Bemerkung zu einer Arbeit des Herrn Müntz

Ohne Zusammenfassung     

A Study of Nanoparticle Aerosol Charging by Monte Carlo Simulations

A Direct Simulation Monte Carlo (DSMC) technique is applied for describing the dynamics of aerosol charging. The method is based on the transformation of known combination coefficients into charging probabilities. Changes in the particle charge distribution are computed as a stochastic game, calculating the time-step after each event. The simulations are validated by comparison with analytical solutions for unipolar aerosol diffusion charging and aerosol photocharging. The advantage of the DSMC method lies in the uncomplicated simulation of multi-dimensional systems that would result in very elaborate population balances. The DSMC method is used for simulation of the photocharging of moderately concentrated bicomponent polydisperse aerosols. By means of this method, the influence of the particle parameters (size, material) on the dynamics of the charge distribution in different size and material fractions has been studied. It is shown that charge separation between size or material fractions can be achieved for aerosol components with dissimilar work functions, while the total aerosol charge is zero.     

Influence of external force on properties and reactivity of disulfide bonds

The mechanochemistry of the disulfide bridge--that is, the influence of an externally applied force on the reactivity of the sulfur-sulfur bond--is investigated by unrestricted Kohn-Sham theory. Specifically, we apply the COGEF (constrained geometry simulates external force) approach to characterize the mechanochemistry of the disulfide bond in three different chemical environments: dimethyl disulfide, cystine, and a 102-atom model of the I27 domain in the titin protein. Furthermore, the mechanism of the thiol-disulfide reduction reaction under the effect of an external force is investigated by considering the COGEF potential for the adduct and transition-state clusters. With the unrestricted Becke-three-parameter-Lee-Yang-Parr (UB3LYP) exchange-correlation functional in the 6-311++G(3df,3pd) orbital basis, the rupture force of dimethyl disulfide is 3.8 nN at a disulfide bond elongation of 35 pm. The interaction with neighboring groups and the effect of conformational rigidity of the protein environment have little influence on the mechanochemical characteristics. Upon stretching, we make the following observations: the diradical character of the disulfide bridge increases; the energy difference between the singlet ground state and low-lying triplet state decreases; and the disulfide reduction is promoted by an external force in the range 0.1-0.4 nN. Our model of the interplay between force and reaction mechanism is in qualitative agreement with experimental observations.     

Ti-STT: a new zeotype shape selective oxidation catalyst

A new zeotype titanium silicate oxidation catalyst with the STT topology has been synthesized from direct synthesis. Ti-STT has a microporous structure with small pore openings, allowing shape selective oxidation catalysis. The isomorphous substitution of Si by Ti in the framework has been confirmed by Raman, FT-IR, UV-VIS and XANES spectroscopies.     

Identification of primary free radicals in trehalose dihydrate single crystals X-irradiated at 10 K

Primary free radical formation in trehalose dihydrate single crystals X-irradiated at 10 K was investigated at the same temperature using X-band Electron Paramagnetic Resonance (EPR), Electron Nuclear Double Resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques. The ENDOR results allowed the unambiguous determination of six proton hyperfine coupling (HFC) tensors. Using the EIE technique, these HF interactions were assigned to three different radicals, labeled R1, R2 and R3. The anisotropy of the EPR and EIE spectra indicated that R1 and R2 are alkyl radicals (i.e. carbon-centered) and R3 is an alkoxy radical (i.e. oxygen-centered). The EPR data also revealed the presence of an additional alkoxy radical species, labeled R4. Molecular modeling using periodic Density Functional Theory (DFT) calculations for simulating experimental data suggests that R1 and R2 are the hydrogen-abstracted alkyl species centered at C5' and C5, respectively, while the alkoxy radicals R3 and R4 have the unpaired electron localized mainly at O2 and O4'. Interestingly, the DFT study on R4 demonstrates that the trapping of a transferred proton can significantly influence the conformation of a deprotonated cation. Comparison of these results with those obtained from sucrose single crystals X-irradiated at 10 K indicates that the carbon situated next to the ring oxygen and connected to the CH(2)OH hydroxymethyl group is a better radical trapping site than other positions.     

Dynamic NMR microscopy measurement of the dynamics and flow partitioning of colloidal particles in a bifurcation

The flow and distribution of Newtonian, polymeric and colloid suspension fluids at low Reynolds numbers in bifurcations has importance in a wide range of disciplines, including microvascular physiology and microfluidic devices. A bifurcation consisting of circular capillaries laser etched into a hard polymer with inlet diameter 2.50 ± 0.01 mm, bifurcating to a small diameter outlet of 0.76 ± 0.01 mm and a large diameter outlet of 1.25 ± 0.01 mm is examined. Four distinct fluids (water, 0.25%wt xanthan gum, 8 and 22%vol hard-sphere colloidal suspensions) are flowed at flow rates from 10 to 30 ml/h corresponding to Reynolds numbers based on the entry flow from 0.001 to 8. PGSE NMR techniques are applied to obtain dynamic images of the fluids inside the bifurcation with spatial resolution of 59 × 59 μm/pixel in plane over a 200-μm-thick slice. Velocity in all three spatial directions is examined to determine the impact of secondary flows and characterize the transport in the bifurcation. The velocity data provide direct measurement of the volumetric distribution of the flow between the two channels as a function of flow rate. Water and the 8% colloidal suspension show a constant distribution with increasing flow rate, the xanthan gum shows an increase in fluid going into the larger outlet with higher flow rate, and the 22% colloidal suspension shows a decrease in fluid entering the larger channel with higher flow rate. For the colloidal particle flow, the distribution of colloid particles down the capillary is determined by examining the spectrally resolved propagator for the oil inside the core–shell particles in a direction perpendicular to the axial flow. Using dynamic magnetic resonance microscopy, the potential for using magnetic resonance for “particle counting” in a microscale bifurcation is thus demonstrated.     

