Hierarchical Structure Formation of Nanoparticulate Spray-Dried

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Hierarchical Structure Formation of Nanoparticulate Spray-Dried Composite Aggregates Sabrina Zellmer,† Georg Garnweitner,† Thomas Breinlinger,‡ Torsten Kraft,‡ and Carsten Schilde*,† †

Institute for Particle Technology, TU Braunschweig, 38106 Braunschweig, Germany and ‡Fraunhofer Institute for Mechanics of Materials IWM, 79108 Freiburg, Germany ABSTRACT The design of hierarchically structured nano- and microparticles of different

sizes, porosities, surface areas, compositions, and internal structures from nanoparticle building blocks is important for new or enhanced application properties of high-quality products in a variety of industries. Spray-drying processes are well-suited for the design of hierarchical structures of multicomponent products. This structure design using various nanoparticles as building blocks is one of the most important challenges for the future to create products with optimized or completely new properties. Furthermore, the transfer of designed nanomaterials to large-scale products with favorable handling and processing can be achieved. The resultant aggregate structure depends on the utilized nanoparticle building blocks as well as on a large number of process and formulation parameters. In this study, structure formation and segregation phenomena during the spray drying process were investigated to enable the synthesis of tailor-made nanostructures with defined properties. Moreover, a theoretical model of this segregation and structure formation in nanosuspensions is presented using a discrete element method simulation. KEYWORDS: spray drying . nanoparticles . aggregate structure . silica . iron oxide . zirconia . multicomponent

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he use of different materials in the form of nanoparticles is important for new or enhanced application properties of high-quality products in the chemical, pharmaceutical, food, and dye industries.1,2 These properties are determined particularly by the size of the particles or by the large specific surface, e.g., attractive forces, mechanical, rheological, and optical properties or other unique characteristics.3,4 In general, the product properties depend on the characteristics of the nanoparticles as much as on further processing steps, particularly the formation of complex nanostructures using nanoparticles as building kit. Besides the well-defined hierarchical multicomponent nanostructures known from the literature, a huge number of industrial products are based on nanoparticulate structures in a more or less complex or distributed way. In many applications, the particles must be suspended in a liquid phase as separately dispersed primary particles or in a certain aggregate size,5 e.g., coatings,6,7 paints, ceramics, composites,8,9 or adhesives. The size, structure, and binding ZELLMER ET AL.

mechanism between the primary particles as well as the mechanical properties of the secondary particles (aggregates after spray drying) determine this redispersion process. In order to select the desired product features and create an economical dispersion process, considerable research efforts are necessary to investigate the chemical and physical processes which take place during particle synthesis as well as their effect on the aggregate structure. Apart from that, spraydrying processes are often used to guarantee the increasing requirements to handle nanoparticulate powders for transport or in further processing steps, e.g., flowability, linting propensity, bulk density.10 The produced aggregate structure is a result of the material itself as well as by a large number of process and formulation parameters. Hence, complex structured microparticles of different sizes, porosities, surface areas, compositions, and internal structures have been designed by research of the aggregate formation processes.11,12 Apart from the nanoparticle synthesis, this structure design of multicomponent-based products using various VOL. XXX

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* Address correspondence to [email protected]. Received for review May 7, 2015 and accepted October 27, 2015. Published online 10.1021/acsnano.5b05220 C XXXX American Chemical Society

