Thomas Kraska

Thomas Kraska
University of Cologne | UOC · Institute of Physical Chemistry

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157
Publications
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2,351
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Publications

Publications (157)
Chapter
Note: This is a chapter of a textbook. We recommend obtaining the complete book, because single chapters not contain the cited literature, cross references, or lists of sysmbols. ABSTRACT: Before discussing phase diagrams and methods for computing them it is necessary to introduce some thermodynamic concepts and functions. As this book does not fo...
Chapter
Note: This is a chapter of a textbook. We recommend obtaining the complete book, because single chapters not contain the cited literature, cross references, or lists of sysmbols. ABSTRACT: In this chapter, phase diagrams of binary fluid mixtures are described and discussed qualitatively (with the barest minimum of equations). It provides a short i...
Book
Fluid mixtures under pressure not only vapor–liquid phase equilibria, but also liquid–liquid and gas–gas equilibria can occur, and also transitions between these. The book explains the main classes of phase diagrams that have been defined for binary mixtures. It then proceeds to describe the thermodynamic equations underlying the phase equilibria a...
Article
An inquiry-based approach is proposed that allows students to develop and understand computer codes for simulations at the particle level. Computational thinking is employed as a low-threshold approach for students without programming experience. The resulting computer code cores are concise and traceable. Here, the topic of the simulations concern...
Article
Die molekulardynamische Simulation ist eine weit verbreitete Methode, die Einblicke in die submikroskopische Ebene von Vielteilchensystemen erlaubt und die Berechnung makroskopischer und struktureller Eigenschaften ermöglicht. Sie kann auch didaktisch hilfreich zur Vermittlung von Wechselwirkungen und den daraus resultierenden molekularen Prozessen...
Article
Während die Quantenchemie seit rund einhundert Jahren in der Fachwissenschaft das Standardmodell der chemischen Bindung darstellt, ist sie bislang nur eingeschränkt im Chemieunterricht angekommen. Insbesondere die Einführung der Welleneigenschaften der Materie wird oft vernachlässigt. Die Komplexität des Themas macht es erforderlich, die Grundlagen...
Article
Die Papierchromatographie ist ein einfaches und ungefährliches Experiment, das sich für die Unterstufe eignet. Neben der Vermittlung der fachlichen Inhalte ermöglicht die Papierchromatographie auch den Einsatz von Modellierung und Simulation, da sich die submikroskopischen Vorgänge so vereinfachen lassen, dass sie für Schülerinnen und Schüler am An...
Article
Der Beobachtung, dass Schülerinnen und Schüler häufig nicht den Beitrag anderer Fächer wie der angewandten Mathematik oder Informatik zur Chemie sehen, kann man versuchen entgegenzuwirken, indem man geeignete mathematische Modellierungen im Chemieunterricht einführt. Dabei kann man sich auf solche Themen fokussieren, die üblicherweise bereits Besta...
Article
Full-text available
The rapid advance in information technology requires further developments in all areas of education. In this context, one should think about going beyond the use of digital media for the mere presentation of scientific content. Interactive computer simulations allow quasi-experimental investigations of scientific phenomena but for students they usu...
Article
In der Schule, aber auch darüber hinaus werden die Fächer aus Sicht der Lernenden häufig isoliert gesehen. Demgegenüber sind in einem Fach auch immer Beiträge anderer Fächer erforderlich. Im Fach Chemie sind dies unter anderem Kenntnisse in angewandter Mathematik. Um möglichst früh den Bezug der Chemie zur Mathematik zu verdeutlichen, können bereit...
Article
Aus der heutigen Realität der Chemie ist die Quantenmechanik nicht mehr wegzudenken. Quantenmechanische Programmpakete ermöglichen Berechnungen in vielen Bereichen der Chemie sowie in den Material‐ und Ingenieurwissenschaften. Zweifellos ist die Quantenmechanik Teil der universitären Ausbildung, nichtsdestotrotz ist es möglich, ihre Grundzüge im Ch...
Article
Full-text available
For supercritical fluids there is a wedge-shaped region called Widom region, where several physico-chemical quantities (e.g. compressibility, heat capacities, density, thermal expansivity, speed of sound) show anomalous behaviour. In this paper, several Widom lines of supercritical CO2 have been computed with the Wagner–Span reference equation of s...
Conference Paper
Full-text available
Based on molecular dynamics simulation of the vapor-liquid interface, the classical thermodynamic spinodal for n-nonane is estimated. The determination of the spinodal is based on properties of the liquid vapor interface. This method relates the tangential pressure component through the vapor*liquid interface to the van derWaals loop in the twophas...
