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Influence of hydrostatic pressure and sound amplitude on the ultrasound induced dispersion and de-agglomeration of nanoparticles
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Ultrasonics Sonochemistry 15 (2008) 517–523 www.elsevier.com/locate/ultsonch
Influence of hydrostatic pressure and sound amplitude on the ultrasound induced dispersion and de-agglomeration of nanoparticles C. Sauter a
a,*
, M.A. Emin b, H.P. Schuchmann a, S. Tavman
b
Institute of Process Engineering in Life Sciences, Department of Food Process Engineering, Karlsruhe University (TH), Kaiserstrasse 12, D-76131 Karlsruhe, Germany b Food Engineering Department, Faculty of Engineering, Ege University, 35100 Izmir, Turkey Received 24 July 2007; received in revised form 23 August 2007; accepted 31 August 2007 Available online 19 September 2007
Abstract In most applications, nanoparticles are required to be in a well-dispersed state prior to commercialisation. Conventional technology for dispersing particles into liquids, however, usually is not sufficient, since the nanoparticles tend to form very strong agglomerates requiring extremely high specific energy inputs in order to overcome the adhesive forces. Besides conventional systems as stirred media mills, ultrasound is one means to de-agglomerate nanoparticles in aqueous dispersions. In spite of several publications on ultrasound emulsification there is insufficient knowledge on the de-agglomeration of nanoparticulate systems in dispersions and their main parameters of influence. Aqueous suspensions of SiO2-particles were stressed up to specific energies EV of 104 kJ/m3 using ultrasound. Ultrasonic de-agglomeration of nanoparticles in aqueous solution is considered to be mainly a result of cavitation. Both hydrostatic pressure of the medium and the acoustic amplitude of the sound wave affect the intensity of cavitation. Furthermore, the presence of gas in the dispersion medium influences cavitation intensity and thus the effectiveness of the de-agglomeration process. In this contribution both, the influence of these parameters on the result of dispersion and the relation to the specific energy input are taken into account. For this, ultrasound experiments were carried out at different hydrostatic pressure levels (up to 10 bars) and amplitude values (64–123 lm). Depending on the optimisation target (time, energy input,. . .) different parameters limit the dispersion efficiency and result. All experimental results can be explained with the specific energy input that is a function of the primary input parameters of the process. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Dispersion; De-agglomeration; Ultrasound
1. Introduction Fine particles in the range up to several nanometers are frequently used in industry because of their large specific surface. This allows the material to fully express its properties. As many applications require aqueous solutions, dispersing nanoparticles in liquids is a key requirement for a diversity of applications in cosmetics, thickening agents, sun creams, pharmaceuticals, catalysts, glasses, silicones,
*
Corresponding author. Tel.: +49 0 721 608 2196. E-mail address: [email protected] (C. Sauter).
1350-4177/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2007.08.010
and paints. However, the interaction forces between the particles (especially van der Waals forces) in relation to gravitational forces increase as the size of the particles decreases. Furthermore, the collision probability of the particles increases as well, since the distance between the particles at constant volume concentration is reduced with decreasing particle size. Nanoparticles thus often form big and very strong agglomerates, requiring exertion of high stresses in order to overcome the adhesion forces. Nanoparticles produced by flame pyrolysis typically are characterized by aggregates and agglomerates, which size can reach up to several hundred nanometers in diameter, formed via van
518
C. Sauter et al. / Ultrasonics Sonochemistry 15 (2008) 517–523
Nomenclature EV P Vc tr xmean
volume specific energy [kJ m3] power [W] volume of the dispersion cell [m3] residence time [s] unimodal mean value of the PCS distribution [nm]
der Waals forces and sintering bonds [1,2]. The processing of nanoparticles most often does not only require a homogeneous dispersion of the dry particles in a liquid phase but also their de-agglomeration and de-aggregation. Mechanical stirring, which is often applied to improve the homogeneity of the dispersion [3], cannot prevent the particles from aggregating or agglomerating. Additional external forces are required as e.g. applied by stirred media mills [4,5]. For several applications ultrasound has been proved suitable for dispersing particles homogeneously in a liquid. Ultrasound application in addition results in cavitation which in turn creates locally high stresses on particles, agglomerates and aggregates. It offers an attractive route for preparing aqueous dispersions of nanoparticles with the possibility to de-aggregate and de-agglomerate particles in parallel to their homogeneous dispersion. Ultrasound can create acoustic cavitation (formation, growth, and implosion of bubbles) in a liquid. When cavitation bubbles collapse small areas of high pressure differences are generated, resulting in micro turbulence and liquid jets [6]. This creates strong forces that act on the agglomerates resulting in their disruption. However, fundamental research into the mechanisms and main parameters of influence on the ultrasound de-agglomeration process is still needed. The scope of this study is the investigation of the influence of process parameters such as amplitude, pressure, retention time and specific energy on the deagglomeration efficiency in ultrasound dispersion processes. 2. Fundamental 2.1. Dispersions, dispersing systems Dispersions are homogeneous suspensions of particles in fluids. The particles used here are smaller than 0.5 lm (nanoparticles), thus they remain in suspension indefinitely, unaffected by gravity. However, nanoparticles tend to form agglomerates thus, exertion of energy is required to breakup these structures. Mechanical energy input may be provided through various dispersing apparatuses, of which the most important are rotor–stator devices, stirred media mills, high pressure systems, and ultrasound. These processes are displayed in Fig. 1. The agglomerates of the pre-mix are disrupted in the dispersing zone by external stresses acting on their interfaces.