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description of the complex surface and structure formation phenomena of multicomponent nanosuspensions. Insoluble particulate suspensions composed of one monomodal distributed particulate material with a specific particle size distribution have to be combined to formulate multicomponent mixtures with various particle sizes, e.g., a bimodal distribution (the volume weighted mean particle sizes as well as a value for the polydispersity3 of the used materials are given in the Supporting Information, Figure 3). Exemplarily for nanosuspensions, the formation of nanoscale substructures of low density particles, multicomponent particles, and microencapsulation is summarized qualitatively by Vehring.11 Mondragon et al. investigated the effect of various formulation parameters and the addition of microparticles on the secondary particle formation and the strength of the resultant spray-dried aggregates.4,12,19 Mezhericher et al. described the time and the radius dependency on the solid volume fraction of nanoparticles, assuming a radially symmetrical droplet structure and gradient in particle concentration in radial direction by comparing continuous species transport (CST) and population balance modeling (PB) with single droplet drying experiments using an acoustic levitator.17,18 Similar to the multicomponent systems described before, the internal structure and arrangement of different particle size classes within an aggregate varies significantly. Indeed, the internal structure of nanoparticulate aggregates is difficult to measure or to describe theoretically. In principle, the evaporation rate depends on the balance of energy required for evaporation and energy transported to the droplet surface.26
28 In the first drying period, when the wet-bulb temperature is reached and the droplet shrinks (shrinkage follows a d2-relation), the droplet surface area decreases linearly with the time.29 In the second drying period a particulate shell is formed. The evaporation through the pores of the shell decreases with increasing shell thickness. A detailed description of the drying stages and their effect on the resultant aggregate morphologies is presented by Mondragon et al.12,29 Furthermore, two fundamental cases for low and high Peclet numbers can be differentiated.11 For low Peclet numbers, the diffusional motion is fast compared to the receding droplet surface, and aggregates without void volume are formed. For high Peclet numbers, the droplet surface moves faster than the dissolved components by diffusional forces and aggregates with void volume are produced. These diffusional forces, especially at low Peclet numbers, are an important factor for the structure formation during spray drying. In addition, the radial distribution of components during the spray drying process depends on several driving forces: The surface activity causes a diffusional flow to the surface. The adsorption of components on the surface, VOL. XXX

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nanoparticles as building blocks is one of the most important challenges for the future to create products with optimized or completely new properties. On this account, during the last five years the research rate of processing nanosuspensions via a spray-drying process increases continuously.13
19 However, investigations using nanoparticulate systems as building kits for a systematic formation of well-defined hierarchical multicomponent nanostructures are infrequent. In the case of structure design using the spraydrying process, the morphology of spray-dried aggregates as a result of various materials and process parameters is presented in the literature. Walzel summarized the effect of different atomizer systems and geometries on the particle size distribution and the morphology of the secondary particles.20 Walten and Mumford divided the formation of various secondary morphologies into three categories with a characteristic drying behavior: skin forming, agglomerate, and crystalline materials.21 Moreover, the complex formation of secondary particles based on multicomponent mixtures is highlighted. Besides the secondary particle morphology, the internal structure and arrangement of different components of a mixture within an aggregate vary significantly and is difficult to describe theoretically. This influence on multicomponent mixtures and their effect on the internal aggregate structure are addressed to a limited extent in the literature. Regarding general spray-drying research, atomization studies, experiments on gas flow patterns and residence times, single droplet drying studies, and theoretical modeling of the drying behavior can be differentiated.21 In principle, the theoretical approaches can be distinguished into lumped parameters and distributed parameters.2 Distributed parameter approaches allow the prediction of different concentrations of the components as a function of the aggregate radius, whereas lumped parameter approaches assume a homogeneous distribution of components within the solid material. A review of existing models which predict the segregation of components during spray drying processes is given by Wang and Langrish.2 Here, e.g., distributed-parameter models of Nijdam et al.,22 Meerdink and Riet,23 Adhikari et al.,24 Morison and Payne,25 or Seydel et al.10 are mentioned. However, these models are not able to describe the substructure formation or segregation processes of multicomponent nanosuspensions or nanosuspensions with various particle size distributions during spray drying. Xiao and Chen performed a multiscale modeling of the surface composition of spray dried two-component nanosuspensions by coupling a molecular-level geometrical interpretation with a continuum diffusion model.1 As a result, an increased surface coverage of the larger sized protein component was obtained experimentally and theoretically. On the basis of these results, Xiao and Chen request a new model for the

