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De-agglomeration of hydrophobic and hydrophilic silica nano-powders in a high shear mixer
Powder Technology 195 (2009) 221–226
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De-agglomeration of hydrophobic and hydrophilic silica nano-powders in a high shear mixer P. Ding, M.G. Orwa, A.W. Pacek ⁎ School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom
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Article history: Received 12 November 2008 Received in revised form 8 June 2009 Accepted 12 June 2009 Available online 21 June 2009 Keywords: Hydrophobic and hydrophilic silica nano-powders De-agglomeration Energy density pH Rheology
a b s t r a c t The effect of energy density, pH and solid concentration on kinetics of de-agglomeration of hydrophobic silica nano-powder in a high shear mixer and on the rheology of resulting suspensions was investigated and compared with de-agglomeration kinetics and rheology of the suspension of hydrophilic silica nano-powder. In both types of nano-powders large aggregates were broken by fracture and erosion. In hydrophobic nanopowder erosion was more pronounced whilst in hydrophilic nano-powder erosion followed initial fracture of large aggregates. At sufficiently high energy input both hydrophobic and hydrophilic aggregates were broken into nano-aggregates but, even at the highest energy input, those nano-aggregates could not have been broken into single nano-particles. Rheology of the suspensions of hydrophobic nano-aggregates strongly depends on pH and on solid concentration whilst rheology of suspensions of hydrophilic nano-powder is rather weakly dependent on those parameters. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Silica nano-particles manufactured by flame hydrolysis of chlorosilanes are hydrophilic with the size between 7 and 40 nm [1,2] but their surface properties are frequently modified by reacting hydroxyl groups with silane coupling agents which leads to hydrophobic surfaces [2,3]. Both types of nano-particles are commonly used in coating industry to improve physical properties of coatings and to facilitate production, storage and application of coats [1]. They have strong effect on rheology of suspensions [4] and are frequently used to enhance dispersion of pigments and to prevent separation in pigments and fillers suspensions [1]. In all those applications silica nano-particles are either added to the pigment suspensions as a dry powder or as a suspension. The quality of paints/fillers depends on the particle size, size distribution, shape and morphology [5]. Therefore, dry silica nano-powders have to be dispersed in aqueous solutions to give homogenous suspension and it is essential that large aggregates inherently present in dry nano-powders are broken into primary silica nano-particles or into nano-aggregates. The mechanism of dispersion of dry, hydrophilic silica nanopowder (AEROSIL®200 V, further refer to as 200 V) in water has been recently investigated by Pacek et al. [4]. They observed both rupture and erosion during de-aggregation and found that sub-micron aggregates (often called primary or hard aggregates) cannot be broken into single nano-particles even at the highest energy input and at the highest repulsive interparticle forces.
⁎ Corresponding author. E-mail address: [email protected] (A.W. Pacek). 0032-5910/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2009.06.003
In this work the kinetics of dispersion of dry hydrophobic silica nano-powder (AEROSIL® R816 further refer to as R816) in water and rheology of resulting suspensions have been investigated and compared with the kinetics and rheology of the suspensions of hydrophilic silica 200 V. According to the manufacturer of silica nanopowders (Degussa) the only difference between R816 and 200 V is the character of the surface of nano-particles whilst the nano-particles size, shape and density are identical. De-agglomeration of both powders has been investigated in the same high shear mixer, at the same energy dissipation rates and energy density, therefore the effect of surface properties on kinetics of de-agglomeration and rheology of resulting suspensions was identified and it is discussed below. 2. Experimental Experimental rig, methodology and procedure are discussed in details elsewhere [4] and they are only briefly summarised below. 2.1. Materials and methods Hydrophobic silica nano-powder (R816) was produced by treating hydrophilic silica nano-powder (200 V) with hexadecysilane and it was supplied as a dry powder. According to manufacturer it had following properties [1]: particles density of 2200 kg/m3, SiO2 content N99.8%, specific surface area of 190 m2/g, average size of 12 nm and the “nature” pH of 4.0–5.5. Zeta potential of nano-particles and nano-aggregates was measured by Zetamaster and size distributions were measured using particle size analyzer Mastersizer 2000 (Malvern Instruments). The
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morphology of aggregates was analysed with an Environmental Scanning Electron Microscope (ESEM, Philips XL30) and rheology of suspensions was measured using a controlled stress rheometer (TA 1000, TA Instruments, UK). Dry nano-powders were dispersed in a batch rotor/stator mixer (L4R from Silverson) with rotor diameter of 0.028 m, rotor height of 0.015 m and a gap between the rotor and the standard disintegrating stator of 0.0005 m. 2.2. Procedure Silica nano-powder was pre-dispersed in water in a glass stirred vessel and pH was adjusted to the required value. Dispersion was transferred to the high shear mixer from which the air was completely excluded. The rotor speed was set to the required value (3000, 5000 or 8000 rpm) and at each speed the dispersion was sheared for four hours. Small samples of dispersion were taken at certain times and aggregates size distributions were measured. 3. Results and Discussion 3.1. Zeta potential Zeta potential of hydrophobic R816 nanoparticles suspended in water was measured over wide range of pH and it is compared with the zeta potential of hydrophilic 200 V nano-particles in Fig. 1. The results show that both R816 and 200 V nano-particles have negatively charged surfaces and the same iso-electric point at pH between 2 and 3. At pH between 3 and 9, the absolute value of zeta potential of R816 was on average 10 mV lower than that of 200 V which is consistent with the data reported by the manufacturer (Degussa). The reduction of zeta potential can be explained by lower concentration of SiOH groups at the surface of R816 nano-particles.
Fig. 2. Transient size distributions in 5 wt.% suspension of hydrophobic nano-powder in water (pH = 4) at energy dissipation rate of 89.3 kW/m3: (●) 0 min, (▼) 10 min, (■) 60 min, (♦) 240 min.
aggregates were already present in the suspension as clearly indicated by the curve at 10 min (Fig. 2), whereas in the case of hydrophilic silica nano-aggregates appeared after more than 30 min of processing (at energy density of 161 MJ/m3). Comparison of transient size distributions of R816 (Fig. 2) with transient size distribution of 200 V (Fig. 1 in
3.2. Effect of energy input on kinetics of de-agglomeration Transient particle size distributions during de-agglomeration of hydrophobic silica nano-powder (R816) at an average energy dissipation rate of 89.3 kWm− 3 are shown in Fig. 2 and the images of dry powder (before de-agglomeration) and nano-aggregates after 4 hours of processing are shown in Fig. 3. The character of the transient size distributions, morphology of dry, hydrophobic nano-powder and structure of nano-aggregates are very similar to size distributions, morphology and structure of hydrophilic nano-powder [4]. However, close comparison of transient distributions of both nano-powders reveals certain differences between kinetics of de-agglomeration of both nano-powders. In the case of hydrophobic silica after 10 min (at energy density of 54 MJ/m3) of processing nano-
Fig. 1. Zeta potential of hydrophobic (R816, solid symbols) and hydrophilic (200 V, empty symbols) silica nano-particles as a function of pH.
