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Über dieses Buch

This book deals with colloidal systems in technical processes and the influence of colloidal systems by technical processes. It explores how new measurement capabilities can offer the potential for a dynamic development of scientific and engineering, and examines the origin of colloidal systems and its use for new products.

The future challenges to colloidal process engineering are the development of appropriate equipment and processes for the production and obtainment of multi-phase structures and energetic interactions in market-relevant quantities. The book explores the relevant processes and for controlled production and how they can be used across all scales.



Fundamentals and Modeling


Colloidal Particles in Thin Liquid Films

This chapter deals with the structuring of Silica nanoparticles in thin liquid films. In the first part of the chapter the particles are kept hydrophilic and their ordering under geometrical confinement in a thin liquid film is described. The thin film is formed between two solid surfaces in Colloidal Probe AFM. The effect of suspension parameters (particle concentration, particle size and salt concentration) and parameters of the outer surfaces (surface potential, roughness and elasticity) on the ordering are studied. In the second part of the chapter the same particles are hydrophobized with short chain amphiphiles. The partially hydrophobic nanoparticles are used to stabilize thin foam films in a Pickering foam. A multiscale approach from bulk solution via macroscopic foams, foam bubbles to the adsorption at the free water/air interface is presented in order to understand the stabilisation of Pickering foams.
Yan Zeng, Sebastian Schön, Adrian Carl, Regine von Klitzing

Colloidal Gels Formed by Dilute Aqueous Dispersions of Surfactant and Fatty Alcohol

Mixtures of surfactants, fatty alcohol as cosurfactant, and water often form gels, even at high dilution. We have investigated highly dilute samples of the system sodium dodecyl sulfate/cetyl alcohol/water (SDS/CA/D2O) at varying SDS/CA ratio. Gel-like samples are obtained only at low SDS/CA ratios. The phase structure and the dynamics of the molecules have been determined by a combination of proton and carbon-13 NMR spectroscopy, cryo-transmission electron microscopy, very-small-angle neutron and x-ray scattering, differential scanning calorimetry, rheology, and pulsed gradient spin echo NMR diffusometry. The gel-like character is found to be caused by jammed uni- and multilamellar vesicles.
Felix Grewe, Jochen Ortmeyer, Roxana Haase, Claudia Schmidt

Resolved Numerical Simulation of Particle Agglomeration

In numerous technical applications collisions between fine particles results in the formation of agglomerates, as for example in colloidal systems. The process of agglomerate formation is rather complex and depends on quite a number of interaction phenomena, such as particle collision rate, fluid dynamic interaction between colliding particles and molecular attraction as well as electrostatic repulsion of the particles. For analysing agglomeration in great detail and for predicting the resulting agglomerate structure the Lattice-Boltzmann method was extended and applied for simulating the motion of resolved primary particles exposed to various flow situations. The temporal variation of agglomerate morphology was characterised using the sphericity, the radius of gyration and the porosity of the convex hull. Two situations were considered where collision and agglomeration are induced by differential settling of a cluster of particles having a pre-scribed size distribution and primary particles are moving in a shear layer whereby also inter-particle collisions are being enforced. For both situations the importance of hydrodynamic interaction in the agglomeration process is emphasised.
M. Ernst, M. Sommerfeld

Small-Molecule Stabilization Mechanisms of Metal Oxide Nanoparticles

The stabilization of nanoparticles to prevent agglomeration is of great importance for their application. To achieve long-term stable particle dispersions that can be stored and processed, and to clarify stabilization mechanisms in detail, the stabilization of metal oxide nanoparticles with small molecules was investigated. Particularly, the adsorption of the stabilizer and thereby the dynamic and kinetic processes on the surface of the metal oxide nanoparticles are essential for the stabilization process. Within this project, particle-stabilizer-solvent-interactions for different particle systems, ITO and ZrO2, were described and influences of the chain length, the stabilizer concentration as well as the binding strength between stabilizer and surface were investigated and modeled. The developed model enables a prediction of the efficiency of the systems and about optimized combinations of stabilizer-particle-solvent systems.
S. Zellmer, C. Grote, T. A. Cheema, G. Garnweitner

