilicate glass sphere (20 µm in diameter from Polysciences) was attached to the tip of the cantilever using Epikote glue according to the method developed by Ducker et al.40 This allows the chemistry of the interacting forces to be controlled and the radius of interaction to be determined. Forces are usually normalized with respect to the radius and reported as force/radius (F/R). The electrolytes used in the AFM experiment were prepared using the same manner as in the aggregation section. Ultraclean water prepared by filtering and passing through activated charcoal and a reverse osmosis membrane then followed by distillation was used for soaking, rinsing, and measurement of interaction forces in water. All preparations of the surfaces were carried out in a laminar flow cabinet. Glassware was soaked in 10 wt % NaOH solution and rinsed thoroughly with ultraclean water. The AFM fluid cell and all fittings were washed thoroughly in distilled AR grade ethanol and rinsed in water. The silica surfaces were not plasma treated (37) van de Hulst, H. C. Light Scattering by Smal Particles; Dover Publications Inc.: New York, 1981. (38) Atkins, D.; Kekicheff, P.; Spalla, O. J. Colloid Interface Sci. 1997, 188, 234-237. (39) Craig, V. S. J.; Hyde, A. M.; Pashley, R. M. Langmuir 1996, 12, 3557-3562. (40) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239-241. (41) Froberg, J. C.; Rojas, O. J.; Claesson, P. M. Int. J. Miner. Process. 1999, 56, 1-30. (42) Higashitani, K.; Vakarelski, I. U.; Ishimura, K. J. Colloid Interface Sci. 2000, 227, 111-118.
Figure 3. Scattering pattern of silica aggregated with 1.5 M KCl and sheared at G ) 257 s-1 and sampled using a peristaltic pump (open diamonds) and a syringe (closed diamonds). (this can influence the surface charge density) to ensure a similar surface to the colloidal silica used in the aggregation experiments. The surface forces between silica surfaces immersed in different electrolytes were measured in water, 1.5 M NaCl and 1.5 M KCl, 0.02, 0.06, and 0.4 mM AlkylTAB (at pH 8), and 0.2, 0.4, 0.5, and 3 mM AlkylTAB (at pH 5), all in the presence of 5 mM sodium tetraborate. The fluid cell was rinsed with water between solution changes. Measurements were conducted several times to check for reproducibility.
Results and Discussion The flocculation of silica particles was investigated as a function of surfactant concentration in solutions of pH 5 and pH 8. All pH 8 and pH 5 solutions contained 5 mM buffer, and all solutions at pH 5 had a small amount of added HCl. In both cases a maximum in flocculation efficiency was observed. Significantly above and below this concentration no flocs were formed. Surfactant concentrations of 0.06 and 0.015 mM were used at pH 8 and a concentration of 0.2 mM was used at pH 5 as these concentrations led to effective flocculation. The range of surfactant concentrations at which flocs formed was 0.17.5 mM for the pH 5 system and 0.015-0.1 mM for the pH 8 system. The concentration of surfactant required to achieve flocculation at the higher pH is approximately an order of magnitude lower. The higher surface charge at pH 8 leads to a greater degree of surfactant adsorption to the silica surface. Therefore it is reasonable to suggest that the surface excess of surfactant adsorbed to the silica surfaces is similar under the flocculation conditions employed at pH 5 and pH 8 despite the bulk concentration being somewhat different. As mentioned earlier, two types of aggregation methods were employed, type I aggregation where the sample is circulated in and out of the scattering cell using a peristaltic pump and type II aggregation where the sample was introduced manually using a syringe. It can be seen from Figure 3 that no significant difference is evident for the two methods of aggregation at moderate shear rates. This is expected as the shearing imposed by the impeller blade was far greater than that exerted by either the peristaltic pump action or the syringe. At lower shear rates (G < 90 s-1) however, it has been seen that the peristaltic pump sheared the aggregates far more than the impeller did, and therefore a syringe was used to deliver the sample in these cases. Most of the scattering patterns shown in this work do not display the typical scattering pattern of fractal aggregates, in that a simple power law dependence of scattered intensity on Q is not observed. Scattering
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Figure 4. Scattering intensity as a function of scattering vector for silica aggregates formed using 1.5 M KCl at a range of shear rates. The shear rates employed were from top to bottom G ) 30 s-1, G ) 90 s-1, G ) 260 s-1, G ) 360 s-1, and G ) 470 s-1.