Fig. 3. Morphology of hydrophobic silica nano-powder: (a) dry nano-powder before de-agglomeration and (b) nano-aggregates after 4 hours of shearing at 89.3 kW/m3.
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[4]) indicates that during first 20 min large agglomerates of hydrophilic silica were broken mainly by fracture whilst in the case of hydrophobic silica fracture and erosion occurred simultaneously. In both powders the median diameters and the shape of size distributions of nano-aggregates (first mode in Fig. 2) practically did not change with the processing time only the volume of nanoaggregates increased and volume of large aggregates decreased. Both Mastersizer (Fig. 2) and ESEM (Fig. 3b) detected nano-aggregates between 50 nm and 1000 nm but single nano-particles (of the order of 12 nm according to manufacturer) were not detected in the suspension. This clearly indicates that sub-micron aggregates (Fig. 3b) could not be broken into single nano-particles even at the highest energy density (1500 MJ/m3) used here. It has been reported that hard (primary) nano-aggregates are frequently formed during manufacturing of fumed silica and that strong chemical/sintering bonds are dominant inter-particle forces [5]. As the transient size distributions are bi-modal, the kinetics of deagglomeration cannot be analysed in terms of an average aggregate size and it requires a full solution of population balance model recently published for a similar problem [6]. However, as the aim of this work is to quantify differences between two powders simplified description has been employed here. The kinetic of de-agglomeration was analyzed in terms of cumulative volume fraction of nano-aggregates (Eq. (1)) and transient median diameter of aggregates larger than 1 µm (Eq. (2)) [7,8]. yðEtÞ = 1− exp½−A1 ðE−Ed Þ
ð1Þ
−β
ð2Þ
d50 ðEÞ = α⋅E
In Fig. 4 transient cumulative volume fractions of nano-aggregates in the 5 wt.% suspensions of hydrophobic (R816, solid symbols) and hydrophilic (200 V, empty symbols) silica nano powders as well as transient median diameters of large agglomerates are compared. Fig. 4a indicates that de-agglomeration depends on both the energy density and on the average energy dissipation rate. There is a certain critical energy dissipation rate below which large agglomerates cannot be broken into nano-aggregates even if the energy density is relatively high. Whilst the hydrophobic (R816) nano-powder was fractured at energy dissipation rate of 4.7 kW/m3 (Fig. 4b) the further erosion/fracture into nano-aggregates was not observed at this energy dissipation rate even at the energy density of the order of 100 MJ/m3 (Fig. 4a). At the same energy density but at energy dissipation rate of 21.7 kW/m3 nearly 50 V% of R816 powder was dispersed into nano-aggregates. However, further increase of energy dissipation rate to 89.3 kW/m3 has negligible effect on the rate of erosion and coefficients in Eq. (1) calculated from experimental data: Ed = 0 and A1 = 0.0055 m3/MJ were the same at both levels of energy dissipation rate. De-agglomeration of hydrophilic (200 V) nanopowder into nano-aggregate required much higher energy dissipation rate. Nano-aggregates were not observed at 4.7 and 21.7 kW/m3 even at energy density as high as 500 MJ/m3. At the same energy density but at energy dissipation rate of 89.3 kW/m3 more than 80 V% of hydrophilic nano-powder was dispersed in nano-aggregates. Deagglomeration of hydrophilic (200 V) nano-powder also required more energy density as indicated by the values of constants in Eq. (1) Ed = 158 MJ/m3 and A1 = 0.0051 m3/MJ. 90 V% hydrophobic (R816) powder was broken into nano-aggregates at energy density of 450 MJ/m3 whereas de-agglomeration of the same mass of hydrophilic powder required 700 MJ/m3. It is worth to notice that whilst in case of hydrophobic nano-powder erosion was observed from very beginning of the process (delay time equal zero), in hydrophilic nanopowder erosion started after nearly 30 min of processing at the highest energy dissipation rate. The transient median diameters of large hydrophobic and hydrophilic aggregates are compared in Fig. 4b. In both cases the size-
Fig. 4. Effect of energy density on: (a) cumulative volume fraction of nano-aggregates and (b) breakage of large aggregates in 5 wt.% suspension ( pH = 4) at different energy dissipation rates; circles — 4.7 MJ/m3; triangles — 21.7 MJ/m3; squares — 89.3 MJ/m3; solid symbols — R816, empty symbols — 200 V; lines — best fit to Eq. (1) and Eq. (2) respectively.
energy model (Eq. (2)) [7,8] seems to correlate the median diameter with energy density rather well. The reduction rate of median diameter of hydrophobic aggregates is much lower than the reduction rate of median diameter of hydrophilic aggregates and constants β in Eq. (2) are equal to 0.18 and 0.27 respectively. All the above results indicate that de-agglomeration of hydrophobic aggregates is controlled by erosion and fracture whereas the initial stage of deagglomeration of hydrophilic aggregates is controlled by fracture. 3.3. Effect of solid concentration on de-agglomeration kinetics and rheology of suspensions Transient cumulative volume fractions of hydrophobic and hydrophilic nano-aggregates at different solid loads are compared in Fig. 5. Cumulative volume fraction of hydrophobic (R816, Fig. 5a) nanoaggregates at 1 wt.% and 5 wt.% practically overlap and at 500 MJm−3 approximately 90 V% of nano-powders was dispersed into nanoaggregates. At these solid concentrations the critical energy density Ed =0 and the erosion constant A1 = 0.0051[m3/MJ]. However, at 10 wt.% of solid, hydrophobic nano-powder could not be dispersed into nanoaggregates even at the highest energy density (1300 MJ/m3). In the case of hydrophilic nano-powder (200 V, Fig. 5b) the cumulative volume fractions of nano-aggregates at 1 and 5 wt.% practically overlap (critical energy density Ed = 138 MJ/m3 and the erosion constant A1 =0.0045 m3/MJ) and 90% of powder was dispersed into nanoaggregates at energy density of approximately 700 MJm−3. The increase of solid load to 10 wt.% led to the increase of critical energy density Ed =170 MJ/m3 and to reduction of erosion constant (A1 = 0.0024 m3/ MJ). Overall efficiency of de-agglomeration was also reduced and 70 V% of
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the increase of solid load to 5 wt.% does not affect erosion rate (Fig. 5b). At 10 wt.% the suspension is slightly shear thinning with viscosity around eight times higher than 1 wt.%. This increase should not affect the de-agglomeration rate (see discussion above and Figs. 5a and 6a). As the solid concentration increases to 10 wt.% more energy is needed to break all the aggregates therefore at constant energy density de-agglomeration rate is lower (Fig. 5b).