Liquid Distribution and Structural Changes During Convective Drying of Gels

Experimental and three-dimensional numerical simulation studies on convective drying of gels are presented in this chapter. As a physical model of a real gel, highly porous particle aggregates are produced by sintering of glass beads inside a graphite mold. A lab-scale X-ray microtomograph is used to perform a series of drying experiments with loose packings of sintered glass beads (mean diameter 700 μm) initially saturated with water. The reconstructed images (voxel size 16 μm) are analyzed to obtain the time evolution of the solid, liquid, and gas phase distributions during convective drying. A computational tool based on the volume-of-fluid approach is developed to simulate the liquid distribution over time at the microscopic scale in this model particle aggregate, which is subjected to convective drying. The simulated liquid phase distributions are found to be in good qualitative agreement with the experimental results. The major physical effect of capillary flow from large pores into small pores is easily recognized: large pores dry out first while small regions of the void space stay saturated with liquid. In addition to these pore-scale studies, resorcinol-formaldehyde (RF) hydrogels are synthesized by sol-gel polycondensation of resorcinol (R) with formaldehyde in the presence of sodium carbonate as a catalyst (C). The mechanical effects (cracks and shrinkage) in RF gels with three different R/C ratios and three different aging times are studied. The results show that the degree of shrinkage drastically increases with decreasing R/C ratio and also that the degree of shrinkage is slightly reduced by longer aging.
Abdolreza Kharaghani, Christoph Kirsch, Thomas Metzger, Evangelos Tsotsas

Large Amplitude Oscillatory Shear Applications for the Characterization of Dispersed Systems

The mechanical properties of dispersed systems, such as suspensions, emulsions and foams, have been studied for many years by oscillatory shear experiments in the linear regime using material functions as for example the frequency dependent storage and loss modulus. In the context of oscillatory shear tests, the linear regime is defined as the range of strain amplitudes where both excitation and response wave signals are sinusoidal and their amplitudes are proportional to each other. In this regime the connection between the material functions and the dispersed system’s microstructure is well understood. However, dispersed systems are often processed or applied at conditions, where the linear regime is easily exceeded and the storage and loss moduli become insufficient to describe the material’s mechanical properties. In addition, the nonlinear regime opens up enhanced characterization possibilities. Consequently, experimental protocols for Large Amplitude Oscillatory Shear (LAOS) have been developed to investigate and quantify this nonlinear behavior. In the following chapter we present basic theoretical descriptions of LAOS experiments, address technical aspects of the technique for systems with low viscosity and discuss three applications to dispersed systems: First, LAOS experiments were used to modify the droplet morphology in a dilute polymer blend which gave information about the ratio of droplet radius to the interfacial tension in the system. Second, LAOS experiments were used to test the predictive capabilities of the schematic MCT model for dense colloidal suspensions under nonlinear deformation. Third, the yielding behavior of a colloidal nanoemulsion gel under oscillatory shear was investigated and the results were combined with a structural analysis using ultra-small angle neutron scattering. The two techniques allowed to propose a detailed microstructural mechanism for yielding of the gel and revealed that large scale inhomogeneities play a significant role for its mechanical properties.
D. Merger, K. Reinheimer, M. Grosso, J. M. Brader, M. Ballauff, J. Kim, M. E. Helgeson, M. Wilhelm

Colloidal Systems with Solid Disperse Phase


Simulating the Restructuring of Colloidal Aggregates

Controlling the structural properties of colloidal aggregates is an active research topic for solid-liquid separation in the food, mining, and wastewater industry, is important for colloidal crystal synthesis, and has attracted attention progressively in the pharmaceutical industry for drug delivery vehicles. For such colloidal processing, the investigation of the restructuring behavior of colloidal aggregates by means of numerical methods has been the research subject for many years, utilizing diverse models for interparticle as well as hydrodynamic interactions. First, proper interparticle force models are required to explain the stability in the simulation of restructuring aggregates. External experimental observations of tangential forces between bonded colloidal particles and the capability of these bonds to support bending moments are included in the proposed model. The resulting two particle contact model is set up with resistances against normal, sliding, and torsional displacement, as well as the newly introduced bending resistance. Second, we investigate two methods for the hydrodynamic interaction of a colloidal particle system in the zero-Reynolds-number regime, Stokesian dynamics and the finite element method. Calculating the long-ranged and many-body nature of hydrodynamic interactions in Stokes flow, Stokesian dynamics is very efficient, while the finite element method provides satisfactory precision of hydrodynamic forces on particles as well as the exact solution of a flow field in and around an aggregate. As the two methods complement each other for a comparison of particle dynamics, their application to restructuring aggregates is described, including numerical setup, origins of calculated particle drag forces, as well as applicability. Through coupling the models for hydrodynamic interactions and the particle contact, the restructuring of colloidal aggregates was finally investigated with the discrete element method. By means of simulating fractal aggregates suspended in shear flows, restructuring rates were studied by tracking mean structural parameters, i.e. the radius of gyration, now with the ability of investigating the full set of contact parameters to restructuring rates. Aggregate restructuring rates from simulations can be transferred to multi-scale formulations, such as involving population balance models, in order to improve the design and processing of colloidal systems.
Vincent Bürger, Eva Schlauch, Volker Becker, Ryohei Seto, Marek Behr, Heiko Briesen