a n c A T
exponent [–] zetapotential [mV] particle concentration [wt.%] amplitude [lm] temperature [°C]
Stirred media mills and rotor–stator-systems are most commonly used systems for the dispersion of nanoparticles (Fig. 1). The power distribution in these systems usually is quite inhomogeneous. Therefore, long mixing or milling times have to be realized and consequently high specific energy inputs are required for an acceptable de-agglomeration result. 2.2. Ultrasound dispersion An ultrasonic wave is a kind of mechanically vibrating wave that requires a transmission medium to be propagated. Application of ultrasound with a certain amplitude value (pA) through a liquid medium at hydrostatic pressure (ph) leads to oscillation of molecules about their mean position. During the compression cycle where the pressure is positive, the average distance between the molecules decreases, whilst during rarefaction the distance increases. When the pressure decreases below a critical value pc (where it occurs under rarefaction cycle, pc = ph pA), such that the average distance between the molecules exceeds the critical molecular distance necessary to hold the liquid intact, the liquid will break down and voids, cavities or bubbles are created. When the pressure increases again at compression cycle of the wave however, cavities, voids, or bubbles are forced to collapse. The continuous process of the production and collapse of these bubbles causes a phenomenon called cavitation [7]. Large amounts of energy get released from total collapse of the bubbles and is transferred to the liquid solutions, thereby creating a mechanical stirring effect with shock waves and micro-jets, which act on the agglomerates resulting in their disruption [7–11]. 2.3. Importance of particles and gas content for cavitation in sonicated liquids The influence of dissolved or dispersed gas on cavitation has already been described in many publications [12–15]. The negative acoustic pressure that is required for the formation of cavitation bubbles in water (<20 bar) is much lower than the theoretically estimated value (approximately 1500 bar) [7]. Reasons for this considerable difference between the theoretical and actual value is the presence
C. Sauter et al. / Ultrasonics Sonochemistry 15 (2008) 517–523
519
Fig. 1. Common mechanical devices for dispersing nanoparticles in aqueous solutions.
Fig. 2. Vapour–gas nuclei in the crevices and recesses of nanoparticle agglomerates, according to Ref. [7].
of weak-spots in the liquid that lowers the liquid’s tensile strength. Presence of gas nuclei in nano bubbles are the main reason for these weak-spots decreasing the cavitation threshold (necessary decrease in pressure for the formation of cavitation bubbles). On the one hand, presence of gas facilitates the onset of cavitation owing to supply the essential weak-spots. On the other hand gas reduces the intensity of the shock wave due to cushioning effects of the gas dissolved in the cavitation bubbles [15]. Furthermore, the presence of particles in the liquid may lower the cavitation threshold because of the particles’ crevices and recesses acting as shelter for the vapour–gas nuclei [7]. The way in which nucleation occurs at these sites is shown in Fig. 2. Trapped vapour–gas nuclei especially become important in case of ultrasound dispersion of nanoparticles as they lead to formation of cavitation in vicinity of particles de-agglomerating them effectively. 2.4. Factors affecting cavitation behaviour Among others, acoustic cavitation can be influenced by hydrostatic pressure, sound amplitude and the nature of
the dissolved gas. The internal pressure of the bubble at the moment of collapse (pm) is approximately ph + pA and the intensity of the bubble collapse is dependent on pm [7]. Higher amplitude values result in an increased probability of cavitation and a more violent collapse of the bubble as the internal pressure pm (= ph + pA) is raised [7,8]. On the other hand, however, sound amplitude cannot be raised indefinitely as the bubble may grow so large that the time available in the adjacent rarefaction cycle will not be sufficient for the collapse. Increasing the external pressure leads to an increase in both the cavitation threshold (pth) and the intensity of bubble collapse [9,16,17]. The influence of the external pressure mainly depends on which of these two mechanisms dominates. Application of external pressure (ph,2) leads to an increased cavitation threshold (pth,2) and so cavitation may be suppressed or the growth of the cavitation bubbles will be insufficient. However, a sufficiently large increase in the amplitude of the applied ultrasonic field (pA,2; discrete line in Fig. 3) can lead to cavitation even at high overpressures, as the region where the local pressure falls below the critical pressure (pc) is enlarged [7]. A shift in hydrostatic pressure (ph,1) to higher values (ph,2) can intensify the cavitation as the intensity of the shock waves due to the collapse of the bubbles is increased at higher pressure levels. 3. Experimental Dispersion experiments were conducted using aqueous nano-scaled siliciumdioxide-suspensions with a variable particle content of up to c = 10 wt.%. The primary particle size of the nanoparticles used (Aerosil 200 VÒ, Degussa, Germany) is 12 nm and the specific surface area is 200 m2/g. As the zeta potential n of these particles in aqueous
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Fig. 3. Relation of local pressure with cavitation (schematic), according to Ref. [11].