3.4. Effect of pH on kinetics of de-agglomeration and rheology of suspensions It has been reported [9] that pH strongly affects both kinetics of wet de-agglomeration of nano-powders and rheology of resulting suspensions by affecting electrostatic charges on the agglomerate surfaces what in turns affects the balance between attractive (van der Waals) and repulsive (electrostatic) interparticle forces. The van der Waals forces are practically independent on pH whilst the electrostatic repulsive forces strongly depend on zeta potential, e.g. on pH. Transient cumulative volume fractions of hydrophobic and hydrophilic nano-aggregates in 5 wt.% suspensions at different pH compared in Fig. 7 indicate that the effect of pH also depends on the character of the particle surface and that it is different for hydrophobic and for hydrophilic nano-particles. Fig. 7a shows that pH practically has no affect on the kinetics of deagglomeration of hydrophobic nano-powder (R816). Transient cumulative size distributions of hydrophobic nano-aggregates at different pH (zeta potential between 0 and −40 mV) practically overlap. The fines generation constant A1 slightly increased from 0.0068 at pH = 4 to 0.0077 at pH = 9 and at all values of pH critical energy density Fig. 5. Transient cumulative volume fraction at different concentration of (a) hydrophobic and (b) hydrophilic nano-aggregates suspended in water; circles — 1 wt.%; triangles — 5 wt.%; squares — 10 wt.%; pH= 4, lines: best fit to Eq. (1).
nano-powder was dispersed into nano-aggregates at 700 MJ/m3 whereas at the same energy density 90 V% of nano-powder was dispersed at two lower solid concentrations. There are two factors contributing to the reduction of deagglomeration rate with the increase of solid concentration. Firstly, as the mass of aggregates increases more energy is needed to break them into nano-aggregates and secondly the drastic change of rheology of suspensions induced by the increase of solid concentration reduce the efficiency of de-agglomeration as illustrated in Fig. 6. 1 wt.% suspension of hydrophobic nano-powder (R816, Fig. 6a) was Newtonian with viscosity slightly higher than viscosity of water (1.26 mPas). As the solid concentration was increased to 5 wt.% the suspension became shear thinning with the viscosity at the highest shear rate one order of magnitude higher than viscosity of 1 wt.% suspension. On the one hand, the higher the viscosity the higher the laminar shear stress on the surface of agglomerates the higher the erosion rate. On the other hand, the increase of viscosity leads to the reduction of the flow intensity through the mixing head therefore to the reduction of number of aggregates exposed to high shear rate and to the reduction of erosion rate. It appears that at 5 wt.% solid both effects cancels out and there is practically no change in erosion rate as shown in Fig. 5a. Further increase of solid content to 10 wt.% leads to extremely shear thinning suspension with viscosity at the highest shear rate three orders of magnitude higher than viscosity of 1 wt.% suspension at the same shear rate. Laminar shear rate also increased but, as the pumping capacity of the high shear mixers is very low it is possible that at such high viscosity the suspension was not pumped through mixing head. The effect of concentration of hydrophilic nano-powder (R200, Fig. 6b) on the rheology of suspension is much weaker. The difference between viscosity at 1 wt.% and 5 wt.% is marginal what explains why
Fig. 6. Effect of solid concentration on rheology of suspensions of (a) hydrophobic and (b) hydrophilic nano-powders after complete de-agglomeration at nature pH; circles — 1 wt.%; triangles — 5 wt.%; squares — 10 wt.%.
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Fig. 8. Effect of pH on rheology of 5 wt.% suspensions of hydrophobic nano-aggregates after de-agglomeration, (●) pH = 3; (▼) pH = 4; (■) pH = 7; (♦) pH = 9.
Fig. 7. Effect of pH on transient cumulative volume fraction of nano-aggregates in 5 wt.% suspensions of (a) hydrophobic and (b) hydrophobic nano-powders; circles — pH = 4; triangles — pH = 7; squares — pH = 9.
Ed = 0 and all the experimental data in Fig. 7a can be approximated by Eq. (1) with A1 = 0.0074 and Ec = 0. The effect of pH on de-agglomeration rate of hydrophilic nano-powder (200 V) is much more pronounced as shown in Fig. 7b. In this case an increases of pH from 4 to 9 drastically speeds up de-agglomeration process by reducing the critical energy density (Ed) from 158 MJ/m3 at pH=4 to 55 MJ/m3 at pH =9 and increasing fines generation constant (A1) from 0.0053 m3/MJ at pH =4 to 0.012 m3/MJ at pH =9. The increase of de-agglomeration rate of hydrophilic nano-powder with the increase of pH can be explained by the increase of the number of silanol groups on the surface of silica nano-aggregates [10]. As pH was increased from 4 to 9, more silanol groups were formed and ionisation of these groups increased negative charge of the surface as indicated by zeta potential (Fig. 1) leading to the increase of electrostatic repulsive force and consequently to the increase of de-agglomeration rate. The lack of the effect of pH on de-agglomeration of hydrophobic nano-powder can be explained by the modification of surface properties during the reaction of hydroxyl groups with silane [2], [3] but, as the details of these reactions are commercially sensitive and were not disclosed by manufacturer detail analysis of the effect of pH on surface properties is not possible. It is also possible that long range attractive hydrophobic forces make kinetics of de-agglomeration less sensitive to the changes of electrostatic forces. Whilst pH drastically affected de-agglomeration rate of hydrophilic nano-powder (200 V) the viscosity of the suspensions were practically independent of pH and at pH between 4 and 9 the 5 wt.% suspensions were water-like (viscosity of 2.5 mPas). It is possible that hydration/structural inter-particle forces induced by the surface silanol groups [4,10] dominate over electrostatic, pH dependent repulsive forces when the concentration of these groups at the surface of hydrophilic silica nano-particles is high.