Gelation, Fragmentation and Reorganization of Precipitated Silica

The polymerization process of silica involves gelation, fragmentation and reorganization. Each of these processes depends on the process parameters and strongly influences the solid formation as well as the particle properties (particle size distribution, structure or firmness). Process parameters like temperature, pH and ionic strength of solution as well as chemical composition and energy dissipation are crucial for the production of silica powder with particular specifications. Characteristic parameters for gelation, fragmentation and reorganization are, among others, the gelation time, gel firmness and fragment sizes in the case of stirred experiments. They depend in different degrees on the process parameters. Different orders of magnitude for gelation time (from a few seconds up to several hours) and gel firmness are obtained by varying the pH and chemical composition of the solution. Modifications in the solid structure appear during the reorganization process, resulting in volume decrease. Thus, the process of reorganization offers the possibility to modify the particle properties even after they have formed. A new method is suggested to reduce the time requirement for the process of reorganization that allows for a faster identification of relevant process parameters. An empirical model is proposed that correlates the data of slow and accelerated process of reorganization. This model predicts the reorganization of a model silica gel for various temperatures.
Sebastian Wilhelm, Matthias Kind

Synthesis, Structure and Mechanics of Nano-Particulate Aggregates

In the most industrial processes nano-sized particles aggregate during their synthesis and the subsequent drying step forming aggregates with sizes in the order of several micrometers. The properties of these aggregates for application or further processing are specified by particle characteristics such as morphology, size, size distribution, bonding mechanism and structure of primary and secondary particles. In this study, the effect of the process parameters during particle synthesis and the following drying step on the structure formation and the resultant product and processing characteristics of precipitated nano-structured silica aggregates were investigated. For this purpose, the educts concentrations, stabilizing additives, mechanical energy input, pH-value and precipitation temperatures were varied during the precipitation process. In addition to the structure formation during precipitation, the resultant micromechanical aggregate properties of spherical silica model aggregates with a well-defined aggregate structure were characterized via nanoindentation and related to the aggregate structure and the interparticulate interaction forces. The micromechanical properties of these model aggregates were modelled depending on their structure using a modified form of the elementary breaking stress model of Rumpf. Since the characterization of particle-particle interactions in the nanometer size range is hardly possible, this effect on the aggregate fracture and deformation behavior was investigated by simulating the nanoindentation measurement of single aggregates using the “discrete element method”.
Carsten Schilde, Arno Kwade

Aggregation and Deformation Induced Reorganisation of Colloidal Suspension

Various mechanisms can lead to colloidal aggregation. Attractive interactions being the most prominent amongst them. In this chapter we describe the synthesis of well defined colloids with a smooth or rough surface, their mechanical characterization, controlled aggregation and the study of the mechanical and structural properties of the colloids and the formed aggregates. Depending on the interplay between the properties of the single colloids, the interactions between the colloids and the structure formed by the colloids, the macroscopic response of the system can greatly change.
Günter K. Auernhammer, Doris Vollmer, Miao Wang, Marcel Roth, Maria D’Acunzi

Fluidization of Highly Concentrated Colloidal Dispersions by Tailoring of Attractive Interactions