exponent (SE) rather than fractal dimension was hence used to quantify the structure of aggregates since the use of the term fractal implies that the structure is scale invariant. The slope of the tangent to the scattered intensity at different Q ranges reflects the structural information about the flocs at the length scales corresponding to those Q ranges. For these systems, the structural scaling is different at different length scales and so, strictly speaking, they cannot be called fractal. The scattering pattern of silica aggregated with 1.5 M KCl as a function of shear rate is plotted in Figure 4. At large Q (5 × 10-3 nm-1 < Q < 10-2 nm-1) which corresponds to a length scale of 100 nm < Rg < 200 nm, the data are linear and the slope reveals a scattering exponent of 3.0 for all shear rates. This indicates the compactness of the floc at short length scales since scattering exponent reflects a kind of “local” fractal dimension. It has been shown in a previous publication that in the images taken using a transmission electron microscope the aggregate consists of densely packed microflocs that are the basic unit to the structure at larger scale.2 These microflocs are very stable as increasing the shear rate does not change the size and structure significantly. The restructuring of these flocs at short length scales seems to be inconsistent with what has been observed in other shear-aggregation processes, where the restructuring normally only influences the large length scale.21,43 It is believed then that for silica aggregated using 1.5 M KCl under shear, break up occurs at large length scales as the hydrodynamic stress is increased but floc restructuring is happening even at length scales much shorter than a micrometer. From Figure 4, it also can be seen that as the applied shear is increased there is a decrease in the scattered intensity at low Q, implying that floc size decreases. Figure 5 summarizes the effect of shear rate on size and structure of aggregates of silica destabilized with 1.5 M KCl. It can be seen that only at G ∼ 30 s-1 do the aggregates have different short- and long-range structures. Very compact flocs with scattering exponents as high as 3 are obtained at all higher shear rates: only the shortrange structure of the floc survives. Figure 5 also shows that the radius of gyration of the aggregates reduces only slowly for shear rates of 257 s-1 and beyond indicating that approximately 1 µm is the size of the microfloc and the equilibrium size of the aggregate which is able to withstand shear without further deformation in structure (43) Lin, M. Y.; Lindsay, H. M.; Weitz, D. A.; Ball, R. C.; Klein, R.; Meakin, P. Phys. Rev., A 1990, 41 (4), 2005-2020.
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Figure 5. Equilibrium radius of gyration (Rg) and the longrange/short-range scattering exponent of silica aggregates in 1.5 M KCl versus shear rate. The notation inside indicates the long-range (if any) and short-range scattering exponents of the aggregate at particular shear rate. For example, notation -/3.0 indicates that the aggregates do not have long-range structure, but the short-range structure has scattering exponent of 3.0.
or reduction in size. At this point, it can be said that two different structures at different length scales are observed. At lower shear rates macroflocs are observed (at low Q), which consist of many dense microflocs (at high Q). A series of aggregation experiments were carried out at various average shear rates from as low as 100 s-1 up to as high as 800 s-1, and the scattering pattern of the resulting aggregates at equilibrium can be seen in Figure 6. The scattering intensity for experiments at shear rates of 180, 270, 380, and 650 s-1 in Figure 6a and the scattering intensity for experiments at shear rates of 32, 110, 200, 430, and 568 s-1 in Figure 6b have been shifted vertically for clarity. From Figure 6, it can be seen that as the shear rate increases the size of the aggregates decreases and the scattering exponent increases. For the pH 8 system, shown in Figure 6a, at shear rates lower than 380 s-1 two different slopes reveal the presence of two different structures at different length scales. At lower Q (large length scale), the aggregate structure was sensitive to the applied shear as can be seen from the increase in scattering exponent from 2.5 to 2.7 as the average shear rate increased from 180 to 380 s-1 (note that at very high Q the primary particles themselves influence the data). The same experimental observation can also be seen for silica aggregation at pH 5, in Figure 6b, where the scattering exponent of the long-range structure increased as the shear rate increased. Figure 7 summarizes the evolution of size and structure of the aggregates formed with addition of KCl or AlkylTAB under different shear rates. One conclusion that can be drawn by comparing the silica aggregation result for silica aggregated with either salt or AlkylTAB is that the aggregates formed with addition of AlkylTAB were much stronger compared to those formed with addition of KCl as can be seen from the size of the surviving aggregate under a particular shear regime. This is possible since in the presence of cationic surfactant, the particles are aggregated through hydrophobic interactions whereas for aggregation with KCl, the particle adhesion is much weaker. The question that now remains is how much stronger is the bond between silica particles when they are aggregated with cationic surfactants compared to when they are aggregated with KCl? With this respect, the dimensionless parameters developed by Selomulya et al.44 which relate the aggregate
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shear range, so the model can be presented as
log
() () d d0
1/SE
d d0
) k log
1/SE
∝
(
)
µ 1 1 ‚ n0 F d0 F2
(
µ 1 1 ‚ n F 2 0 d F 0
)
(6) (7)
where k is a constant which is the slope of the log-log plot of floc factor versus aggregation factor. This constant is dependent on the geometry of the system. To simplify the analysis, the same number of silica particles was used in both the electrolyte and surfactant systems. This makes the term n0 and d0 on the right-hand side of eq 7 cancel out when the ratio of the floc factors is taken for the two systems as the primary particle size is the same in both cases. The density and the viscosity in both systems are also assumed to be the same. Since the systems are kinematically and dynamically similar, the constant for both sides is the same, therefore it is valid to write
(d/d0)11/SE1 1/SE2
(d/d0)2
Figure 6. Scattering pattern of silica aggregated with (a) 0.06 mM AlkylTAB (pH ) 8, 5 mM Borax) and (b) 0.2 mM AlkylTAB (pH ) 5, 5 mM Borax) under various shear rates.