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Ei ¼

cS, i e(0:5Pe) ¼ cm, i 3 3 βi

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e.g. a monolayer of surfactants, surface active agents or proteins, is preferred. Due to the evaporation of the solvent the droplet size shrinks, which leads to an increase of the solute/particle concentration on the surface. Assuming that segregation during spraydrying cannot take place when a dense particulate layer is already formed in the second drying period, segregation occurs in the first drying period. The increasing solute/particle concentration causes a diffusional flux away from the droplet surface in the first drying period according to Fick's first law.30,31 As a simplified model for the radial distribution of different components in the absence of internal convection, the distribution of components is described by a nonlinear diffusion equation. For constant diffusion coefficients and negligible interactions between the solutes/ particles, the diffusion can be described by Fick's second law of motion in a one-radial symmetric form.10,11,32,33 As a result, the surface concentration cs,i of component i, in relation to the average concentration in the droplet cm,i, can be calculated as Ei. The evaporation rate k and the parameter βi are a function of the Peclet number (Pe):11

Figure 1. SEM images of aggregate system produced by spray drying (mean primary particle sizes of the particle volume distribution of 200 and 650 nm; left, aggregates; right, aggregate surfaces).

(1)

K kB T kB T and Di ¼ ∼ 8Di 6πηR0 ηxi cS, i Pei Pei 2 Pei 3 þ þ f approximation : Ei ¼ ¼ 1þ cm, i 5 100 4000

f with Pei ¼

For spray drying of nanosuspensions, no crystallization, precipitation, or polymerization processes on the droplet surface are observed. The surface concentration of nanoparticles according to eq 1 is inversely proportional to the diffusion coefficient Di (Stokes
Einstein equation34). Consequently, according to eq 1, nanoparticles with a larger particle diameter have to be located in a higher concentration on the particle surface, whereas smaller particles remain predominantly in the center of the spray-dried aggregates. A similar theoretical dependency is presented in the more complex model of Seydel.10 However, the particles could be moved by convectional flow within the droplets and the one-radial symmetric equation is nonapplicable as could be observed for emulsions.35 Moreover, an increasing volume of the disperse phase as well as the reduction of the diffusion of nanoparticles (compared to macromolecules) leads to an internal void volume (Peclet numbers larger than 1). The surface moves faster than the dissolved components move driven by diffusional forces. RESULTS AND DISCUSSION For stabilized suspensions with high repulsive interaction forces and low adhesion, a dense particle packing on the aggregate surface is typically observed (see Figure 1).36 Figure 2 shows the formation of silica ZELLMER ET AL.

Figure 2. Model aggregate systems composed of two fractions of monodisperse silica nanoparticles with different weight contents (left, aggregates; right, aggregate surfaces in higher magnification).

model aggregates composed of two fractions of monodispersed silica nanoparticles (mean particle sizes of 100 and 650 nm, respectively) with different weight contents and without surface modification of the primary particles (stable bimodal particle size distribution is given in the Supporting Information, Figure 3). In the utilized dispersion, the silica particles are electrostatically stabilized, showing a ζ potential of around
50 mV (Supporting Information, Figure 2). Using a weight content of 40 wt % of fine nanoparticle fraction (100 nm), the coarse nanoparticle fraction (650 nm) is not visible at the aggregate surface. With decreasing weight content of the fine nanoparticle fraction the visible fraction of the coarse nanoparticles on the aggregate surface increases. Thus, the particle fraction segregation depends on different particle sizes during the spray drying process. In contrast to the expected theoretical considerations resulting from eq 1, the smaller particle size fraction shows a strong increase and the coarse fraction shows a VOL. XXX

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Figure 3. Cross-section images of aggregates composed of two fractions of monodisperse silica nanoparticles with different weight contents and surface modifications (TODS and E3MS).