Mode coupling theory (MCT) predicts fluid states of colloidal dispersions at particle volume fractions ϕ well above the hard sphere (HS) colloidal glass transition due to weak attractive interactions among particles. This opens a versatile, new route to manufacture highly concentrated, freely flowing dispersions with narrow particle size distribution. Our investigations are based on two model systems: polystyrene (PS)-microgel particles suspended in an isorefractive organic solvent and an aqueous polymer dispersion based on a well-stabilized, commercial polymer latex. Both systems exhibit hard sphere type flow behavior with a divergence of zero-shear viscosity at ϕ = 0.58. Suspensions were fluidized via addition of non-adsorbing polymers to the continuous phase, thus introducing weak depletion attraction among particles. The index-matched microgel system was used to study phase behavior as a function of particle and polymer concentration as well as polymer to particle size ratio and particle rigidity. A tight correlation between structural relaxation times from dynamic light scattering (DSL) experiments and rheological data was found. Fluid states were observed at particle loadings close to ϕ = 0.7 and a minimum viscosity has been achieved at polymer concentrations below the overlap concentration c*. Low viscosity values at particle loadings beyond ϕ = 0.58 could so far only be obtained for dispersions with bi- or multimodal particle size distribution. Flow curves obtained here for monomodal dispersions fluidized due to weak attractive interactions are similar to those of commercial dispersions with broad particle size distribution, demonstrating the competitive strength of the new concept. Sharply monodisperse aqueous polymer dispersions were used to demonstrate that beyond the predictions of MCT, also densely packed, crystalline suspensions can be fluidized upon adding small amounts of non-adsorbing polymer. A microfluidic flow channel attached to an inverted fluorescence microscope was used to study the true flow profiles of suspensions doped with size-matched fluorescent tracer particles. Reducing the range of weak depletion attraction by reducing the size of free, non-adsorbing polymer extended the fluidized region to even higher particle loadings of about ϕ = 0.72—in qualitative agreement with MCT predictions. However, an increase of the microgel crosslink density from 1:50 to 1:10 reduced the fluidized region significantly to about the effect observed with hard sphere-like PMMA dispersions. Particle softness and osmotic deswelling are discussed as possible origins of the exceptionally effective depletion fluidization in case of 1:50 crosslinked microgels. To enable similar studies combining DLS and rheology on model systems which are closer to aqueous, technical dispersions, perfluoroacrylate particles sterically stabilized with polyethylene-glycol (PEG) chains have been synthesized and characterized. A first study indicates that such dispersions can be refractive index matched in aqueous media and undergo a glass transition, thereby exposing dynamics in DLS which are quite analogous to that seen in so far studied model systems. The potential of fluidizing such dispersions at high particle loading by addition of free PEG or other depletants will be systematically explored in future work.
E. Bartsch, D. Burger, S. Burger, J. Gisin, R. Schneider, O. Thorwarth, J. Vesaratchanon, C. Weis, M. Wiemann, N. Willenbacher

Process Engineering of Nanoparticles Below 20 nm—A Fundamental Discussion of Characterization, Particle Formation, Stability and Post Processing

In the following chapter fundamental aspects that have to be considered during the processing of small nanoparticles will be addressed. We investigated quantum confined manganese doped ZnS, ZnO, PbS and PbSe semiconductor nanoparticles, so-called quantum dots (QDs) and silver noble metal nanorods. All materials were chosen due to their technical relevance for future applications in the emerging fields of solar cells, sensors and diagnostics as well as due to the possibility of their in situ characterization by UV/Vis absorbance spectroscopy. After a brief introduction to the specific prospects and challenges of these materials we will focus on the important processing issues that need to be solved for producing these particles at high quality on a larger scale: (i) the modelling of particle formation including nucleation, growth and ripening based on a mechanistic understanding and on experimentally derived data on solubility and surface energies, (ii) the stabilization of nanoparticles not only against agglomeration but also against shape changes and (iii) classification. The latter is realized by size selective precipitation which allows surprisingly sharp separations (κ = 0.75) of particles with only a few nm in diameter. Although the extremely small particle sizes (feed PSD between 1.5 and 3 nm), classification results were successfully analyzed by well-known concepts from particle technology. Our results are seen to be an essential contribution to colloidal processing. They enable a future optimization of process parameters by a knowledge-based design strategy that can be applied within continuous as well as automatized batch reactor concepts.
Doris Segets, Wolfgang Peukert

Colloidal Systems with Liquid Disperse Phase


Thermodynamic Models for the Adsorption of Alkyl Trimethyl Ammonium Bromides at the Water/Hexane Interface

Based on surface/interfacial tension isotherms measured for the homologous series of alkyl trimethyl ammonium bromides (CnTAB) using the drop profile analysis tensiometry the adsorption behavior at three different liquid-fluid interfaces is discussed: solution/air, solution/hexane vapor and solution/hexane bulk liquid. The adsorption behavior can be described by different models. In the presence of hexane molecules (as a bulk liquid or as vapor in the air phase) the adsorption of the CnTAB molecules can be best described by a competitive adsorption with hexane molecules. This competitive thermodynamic model can be applied successfully to all three interfaces.
N. Mucic, A. Javadi, J. Krägel, M. Karbaschi, E. V. Aksenenko, V. B. Fainerman, R. Miller