Fig. 4. Dispersing and de-agglomerating nanoparticles by ultrasound: experimental set-up.
solution lies in the order of 35 mV, the particles are stabilized by electrostatic forces against re-agglomeration. Transducer model UIP 1000 (Dr. Hielscher Gmbh, Germany) was used for ultrasound generation at a power input of 1000 Watt and a frequency f of 20 kHz. The amplitude of the sound wave can be adjusted from 20% to 100% of the maximum (123 lm). Using ultrasound in batch mode (Fig. 4), the pre-mixed suspensions were stressed up to a specific energy of EV = 104 kJ/m3. The circulating suspension was continuously cooled using a cryostat (T = 20 °C). To investigate the influence of hydrostatic pressure on the de-agglomeration efficiency by ultrasound, the local pressure of the system was raised up to 10 bars applying compressed air. A wattmeter was used to measure the power input in order to calculate the specific energy Eq. (1): EV ¼
P tr Vc
The nanoparticle dispersions produced were characterized by the mean particle size which is one of the most important parameters to evaluate dispersion quality. Particle size distributions were measured by dynamic light scattering (PCS, Coulter N4, Beckman Coulter, Germany). As the modality of the particle size distribution cannot be determined by dynamic light scattering, additional measurements by laser diffraction (LS 230, Beckman Coulter, Germany) including polarization intensity differential scattering (PIDS) technology were implemented.
4. Results and discussion 4.1. Influence of the sound amplitude
ð1Þ
with EV = specific energy, P = power, Vc = Volume of the cell and tr = residence time.
Dispersions were prepared with propagation of ultrasound at various amplitudes. Fig. 5 gives the mean particle sizes of the dispersions as a function of retention time for
C. Sauter et al. / Ultrasonics Sonochemistry 15 (2008) 517–523
Over-pressure: 0 bar Amplitude of ultrasound 64 μm 85 μm 123 μm
Particle size xmean / nm
220 210 200 190
230
200 190 180 170 160
Process mode: batch Process volume: 350 ml 5 wt.-% Aerosil 200V 10
220
Over-pressure: 3.5 bar Amplitude of ultrasound 64 μm 85 μm 123 μm
200 190 180 170
200 190 180 170 160 150
160 140
150
130
Process mode: batch Process volume: 350 ml 5 wt.-% Aerosil 200V
1
Process mode: batch Process volume: 350 ml 5 wt.-% Aerosil 200V 1
Amplitude: 85 μm Static overpressure Δ p 0 bar 3.5 bar 5 bar
210
Particle size xmean / nm
210
130
10
Retention time t / min
Retention time t / min
140
Process mode: batch Process volume: 350 ml 5 wt.-% Aerosil 200V
1
1
Particle size xmean / nm
210
180 170
Amplitude: 64 μm Static overpressure Δ p 0 bar 3.5 bar 6 bar 8 bar 10 bar
220
Particle size xmean / nm
230
521
10
Retention time t / min 10
Retention time t / min Fig. 5. Influence of the sound amplitude on the de-agglomeration of nanoparticle agglomerates at a hydrostatic overpressure of (a) 0 bar and (b) 3.5 bar.
different values of amplitude at two different hydrostatic pressure levels: 0 bar and 3.5 bar. In principal, there is a decrease in particle size with increasing retention time due to the increase in energy dissipation which is responsible for de-agglomeration. Higher amplitude values should increase the intensity of the bubble collapse and consequently the de-agglomeration of nanoparticle agglomerates [7]. Contrary to these theoretical expectations, no significant effect of the amplitude on the efficiency of the de-agglomeration was found (Fig. 5a). However, at higher pressure levels (here: 3.5 bar, Fig. 5b) the effect predicted by theory is more obvious. At higher hydrostatic pressure values the area, where cavitation can take place is enlarged, resulting in an improved result of dispersion. 4.2. Influence of hydrostatic pressure Cavitation as energy dissipating source depends on the hydrostatic pressure of the medium (pm = ph + pA). Fig. 6
Fig. 6. Influence of static overpressure on the de-agglomeration of nanoparticles by ultrasound at a sound amplitude of (a) A = 64 lm and (b) A = 85 lm.
depicts the mean particle sizes of dispersions as function of retention time for different values of pressure at constant amplitudes A of 64 lm and 85 lm, respectively. Increasing the hydrostatic pressure leads to an increase in both the cavitation threshold and the intensity of bubble collapse. Thus, influence of applied external pressure depends on which of these two mechanisms dominates. At an amplitude of A = 64 lm low overpressures of up to 3.5 bars improve the de-agglomeration process (Fig. 6a), as the pressure intensifies the collapse of bubbles. However, a further increase of pressure has a negative influence on the deagglomeration process, as the cavitation threshold is increased too. Furthermore, when the pressure was elevated up to 10 bars, formation of bubbles stopped due to lack of negative pressure below the critical value. A sufficient large increase in the sound amplitude of the applied ultrasonic field can lead to cavitation even at high overpressures. Fig. 6b illustrates, that the application of a certain overpressure results in an increased effectiveness of the de-agglomeration process, using higher amplitude values. In conclusion, overpressure may improve the ultrasonic de-agglomeration process as long as the sound amplitude is
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C. Sauter et al. / Ultrasonics Sonochemistry 15 (2008) 517–523 230
200 190
210
Particle size xmean / nm
Particle size xmean / nm
210
180 170 160 150 140 130
200 190 180 170
Process mode: batch Process volume: 350 ml 5 wt.-% Aerosil 200V xmean / nm = C*(KJm-3)-0.1 10
160
100
Degassed pre-mix Non-treated pre-mix 1
1000
10
Retention time t / min
-3
Specific energy EV / MJm
Fig. 7. Result of de-agglomeration by ultrasound for different hydrostatic pressures and amplitude values: Mean particle sizes as a function of the specific energy input applied.