In case of R816 situation was opposite with pH having relatively small effect on de-agglomeration rate but very strong effect on the rheology of the suspension as shown in Fig. 8. It appears that the strong effect of pH can be explained within the framework DLVO theory developed for lyophobic systems [11,12]. At pH = 3 (absolute value of the zeta potential b10 mV), electrostatic repulsive forces are small and the rheology is controlled by attractive van der Waals forces leading to very viscous and shear thinning suspension. At pH = 7, absolute value of zeta potential increases to 30 mV therefore interparticles repulsive force also increase that leads to a drastic reduction of apparent viscosity of suspension. Further increase of pH to 9 leads to much smaller increase of absolute value of zeta potential to 37 mV therefore the reduction of viscosity is also rather small. The changes of rheology of the suspension of hydrophobic nanopowder were quantified using power law model: n−1
η = k⋅γ˙
ð3Þ
and the consistency constants (k) and power law index (n) calculated from flow curves shown Fig. 8 are summarised in Table 1. As pH increases from 3 to 9 the rheology of suspension gradually changes from strongly non-Newtonian and shear thinning with a very high viscosity at pH = 3 to Newtonian water like at pH = 9. The reduction of pH from 9 to 3 leads to an increase of viscosity and to reduction of power law index. Similar effect of pH on the rheology of other types of nano-particles has been recently reported by the authors [9]. 4. Conclusions De-agglomeration of both hydrophobic (R816) and hydrophilic (200 V) silica nano-powders follows similar general pattern. Fracture and erosion leading to bi-modal transient size distributions of aggregates was observed in both powders. The qualitative analysis and comparison of the kinetics of de-agglomeration carried out in terms of median diameters of large aggregates and cumulative volume fractions of nano-aggregates revealed that initially erosion dominated
Table 1 Parameters in the power-law model of 5wt% suspension of hydrophobic R816 nanopowder. pH k [Pasn] n r2
3 8.871 0.0946 0.9999
4 5.8790 0.1007 0.9996
7 0.0110 0.8316 0.9459
9 4.24×100.9168 1.000
3
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in hydrophobic nano-powder and fracture in hydrophilic nanopowder. In both nano-powders nano-aggregates could not be broken into single nano-particles even at the highest energy density and the highest repulsive interparticle forces. Experimental results clearly show that the erosion of hydrophobic nano-aggregates occurs at much lower energy dissipation rates and energy density than the erosion of hydrophilic aggregates. In other words, less energy is required to disperse a unit mass of hydrophobic nano-powder than to disperse a unit mass of hydrophilic nano-powder. Very similar results were reported by manufacturer [1]. Those results are rather unexpected because in hydrophilic nano-powder van der Walls force is the only one attractive inter-particle force whereas in hydrophobic nanopowder there is also strong hydrophobic long range attractive force [13–15] The presence of extra attractive force should made deagglomeration of hydrophobic silica nano-powder more energy demanding than de-aggregation of hydrophilic silica which clearly is not a case. It is very difficult to explain this disagreement between theory and experiments and one possible explanation is that the interaction between silica nano-particles cannot be described using standard DVLO related models as discussed by authors in previous work [4]. It has also been suggested in literature [16], [17] that coagulation of silica nano-particles is strongly dependent on alkaline cations and pH and simultaneous effect of those on aggregation/deaggregation kinetics is well outside standard DVLO model. The solid concentration has opposite effect on erosion in both powders. The increase of the concentration of hydrophilic nanopowder (200 V) in the suspension from 1 wt.% to 10 wt.% leads to relatively small reduction of the erosion efficiency and nano-powder can be dispersed into nano-aggregates. At 10w/w% hydrophobic nano-powder (R816) could not be dispersed into nano-aggregates as the erosion efficiency dropped to zero. There is very strong effect of pH on erosion efficiency in hydrophilic nano-powder (200 V) where the increase of pH from 4 to 9 caused a drastic increase of erosion efficiency, whilst similar increase of pH in the suspension of hydrophobic nano-powder has practically no effect on the erosion of nano-aggregates. The rheology of hydrophobic nano-aggregates strongly depends on pH and on solid concentration whilst rheology of suspension of hydrophilic nano-powder is rather weakly dependent on those parameters.
Department of Chemical Eng; Poznan University of Technology, Inst. of Chemical Technology and Eng; Rockfield Software Limited; Unilever UK Port Sunlight, Warsaw University of Technology, Department of Chemical and Process Eng.
Acknowledgement
Glossary
This work is a part of PROFORM (“Transforming Nano-particles into Sustainable Consumer Products Through Advanced Product and Process Formulation” EC Reference NMP4-CT-2004-505645) project which is partially funded by the 6th Framework Programme of EC. The contents of this paper reflect only the authors’ view. The authors gratefully acknowledge the useful discussions held with other partners of the Consortium: Bayer Technology Services GmbH; BHR Group Limited; Centre for Computational Continuum Mechanics (C3M); Karlsruhe University, Inst. of Food Process Eng; Loughborough University,
A1 d0.5 E Ed k n r2 y(t) α β
References [1] Degussa, 2006, Fine Particles, Technical Bulletin, No 18. [2] Tadao Sugimoto, 2000, Fine Particles, Synthesis, Characterization and Mechanism of Growth, Surfactant Science, Marcel Dekker Inc, New York, Series vol 92. [3] T.J. Graule, E. Barna, B. Bommer, J. Kursteiner, A. Vital, O. Trzebiatowski, B. Schmid, J. van Leeuwen, W. Koch, Synthesis of spherical, non-aggregated silica nanoparticles for nanocomposite coatings, Kgk-Kautschuk Gummi Kunststoffe 58 (2005) 252–255. [4] A.W. Pacek, P. Ding, A.T. Utomo, Effect of energy density, pH and temperature on de-aggregation in nano-particles/water suspensions in high shear mixer, Powder Technology 173 (2007) 203–210. [5] S. Tsantilis, S.E. Pratsinis, Soft- and hard-agglomerate aerosols made at high temperatures, Langmuir 20 (2004) 5933–5939. [6] J. Baldyga, W. Orciuch, L. Makowski, K. Malik, G. Ozcan-Taskin, W. Eagles, G. Padron, Dispersion of nanoparticle clusters in a rotor-stator mixer, Industrial & Engineering Chemistry Research 47 (2008) 3652–3663. [7] P. Ding, A.W. Pacek, De-agglomeration of goethite nano-particles using ultrasonic comminution device, Powder Technology 187 (2008) 1–10. [8] K.A. Kusters, S.E. Pratsinis, S.G. Thoma, D.M. Smith, Energy-Size Reduction Laws for Ultrasonic Fragmentation, Powder Technology 80 (1994) 253–263. [9] P. Ding, A.W. Pacek, Effect of pH on deagglomeration and rheology/morphology of aqueous suspensions of goethite nano-powder, Journal of Colloid and Interface Science (2008), doi:10.1016/j.jcis.2008.04.077. [10] R.K. Iler, The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface properties, and Biochemistry, Wiley, New York, 1979. [11] Derjaguin, B. V. and Landau L. D., 1941, Acta Phys.-Chim.URSS, 14, 633-662. [12] E.J.W. Verwey, J.Th.G. Overbeek, Theory of the Stability of Lyophobic Colloids, Theory of the Stability of Lyophobic Colloids, Elsevier, Amsterdam, 1948. [13] J. Israelachvili, Intermolecular & Surface Forces, Academic Press Limited, 1992. [14] H.K. Christenson, P.M. Claesson, Direct measurements of the force between hydrophobic surfaces in water, Advances in Colloid and Interface Science 91 (2001) 391–436. [15] J.C. Eriksson, U. Henriksson, A. Kumpulainen, Hydrophobic attraction forces in asymmetric aqueous films between hydrophobized mica/bare mica surfaces, Colloids and Surfaces A-Physicochemical and Engineering Aspects 282 (2006) 79–83. [16] J. Depasse, Coagulation of colloidal silica by alkaline cations: Surface dehydration or interparticle bridging, Journal of Colloid and Interface Science 194 (1997) 260–262. [17] J. Depasse, Simple experiments to emphasize the main characteristics of the coagulation of silica hydrosols by alkaline cations; Application to the analysis of the model of Colic et al, Journal of Colloid and Interface Science 220 (1999) 174–176.