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strong decrease in their respective concentrations at the surface of the aggregate. Moreover, the mean particle size of the visible coarse primary particles at the aggregate surface at low weight contents of the finer nanoparticles seems to be smaller than 650 nm. On one hand, the silica nanoparticle fractions are produced with a certain particle size distribution, which are also segregated within the particle fraction itself leading to an increased surface concentration of the smaller nanoparticles of the coarse fraction. On the other hand, the cross-section of the coarse nanoparticles is possibly masked by the smaller ones. As expected, this leads to significant differences in the resulting aggregate pore size distribution and porosity (exemplarily shown for various mixtures in the Supporting Information, Figure 7). This distribution of the two nanoparticle fractions as a function of the secondary particle radius is depicted in Figure 3. The cross-section of aggregates composed of two fractions of monodisperse silica nanoparticles with different weight contents and surface modifications is presented. Due to the radial distribution of the two differently sized silica fractions, the driving forces for segregation seems to be independent of the surface modification as long as the suspension is ideally stabilized and the adhesive forces of particles in contact during the second drying period are low. Segregation occurs where the nanosuspension is highly stabilized, e.g. using similar surface charges. In Figure 3 (right) a radial distribution within the coarse nanoparticle fraction can also be distinguished. For this reason, particle sizes visible at the surface of secondary particles in the scanning electron microscopy pictures (SEM) are usually not representative of the mean primary particle size within spray-dried aggregates. Figure 4 shows multicomponent aggregates, each composed of silica, zirconia, or iron oxide nanoparticles, prepared by mixing a dispersion of monodisperse silica nanoparticles with zirconia or iron oxide nanoparticles with different particle sizes. While the size of the ZrO2/Fe2O3 nanoparticles was kept constant, for the SiO2 nanoparticles both the fine (100 nm) and the coarse (650 nm) systems were used. For a highly stabilized nanosuspension, the segregation effects seem to be independent of the particle material but rather dependent on their disperse properties, the particle interactions, as well as on the resultant suspension viscosity. According to these results, new tailor-made nanostructures can be produced with defined structure properties. As an example, Figure 5 shows porous silica secondary particles with various primary particle sizes and contents of spherical pores. The aggregates were produced via spray drying of silica and polystyrene nanoparticles (DR 3945, Dow Chemical Co.). Consequently, using a subsequent tempering step at 400 °C (carbon coating on the silica particle surface) or 700 °C (complete removal of polymer material) the polystyrene

Figure 4. Multicomponent aggregates composed of one type of monodisperse silica nanoparticles (small and large) as well as zirconia or iron oxide nanoparticles.

Figure 5. Porous silica aggregates with two different primary particle sizes and spherical pores introduced by incorporation and removal of polystyrene nanoparticles with different particle sizes.

particles were removed. A similar approach was used by Arutanti et al.13 Depending on the weight content of polystyrene particles and the size ratio of the polystyrene and silica nanoparticles a variable porosity on the particle surface and within the aggregates can be achieved (corresponding porosities and pore size distributions are given in the Supporting Information, Figure 8). In addition to the experimental investigations on the formation of nanostructured aggregates during the spray-drying process, numerical simulations to understand the mechanism of structure formation and particle segregation during spray drying were used. For that purpose, the discrete element method (DEM) was applied,37 which is appropriate for the VOL. XXX

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ARTICLE Figure 6. (Left) Repulsive particle interactions within the fluid phase for the different silica particle sizes. (Middle) Segregation during the DEM simulation for different ratios of 100 and 650 nm silica nanoparticles with a total mass fraction of 5 wt %. (Right) Corresponding aggregate surface after spray drying at similar ratios of 100 and 650 nm and a total mass fraction of 5 wt %.