high enough. One drawback of the application of static overpressure and higher sound amplitudes is the power required being much higher at elevated hydrostatic pressure. Specific energy is a parameter combining both power density and retention time which are the most crucial parameters influencing the de-agglomeration process. Fig. 7 shows the mean particle sizes of nanoparticles deagglomerated in aqueous dispersion as a function of the specific energy applied. The agglomerate size is reduced with increasing specific energy EV. Regarding the specific energy input required, neither the amplitude of the ultrasonic field, nor the over-pressure show any impact on the de-agglomeration efficiency within the analysed range (Fig. 7). The increase in de-agglomeration efficiency (as seen in Fig. 6a and b) is caused by the increase in specific energy input in both cases. Eq. (2) gives the process function describing the decrease in mean particle size with increasing power input per dispersion volume (pv) and increasing residence time tv. The characteristic exponents a1 and a2 in Eq. (2) are frequently of the same size and can be taken as one single parameter a. Hence the de-agglomeration process can be described by the value of the specific energy EV allowing to compare different types of dispersion equipments [18]. 1 a2 ðP v tr Þa Ea xmean a P a v tr V
Process mode: batch Process volume: 350 ml Hydrostatic pressure: 3.5 bar Amplitude: 64 μm 5 wt.-% Aerosil 200V
220
Amplitude of ultrasound 64 μm 85 μm at 0 bar 123 μm 64 μm 85 μm at 3.5 bar 123 μm
220
ð2Þ
For the ultrasound de-agglomeration experiments of Aerosil 200 V, the exponent a was found to be in the range of 0.1, being a measure for the extremely high attractive forces that have to be overcome in de-agglomeration. 4.3. Influence of the gas content Another factor affecting ultrasonic de-agglomeration of particles is the gas content of the dispersion. In order to
Fig. 8. Influence of the gas content of the pre-mix on de-agglomeration process.
investigate the influence of gas, degassed and non-degassed pre-mixes were prepared and de-agglomerated by ultrasound at constant pressure and amplitude values. Fig. 8 shows that the presence of gas in the dispersion favours the de-agglomeration of nanoparticles. The removal of gas from the medium reduces the existence of nuclei and therefore it becomes more difficult to cavitate the liquid. Regarding the results of the degassed pre-mixes, there is a sudden reduction in the particle size during the first few minutes similar to the overpressure results depicted before in Fig. 6a. 5. Conclusions Ultrasound dispersion is an efficient method to obtain finely dispersed nanoparticles in aqueous solutions as cavitation is responsible for a high energy input resulting in nanoparticle de-agglomeration. The dissipated energy due to the collapse of cavitation bubbles is a function of the hydrostatic pressure of the dispersion and the acoustic amplitude. The experimental studies of this work show that depending on the main focus of the dispersion process, energy input, process time or dispersion result, different parameter settings have to be chosen. However, in terms of the specific energy input, neither the amplitude nor the pressure show a significant influence on the de-agglomeration process, as the power consumption increases for higher values of sound amplitude and hydrostatic pressure. The gas content of the pre-mix influences the cavitation intensity, as a lower gas content reduces the available nuclei and cavitation intensity is reduced. This is an important effect as the trapped vapour–gas nuclei within the agglomerates favour the ultrasound de-agglomeration of nanoparticles.
C. Sauter et al. / Ultrasonics Sonochemistry 15 (2008) 517–523
Acknowledgements This study was carried out within the project PROFORM (‘‘/Transforming Nanoparticles into Sustainable Consumer Products Through Advanced Product and Process Formulation/’’ EC Reference NMP4-CT-2004505645) which was 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: BHR Group Limited; Bayer Technology Services GmbH; University of Loughborough; Unilever UK Central Resources Ltd.; Birmingham University School of Engineering; Warsaw University of Technology, Department of Chemical and Process Engineering; Poznan University of Technology, Institute of Chemical Technology and Engineering; Rockfield Software Limited; Centre for Continuum Mechanics. References [1] H. Barthel, L. Ro¨sch, J. Weis, Fumed Silica – Production, Properties, and Applications, Silicon chemistry II: From Molecules to Materials, VCH, Weinheim, 1996, p. 761. [2] S.E. Pratsinis, S. Tsantilis, Soft- and hard-agglomerates made at high temperatures, Langmuir 20 (2004) 5933–5939. [3] I.A. Ibrahim, F.A. Mohamed, E.J. Lavernia, Processing techniques for particulate-reinforced metal aluminium matrix composites, Journal of Material Science 26 (1991) 5965–5978.