γ̇
η
Constant defined in Eq. (1) [m3/MJ] Median diameter [µm] Energy density [MJ/m3] Critical energy density defined in Eq. (1) [MJ/m3] Consistency constant [Pa⋅sn] Power law index [–] Coefficient of determination [–] Cumulative volume fraction of fines [–] Constant defined in eq. (2) [(MJ)βm1−3β] Constant defined in Eq. (2) [−] Shear rate [s− 1] Viscosity [Pa⋅s]
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De-agglomeration of hydrophobic and hydrophilic silica nano-powders in a high shear mixer P. Ding, M.G. Orwa, A.W. Pacek ⁎ School of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom
a r t i c l e
i n f o
Article history: Received 12 November 2008 Received in revised form 8 June 2009 Accepted 12 June 2009 Available online 21 June 2009 Keywords: Hydrophobic and hydrophilic silica nano-powders De-agglomeration Energy density pH Rheology
a b s t r a c t The effect of energy density, pH and solid concentration on kinetics of de-agglomeration of hydrophobic silica nano-powder in a high shear mixer and on the rheology of resulting suspensions was investigated and compared with de-agglomeration kinetics and rheology of the suspension of hydrophilic silica nano-powder. In both types of nano-powders large aggregates were broken by fracture and erosion. In hydrophobic nanopowder erosion was more pronounced whilst in hydrophilic nano-powder erosion followed initial fracture of large aggregates. At sufficiently high energy input both hydrophobic and hydrophilic aggregates were broken into nano-aggregates but, even at the highest energy input, those nano-aggregates could not have been broken into single nano-particles. Rheology of the suspensions of hydrophobic nano-aggregates strongly depends on pH and on solid concentration whilst rheology of suspensions of hydrophilic nano-powder is rather weakly dependent on those parameters. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Silica nano-particles manufactured by flame hydrolysis of chlorosilanes are hydrophilic with the size between 7 and 40 nm [1,2] but their surface properties are frequently modified by reacting hydroxyl groups with silane coupling agents which leads to hydrophobic surfaces [2,3]. Both types of nano-particles are commonly used in coating industry to improve physical properties of coatings and to facilitate production, storage and application of coats [1]. They have strong effect on rheology of suspensions [4] and are frequently used to enhance dispersion of pigments and to prevent separation in pigments and fillers suspensions [1]. In all those applications silica nano-particles are either added to the pigment suspensions as a dry powder or as a suspension. The quality of paints/fillers depends on the particle size, size distribution, shape and morphology [5]. Therefore, dry silica nano-powders have to be dispersed in aqueous solutions to give homogenous suspension and it is essential that large aggregates inherently present in dry nano-powders are broken into primary silica nano-particles or into nano-aggregates. The mechanism of dispersion of dry, hydrophilic silica nanopowder (AEROSIL®200 V, further refer to as 200 V) in water has been recently investigated by Pacek et al. [4]. They observed both rupture and erosion during de-aggregation and found that sub-micron aggregates (often called primary or hard aggregates) cannot be broken into single nano-particles even at the highest energy input and at the highest repulsive interparticle forces.
⁎ Corresponding author. E-mail address: [email protected] (A.W. Pacek). 0032-5910/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2009.06.003
In this work the kinetics of dispersion of dry hydrophobic silica nano-powder (AEROSIL® R816 further refer to as R816) in water and rheology of resulting suspensions have been investigated and compared with the kinetics and rheology of the suspensions of hydrophilic silica 200 V. According to the manufacturer of silica nanopowders (Degussa) the only difference between R816 and 200 V is the character of the surface of nano-particles whilst the nano-particles size, shape and density are identical. De-agglomeration of both powders has been investigated in the same high shear mixer, at the same energy dissipation rates and energy density, therefore the effect of surface properties on kinetics of de-agglomeration and rheology of resulting suspensions was identified and it is discussed below. 2. Experimental Experimental rig, methodology and procedure are discussed in details elsewhere [4] and they are only briefly summarised below. 2.1. Materials and methods Hydrophobic silica nano-powder (R816) was produced by treating hydrophilic silica nano-powder (200 V) with hexadecysilane and it was supplied as a dry powder. According to manufacturer it had following properties [1]: particles density of 2200 kg/m3, SiO2 content N99.8%, specific surface area of 190 m2/g, average size of 12 nm and the “nature” pH of 4.0–5.5. Zeta potential of nano-particles and nano-aggregates was measured by Zetamaster and size distributions were measured using particle size analyzer Mastersizer 2000 (Malvern Instruments). The
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morphology of aggregates was analysed with an Environmental Scanning Electron Microscope (ESEM, Philips XL30) and rheology of suspensions was measured using a controlled stress rheometer (TA 1000, TA Instruments, UK). Dry nano-powders were dispersed in a batch rotor/stator mixer (L4R from Silverson) with rotor diameter of 0.028 m, rotor height of 0.015 m and a gap between the rotor and the standard disintegrating stator of 0.0005 m. 2.2. Procedure Silica nano-powder was pre-dispersed in water in a glass stirred vessel and pH was adjusted to the required value. Dispersion was transferred to the high shear mixer from which the air was completely excluded. The rotor speed was set to the required value (3000, 5000 or 8000 rpm) and at each speed the dispersion was sheared for four hours. Small samples of dispersion were taken at certain times and aggregates size distributions were measured. 3. Results and Discussion 3.1. Zeta potential Zeta potential of hydrophobic R816 nanoparticles suspended in water was measured over wide range of pH and it is compared with the zeta potential of hydrophilic 200 V nano-particles in Fig. 1. The results show that both R816 and 200 V nano-particles have negatively charged surfaces and the same iso-electric point at pH between 2 and 3. At pH between 3 and 9, the absolute value of zeta potential of R816 was on average 10 mV lower than that of 200 V which is consistent with the data reported by the manufacturer (Degussa). The reduction of zeta potential can be explained by lower concentration of SiOH groups at the surface of R816 nano-particles.
Fig. 2. Transient size distributions in 5 wt.% suspension of hydrophobic nano-powder in water (pH = 4) at energy dissipation rate of 89.3 kW/m3: (●) 0 min, (▼) 10 min, (■) 60 min, (♦) 240 min.
aggregates were already present in the suspension as clearly indicated by the curve at 10 min (Fig. 2), whereas in the case of hydrophilic silica nano-aggregates appeared after more than 30 min of processing (at energy density of 161 MJ/m3). Comparison of transient size distributions of R816 (Fig. 2) with transient size distribution of 200 V (Fig. 1 in
3.2. Effect of energy input on kinetics of de-agglomeration Transient particle size distributions during de-agglomeration of hydrophobic silica nano-powder (R816) at an average energy dissipation rate of 89.3 kWm− 3 are shown in Fig. 2 and the images of dry powder (before de-agglomeration) and nano-aggregates after 4 hours of processing are shown in Fig. 3. The character of the transient size distributions, morphology of dry, hydrophobic nano-powder and structure of nano-aggregates are very similar to size distributions, morphology and structure of hydrophilic nano-powder [4]. However, close comparison of transient distributions of both nano-powders reveals certain differences between kinetics of de-agglomeration of both nano-powders. In the case of hydrophobic silica after 10 min (at energy density of 54 MJ/m3) of processing nano-
Fig. 1. Zeta potential of hydrophobic (R816, solid symbols) and hydrophilic (200 V, empty symbols) silica nano-particles as a function of pH.