Article
Full-text available
Vapor-liquid nucleation in the binary system n-nonane/methane is investigated by molecular dynamics simulation. The supersaturation is achieved by cooling down the system during the expansion in order to closely mimic the real process. Binary clusters formed by nucleation are frequently inhomogeneous objects in which components are not well mixed....
Article
Full-text available
Interfacial pressure and density profiles are calculated from molecular dynamics and lattice Boltzmann simulations of a liquid film in equilibrium with its vapor. The set of local values of tangential pressure and density along an interface exhibits a van der Waals-type loop; starting from the stable vapor bulk phase one passes through metastable a...
Article
The limit of overheating or expanding is an important property of liquids, which is relevant for the design and safety assessment of processes involving pressurized liquids. In this work, the thermodynamic stability limit—the so-called spinodal—of water is calculated by molecular dynamics computer simulation, using the molecular potential model of...
Article
Full-text available
Based on molecular dynamics simulation of the vapor-liquid interface, the classical thermodynamic spinodal for n-nonane is estimated using an earlier developed method. The choice of n-nonane as investigated molecule originates from the question whether a deviation from the spherical symmetry of a molecule affects the prediction of the stability lim...
Article
Full-text available
Binary clusters formed by vapor-liquid nucleation are frequently nonhomogeneous objects in which components are not well mixed. The structure of a cluster plays an important role in nucleation and cluster growth. We demonstrate structuring effects by studying high-pressure nucleation and cluster growth in the n-nonane/methane mixture by means of m...
Article
Full-text available
Vapour pressure curves and stability lines can be extended beyond the critical points into the supercritical domain by so-called Widom lines, along which some thermodynamic property undergoes a rapid change and liquid-like behaviour turns to vapour-like one. Knowledge about such lines is therefore important for thermohydraulic calculations and de...
Article
Full-text available
We report results on nucleation, growth, and structure formation of methane/n-nonane clusters in an expanding system investigated by molecular dynamics simulation. From bulk phase equilibria data, it is expected that the concentration of the less volatile substance n-nonane in the clusters is very high. However, analyses of experimental data in the...
Book
In fluid (liquid, gas, and/or supercritical) mixtures at elevated pressures, the simultaneous occurence of vapour–liquid, liquid–liquid, and sometimes also various solid–fluid equilibria can lead to rather complicated phase behavior. The book begins with a qualitative, richly illustrated overview over the experimentally known fluid phase diagram cl...
Article
A united-atom potential model for naproxen suitable for molecular dynamics (MD) simulation has been developed. The charge distribution is approximated by point charges obtained from ab initio calculations using the CHELPG method. Also the intramolecular interactions such as bond and angle vibration, and the torsion potential are obtained from ab in...
Article
Vapor-liquid nucleation and growth kinetics of methanol clusters were investigated by molecular dynamics simulations. Supersaturated states were generated by temperature quenches of a stable gas phase. The initial methanol vapor phase density varied from 0.056 to 0.446 mol/dm3, the target temperature after the quench ranged from 250 K to 290 K. The...
Article
The structures of ion clusters have been investigated for the restricted primitive model in the low density limit. The calculations have been accomplished by a Monte Carlo integration over the probability function of the cluster configuration. The size of the clusters ranges from dimer to octamer. Properties related to the structure, such as the nu...
Article
The formation of pharmaceutical particles by the rapid expansion of a supercritical solution is investigated by molecular dynamics simulation. As a pharmaceutical model substance naproxen, a pain reliever and anti-inflammatory drug, is used. The expansion process is modeled in the simulation method by stepwise increasing the size of the simulation...
Article
The poor dissolution behaviour of solid drugs in biological environment leads to a low bioavailability. However, the dissolution rate of such drugs can be enhanced dramatically by reduction of the particle size. At present, supercritical fluid based particle size reduction processes are gaining in importance in pharmaceutical technology. For the de...
Article
Nuclear Engineering and Design j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / n u c e n g d e s a b s t r a c t During a loss of coolant accident (LOCA) the pressure of the coolant can drop significantly in the vicinity of the leak. It will be shown that unlike in pressurized water reactors (PWRs) where this pressur...
Conference Paper
With a recently developed method (J. Phys. Chem. C 113, 19028-19038, 2009) we study the formation of Naproxen particles by expansion of a supercritical solution (RESS) by means of molecular dynamics simulation. For this investigation we propose a new potential model for Naproxen. The employed van der Waals parameters are based on the TraPPE model....