≈
properties and aggregation conditions will be used. As pointed out earlier, a linear relationship between the floc factor and aggregation factor was obtained for a certain (44) Selomulya, C. The Effect of Shear on Flocculation and Floc Size/ Structure. In School of Chemical Engineering and Industrial Chemistry; University of New South Wales: Sydney, Australia, 2001.
(8)
where 1 and 2 are the electrolyte and surfactant system, respectively. Since the interparticle bonding force is the force of interest, the scattering exponent of the short length scale will be the appropriate reference to be used because it is the closest enclosed structure at which particles are interacting. Also, this can be explained further by considering that if it was the scattering exponent of the long length scale structure used as the reference, the force of interest will not be limited to that between adjacent particles but also include the force between particles in adjacent microflocs. For the two systems having the same floc factor, eq 8 can be further simplified to
F 1 1 ≈ F2 2
Figure 7. Evolution of floc size and structure of silica aggregated with 1.5 M KCl (diamonds) or AlkylTAB at pH 8 (squares) and pH 5 (triangles) as a function of shear rate. The notation inside indicates the long-range (if any) and shortrange scattering exponents of the aggregate at particular shear rate. For example, notation -/3.0 indicates that the aggregates do not have long-range structure, but the short-range structure has scattering exponent of 3.0.
F1 2 F 2 1
(9)
Equation 9 implies that the ratio of energy dissipation rate (or the applied shear rates) in two systems that gives rise to the same floc factor yields the relative interparticle bonding force for the systems. By use of the floc properties (Rg and scattering exponent) and the applied shear rate, the plot of floc factor versus inverse energy dissipation rate is shown in Figure 8. For a given energy dissipation rate, a greater floc factor indicates a more open structure and therefore a greater bond strength. It can be seen from Figure 8 that there is no significant difference in the floc factor for the two different surfactant concentrations trialed at pH 8. A small reduction in bond strength is observed for the pH 5 system and a much greater reduction for the KCl system. The relative bond strength can be determined by comparing the energy dissipation rates that give rise to the same floc factor in different systems, according to eq 9. It is necessary to point out at this stage that the dimensional analysis will only give the relative bond strength between two systems, not the actual bond strength. If we assign the bond strength in the electrolyte system the value F, then the bond strength in the pH 5 system is found to be 145F and in the pH 8 systems to be 800F. With these determined relative bond strengths, the floc factor versus aggregation factor is plotted in Figure 9. There is a definite linear relationship between the dimensionless aggregate properties and the particle/fluid
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Langmuir, Vol. 20, No. 15, 2004
Figure 8. Floc factor versus the inverse energy dissipation rate for silica aggregated in a baffled mixing tank, using 1.5 mM KCl (diamond), 0.015 mM (gray squares) and 0.06 mM (closed squares) are AlkylTAB at pH 8 with 5 mM Borax and 0.2 mM AlkylTAB at a pH of 5 (triangles).