Electromembrane extraction from biological fluids

Electro-assisted extraction of ionic drugs from biological fluids through a supported liquid membrane (SLM) and into an aqueous acceptor solution was recently introduced as a new sample preparation technique termed electromembrane extraction (EME). The applied electrical potential across the SLM has typically been in the range of 1-300 V. Successful extractions have been demonstrated even with common batteries (9 V) instead of a power supply. The chemical composition of the SLM has been crucial for the selectivity and for the recoveries of the extraction. Compared to other liquid-phase microextraction techniques (LPME), extraction times have been reduced by a factor of 6-17, and successful extractions have been obtained at extraction times of 1-5 min, and even down to a few seconds with online microfluidic EME devices. The technique has provided very efficient sample clean-up and has been found well suited for the extraction of sample sizes in the low μL range. Extractions have been performed with both rod-shaped hydrophobic porous fibers and with flat hydrophobic porous sheets as SLM support. The technique has been successfully downscaled into the micro-chip format. The nature of the SLM has been tuned for extraction of drugs with different polarity allowing extractions to be tailored for specific applications depending on the analyte of interest. The technique has been found to be compatible with a wide range of biological fluids and extraction of drugs directly from untreated human plasma and whole blood has been demonstrated. EME selectively extracts the compounds from the complex biological sample matrix as well as allowing concentration of the drugs. With home-built equipment fully acceptable validation results have been obtained.     

High order numerical approximation of minimal surfaces

We present an algorithm for finding high order numerical approximations of minimal surfaces with a fixed boundary. The algorithm employs parametrization by high order polynomials and a linearization of the weak formulation of the Laplace–Beltrami operator to arrive at an iterative procedure to evolve from a given initial surface to the final minimal surface. For the steady state solution we measure the approximation error in a few cases where the exact solution is known. In the framework of parametric interpolation, the choice of interpolation points (mesh nodes) is directly affecting the approximation error, and we discuss how to best update the mesh on the evolutionary surface such that the parametrization remains smooth. In our test cases we may achieve exponential convergence in the approximation of the minimal surface as the polynomial degree increases, but the rate of convergence greatly differs with different choices of mesh update algorithms. The present work is also of relevance to high order numerical approximation of fluid flow problems involving free surfaces.     

Drop-to-drop microextraction across a supported liquid membrane by an electrical field under stagnant conditions

Electromembrane extraction (EME) of basic drugs from 10 μL sample volumes was performed through an organic solvent (2-nitrophenyl octyl ether) immobilized as a supported liquid membrane (SLM) in the pores of a flat polypropylene membrane (25 μm thickness), and into 10 μL 10 mM HCl as the acceptor solution. The driving force for the extractions was 3–20 V d.c. potential sustained over the SLM. The influence of the membrane thickness, extraction time, and voltage was investigated, and a theory for the extraction kinetics is proposed. Pethidine, nortriptyline, methadone, haloperidol, and loperamide were extracted from pure water samples with recoveries ranging between 33% and 47% after only 5 min of operation under totally stagnant conditions. The extraction system was compatible with human urine and plasma samples and provided very efficient sample pretreatment, as acidic, neutral, and polar substances with no distribution into the organic SLM were not extracted across the membrane. Evaluation was performed for human urine, providing linearity in the range 1–20 μg/mL, and repeatability (RSD) in average within 12%.     

Representation of molecular structure using quantum topology with inductive logic programming in structure–activity relationships

The requirement of aligning each individual molecule in a data set severely limits the type of molecules which can be analysed with traditional structure activity relationship (SAR) methods. A method which solves this problem by using relations between objects is inductive logic programming (ILP). Another advantage of this methodology is its ability to include background knowledge as 1st-order logic. However, previous molecular ILP representations have not been effective in describing the electronic structure of molecules. We present a more unified and comprehensive representation based on Richard Bader's quantum topological atoms in molecules (AIM) theory where critical points in the electron density are connected through a network. AIM theory provides a wealth of chemical information about individual atoms and their bond connections enabling a more flexible and chemically relevant representation. To obtain even more relevant rules with higher coverage, we apply manual postprocessing and interpretation of ILP rules. We have tested the usefulness of the new representation in SAR modelling on classifying compounds of low/high mutagenicity and on a set of factor Xa inhibitors of high and low affinity.