simulation of particulate systems and describes the particles as autonomous individuals of spherical shape that interact with each other via particle
particle contact forces. The diffusional flux away from the droplet surface caused by the increasing solute/ particle concentration with receding droplet surface is considered in the DEM simulation; hence, an undirected diffusion force acting on each particle was realized. The particle concentration was very low, and the particles were ideal electrostatically stabilized within the suspension. Hence, the flux of the droplet surface due to the surface activity was not considered in the simulation. A summary of various contact force models as well as an overview on the simulation of aggregates using DEM is given in a previous study.38 In general, the influence of the fluid flow field on the motion of the particles and the motion of the particles on the fluid flow field has to be taken into account. However, in the case of spray drying only small internal flow is expected as the primary driving force of granule formation out of the suspension droplets is the receding droplet surface. Therefore, a simplified coupling approach with a prescribed analytic description of the fluid phase similar to Breinlinger and Kraft39 was used. For the simulation of the spray-drying process and resultant structures of the aggregates as shown in Figure 2, the formation of very small aggregates was simulated at similar ratios and particle sizes and total mass contents of 5 wt %. In general, the simulation of the formation of the whole aggregate is limited by the number of particles. Hence, the simulated droplet size has to be reduced and only the simulation of smaller aggregates could be realized. However, according to the SEM pictures similar aggregate structures almost independent of the aggregate sizes were obtained in the experiments. ZELLMER ET AL.

Figure 7. (Top) Segregation during the DEM simulation for 60 nm (15 wt %) and 650 nm (85 wt %) silica nanoparticles with a total volume fraction of 30 vol %. (Bottom) aggregate surface in higher magnification after spray drying of 60 nm (15 wt %) and 650 nm (85 wt %) silica nanoparticles with a total volume fraction of 30 vol %.

For the particle
particle interactions, the experimentally determined interaction potentials (Figure 6, left; ζ potential measured via DT 1200 from Dispersion Technologies; see also Figure 2 of the Supporting Information) were used to fit a classic repulsive DEM contact model.40 For stabilized suspensions, this technique allows reasonable simulation time steps and has been shown to reproduce accurate results in other applications of microscopic drying simulations,39 without the explicit incorporation of the entire extended version of the DLVO-theory.41
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ARTICLE Figure 8. Aggregate structure building kit for the spray drying fabrication of different hierarchical structures from stabilized nanosuspensions.

Due to the short drying times and the small primary particle sizes, gravity was not considered within the DEM model. In this work, only the constant rate period of droplet drying was considered, which corresponds to a constant velocity of the receding surface. The particle
surface interaction force was modeled using the capillary force given by Breinlinger et al.36 The repulsive particle interactions within the fluid phase as well as the segregation during the simulation are presented in Figure 6 (left, repulsive particle interaction potential; middle, segregation of the resultant aggregates after the DEM simulation; right, corresponding aggregate surfaces at similar mass ratios and particle sizes). The simulation results show a clear trend for the segregation of small particle fractions at the aggregate surface. The aggregate surface is ZELLMER ET AL.

largely covered by small particles due to the Brownian motion during simulation. Both droplets and granules show approximately rotational symmetry for the simulation of the aggregate formation at high volume fractions (up to 30 vol %). Thus, the simulation of only a column of the drying suspension, which resembles a radial segment of the droplet close to the surface as seen in the top of Figure 7, was considered. Similar to the results shown at small solids content, the aggregate surface is largely covered by small particles as seen in Figure 7 and the average particle size in the outer regions is reduced. For instance, in a zone the thickness of approximately one large particle the average particle volume is about 52% smaller than the global average, while in the inner region it is about 138% larger, which corresponds to a VOL. XXX

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CONCLUSIONS In summary, a hierarchical structure formation and segregation of nanoparticles can be observed during the spray-drying process of composite systems from mixed nanoparticles dispersions. The predominant mechanism for the segregation of the particles depends on the size and specifically on the size ratio of the primary particles. For electrostatically stabilized colloidal particle dispersions, this structure formation is independent of the surface modification and the material as long as the different components show high repulsion by similar surface charge. Since