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[4] A. Kwade, Autogenzerkleinerung von Kalkstein in Ru¨hrwerkskugelmu¨hlen, Ph.D. Thesis, University of Braunschweig, 1997. [5] F. Mu¨ller, W. Peukert, R. Polke, F. Stenger, Dispersing nanoparticles in liquids, Mineral Processing 74S (2004) 31–41. [6] O. Behrend, Mechanisches Emulgieren mit Ultraschall, Ph.D. Thesis, University of Karlsruhe, 2002. [7] T.J. Mason, J.P. Lorimer, Applied Sonochemistry, Weinheim, WileyVCH, 2002. [8] Y.T. Sha, A.B. Pandit, V.S. Moholkar, Cavitation Reaction Engineering, Kluwer Academic, 1999. [9] C. Bondy, K. So¨llner, On the mechanism of emulsification by ultrasonic waves, Transactions of the Faraday Society 31 (1935) 835–842. [10] S.A. Neduzhii, Investigation of emulsification brought on by sonic and ultrasonic oscillations, Soviet Physics Acoustics 7 (1962) 221. [11] W. Lauterborn, C.D. Ohl, Cavitation bubble dynamics, Ultrasonic Sonochemistry 4 (1997) 65–75. [12] E.N. Harvey, W.D. McElroy, A.H. Whitely, On cavity formation in water, Journal of Applied Physics 18 (1947) 162–172. [13] M.G. Sirotyuk, Effect of the temperature and gas content of the liquid on cavitation processes, Soviet Physics—Acoustics 12 (1966) 67–77. [14] C. Bondy, K. So¨llner, The influence of gasses on mercury emulsions prepared by ultrasonic waves, Transactions of the Faraday Society 31 (1935) 843–846. [15] O. Behrend, H. Schubert, Influence of hydrostatic pressure and gas content on continuous ultrasound emulsification, Ultrasonics Sonochemistry 8 (2002) 271–276. [16] R.S. Silver, The theory of stress due to collapse of vapour bubbles in a liquid, Engineering 154 (1942) 501. [17] M.G. Sirotyuk, Ultrasonic cavitation process at elevated hydrostatic pressures, Soviet Physics—Acoustics 12 (1966) 199–204. [18] H. Karbstein, Untersuchungen zum Herstellen und Stabilisieren von ¨ l-in-Wasser-Emulsionen, Ph.D. Thesis, University of Karlsruhe, 1994. O
Ultrasonics Sonochemistry 15 (2008) 517–523 www.elsevier.com/locate/ultsonch
Influence of hydrostatic pressure and sound amplitude on the ultrasound induced dispersion and de-agglomeration of nanoparticles C. Sauter a
a,*
, M.A. Emin b, H.P. Schuchmann a, S. Tavman
b
Institute of Process Engineering in Life Sciences, Department of Food Process Engineering, Karlsruhe University (TH), Kaiserstrasse 12, D-76131 Karlsruhe, Germany b Food Engineering Department, Faculty of Engineering, Ege University, 35100 Izmir, Turkey Received 24 July 2007; received in revised form 23 August 2007; accepted 31 August 2007 Available online 19 September 2007
Abstract In most applications, nanoparticles are required to be in a well-dispersed state prior to commercialisation. Conventional technology for dispersing particles into liquids, however, usually is not sufficient, since the nanoparticles tend to form very strong agglomerates requiring extremely high specific energy inputs in order to overcome the adhesive forces. Besides conventional systems as stirred media mills, ultrasound is one means to de-agglomerate nanoparticles in aqueous dispersions. In spite of several publications on ultrasound emulsification there is insufficient knowledge on the de-agglomeration of nanoparticulate systems in dispersions and their main parameters of influence. Aqueous suspensions of SiO2-particles were stressed up to specific energies EV of 104 kJ/m3 using ultrasound. Ultrasonic de-agglomeration of nanoparticles in aqueous solution is considered to be mainly a result of cavitation. Both hydrostatic pressure of the medium and the acoustic amplitude of the sound wave affect the intensity of cavitation. Furthermore, the presence of gas in the dispersion medium influences cavitation intensity and thus the effectiveness of the de-agglomeration process. In this contribution both, the influence of these parameters on the result of dispersion and the relation to the specific energy input are taken into account. For this, ultrasound experiments were carried out at different hydrostatic pressure levels (up to 10 bars) and amplitude values (64–123 lm). Depending on the optimisation target (time, energy input,. . .) different parameters limit the dispersion efficiency and result. All experimental results can be explained with the specific energy input that is a function of the primary input parameters of the process. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Dispersion; De-agglomeration; Ultrasound
1. Introduction Fine particles in the range up to several nanometers are frequently used in industry because of their large specific surface. This allows the material to fully express its properties. As many applications require aqueous solutions, dispersing nanoparticles in liquids is a key requirement for a diversity of applications in cosmetics, thickening agents, sun creams, pharmaceuticals, catalysts, glasses, silicones,
*
Corresponding author. Tel.: +49 0 721 608 2196. E-mail address: [email protected] (C. Sauter).