Figure 9. Plot of floc factor versus aggregation factor on a log-log scale for both electrolyte and surfactant system. The bonding force of the silica particles in 1.5 M KCl is assigned to be “F”. The interparticle bonding force for the pH 8 surfactant systems was then determined to be 800F. The bonding force for the pH 5 surfactant system was determined to be 145F. That is, the bonding force in the surfactant systems at pH 8 and pH 5 were found to be 800 and 145 times stronger than the binding force in the 1.5 M KCl system, respectively.
properties for silica particles aggregated with KCl and AlkylTAB for the conditions studied and using the above relative bond strengths all the data falls on the same line. As seen above, there is a trend of increasing overall floc factor as the aggregation factor increases. This means that for the same floc structure, primary particle size, and hydrodynamic conditions, the floc size will increase as the interparticle force (F) and/or the number of primary particles (n0) increases. In contrast, if the size of the primary particle (d0) and/or energy dissipation rate () increases, the equilibrium aggregate size will decrease. The increase in primary particle size (lower d/d0) decreases the resistance of the floc toward breakage as it is more susceptible to hydrodynamic drag and gravity forces as has been reported in the literature.11,44,45 The relationship shown in Figure 9 also indicates that a larger floc size can result from more compact aggregates. This can be (45) Selomulya, C.; Amal, R.; Bushell, G.; Waite, T. D. J. Colloid Interface Sci. 2001, 236, 67-77.
Hermawan et al.
Figure 10. Interaction between silica surfaces in 1.5 M NaCl and 5 mM sodium borate buffer solution. The square (0×ff) represents the interaction on approach and the triangle (4) represents interaction on retraction. The retraction shows a primary adhesion “pull off” (arrow) and then evidence of polymer contamination. This appears to be ubiquitous in the presence of the Borax buffer.
explained by the higher number of interparticle contacts for compact aggregates, which makes them stronger compared to loose aggregates. Moreover, increasing the interparticle bonding force will be manifest by an increase in floc size (d/d0) for the same floc structure (same SE) at a given shear condition. This means that flocs could have the same size but a less compact structure when they have a stronger bonding force. Meanwhile, if the shear rate is increased, which translates to an increase in energy dissipation rate, more compact aggregates and/or smaller aggregates will result. Extensive study has been done in the past to measure the forces between silica particles and surfaces in various different solutions of electrolyte and surfactant on approach29,39,46-48 using AFM. It is believed that the force that is most suitable to be compared to the interparticle forces estimated using the dimensionless model is the adhesive force, Fad, as this is the force that must be overcome in order to separate primary particles and reduce the floc size, as occurs when the flocs are sheared. The adhesive force is defined as the force required to separate surfaces from contact. It is essential to note that adhesive forces can be influenced significantly by the local properties of surfaces at contact, such as surface roughness and contamination. Figure 10 shows the interaction force curves between silica surfaces (silica spherical borosilicate particle and an oxidized silicon wafer) in 1.5 M aqueous electrolyte solution in the presence of 5 mM sodium borate buffer. The data for forces measured in highly purified NaCl is presented as all force curves obtained using KCl exhibited higher levels of contamination. As the silica surfaces approach, no interaction is observed until a surface separation of ∼10 nm. An attraction is evident from here to contact. The Debye length at this concentration is ∼0.25 nm; therefore any electrostatic repulsive force is highly shielded. It has been shown in many studies that the behavior between silica surfaces at very small separations does not follow the DLVO theory because the predicted van der Waals attraction is overcome by an additional repulsion force.40,49 Some researchers42,49-52 have identified (46) Rutland, M. W.; Parker, J. L. Langmuir 1994, 10, 1110-1121. (47) Toikka, G.; Hayes, R. A. J. Colloid Interface Sci. 1997, 191, 102109. (48) Yaminsky, V.; Yaminsky, F. J.; Ninham, B. W. Langmuir 1996, 12, 3531-3535.