Chapter
Full-text available
Condensed matters (liquids, glasses and solids) can be overheated or stretched only up to a limit. Within mean-field approximation, this limit is the so-called spinodal. This is the final limit for overheating, and therefore it is a very important quantity for safety calculations wherever high pressure- high temperature liquids are involved. In tem...
Article
The formation of naphthalene particles by expansion of a supercritical solution is investigated by molecular dynamics simulation. Supercritical carbon dioxide is chosen as solvent. A method for the successive expansion of the simulation box is developed which allows expanding the system very closely to the adiabatic curve obtained form a reference...
Article
Full-text available
The influence of the molar mass of a carrier gas on the formation of nanoparticles in the vapor phase is investigated. The function of the carrier gas atmosphere is the regulation of the particle temperature by collisions with the cluster surface. The aim of this work is to optimize the carrier gas in a simulation in order to mimic a large amount o...
Article
In the context of the investigation of particle formation, a potential model by means of the embedded atom method is developed for the hexagonal close packed metal zinc. This type of model includes many-body interactions caused by delocalised electrons in metals. The effective core charge as function of the distance is calculated here by an integra...
Article
Full-text available
Liquid−liquid miscibility in binary and ternary fluids has been studied to a great extent by Professor Schneider. One of the important results of his studies was the description of the so-called immiscibility holes or islands in type IV systems. In these holes, the liquid−liquid coexistence curve virtually disappears; i.e., it goes below the vapor...
Article
The limit of metastability, the so-called spinodal, is calculated for pure carbon dioxide by molecular dynamics simulation. The determination of the spinodal is based on properties of the liquid vapor interface using a recently developed method. This method relates the tangential pressure component through the vapor-liquid interface to the van der...
Conference Paper
Rapid Expansion from Supercritical Solution (RESS) is a method for the production of small particles down to the nanometre size for various substances including pharmaceuticals. The substance is dissolved in a supercritical solvent, typically carbon dioxide, and then expanded in a nozzle. This leads to lowering of the solubility and hence to a very...
Article
A new correlation method for the surface tension of fluids is proposed, which is based on friction theory applied to the interface of a two-phase system. The substance properties enter the model by a regular equation of state. Here we derive the method and test it with the Lennard-Jones 12-6 fluid as the reference system using molecular dynamics si...
Article
Unlike gases, liquids can be overheated or stretched only up to a limit. The determination of the mean-field thermodynamic stability limit—the so-called spinodal—is a very difficult theoretical and a more-or-less impossible experimental task. Based on a recent semi-empirical method, the spinodal pressure of helium-4 at given temperature is determin...
Article
Upon heating or isotropic expansion, condensed matters (solids and liquids) can reach their stability limits, where the system must be stabilized by the appearance of a second phase. During isotropic expansion, the pressure can reach negative values. In auxetic materials, volume expansion is an inherent way of behavior, therefore, negative-pressure...
Article
Ostwald ripening is an important growth process in many scientific disciplines ranging from material science, geology, biophysics, and product formulation. Here ripening of argon clusters in a vapor phase is observed directly in constant energy molecular dynamics simulations serving as a model system for large-time scale ripening processes. Startin...
Conference Paper
We perform molecular dynamics simulation with a new parameterisation of the embedded atom method [1] in order to investigate the formation of zinc nano-cluster. First we focus on the homogeneous nucleation in supersaturated zinc vapour simulation [2]. Argon is added to the vapour phase, which acts as inert gas thermostat to remove the latent heat f...
Article
Full-text available
With isotropic tri-axial stretching (negative pressure) and/or with heating, the thermodynamic stability limit (spinodal) of condensed matter—like solids, liquids and glasses—can be reached. In this paper, we analyse and compare the spinodal strength (i.e. the negative pressure necessary to reach the spinodal) of liquids, solids and glasses. Some e...
Conference Paper
Full-text available
Zink Nanopartikel sind von Interesse sowohl bei technischen Prozessen wie der solarthermischen Gewinnung von Wasserstoff als auch hinsichtlich grundlegender Fragestellungen wie die Strukturbildung in hcp Metallen, die eine starke Abweichung von dem idealen Verhältnis der Gitterkonstanten haben. Wir haben uns mit der Bildung von Zink Nanopartikeln a...
Article
Full-text available
We present an overview of the current status of experimental, theoretical, molecular dynamics (MD), and density functional theory (DFT) studies of argon vapor-to-liquid nucleation. Since the experimental temperature-supersaturation domain does not overlap with the corresponding MD and DFT domains, separate comparisons have been made: theory versus...
