Showing papers by "Peter Schurtenberger published in 2011"

Cluster-Driven Dynamical Arrest in Concentrated Lysozyme Solutions

Abstract: We present a detailed experimental and numerical study of the structural and dynamical properties of salt-free lysozyme solutions. In particular, by combining small-angle X-ray scattering (SAXS) data with neutron spin echo (NSE) and rheology experiments, we are able to identify that an arrest transition takes place at intermediate densities, driven by the slowing down of the cluster motion. Using an effective pair potential among proteins, based on the combination of short-range attraction and long-range repulsion, we account remarkably well for the peculiar volume fraction dependence of the effective structure factor measured by SAXS. We show that a transition from a monomer to a cluster-dominated fluid happens at volume fractions larger than phi greater than or similar to 0.05 where the close agreement between NSE measurements and Brownian dynamics simulations confirms the transient nature of the clusters. Clusters even stay transient above the geometric percolation found in simulation at phi > 0.15, though NSE reveals a cluster lifetime that becomes increasingly large and indicates a divergence of the diffusivity at phi greater than or similar to 0.26. Macroscopic measurements of the viscosity confirm this transition where the long-lived-nature of the clusters is at the origin of the simultaneous dynamical arrest at all length scales. (Less)

The Largest Synthetic Structure with Molecular Precision: Towards a Molecular Object

Abstract: Pushing the limits: A 200A - 10 Da structurally defined, linear macromolecule (PG5) has a molar mass, cross-section dimension, and cylindrical shape that are comparable to some naturally occurring objects, such as amyloid fibrils or certain plant viruses. The macromolecule is resistant against flattening out on a surface; the picture shows PG5 embracing the tobacco mosaic virus (TMV).

The pH induced sol-gel transition in skim milk revisited. A detailed study using time-resolved light and x-ray scattering experiments.

Abstract: We present a detailed study of the evolution of the size, structure and stability of casein micelles upon acidification of skim milk typically applied in yogurt-making processes using a combination of time-resolved light and small-angle X-ray scattering experiments. While most of the available light scattering studies on casein acidification have been restricted to transparent and therefore highly diluted samples, we now profit from a newly developed multiangle 3D light scattering instrument, which allows for time-resolved measurements in highly turbid samples. Our experiments clearly demonstrate the presence of two parallel pH-dependent processes, micellar reassembly and aggregation. Using a systematic investigation of the effect of casein concentration, acidification rate, and ionic strength, we are able to decouple these two processes and obtain detailed information about the pH-induced restructuration of the casein micelle structure that occurs prior to destabilization. Moreover, our experiments also unambiguously demonstrate that these micellar reassembly processes are highly concentration dependent, and that typical light scattering studies conducted under highly diluted conditions are resulting in findings that may not be relevant for the situation encountered in industrial processes at higher concentrations. Experiments conducted with covalently cross-linked micelles, where the pH-induced reassembly has been suppressed, further confirm our findings.

Magnetic properties of silica coated spindle-type hematite particles

Abstract: Magnetic properties of particles are generally determined from randomly oriented ensembles and the influence of the particle orientation on the magnetic response is neglected. Here, we report on the magnetic characterization of anisotropic spindle-type hematite particles. The easy axis of magnetization is within the basal plane of hematite, which is oriented perpendicular to the spindle axis. Two standard synthesis routes are compared and the effects of silica coating and particle orientation on the magnetic properties are investigated. Depending on the synthesis route we find fundamentally different magnetic behavior compatible with either single domain particles or superparamagnetic sub-units. Furthermore, we show that silica coating reduces the mean blocking temperature to nearly room temperature. The mechanical stress induced by the silica coating appears to reduce the magnetic coupling between the sub-units.

Phase separation and dynamical arrest for particles interacting with mixed potentials—the case of globular proteins revisited

Abstract: We examine the applicability of the extended law of corresponding states (ELCS) to equilibrium and non equilibrium features of the state diagram of the globular protein lysozyme. We provide compelling evidence that the ELCS correctly reproduces the location of the binodal for different ionic strengths, but fails in describing the location of the arrest line. We subsequently use Mode Coupling Theory (MCT) to gain additional insight into the origin of these observations. We demonstrate that while the critical point and the connected binodal and spinodal are governed by the integral features of the interaction potential described by the normalized second virial coefficient, the arrest line is mainly determined by the attractive well depth or bond strength.

Hydrodynamic properties of magnetic nanoparticles with tunable shape anisotropy: prediction and experimental verification.

Abstract: We describe the characterization of the hydrodynamic properties of anisotropic magnetic nanoparticles using a combination of transmission electron microscopy (TEM) and dynamic as well as depolarized dynamic light scattering (DLS/DDLS). The particles used are nearly monodisperse hematite spindles with an average length of 280 nm and a minor axis of 57 nm, coated with a layer of silica of variable thickness that allows us to tune the particle aspect ratio between 5 and 2. Their geometrical dimensions can thus be determined easily and quantitatively from TEM. Moreover, their size is ideal to employ DLS and DDLS to measure the translational and rotational diffusion coefficients D-T and D-R, while the presence of a magnetic core opens a plethora of opportunities for future studies and applications. We demonstrate that we can successfully predict the hydrodynamic properties of the different particles based on a TEM characterization of their size distribution and using established theoretical models for the hydrodynamic properties of anisotropic particles. When compared with the theoretical predictions, our light scattering measurements are in quantitative agreement. This agreement between theory and experiment is achieved without having to invoke any adjustable free parameter, as the TEM results are used to calculate the corresponding diffusion coefficients on an absolute scale We demonstrate that this is achieved due to a new and simple method for the statistical weighting of the TEM information, and the use of the correct hydrodynamic models for the observed particle shape. In addition, we also demonstrate an enhanced sensitivity of the rotational diffusion for the surface properties of ellipsoidal nanoparticles, and point out that this may serve as an ideal tool toward characterizing functionalized surfaces. (Less)