MATERIALS AND METHODS In this study, almost monodisperse silica nanoparticles of different particle sizes between 60 and 650 nm were produced via the Stöber synthesis46 by varying the ammonium hydroxide concentration and purified according to Jaeger.47 The monodisperse zirconia nanoparticles of 5 nm and iron oxide nanoparticles of 8 nm were synthesized via the nonaqueous synthesis (zirconia: Zr(IV) n-propoxide in 70 wt % 1-propanol in benzyl alcohol and iron oxide: iron(III) acetylacetonate in triethylene glycol) according to our previous reports (DLS measurements and TEM images are shown in the Supporting Information, Figure 4).48
50 The particle sizes were characterized via dynamic light scattering (Zetasizer Nano ZS, Malvern, DLS measurements are shown in the Supporting Information, Figure 1). Furthermore, the silica particles were modified using ethoxytrimethylsilane (E3MS) and 2-[2-(2-methoxyethoxy)ethoxy]acetic acid (TODS) in water. The ZrO2 nanoparticles were stabilized with TODS in water. Due to the hydrophilic reaction medium TEG used for the synthesis of iron oxide nanoparticles, the particles are stable without the addition of any stabilizer molecules in water. The model aggregates were produced via the spray-drying process. Instead, nanoparticle dispersions with a solid content of 5 wt % were spray-dried at a drying temperature of 100 °C and separated with a cyclone (spray dryer 2M8-Trix system, ProCepT Inc.). The values for the solids contents of the multicomponents mixtures are related to the total solids contents of the nanosuspensions of 5 wt %. The average aggregate sizes for all investigated systems were measured in the range of 10 μm via laser diffraction (Helos, Sympatec). Furthermore, scanning electron microscopy (SEM, LEO 1550, Zeiss) was used to characterize the internal structure of the secondary particles. As an example, the spray-dried silica aggregates with mean primary particle sizes of 200 and 650 nm are shown in Figure 1.

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in industrial processes nanoparticles with a certain particle size distribution are typically produced and then spray-dried to obtain granular products for better handling, segregation within the secondary particles always takes place. Hence, mean particle sizes, measured via scanning electron microscopy, are usually not representative for the mean primary particle size of spray-dried nanosupensions. However, by an understanding and control of the segregation phenomena during the spray drying process, new tailor-made nanostructures can be produced with defined properties (e.g., in regard to the structure, controlled release, catalytic effects, chromatographic properties, densities, etc.). In Figure 8, various possible structures which can be realized via spray drying of well-stabilized nanosuspensions are shown. A first theoretical modeling of this segregation and structure formation of spherical particles with different particle size distributions via spray drying was presented using a discrete element method simulation. The main reason for the structure formation of insoluble particles during spray drying is the diffusion of particles, which increases linearly with decreasing particle sizes according to the Stokes
Einstein equation. Taking various charged nano- and microparticle components into consideration, an even more promising building kit for a hierarchical structure formation via the spray drying processes can be realized.

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concentration of small particles in the surface region. Comparing the simulation results in Figure 7 with the experimental image, it can be seen that they are in good qualitative agreement as both show a tendency for the segregation of small particles at the aggregate surface at high solids content as well. As a result of the simulation, the increasing concentration of small particles on the aggregate surface is caused by the increasing diffusion with decreasing particle size similar to the granular convection/kinetic segregation effects of bulk solids. This segregation effect during spray drying is less pronounced for larger particles, e.g. in the micrometer size range.45 The dense packing of the nanoparticles can be explained by the repulsive electrostatic interaction potential.

The operating parameters were set constant in all experiments. Moreover, no substantial change in the suspension viscosity was observed since the nanoparticle volume fraction was very low. Thus, a similar initial droplet size distribution at constant operating parameters was guaranteed. For comparison, the mean aggregate sizes at various operating parameters are shown additionally (Supporting Information, Table 1). Conflict of Interest: The authors declare no competing financial interest. Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b05220. Figures showing particle size distributions, TEM/SEM pictures of nanoparticles, ζ ptoential, effect of process parameters on aggregate sizes, and aggregate porosities (PDF) Acknowledgment. The SEM pictures were kindly taken by the Peter Pfeiffer, Institute of Material Science, TU Braunschweig.

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