1350-4177/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2007.08.010
and paints. However, the interaction forces between the particles (especially van der Waals forces) in relation to gravitational forces increase as the size of the particles decreases. Furthermore, the collision probability of the particles increases as well, since the distance between the particles at constant volume concentration is reduced with decreasing particle size. Nanoparticles thus often form big and very strong agglomerates, requiring exertion of high stresses in order to overcome the adhesion forces. Nanoparticles produced by flame pyrolysis typically are characterized by aggregates and agglomerates, which size can reach up to several hundred nanometers in diameter, formed via van
518
C. Sauter et al. / Ultrasonics Sonochemistry 15 (2008) 517–523
Nomenclature EV P Vc tr xmean
volume specific energy [kJ m3] power [W] volume of the dispersion cell [m3] residence time [s] unimodal mean value of the PCS distribution [nm]
der Waals forces and sintering bonds [1,2]. The processing of nanoparticles most often does not only require a homogeneous dispersion of the dry particles in a liquid phase but also their de-agglomeration and de-aggregation. Mechanical stirring, which is often applied to improve the homogeneity of the dispersion [3], cannot prevent the particles from aggregating or agglomerating. Additional external forces are required as e.g. applied by stirred media mills [4,5]. For several applications ultrasound has been proved suitable for dispersing particles homogeneously in a liquid. Ultrasound application in addition results in cavitation which in turn creates locally high stresses on particles, agglomerates and aggregates. It offers an attractive route for preparing aqueous dispersions of nanoparticles with the possibility to de-aggregate and de-agglomerate particles in parallel to their homogeneous dispersion. Ultrasound can create acoustic cavitation (formation, growth, and implosion of bubbles) in a liquid. When cavitation bubbles collapse small areas of high pressure differences are generated, resulting in micro turbulence and liquid jets [6]. This creates strong forces that act on the agglomerates resulting in their disruption. However, fundamental research into the mechanisms and main parameters of influence on the ultrasound de-agglomeration process is still needed. The scope of this study is the investigation of the influence of process parameters such as amplitude, pressure, retention time and specific energy on the deagglomeration efficiency in ultrasound dispersion processes. 2. Fundamental 2.1. Dispersions, dispersing systems Dispersions are homogeneous suspensions of particles in fluids. The particles used here are smaller than 0.5 lm (nanoparticles), thus they remain in suspension indefinitely, unaffected by gravity. However, nanoparticles tend to form agglomerates thus, exertion of energy is required to breakup these structures. Mechanical energy input may be provided through various dispersing apparatuses, of which the most important are rotor–stator devices, stirred media mills, high pressure systems, and ultrasound. These processes are displayed in Fig. 1. The agglomerates of the pre-mix are disrupted in the dispersing zone by external stresses acting on their interfaces.
a n c A T
exponent [–] zetapotential [mV] particle concentration [wt.%] amplitude [lm] temperature [°C]
Stirred media mills and rotor–stator-systems are most commonly used systems for the dispersion of nanoparticles (Fig. 1). The power distribution in these systems usually is quite inhomogeneous. Therefore, long mixing or milling times have to be realized and consequently high specific energy inputs are required for an acceptable de-agglomeration result. 2.2. Ultrasound dispersion An ultrasonic wave is a kind of mechanically vibrating wave that requires a transmission medium to be propagated. Application of ultrasound with a certain amplitude value (pA) through a liquid medium at hydrostatic pressure (ph) leads to oscillation of molecules about their mean position. During the compression cycle where the pressure is positive, the average distance between the molecules decreases, whilst during rarefaction the distance increases. When the pressure decreases below a critical value pc (where it occurs under rarefaction cycle, pc = ph pA), such that the average distance between the molecules exceeds the critical molecular distance necessary to hold the liquid intact, the liquid will break down and voids, cavities or bubbles are created. When the pressure increases again at compression cycle of the wave however, cavities, voids, or bubbles are forced to collapse. The continuous process of the production and collapse of these bubbles causes a phenomenon called cavitation [7]. Large amounts of energy get released from total collapse of the bubbles and is transferred to the liquid solutions, thereby creating a mechanical stirring effect with shock waves and micro-jets, which act on the agglomerates resulting in their disruption [7–11]. 2.3. Importance of particles and gas content for cavitation in sonicated liquids The influence of dissolved or dispersed gas on cavitation has already been described in many publications [12–15]. The negative acoustic pressure that is required for the formation of cavitation bubbles in water (<20 bar) is much lower than the theoretically estimated value (approximately 1500 bar) [7]. Reasons for this considerable difference between the theoretical and actual value is the presence
C. Sauter et al. / Ultrasonics Sonochemistry 15 (2008) 517–523
519
Fig. 1. Common mechanical devices for dispersing nanoparticles in aqueous solutions.
Fig. 2. Vapour–gas nuclei in the crevices and recesses of nanoparticle agglomerates, according to Ref. [7].