Fig. 3. Morphology of hydrophobic silica nano-powder: (a) dry nano-powder before de-agglomeration and (b) nano-aggregates after 4 hours of shearing at 89.3 kW/m3.
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[4]) indicates that during first 20 min large agglomerates of hydrophilic silica were broken mainly by fracture whilst in the case of hydrophobic silica fracture and erosion occurred simultaneously. In both powders the median diameters and the shape of size distributions of nano-aggregates (first mode in Fig. 2) practically did not change with the processing time only the volume of nanoaggregates increased and volume of large aggregates decreased. Both Mastersizer (Fig. 2) and ESEM (Fig. 3b) detected nano-aggregates between 50 nm and 1000 nm but single nano-particles (of the order of 12 nm according to manufacturer) were not detected in the suspension. This clearly indicates that sub-micron aggregates (Fig. 3b) could not be broken into single nano-particles even at the highest energy density (1500 MJ/m3) used here. It has been reported that hard (primary) nano-aggregates are frequently formed during manufacturing of fumed silica and that strong chemical/sintering bonds are dominant inter-particle forces [5]. As the transient size distributions are bi-modal, the kinetics of deagglomeration cannot be analysed in terms of an average aggregate size and it requires a full solution of population balance model recently published for a similar problem [6]. However, as the aim of this work is to quantify differences between two powders simplified description has been employed here. The kinetic of de-agglomeration was analyzed in terms of cumulative volume fraction of nano-aggregates (Eq. (1)) and transient median diameter of aggregates larger than 1 µm (Eq. (2)) [7,8]. yðEtÞ = 1− exp½−A1 ðE−Ed Þ
ð1Þ
−β
ð2Þ
d50 ðEÞ = α⋅E
In Fig. 4 transient cumulative volume fractions of nano-aggregates in the 5 wt.% suspensions of hydrophobic (R816, solid symbols) and hydrophilic (200 V, empty symbols) silica nano powders as well as transient median diameters of large agglomerates are compared. Fig. 4a indicates that de-agglomeration depends on both the energy density and on the average energy dissipation rate. There is a certain critical energy dissipation rate below which large agglomerates cannot be broken into nano-aggregates even if the energy density is relatively high. Whilst the hydrophobic (R816) nano-powder was fractured at energy dissipation rate of 4.7 kW/m3 (Fig. 4b) the further erosion/fracture into nano-aggregates was not observed at this energy dissipation rate even at the energy density of the order of 100 MJ/m3 (Fig. 4a). At the same energy density but at energy dissipation rate of 21.7 kW/m3 nearly 50 V% of R816 powder was dispersed into nano-aggregates. However, further increase of energy dissipation rate to 89.3 kW/m3 has negligible effect on the rate of erosion and coefficients in Eq. (1) calculated from experimental data: Ed = 0 and A1 = 0.0055 m3/MJ were the same at both levels of energy dissipation rate. De-agglomeration of hydrophilic (200 V) nanopowder into nano-aggregate required much higher energy dissipation rate. Nano-aggregates were not observed at 4.7 and 21.7 kW/m3 even at energy density as high as 500 MJ/m3. At the same energy density but at energy dissipation rate of 89.3 kW/m3 more than 80 V% of hydrophilic nano-powder was dispersed in nano-aggregates. Deagglomeration of hydrophilic (200 V) nano-powder also required more energy density as indicated by the values of constants in Eq. (1) Ed = 158 MJ/m3 and A1 = 0.0051 m3/MJ. 90 V% hydrophobic (R816) powder was broken into nano-aggregates at energy density of 450 MJ/m3 whereas de-agglomeration of the same mass of hydrophilic powder required 700 MJ/m3. It is worth to notice that whilst in case of hydrophobic nano-powder erosion was observed from very beginning of the process (delay time equal zero), in hydrophilic nanopowder erosion started after nearly 30 min of processing at the highest energy dissipation rate. The transient median diameters of large hydrophobic and hydrophilic aggregates are compared in Fig. 4b. In both cases the size-
Fig. 4. Effect of energy density on: (a) cumulative volume fraction of nano-aggregates and (b) breakage of large aggregates in 5 wt.% suspension ( pH = 4) at different energy dissipation rates; circles — 4.7 MJ/m3; triangles — 21.7 MJ/m3; squares — 89.3 MJ/m3; solid symbols — R816, empty symbols — 200 V; lines — best fit to Eq. (1) and Eq. (2) respectively.
energy model (Eq. (2)) [7,8] seems to correlate the median diameter with energy density rather well. The reduction rate of median diameter of hydrophobic aggregates is much lower than the reduction rate of median diameter of hydrophilic aggregates and constants β in Eq. (2) are equal to 0.18 and 0.27 respectively. All the above results indicate that de-agglomeration of hydrophobic aggregates is controlled by erosion and fracture whereas the initial stage of deagglomeration of hydrophilic aggregates is controlled by fracture. 3.3. Effect of solid concentration on de-agglomeration kinetics and rheology of suspensions Transient cumulative volume fractions of hydrophobic and hydrophilic nano-aggregates at different solid loads are compared in Fig. 5. Cumulative volume fraction of hydrophobic (R816, Fig. 5a) nanoaggregates at 1 wt.% and 5 wt.% practically overlap and at 500 MJm−3 approximately 90 V% of nano-powders was dispersed into nanoaggregates. At these solid concentrations the critical energy density Ed =0 and the erosion constant A1 = 0.0051[m3/MJ]. However, at 10 wt.% of solid, hydrophobic nano-powder could not be dispersed into nanoaggregates even at the highest energy density (1300 MJ/m3). In the case of hydrophilic nano-powder (200 V, Fig. 5b) the cumulative volume fractions of nano-aggregates at 1 and 5 wt.% practically overlap (critical energy density Ed = 138 MJ/m3 and the erosion constant A1 =0.0045 m3/MJ) and 90% of powder was dispersed into nanoaggregates at energy density of approximately 700 MJm−3. The increase of solid load to 10 wt.% led to the increase of critical energy density Ed =170 MJ/m3 and to reduction of erosion constant (A1 = 0.0024 m3/ MJ). Overall efficiency of de-agglomeration was also reduced and 70 V% of
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the increase of solid load to 5 wt.% does not affect erosion rate (Fig. 5b). At 10 wt.% the suspension is slightly shear thinning with viscosity around eight times higher than 1 wt.%. This increase should not affect the de-agglomeration rate (see discussion above and Figs. 5a and 6a). As the solid concentration increases to 10 wt.% more energy is needed to break all the aggregates therefore at constant energy density de-agglomeration rate is lower (Fig. 5b).
