Chemistry of iron nitrate-based precursor solutions for spray-flame synthesis.

Understanding the chemistry of iron-based metal-organic precursor solutions for spray-flame synthesis is a key step to developing inexpensive and large scale applications for gas-phase synthesized, nano-sized iron oxide particles. Owing to the large variety of available organic solvents and iron compounds, the choice of a suitable precursor-solvent pair is challenging. Systematic investigations of the precursor chemistry of iron-based systems are currently not available. This work aims at filling this gap by providing a detailed spectroscopic analysis of mixtures containing iron(iii) nitrate nonahydrate and alkyl alcohols (C2-4). Moreover, the impact of adding 2-ethylhexanoic acid is explored. The FTIR spectra reveal the formation of carboxylates and allow deriving information about the coordination of the metal-carboxylate complexes. The stability of the precursor solutions is investigated by monitoring precipitation phenomena and turbidity. Furthermore, gas chromatography is employed to provide additional information on oxidation products and esters as well as to aid the interpretation of the FTIR data. It is found that the formation of esters has an enhancing effect on iron sorption and, thus, it promotes precursor stability.

Cysteines in ß-lactoglobulin affects its interfacial adsorption and protein film stabilization.

HYPOTHESIS: Disulfide bonds in proteins are strong chemical bonds forming the secondary and tertiary structure like in the dairy protein ß-lactoglobulin. We hypothesize that the partial or complete removal of disulfide bonds affects the structural rearrangement of proteins caused by intra- and intermolecular interactions that in turn define the interfacial activity of proteins at oil/water interfaces. The experimental and numerical investigations contribute to the mechanistic understanding of the structure-function relationship, especially for the interfacial adsorption behavior of proteins. EXPERIMENTAL AND NUMERICAL: Systematically, the 5 cysteines of ß-lactoglobulin were recombinantly exchanged by alanine. First, the protein structure of the variants in bulk was analyzed with Fourier-transform-infrared-spectroscopy and molecular dynamic simulations. Second, the structural changes after adsorption to the interface have been also analyzed by molecular dynamic simulations. The adsorption behavior was investigated by pendant drop analysis and the interfacial film properties by dilatational rheology. FINDINGS: The structural flexibility of ß-lactoglobulin with no cysteines encourages its unfolding at the interface, and accelerates the interfacial protein film formation that results in more visco-elastic films in comparison to the reference.

Characterization of ß-lactoglobulin adsorption on silica membrane pore surfaces and its impact on membrane emulsification processes.

Protein adsorption plays a key role in membrane fouling in liquid processing, but the specific underlying molecular mechanisms of ß-lactoglobulin adsorption on ceramic silica surfaces in premix membrane emulsification have not been investigated yet. In this study, we aimed to elucidate the ß-lactoglobulin adsorption and its effect on the premix membrane emulsification of ß-lactoglobulin-stabilized oil-in-water emulsions. In particular, the conformation, molecular interactions, layer thickness, surface energy of the adsorbed ß-lactoglobulin and resulting droplet size distribution are investigated in relation to the solvent properties (aggregation state of ß-lactoglobulin) and the treatment of the silica surface (hydrophilization). The ß-lactoglobulin adsorption is driven by attractive electrostatic interactions between positively charged amino acid residues, i.e., lysin and negatively charged silanol groups, and is stabilized by hydrophobic interactions. The strong negative charges of the treated silica surfaces result in a high apparent layer thickness of ß-lactoglobulin. Although the conformation of the adsorbed ß-lactoglobulin layer varies with membrane treatment and the solvent properties, the ß-lactoglobulin adsorption offsets the effect of hydrophilization of the membrane so that the surface energies after ß-lactoglobulin adsorption are comparable. The resulting droplet size distribution of oil-in-water emulsions produced by premix membrane emulsification are similar for treated and untreated silica surfaces.

Protein conformational changes at the oil/water-interface induced by premix membrane emulsification.

We present combined experimental and modelling evidence that ß-lactoglobulin proteins employed as stabilizers of oil/water emulsions undergo minor but significant conformational changes during premix membrane emulsification processes. Circular Dichroism spectroscopy and Molecular Dynamics simulations reveal that the native protein structure is preserved as a metastable state after adsorption at stress-free oil/water interfaces. However, the shear stress applied to the oil droplets during their fragmentation in narrow membrane pores causes a transition into a more stable, partially unfolded interfacial state. The protein's ß-sheet content is reduced by up to 8% in a way that is largely independent of the pressure applied during emulsification, and is driven by an increase of contacts between the oil and hydrophobic residues at the expense of structural order within the protein core.

Structure and adsorption behavior of high hydrostatic pressure-treated ß-lactoglobulin.

HYPOTHESIS: High hydrostatic pressure treatment causes structural changes in interfacial-active ß-lactoglobulin (ß-lg). We hypothesized that the pressure-induced structural changes affect the intra- and intermolecular interactions which determine the interfacial activity of ß-lg. The conducted experimental and numerical investigations could contribute to the mechanistic understanding of the adsorption behavior of proteins in food-related emulsions. EXPERIMENTS: We treated ß-lg in water at pH 7 with high hydrostatic pressures up to 600 MPa for 10 min at 20 °C. The secondary structure was characterized with Fourier-transform infrared spectroscopy (FTIR) and circular dichroism (CD), the surface hydrophobicity and charge with fluorescence-spectroscopy and ζ-potential, and the quaternary structure with membrane-osmometry, analytical ultracentrifugation (AUC) and mass spectrometry (MS). Experimental analyses were supported through molecular dynamic (MD) simulations. The adsorption behavior was investigated with pendant drop analysis. FINDINGS: MD simulation revealed a pressure-induced molten globule state of ß-lg, confirmed by an unfolding of ß-sheets with FTIR, a stabilization of α-helices with CD and loss in tertiary structure induced by an increase in surface hydrophobicity. Membrane-osmometry, AUC and MS indicated the formation of non-covalently linked dimers that migrated slower through the water phase, adsorbed more quickly due to hydrophobic interactions with the oil, and lowered the interfacial tension more strongly than reference ß-lg.