of weak-spots in the liquid that lowers the liquid’s tensile strength. Presence of gas nuclei in nano bubbles are the main reason for these weak-spots decreasing the cavitation threshold (necessary decrease in pressure for the formation of cavitation bubbles). On the one hand, presence of gas facilitates the onset of cavitation owing to supply the essential weak-spots. On the other hand gas reduces the intensity of the shock wave due to cushioning effects of the gas dissolved in the cavitation bubbles [15]. Furthermore, the presence of particles in the liquid may lower the cavitation threshold because of the particles’ crevices and recesses acting as shelter for the vapour–gas nuclei [7]. The way in which nucleation occurs at these sites is shown in Fig. 2. Trapped vapour–gas nuclei especially become important in case of ultrasound dispersion of nanoparticles as they lead to formation of cavitation in vicinity of particles de-agglomerating them effectively. 2.4. Factors affecting cavitation behaviour Among others, acoustic cavitation can be influenced by hydrostatic pressure, sound amplitude and the nature of
the dissolved gas. The internal pressure of the bubble at the moment of collapse (pm) is approximately ph + pA and the intensity of the bubble collapse is dependent on pm [7]. Higher amplitude values result in an increased probability of cavitation and a more violent collapse of the bubble as the internal pressure pm (= ph + pA) is raised [7,8]. On the other hand, however, sound amplitude cannot be raised indefinitely as the bubble may grow so large that the time available in the adjacent rarefaction cycle will not be sufficient for the collapse. Increasing the external pressure leads to an increase in both the cavitation threshold (pth) and the intensity of bubble collapse [9,16,17]. The influence of the external pressure mainly depends on which of these two mechanisms dominates. Application of external pressure (ph,2) leads to an increased cavitation threshold (pth,2) and so cavitation may be suppressed or the growth of the cavitation bubbles will be insufficient. However, a sufficiently large increase in the amplitude of the applied ultrasonic field (pA,2; discrete line in Fig. 3) can lead to cavitation even at high overpressures, as the region where the local pressure falls below the critical pressure (pc) is enlarged [7]. A shift in hydrostatic pressure (ph,1) to higher values (ph,2) can intensify the cavitation as the intensity of the shock waves due to the collapse of the bubbles is increased at higher pressure levels. 3. Experimental Dispersion experiments were conducted using aqueous nano-scaled siliciumdioxide-suspensions with a variable particle content of up to c = 10 wt.%. The primary particle size of the nanoparticles used (Aerosil 200 VÒ, Degussa, Germany) is 12 nm and the specific surface area is 200 m2/g. As the zeta potential n of these particles in aqueous
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Fig. 3. Relation of local pressure with cavitation (schematic), according to Ref. [11].
Fig. 4. Dispersing and de-agglomerating nanoparticles by ultrasound: experimental set-up.
solution lies in the order of 35 mV, the particles are stabilized by electrostatic forces against re-agglomeration. Transducer model UIP 1000 (Dr. Hielscher Gmbh, Germany) was used for ultrasound generation at a power input of 1000 Watt and a frequency f of 20 kHz. The amplitude of the sound wave can be adjusted from 20% to 100% of the maximum (123 lm). Using ultrasound in batch mode (Fig. 4), the pre-mixed suspensions were stressed up to a specific energy of EV = 104 kJ/m3. The circulating suspension was continuously cooled using a cryostat (T = 20 °C). To investigate the influence of hydrostatic pressure on the de-agglomeration efficiency by ultrasound, the local pressure of the system was raised up to 10 bars applying compressed air. A wattmeter was used to measure the power input in order to calculate the specific energy Eq. (1): EV ¼
P tr Vc
The nanoparticle dispersions produced were characterized by the mean particle size which is one of the most important parameters to evaluate dispersion quality. Particle size distributions were measured by dynamic light scattering (PCS, Coulter N4, Beckman Coulter, Germany). As the modality of the particle size distribution cannot be determined by dynamic light scattering, additional measurements by laser diffraction (LS 230, Beckman Coulter, Germany) including polarization intensity differential scattering (PIDS) technology were implemented.
4. Results and discussion 4.1. Influence of the sound amplitude
ð1Þ
with EV = specific energy, P = power, Vc = Volume of the cell and tr = residence time.
Dispersions were prepared with propagation of ultrasound at various amplitudes. Fig. 5 gives the mean particle sizes of the dispersions as a function of retention time for
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Over-pressure: 0 bar Amplitude of ultrasound 64 μm 85 μm 123 μm
Particle size xmean / nm
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Over-pressure: 3.5 bar Amplitude of ultrasound 64 μm 85 μm 123 μm
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Amplitude: 64 μm Static overpressure Δ p 0 bar 3.5 bar 6 bar 8 bar 10 bar
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10
Retention time t / min 10
Retention time t / min Fig. 5. Influence of the sound amplitude on the de-agglomeration of nanoparticle agglomerates at a hydrostatic overpressure of (a) 0 bar and (b) 3.5 bar.
different values of amplitude at two different hydrostatic pressure levels: 0 bar and 3.5 bar. In principal, there is a decrease in particle size with increasing retention time due to the increase in energy dissipation which is responsible for de-agglomeration. Higher amplitude values should increase the intensity of the bubble collapse and consequently the de-agglomeration of nanoparticle agglomerates [7]. Contrary to these theoretical expectations, no significant effect of the amplitude on the efficiency of the de-agglomeration was found (Fig. 5a). However, at higher pressure levels (here: 3.5 bar, Fig. 5b) the effect predicted by theory is more obvious. At higher hydrostatic pressure values the area, where cavitation can take place is enlarged, resulting in an improved result of dispersion. 4.2. Influence of hydrostatic pressure Cavitation as energy dissipating source depends on the hydrostatic pressure of the medium (pm = ph + pA). Fig. 6
Fig. 6. Influence of static overpressure on the de-agglomeration of nanoparticles by ultrasound at a sound amplitude of (a) A = 64 lm and (b) A = 85 lm.