Simple model for the growth behaviour of mixed lecithin–bile salt micelles

Abstract: Mixed lecithin–bile salt micelles are known to have a cylindrical or worm-like structure. We investigated their shape, length, flexibility and cross-sectional structure using small-angle neutron scattering (SANS). A broad range of sample compositions was studied varying both the total amphiphile concentration and the molar ratio of bile salt (sodium taurochenodeoxycholate, NaTCDC) to lecithin (egg yolk phosphatidylcholine, EYL). The length of the micelles was quantitatively linked to the micellar composition by introducing a simple model. The model takes into account the partitioning of lecithin and bile salt between the bulk, cylindrical parts and the end caps of the micelles. The model also sheds light on the organization of the micelles, both in their cylindrical regions and end caps.

Phase separation in binary eye lens protein mixtures

Abstract: Liquid–liquid phase separation occurs in young mammalian eye lenses and in concentrated solutions of isolated eye lens proteins, and has been linked to some forms of cataract. Here we study theoretically the protein compositions and cloud temperatures of two separated equilibrium phases that form out of concentrated mixtures of model proteins, chosen to have properties similar to those which reproduce experimental data on mixtures of two of the prevalent mammalian eye lens proteins, γ- and α-crystallin. We use a thermodynamic perturbation theory that has previously been shown to provide a quantitative model for key features of the experimentally observed neutron scattering, phase boundary and tie line data, and that is also consistent with corresponding model, coarse-grained molecular dynamics simulations. In so doing we find an extremely sensitive dependence of protein partitioning on mutual attraction that is likely to have implications for many other protein, colloid, and other soft condensed matter systems. Previously, we found that a model square well attraction between the proteins of well depth uαγ ≈ 0.5 kBT protects concentrated γ-α mixtures against thermodynamic instability and is thus essential for their transparency. Furthermore, the dependence of the mixture phase separation on uαγ was found to be highly non-monotonic, in that either weakening or increasing uαγ by 0.5 kBT can lead to considerably enhanced phase separation that occurs at much higher temperatures. In the present work we show that the compositions of the separated protein phases are even more dramatically sensitive to the magnitude of uαγ. Specifically, increasing uαγ by just 0.2 kBT can change the phase separation of α–γ mixtures from one that is primarily compositional in nature to one of protein density separation, in which the two phases in equilibrium differ principally in overall protein concentration. Further, for the square-well widths investigated, we find that the phase separation properties change relatively rapidly in response to changes in square well depth, in comparison with their response to changes in the diameter ratio of the model proteins. We discuss potential ways in which sensitive connections between changes in molecular attraction and their macroscopic consequences, a hallmark of concentrated liquid mixtures, can lead to potential molecular mechanisms for hereditary and other forms of cataract, and can be applied to other colloidal and physiological systems.

Single Step Hybrid Coating Process to Enhance the Electrosteric Stabilization of Inorganic Particles

Abstract: We report on a single-step coating process and the resulting colloidal stability of silica-coated spindle-type hematite nanoparticles (NPs) decorated with a layer of poly(acrylic acid) (PAA) polyelectrolyte chains that are partially incorporated into the silica shell. The stability of PAA coated NPs as a function of pH and salt concentration in water was compared to bare hematite particles and simple silica-coated hematite NPs, studying their electrophoretic mobility and the hydrodynamic radius by dynamic light scattering. Particles coated with this method were found to be more stable upon the addition of salt at pH 7, and their aggregation at the pH of the isoelectric point is reversible. The hybrid coating appears to increase the colloidal stability in aqueous media due to the combination of the decrease of the isoelectric point and the electrosteric stabilization. This coating method is not limited to hematite particles but can easily be adapted to any silica-coatable particle.

Catalytic Hydrogenation Using Abnormal N-Heterocyclic Carbene Palladium Complexes: Catalytic Scope and Mechanistic Insights

Abstract: Palladium complexes containing abnormally bound C4-bound dicarbene ligands have been exploited for catalytic alkene hydrogenation. Comparison to normally C2-bound homologues indicates that the carbene bonding mode critically influences the catalytic activity. Good catalytic performance in the hydrogenation of cis-disubstituted olefins and non-isomerizable terminal olefins under mild conditions (RT, 0.1 MPa H-2) only occurs when the carbene is abnormally bound to the palladium center. Detailed mechanistic investigations using dynamic light scattering in connection with time-dependent analysis of conversions, and also performance of substoichiometric catalytic experiments provide evidence that the catalysis is heterogeneous and that the abnormally bound carbene ligand has the role of an activator.

Work hardening of soft glassy materials, or a metallurgist’s view of peanut butter

Abstract: The non-linear rheology of colloidal gels, glasses, and pastes is rather complex. For instance, colloidal glasses and pastes yield at a certain strain, followed by shear thinning. A detailed understanding of the yielding point, and what causes it, is lacking. Here, we perform constant shear rate rheology on a pre-sheared material, smooth peanut butter, which is a glassy colloidal paste. Controlling the ageing appears to control the yield stress value, whose associated stress scales with the applied pre-shear with a power law of 1/2. This corresponds to the governing formula for the work hardening mechanism in metals, from which an analogy is drawn.