Engineering considerations on extrusion-based bioprinting: interactions of material behavior, mechanical forces and cells in the printing needle.

Systematic analysis of the extrusion process in 3D bioprinting is mandatory for process optimization concerning production speed, shape fidelity of the 3D construct and cell viability. In this study, we applied numerical and analytical modeling to describe the fluid flow inside the printing head based on a Herschel-Bulkley model. The presented analytical calculation method nicely reproduces the results of Computational Fluid Dynamics simulation concerning pressure drop over the printing head and maximal shear parameters at the outlet. An approach with dimensionless flow parameter enables the user to adapt rheological characteristics of a bioink, the printing pressure and needle diameter with regard to processing time, shear sensitivity of the integrated cells, shape fidelity and strand dimension. Bioinks consist of a blend of polymers and cells, which lead to a complex fluid behavior. In the present study, a bioink containing alginate, methylcellulose and agarose (AMA) was used as experimental model to compare the calculated with the experimental pressure gradient. With cultures of an immortalized human mesenchymal stem cell line and plant cells (basil) it was tested how cells influence the flow and how mechanical forces inside the printing needle affect cell viability. Influences on both sides increased with cell (aggregation) size as well as a less spherical shape. This study contributes to a systematic description of the extrusion-based bioprinting process and introduces a general strategy for process design, transferable to other bioinks.

Conformational state and charge determine the interfacial stabilization process of beta-lactoglobulin at preoccupied interfaces.

Amphiphilic properties enable proteins like ß-lactoglobulin to stabilize oil/water-interfaces and provide stability in food-related emulsions. During emulsification, the protein undergoes three stages: (I) migration through bulk phase, (II) adsorption, and (III) interfacial rearrangement at the oil/water-interface - the kinetics of which require further research. Therefore, the aim of our study was the analytical and computational investigation of stage (I) and (II) as a function of the interfacial preoccupation, conformational state and charge of ß-lactoglobulin. For this purpose, the adsorption of ß-lactoglobulin (at pH 7, pH 7 containing 0.1 M NaCl, and pH 9) at increasingly preoccupied oil/water-interfaces has been compared through measuring interfacial tension and ζ-potential and through running molecular dynamics simulations. With increasing interfacial preoccupation, (I) the migration via lag time increased and (II) the adsorption rate decreased. The (II) adsorption rate was highest for ß-lactoglobulin containing NaCl, due to dense packing and electrostatic screening. ß-lactoglobulin at pH 7 reached a lower adsorption rate than the more negatively charged ß-lactoglobulin at pH 9, due to exposure of hydrophobic regions that had a greater effect on adsorption rates than electrostatic repulsion. Our research contributes to a profound understanding of the interfacial stabilization mechanism of proteins at oil/water-interfaces, necessary to characterise and control emulsification processes.

Screening Precursor-Solvent Combinations for Li4Ti5O12 Energy Storage Material Using Flame Spray Pyrolysis.

The development and industrial application of advanced lithium based energy-storage materials are directly related to the innovative production techniques and the usage of inexpensive precursor materials. Flame spray pyrolysis (FSP) is a promising technique that overcomes the challenges in the production processes such as scalability, process control, material versatility, and cost. In the present study, phase pure anode material Li4Ti5O12 (LTO) was designed using FSP via extensive systematic screening of lithium and titanium precursors dissolved in five different organic solvents. The effect of precursor and solvent parameters such as chemical reactivity, boiling point, and combustion enthalpy on the particle formation either via gas-to-particle (evaporation/nucleation/growth) or via droplet-to-particle (precipitation/incomplete evaporation) is discussed. The presence of carboxylic acid in the precursor solution resulted in pure (>95 mass %) and homogeneous LTO nanoparticles of size 4-9 nm, attributed to two reasons: (1) stabilization of water sensitive metal alkoxides precursor and (2) formation of volatile carboxylates from lithium nitrate evidenced by attenuated total reflection Fourier transform infrared spectroscopy and single droplet combustion experiments. In contrast, the absence of carboxylic acids resulted in larger inhomogeneous crystalline titanium dioxide (TiO2) particles with significant reduction of LTO content as low as ∼34 mass %. In-depth particle characterization was performed using X-ray diffraction with Rietveld refinement, thermogravimetric analysis coupled with differential scanning calorimetry and mass spectrometry, gas adsorption, and vibrational spectroscopy. High-resolution transmission electron microscopy of the LTO product revealed excellent quality of the particles obtained at high temperature. In addition, high rate capability and efficient charge reversibility of LTO nanoparticles demonstrate the vast potential of inexpensive gas-phase synthesis for energy-storage materials.

Dynamic forces on agglomerated particles caused by high-intensity ultrasound.

In this paper the acoustic forces on particles and agglomerates caused by high-intensity ultrasound in gaseous atmosphere are derived by means of computational fluid dynamics (CFD). Sound induced forces cause an oscillating stress scenario where the primary particles of an agglomerate are alternatingly pressed together and torn apart with the frequency of the applied wave. A comparison of the calculated acoustic forces with respect to the inter particle adhesion forces from Van-der-Waals and liquid bridge interactions reveals that the separation forces may reach the same order of magnitude for 80 µm sized SiO2-particles. Hence, with finite probability acoustically agitated gases may de-agglomerate/disperse solid agglomerate structures. This effect is confirmed by dispersion experiments in an acoustic particle levitation setup.