3.4. Effect of pH on kinetics of de-agglomeration and rheology of suspensions It has been reported [9] that pH strongly affects both kinetics of wet de-agglomeration of nano-powders and rheology of resulting suspensions by affecting electrostatic charges on the agglomerate surfaces what in turns affects the balance between attractive (van der Waals) and repulsive (electrostatic) interparticle forces. The van der Waals forces are practically independent on pH whilst the electrostatic repulsive forces strongly depend on zeta potential, e.g. on pH. Transient cumulative volume fractions of hydrophobic and hydrophilic nano-aggregates in 5 wt.% suspensions at different pH compared in Fig. 7 indicate that the effect of pH also depends on the character of the particle surface and that it is different for hydrophobic and for hydrophilic nano-particles. Fig. 7a shows that pH practically has no affect on the kinetics of deagglomeration of hydrophobic nano-powder (R816). Transient cumulative size distributions of hydrophobic nano-aggregates at different pH (zeta potential between 0 and −40 mV) practically overlap. The fines generation constant A1 slightly increased from 0.0068 at pH = 4 to 0.0077 at pH = 9 and at all values of pH critical energy density Fig. 5. Transient cumulative volume fraction at different concentration of (a) hydrophobic and (b) hydrophilic nano-aggregates suspended in water; circles — 1 wt.%; triangles — 5 wt.%; squares — 10 wt.%; pH= 4, lines: best fit to Eq. (1).
nano-powder was dispersed into nano-aggregates at 700 MJ/m3 whereas at the same energy density 90 V% of nano-powder was dispersed at two lower solid concentrations. There are two factors contributing to the reduction of deagglomeration rate with the increase of solid concentration. Firstly, as the mass of aggregates increases more energy is needed to break them into nano-aggregates and secondly the drastic change of rheology of suspensions induced by the increase of solid concentration reduce the efficiency of de-agglomeration as illustrated in Fig. 6. 1 wt.% suspension of hydrophobic nano-powder (R816, Fig. 6a) was Newtonian with viscosity slightly higher than viscosity of water (1.26 mPas). As the solid concentration was increased to 5 wt.% the suspension became shear thinning with the viscosity at the highest shear rate one order of magnitude higher than viscosity of 1 wt.% suspension. On the one hand, the higher the viscosity the higher the laminar shear stress on the surface of agglomerates the higher the erosion rate. On the other hand, the increase of viscosity leads to the reduction of the flow intensity through the mixing head therefore to the reduction of number of aggregates exposed to high shear rate and to the reduction of erosion rate. It appears that at 5 wt.% solid both effects cancels out and there is practically no change in erosion rate as shown in Fig. 5a. Further increase of solid content to 10 wt.% leads to extremely shear thinning suspension with viscosity at the highest shear rate three orders of magnitude higher than viscosity of 1 wt.% suspension at the same shear rate. Laminar shear rate also increased but, as the pumping capacity of the high shear mixers is very low it is possible that at such high viscosity the suspension was not pumped through mixing head. The effect of concentration of hydrophilic nano-powder (R200, Fig. 6b) on the rheology of suspension is much weaker. The difference between viscosity at 1 wt.% and 5 wt.% is marginal what explains why
Fig. 6. Effect of solid concentration on rheology of suspensions of (a) hydrophobic and (b) hydrophilic nano-powders after complete de-agglomeration at nature pH; circles — 1 wt.%; triangles — 5 wt.%; squares — 10 wt.%.
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Fig. 8. Effect of pH on rheology of 5 wt.% suspensions of hydrophobic nano-aggregates after de-agglomeration, (●) pH = 3; (▼) pH = 4; (■) pH = 7; (♦) pH = 9.
Fig. 7. Effect of pH on transient cumulative volume fraction of nano-aggregates in 5 wt.% suspensions of (a) hydrophobic and (b) hydrophobic nano-powders; circles — pH = 4; triangles — pH = 7; squares — pH = 9.
Ed = 0 and all the experimental data in Fig. 7a can be approximated by Eq. (1) with A1 = 0.0074 and Ec = 0. The effect of pH on de-agglomeration rate of hydrophilic nano-powder (200 V) is much more pronounced as shown in Fig. 7b. In this case an increases of pH from 4 to 9 drastically speeds up de-agglomeration process by reducing the critical energy density (Ed) from 158 MJ/m3 at pH=4 to 55 MJ/m3 at pH =9 and increasing fines generation constant (A1) from 0.0053 m3/MJ at pH =4 to 0.012 m3/MJ at pH =9. The increase of de-agglomeration rate of hydrophilic nano-powder with the increase of pH can be explained by the increase of the number of silanol groups on the surface of silica nano-aggregates [10]. As pH was increased from 4 to 9, more silanol groups were formed and ionisation of these groups increased negative charge of the surface as indicated by zeta potential (Fig. 1) leading to the increase of electrostatic repulsive force and consequently to the increase of de-agglomeration rate. The lack of the effect of pH on de-agglomeration of hydrophobic nano-powder can be explained by the modification of surface properties during the reaction of hydroxyl groups with silane [2], [3] but, as the details of these reactions are commercially sensitive and were not disclosed by manufacturer detail analysis of the effect of pH on surface properties is not possible. It is also possible that long range attractive hydrophobic forces make kinetics of de-agglomeration less sensitive to the changes of electrostatic forces. Whilst pH drastically affected de-agglomeration rate of hydrophilic nano-powder (200 V) the viscosity of the suspensions were practically independent of pH and at pH between 4 and 9 the 5 wt.% suspensions were water-like (viscosity of 2.5 mPas). It is possible that hydration/structural inter-particle forces induced by the surface silanol groups [4,10] dominate over electrostatic, pH dependent repulsive forces when the concentration of these groups at the surface of hydrophilic silica nano-particles is high.