Floc Strength Characterization Technique
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Table 1. Adhesion Force Measured in AlkylTAB Solutions of 5 mM Sodium Borate Buffer solution pH
AlkylTAB concn (mM)
adhesive force (nN)
8 8 8 5 5 5 5
0.02 0.06 0.4 0.2 0.4 0.5 3.0
146 86 30 58 20 0 0
and described this force as the repulsive hydration force. However at these high concentrations of electrolyte it has been proposed that the hydration force is removed allowing the van der Waals attraction to dominate. On this scale the surface roughness will play a role and this precludes any serious comparison with the theoretically calculated van der Waals force. Suffice to say the theoretical van der Waals attraction between silica surfaces in water is several times larger than the observed attraction. The force on separation shows a primary adhesion. When this is exceeded, the surfaces jump to a separation of 8 nm before multiple other smaller attractive features are seen. This is attributed to a low level of polymer contamination. Contaminants present in the system are likely to contribute to the adhesive force and may be significant when the adhesive force is small. Several repeat measurements all showed evidence of contamination. Low levels of contaminant are difficult to preclude when high concentrations of salt are used, in particular the sodium borate buffer poses a high contamination risk. An alternate explanation for the observed adhesion is due to the adsorption of cations onto the silica surface. These cations can act as a bridge between the two silica surfaces. However the range and nature of the adhesive force are more suggestive of contamination as the cationic bridge should act at only very short ranges. It cannot be ruled out that the multiple attractive features are due to silica hairs; however, this seems unlikely given the range of the interaction. The measured force on retraction therefore overestimates the adhesive force between silica surfaces that have been destabilized by 1.5 M electrolyte. It is likely that contamination would also be present in the colloidal silica dispersions that have been used in the flocculation study, but the much greater surface area of silica available in flocculation studies minimizes the influence of small amounts of contaminant. In the flocculation studies, the large surface area of the colloidal material will deplete the bulk concentration of the surfactant. Additionally, the electrolyte concentrations and pH adjustment in the AFM studies will not exactly match that of the flocculation studies, and finally the nature of the silica surfaces used may differ. These will all influence the surface excess of surfactant and thereby the adhesive force. Therefore a number of concentrations of AlkylTAB in the region of the concentration used in the flocculation studies were investigated by AFM so as to ascertain the maximal adhesion force comparable to the flocculation studies. This should correspond to the most stable floc structures. The adhesive interactions are summarized in Table 1. We can now compare the ratio of the adhesive forces that we have measured using the AFM with those (49) Chapel, J. P. Langmuir 1994, 10, 4237-4243. (50) Depasse, J. J. Colloid Interface Sci. 1997, 194, 260-262. (51) Franks, G. V.; Marechal, P. S.; Jameson, G. J. Effect of Monovalent Salt type on the Zeta Potensials of Alkane Particles. In Chemeca; Perth, Australia, 2000. (52) Franks, G. V.; Johnson, S. P.; Scales, P. J.; Healy, T. W.; Boger, D. V. Langmuir 1999, 15, 4411-4420.
Table 2. Comparison of the Ratio of Adhesive Forces Predicted by Dimensional Analysis Compared to Those Measured by AFM system
adhesive force, nN
AFM ratioa
dimensional analysis ratioa
salt AlkylTAB pH 8 AlkylTAB pH 5
0.81 146 58
1 180 71
1 800 145
a Adhesion in AlkylTAB system/adhesion in high electrolyte system.
calculated using the dimensionless model and the flocculation data for AlkylTAB at pH 5 and pH 8 versus the 1.5 M electrolyte system. As the attraction upon separation in the high salt system is most likely due to a low level of polymer contamination, it is more accurate to use the approach data for the electrolyte system. The ratios determined in this manner are presented in Table 2. The ratio determined using the dimensionless model from flocculation data for AlkylTAB at pH 5 is 145, whereas that determined by the force measurements is ∼2 times less at 71. The agreement between the dimensionless model is slightly less good for the flocculation data for AlkylTAB at pH 8 is 800, whereas that determined by the force measurements is a factor of ∼4.4 less at 180. The agreement between the methods is reasonable when the difference between the silica surfaces, particle sizes, and techniques are considered. Additionally, when the adhesion force is large, the surfaces in the AFM are separated with an increased peeling action due to the bending of the cantilever spring. This is likely to underestimate the larger adhesive forces in comparison to the smaller adhesive forces. Direct force measurements in these systems that contain mixed surfactants, high levels of salt, and buffer are challenging. Usually highly contrived, idealized systems are adopted for such studies, enabling the level of contaminants to be held at very low levels. For more industrially relevant flocculating systems it is likely that the dimensionless model develop by Selomulya et al.45 is a preferable measure of relative bond strength or interparticle force. Conclusion A new approach based on dimensional analysis developed by Selomulya et al.22 which involves fluid, particle, and floc properties, and the flocculation conditions have been used to estimate the floc strength between systems. Silica particles were aggregated using both 1.5 M electrolyte and AlkylTAB of different concentrations and pH, and the bonding forces were estimated and compared. It was found from the analysis that the bonding force of silica particles in 0.06 mM AlkylTAB buffered at pH 8 using 5 mM sodium borate buffer, is approximately 800 times stronger than that formed in the salt system, while a ratio of 145 times stronger was found for the bonding forces in 0.2 mM AlkylTAB (pH 5). Attempts were made to verify this bonding force by measuring the adhesive force using an atomic force microscope and similar results were obtained. The presence of even very low levels of contaminant in the system greatly hamper the AFM measurements, highlighting the utility of the dimensionless model for determining relative bond strengths of flocs in complex systems. Acknowledgment. The author would like to acknowledge Mr. Gan Wee Yong for helping with some of the aggregation and characterization experiments, and Dr. Cordelia Selomulya for her valuable input. LA049651P