depicts the mean particle sizes of dispersions as function of retention time for different values of pressure at constant amplitudes A of 64 lm and 85 lm, respectively. Increasing the hydrostatic pressure leads to an increase in both the cavitation threshold and the intensity of bubble collapse. Thus, influence of applied external pressure depends on which of these two mechanisms dominates. At an amplitude of A = 64 lm low overpressures of up to 3.5 bars improve the de-agglomeration process (Fig. 6a), as the pressure intensifies the collapse of bubbles. However, a further increase of pressure has a negative influence on the deagglomeration process, as the cavitation threshold is increased too. Furthermore, when the pressure was elevated up to 10 bars, formation of bubbles stopped due to lack of negative pressure below the critical value. A sufficient large increase in the sound amplitude of the applied ultrasonic field can lead to cavitation even at high overpressures. Fig. 6b illustrates, that the application of a certain overpressure results in an increased effectiveness of the de-agglomeration process, using higher amplitude values. In conclusion, overpressure may improve the ultrasonic de-agglomeration process as long as the sound amplitude is
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200 190
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Particle size xmean / nm
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Process mode: batch Process volume: 350 ml 5 wt.-% Aerosil 200V xmean / nm = C*(KJm-3)-0.1 10
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Degassed pre-mix Non-treated pre-mix 1
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Retention time t / min
-3
Specific energy EV / MJm
Fig. 7. Result of de-agglomeration by ultrasound for different hydrostatic pressures and amplitude values: Mean particle sizes as a function of the specific energy input applied.
high enough. One drawback of the application of static overpressure and higher sound amplitudes is the power required being much higher at elevated hydrostatic pressure. Specific energy is a parameter combining both power density and retention time which are the most crucial parameters influencing the de-agglomeration process. Fig. 7 shows the mean particle sizes of nanoparticles deagglomerated in aqueous dispersion as a function of the specific energy applied. The agglomerate size is reduced with increasing specific energy EV. Regarding the specific energy input required, neither the amplitude of the ultrasonic field, nor the over-pressure show any impact on the de-agglomeration efficiency within the analysed range (Fig. 7). The increase in de-agglomeration efficiency (as seen in Fig. 6a and b) is caused by the increase in specific energy input in both cases. Eq. (2) gives the process function describing the decrease in mean particle size with increasing power input per dispersion volume (pv) and increasing residence time tv. The characteristic exponents a1 and a2 in Eq. (2) are frequently of the same size and can be taken as one single parameter a. Hence the de-agglomeration process can be described by the value of the specific energy EV allowing to compare different types of dispersion equipments [18]. 1 a2 ðP v tr Þa Ea xmean a P a v tr V
Process mode: batch Process volume: 350 ml Hydrostatic pressure: 3.5 bar Amplitude: 64 μm 5 wt.-% Aerosil 200V
220
Amplitude of ultrasound 64 μm 85 μm at 0 bar 123 μm 64 μm 85 μm at 3.5 bar 123 μm
220
ð2Þ
For the ultrasound de-agglomeration experiments of Aerosil 200 V, the exponent a was found to be in the range of 0.1, being a measure for the extremely high attractive forces that have to be overcome in de-agglomeration. 4.3. Influence of the gas content Another factor affecting ultrasonic de-agglomeration of particles is the gas content of the dispersion. In order to
Fig. 8. Influence of the gas content of the pre-mix on de-agglomeration process.
investigate the influence of gas, degassed and non-degassed pre-mixes were prepared and de-agglomerated by ultrasound at constant pressure and amplitude values. Fig. 8 shows that the presence of gas in the dispersion favours the de-agglomeration of nanoparticles. The removal of gas from the medium reduces the existence of nuclei and therefore it becomes more difficult to cavitate the liquid. Regarding the results of the degassed pre-mixes, there is a sudden reduction in the particle size during the first few minutes similar to the overpressure results depicted before in Fig. 6a. 5. Conclusions Ultrasound dispersion is an efficient method to obtain finely dispersed nanoparticles in aqueous solutions as cavitation is responsible for a high energy input resulting in nanoparticle de-agglomeration. The dissipated energy due to the collapse of cavitation bubbles is a function of the hydrostatic pressure of the dispersion and the acoustic amplitude. The experimental studies of this work show that depending on the main focus of the dispersion process, energy input, process time or dispersion result, different parameter settings have to be chosen. However, in terms of the specific energy input, neither the amplitude nor the pressure show a significant influence on the de-agglomeration process, as the power consumption increases for higher values of sound amplitude and hydrostatic pressure. The gas content of the pre-mix influences the cavitation intensity, as a lower gas content reduces the available nuclei and cavitation intensity is reduced. This is an important effect as the trapped vapour–gas nuclei within the agglomerates favour the ultrasound de-agglomeration of nanoparticles.
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Acknowledgements This study was carried out within the project PROFORM (‘‘/Transforming Nanoparticles into Sustainable Consumer Products Through Advanced Product and Process Formulation/’’ EC Reference NMP4-CT-2004505645) which was 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: BHR Group Limited; Bayer Technology Services GmbH; University of Loughborough; Unilever UK Central Resources Ltd.; Birmingham University School of Engineering; Warsaw University of Technology, Department of Chemical and Process Engineering; Poznan University of Technology, Institute of Chemical Technology and Engineering; Rockfield Software Limited; Centre for Continuum Mechanics. References [1] H. Barthel, L. Ro¨sch, J. Weis, Fumed Silica – Production, Properties, and Applications, Silicon chemistry II: From Molecules to Materials, VCH, Weinheim, 1996, p. 761. [2] S.E. Pratsinis, S. Tsantilis, Soft- and hard-agglomerates made at high temperatures, Langmuir 20 (2004) 5933–5939. [3] I.A. Ibrahim, F.A. Mohamed, E.J. Lavernia, Processing techniques for particulate-reinforced metal aluminium matrix composites, Journal of Material Science 26 (1991) 5965–5978.
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