Filled Vesicles Formed by Phase Transfer of Emulsions or Microemulsions

Vesicles or liposomes are of great interest as drug delivery system or simple model for cell membranes. In biological environments vesicles are capable of transporting messenger molecules in high concentrations within a cell. For industrial applications, it is necessary to produce vesicles which are unilamellar, monodisperse, easy to adjust in size, and which can be filled with various types of active compounds. Particularly the defined filling of these tiny compartments has not yet been brought to a large scale. Our research project within the DFG-priority program 1273 (Colloid Process Engineering) was focused on a new method, which can easily be used for the continuous production of such colloidal particles. Moreover, the novel approach allows us to use a large variety of incorporated ingredients. The high encapsulation efficiency in addition with the flexible synthesis facilitates the utilization as a drug carrier system. On grounds of the interesting structure, consisting of an unilamellar surfactant shell, which is swollen with oil, and an enclosed aqueous reservoir (core), the produced colloidal particles may alternatively be denoted as a special case of water-in-water-emulsions. The synthesis of these particles occurred in three steps. First, a water phase was covered by an oil phase containing surfactants or lipids. A water-in-oil emulsion or microemulsion was then added to the oil phase. In the third step the phase transfer of aqueous droplets from the oil phase into the underlying water phase was stimulated by sedimentation, flow, electric forces or centrifugation processes. During this phase transition a small amount of the organic solvent was entrapped in the ultra-thin membranes and influenced the properties of the filled, vesicular structures. The thin layer of organic solvents reduced the diffusion processes from the core of the vesicles into the surrounding water phase. This might be of special advantage for the encapsulation of water soluble ingredients as drugs or other interesting compounds. It also offers the opportunity, to store oil soluble substances in the swollen membranes of the vesicles. On the other hand the thin oil layers surrounding the vesicles induced creaming processes and influenced the stability of these aggregates. For all applied experimental techniques we systematically measured the encapsulation capacity, the size of the filled vesicles, the amount of entrapped oil within the membranes and the stability of these aggregates. It turned out, that the jet-stream and the electrospray technique provided the best results concerning long-term stability, vesicle production and encapsulation efficiency. Due to the broad spectrum of different applications, we could use the phase-transfer process for the production of tailor-made, filled and swollen vesicles, which showed interesting properties.
Christian Strötges, Evelin Schmitte, Heinz Rehage

New Process Routes


Continuous Preparation of Polymer/Inorganic Composite Nanoparticles via Miniemulsion Polymerization

Composite nanostructured particles can be produced by polymerization of monomer miniemulsion droplets loaded with inorganic nanoparticles. The article gives an overview on the development of a scalable continuous process for the production of such hybrid nanoparticles via miniemulsion polymerization. Different possibilities for the necessary surface modification of the inorganic material are discussed in detail. Furthermore, the influence of the surfactant concentration on the droplet size after emulsification as well as on the nucleation mechanisms during polymerization is highlighted. Possible process routes for the emulsification of the nanoparticle-loaded monomer phase are compared taking into account different process and material parameters, such as energy consumption, abrasion, dispersed phase viscosity, inorganic particle load and size, and morphology of the resulting hybrid particles. The possibility of an industrial implementation via an integrated high pressure homogenization process and a subsequent continuous polymerization are presented.
Tobias Merkel, Lena L. Hecht, Alexander Schoth, Caroline Wagner, Rafael Muñoz-Espí, Katharina Landfester, Heike P. Schuchmann

Process Development of a Liquid-Liquid Phase Transfer of Colloidal Particles for Production of High-Quality Organosols

The emphasis of the study presented is on a new process of particle extraction to transfer magnetite nanoparticles from an aqueous into an immiscible organic phase directly through the liquid-liquid interface. For the production of high-quality organosols, stabilized colloidal and functionalized particles are required in a liquid organic phase. The mechanism of phase transfer is initiated by adsorption and chemical binding of surfactants (fatty acids) at the particle surface. The resulting physico-chemical dispersion of the hydrophobically modified particles leads to the formation of the stabilized organic colloid, or organosol. The aim here is to demonstrate the entire chain of the transfer process in a continuous miniplant, which comprises particle synthesis, conditioning, and transfer, and which uses a drop column for extraction and as a transfer device. Based on the investigation of the governing principles and the material parameters, the results obtained for the transfer kinetics in the individual contact devices (centrifuge, single-drop column, and drop column for different operations) are used for the dimensioning of the entire process chain.
Jacqueline V. Erler, Stefanie Machunsky, Steffen Franke, Philipp Grimm, Hans-Joachim Schmid, Urs A. Peuker
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