Article
Full-text available
Interfacial pressure and density profiles are calculated from molecular dynamics and lattice Boltzmann simulations of a liquid film in equilibrium with its vapor. The set of local values of tangential pressure and density along an interface exhibits a van der Waals-type loop; starting from the stable vapor bulk phase one passes through metastable a...
Conference Paper
Zink Nanopartikel sind von Interesse sowohl bei technischen Prozessen wie der solarthermischen Gewinnung von Wasserstoff als auch hinsichtlich grundlegender Fragestellungen wie die Strukturbildung in hcp Metallen, die eine starke Abweichung von dem idealen Verhältnis der Gitterkonstanten haben. Wir haben uns mit der Bildung von Zink Nanopartikeln a...
Conference Paper
We investigate the homogeneous nucleation of supersaturated zinc vapour by non-equilibrium molecular dynamics simulation [1]. Argon is added to the vapour phase, which acts as inert gas thermostat to remove the latent head from the forming clusters. Our simulations cover a temperature range from 400 K to 800 K and a supersaturation ranging from log...
Conference Paper
Full-text available
We perform molecular dynamics simulation with a recently developed new parameterisation of the embedded atom method (EAM) in order to investigate zinc nano-cluster. Nano-clusters exhibit differences in several properties compared to the bulk phase. In this context zinc is especially interesting because it is a hcp metal with a large deviation of th...
Article
Small clusters exhibit different behaviors in many physical properties compared to bulk systems. A longstanding problem in cluster science is the question how small clusters solidify. Here, we analyze the solidification of small Fe0.5Pt0.5 clusters by molecular dynamics simulation. We use a carrier gas thermostat in the simulations to mimic the exp...
Article
Full-text available
Homogeneous nucleation and growth of zinc from supersaturated vapor are investigated by nonequilibrium molecular dynamics simulations in the temperature range from 400 to 800 K and for a supersaturation ranging from log S=2 to 11. Argon is added to the vapor phase as carrier gas to remove the latent heat from the forming zinc clusters. A new parame...
Article
Heterogeneous nucleation and growth of supersaturated argon vapor at polyethylene surfaces is investigated using molecular dynamics simulation. The specific system is chosen as model for high wettable systems. The simulations are conducted in a nonequilibrium ensemble which includes heat transfer during the condensation process. Temporary density a...
Article
Full-text available
The formation of metal nano-particles in a homogeneous vapour phase and in a polymer matrix is investigated by molecular dynamics simulations. The different types of particle formation and growth are investigated specially with respect to the formation of atomic structure and particle morphology. We find that the structure formation proceeds via li...
Article
An easy applicable and accurate method for the correlation of the infinite dilution diffusion coefficient of organic solutes in supercritical fluids is proposed. The method is applicable to all supercritical fluids; in this work, however, only supercritical carbon dioxide is considered. The infinite dilution diffusion coefficient is calculated from...
Article
The structure analysis of atom configurations obtained from molecular simulation is usually conducted by the radial pair correlation function. It describes the probability of finding an atom in a certain distance to a reference atom. By comparison with the radial pair correlation functions of perfectly ordered crystal structures one can often deter...
Article
Full-text available
The formation of platinum nano-particles on a polyethylene substrate is investigated by molecular dynamics simulation. As initial configuration, a polymer film is put in contact with a supersaturated platinum vapour. Argon is added in the vapour phase as carrier gas that transfers heat from the vapour phase to the polymer surface. The simulations p...
Article
Homogeneous nucleation and growth from binary metal vapour is investigated by molecular dynamics simulation. It is focused here mainly on the iron-platinum system with a mole fraction of 0.5. The simulations are started in the highly supersaturated vapour phase. Argon is added as carrier gas removing the heat of condensation from the forming cluste...
Chapter
The formation of particles and aerosols is an important step in several industrial as well as environmental processes. Its formation can be an undesired phenomenon, for example, in thermal separation processes, in turbines or the discharge of environmentally problematic aerosols from industrial processes. However, it can also be the central process...
Chapter
Liquids below the vapor pressure are metastable. A part of this metastable region is located at negative pressure. In spite of this metastability, these states can be reached experimentally and their lifetime can be long enough for a lot of experimental investigations. Homogeneous solution of partially miscible liquids can exhibit liquid-liquid pha...
Article
In this work we report on two different high-pressure phase equilibria. The first is the solubility of a low-volatile solid in a supercritical phase. These are two anthraquinone dyes 1,4-bis-(hexadecylamino)-9,10-anthraquinone and 1,4-bis-(dodecylamino)-9, 10-anthraquinone dissolved in CO2 and N2O. Secondly, the partition of an infinitely diluted h...

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