In case of R816 situation was opposite with pH having relatively small effect on de-agglomeration rate but very strong effect on the rheology of the suspension as shown in Fig. 8. It appears that the strong effect of pH can be explained within the framework DLVO theory developed for lyophobic systems [11,12]. At pH = 3 (absolute value of the zeta potential b10 mV), electrostatic repulsive forces are small and the rheology is controlled by attractive van der Waals forces leading to very viscous and shear thinning suspension. At pH = 7, absolute value of zeta potential increases to 30 mV therefore interparticles repulsive force also increase that leads to a drastic reduction of apparent viscosity of suspension. Further increase of pH to 9 leads to much smaller increase of absolute value of zeta potential to 37 mV therefore the reduction of viscosity is also rather small. The changes of rheology of the suspension of hydrophobic nanopowder were quantified using power law model: n−1
η = k⋅γ˙
ð3Þ
and the consistency constants (k) and power law index (n) calculated from flow curves shown Fig. 8 are summarised in Table 1. As pH increases from 3 to 9 the rheology of suspension gradually changes from strongly non-Newtonian and shear thinning with a very high viscosity at pH = 3 to Newtonian water like at pH = 9. The reduction of pH from 9 to 3 leads to an increase of viscosity and to reduction of power law index. Similar effect of pH on the rheology of other types of nano-particles has been recently reported by the authors [9]. 4. Conclusions De-agglomeration of both hydrophobic (R816) and hydrophilic (200 V) silica nano-powders follows similar general pattern. Fracture and erosion leading to bi-modal transient size distributions of aggregates was observed in both powders. The qualitative analysis and comparison of the kinetics of de-agglomeration carried out in terms of median diameters of large aggregates and cumulative volume fractions of nano-aggregates revealed that initially erosion dominated
Table 1 Parameters in the power-law model of 5wt% suspension of hydrophobic R816 nanopowder. pH k [Pasn] n r2
3 8.871 0.0946 0.9999
4 5.8790 0.1007 0.9996
7 0.0110 0.8316 0.9459
9 4.24×100.9168 1.000
3
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in hydrophobic nano-powder and fracture in hydrophilic nanopowder. In both nano-powders nano-aggregates could not be broken into single nano-particles even at the highest energy density and the highest repulsive interparticle forces. Experimental results clearly show that the erosion of hydrophobic nano-aggregates occurs at much lower energy dissipation rates and energy density than the erosion of hydrophilic aggregates. In other words, less energy is required to disperse a unit mass of hydrophobic nano-powder than to disperse a unit mass of hydrophilic nano-powder. Very similar results were reported by manufacturer [1]. Those results are rather unexpected because in hydrophilic nano-powder van der Walls force is the only one attractive inter-particle force whereas in hydrophobic nanopowder there is also strong hydrophobic long range attractive force [13–15] The presence of extra attractive force should made deagglomeration of hydrophobic silica nano-powder more energy demanding than de-aggregation of hydrophilic silica which clearly is not a case. It is very difficult to explain this disagreement between theory and experiments and one possible explanation is that the interaction between silica nano-particles cannot be described using standard DVLO related models as discussed by authors in previous work [4]. It has also been suggested in literature [16], [17] that coagulation of silica nano-particles is strongly dependent on alkaline cations and pH and simultaneous effect of those on aggregation/deaggregation kinetics is well outside standard DVLO model. The solid concentration has opposite effect on erosion in both powders. The increase of the concentration of hydrophilic nanopowder (200 V) in the suspension from 1 wt.% to 10 wt.% leads to relatively small reduction of the erosion efficiency and nano-powder can be dispersed into nano-aggregates. At 10w/w% hydrophobic nano-powder (R816) could not be dispersed into nano-aggregates as the erosion efficiency dropped to zero. There is very strong effect of pH on erosion efficiency in hydrophilic nano-powder (200 V) where the increase of pH from 4 to 9 caused a drastic increase of erosion efficiency, whilst similar increase of pH in the suspension of hydrophobic nano-powder has practically no effect on the erosion of nano-aggregates. The rheology of hydrophobic nano-aggregates strongly depends on pH and on solid concentration whilst rheology of suspension of hydrophilic nano-powder is rather weakly dependent on those parameters.
Department of Chemical Eng; Poznan University of Technology, Inst. of Chemical Technology and Eng; Rockfield Software Limited; Unilever UK Port Sunlight, Warsaw University of Technology, Department of Chemical and Process Eng.
Acknowledgement
Glossary
This work is a part of PROFORM (“Transforming Nano-particles into Sustainable Consumer Products Through Advanced Product and Process Formulation” EC Reference NMP4-CT-2004-505645) project which is partially funded by the 6th Framework Programme of EC. The contents of this paper reflect only the authors’ view. The authors gratefully acknowledge the useful discussions held with other partners of the Consortium: Bayer Technology Services GmbH; BHR Group Limited; Centre for Computational Continuum Mechanics (C3M); Karlsruhe University, Inst. of Food Process Eng; Loughborough University,
A1 d0.5 E Ed k n r2 y(t) α β
References [1] Degussa, 2006, Fine Particles, Technical Bulletin, No 18. [2] Tadao Sugimoto, 2000, Fine Particles, Synthesis, Characterization and Mechanism of Growth, Surfactant Science, Marcel Dekker Inc, New York, Series vol 92. [3] T.J. Graule, E. Barna, B. Bommer, J. Kursteiner, A. Vital, O. Trzebiatowski, B. Schmid, J. van Leeuwen, W. Koch, Synthesis of spherical, non-aggregated silica nanoparticles for nanocomposite coatings, Kgk-Kautschuk Gummi Kunststoffe 58 (2005) 252–255. [4] A.W. Pacek, P. Ding, A.T. Utomo, Effect of energy density, pH and temperature on de-aggregation in nano-particles/water suspensions in high shear mixer, Powder Technology 173 (2007) 203–210. [5] S. Tsantilis, S.E. Pratsinis, Soft- and hard-agglomerate aerosols made at high temperatures, Langmuir 20 (2004) 5933–5939. [6] J. Baldyga, W. Orciuch, L. Makowski, K. Malik, G. Ozcan-Taskin, W. Eagles, G. Padron, Dispersion of nanoparticle clusters in a rotor-stator mixer, Industrial & Engineering Chemistry Research 47 (2008) 3652–3663. [7] P. Ding, A.W. Pacek, De-agglomeration of goethite nano-particles using ultrasonic comminution device, Powder Technology 187 (2008) 1–10. [8] K.A. Kusters, S.E. Pratsinis, S.G. Thoma, D.M. Smith, Energy-Size Reduction Laws for Ultrasonic Fragmentation, Powder Technology 80 (1994) 253–263. [9] P. Ding, A.W. Pacek, Effect of pH on deagglomeration and rheology/morphology of aqueous suspensions of goethite nano-powder, Journal of Colloid and Interface Science (2008), doi:10.1016/j.jcis.2008.04.077. [10] R.K. Iler, The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface properties, and Biochemistry, Wiley, New York, 1979. [11] Derjaguin, B. V. and Landau L. D., 1941, Acta Phys.-Chim.URSS, 14, 633-662. [12] E.J.W. Verwey, J.Th.G. Overbeek, Theory of the Stability of Lyophobic Colloids, Theory of the Stability of Lyophobic Colloids, Elsevier, Amsterdam, 1948. [13] J. Israelachvili, Intermolecular & Surface Forces, Academic Press Limited, 1992. [14] H.K. Christenson, P.M. Claesson, Direct measurements of the force between hydrophobic surfaces in water, Advances in Colloid and Interface Science 91 (2001) 391–436. [15] J.C. Eriksson, U. Henriksson, A. Kumpulainen, Hydrophobic attraction forces in asymmetric aqueous films between hydrophobized mica/bare mica surfaces, Colloids and Surfaces A-Physicochemical and Engineering Aspects 282 (2006) 79–83. [16] J. Depasse, Coagulation of colloidal silica by alkaline cations: Surface dehydration or interparticle bridging, Journal of Colloid and Interface Science 194 (1997) 260–262. [17] J. Depasse, Simple experiments to emphasize the main characteristics of the coagulation of silica hydrosols by alkaline cations; Application to the analysis of the model of Colic et al, Journal of Colloid and Interface Science 220 (1999) 174–176.
γ̇
η
Constant defined in Eq. (1) [m3/MJ] Median diameter [µm] Energy density [MJ/m3] Critical energy density defined in Eq. (1) [MJ/m3] Consistency constant [Pa⋅sn] Power law index [–] Coefficient of determination [–] Cumulative volume fraction of fines [–] Constant defined in eq. (2) [(MJ)βm1−3β] Constant defined in Eq. (2) [−] Shear rate [s− 1] Viscosity [Pa⋅s]