journal articles

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2015

  • F. Römer and D. A. Fedosov, “Dense brushes of stiff polymers or filaments in fluid flow” EPL (Europhysics Letters) 109, 68001 (2015). doi:10.1209/0295-5075/109/68001
    [BibTeX] [Abstract] [Download PDF]

    Dense filamentous brush-like structures are present in many biological interfacial systems ( e.g. , glycocalyx layer in blood vessels) to control their surface properties. Such structures can regulate the softness of a surface and modify fluid flow. In this letter, we propose a theoretical model which predicts quantitatively flow-induced deformation of a dense brush of stiff polymers or filaments, whose persistence length is larger or comparable to their contour length. The model is validated by detailed mesoscopic simulations and characterizes different contributions to brush deformation including hydrodynamic friction due to flow and steric excluded-volume interactions between grafted filaments. This theoretical model can be used to describe the effect of a stiff-polymer brush on fluid flow and to aid in the quantification of experiments.

    @Article{EPL-2015,
    Title = {Dense brushes of stiff polymers or filaments in fluid flow},
    Author = {F. R\"omer and D. A. Fedosov},
    Journal = {EPL (Europhysics Letters)},
    Year = {2015},
    Number = {6},
    Pages = {68001},
    Volume = {109},
    Abstract = {Dense filamentous brush-like structures are present in many biological interfacial systems ( e.g. , glycocalyx layer in blood vessels) to control their surface properties. Such structures can regulate the softness of a surface and modify fluid flow. In this letter, we propose a theoretical model which predicts quantitatively flow-induced deformation of a dense brush of stiff polymers or filaments, whose persistence length is larger or comparable to their contour length. The model is validated by detailed mesoscopic simulations and characterizes different contributions to brush deformation including hydrodynamic friction due to flow and steric excluded-volume interactions between grafted filaments. This theoretical model can be used to describe the effect of a stiff-polymer brush on fluid flow and to aid in the quantification of experiments.},
    Doi = {10.1209/0295-5075/109/68001},
    Owner = {frank},
    Timestamp = {2015.03.17},
    Url = {http://wgserve.de/fr/wp-content/papercite-data/pdf/EPL_109_68001_(2015).pdf}
    }

2013

  • F. Bresme and F. Römer, “Heat transport in liquid water at extreme pressures: A non equilibrium molecular dynamics study” Journal of Molecular Liquids 185, 1-7 (2013). doi:10.1016/j.molliq.2012.09.013
    [BibTeX] [Abstract]

    We investigate the structure and heat transport of liquid water at high pressures and temperatures, 1–50 kbar and 300–600 K, i.e., in a region of the phase diagram that is challenging for experimental investigations. Using equilibrium and non equilibrium molecular dynamics simulations and the TIP4P/2005 water model, we compute the structure and thermal conductivity of liquid water. At extreme pressures, 20–50 kbar the tetrahedral order characteristic of the hydrogen bonded network is severely disrupted, and the liquid radial distribution function becomes very similar to that of simple liquids. At these extreme conditions the thermal conductivity does not feature an anomalous behavior, and decreases with temperature as observed in a wide range of simple liquids. The dependence of the thermal conductivity with temperature and pressure follows experimental observations, and we find that it can be accurately predicted in terms of the liquid isothermal compressibility, by using a modified Leibfried–Schlömann equation. We also analyze whether the thermal conductivity follows the T scaling behavior characteristic of hard sphere fluids, a behavior that has been suggested following the analysis of high pressure and high temperature experimental data. Upon close inspection we find clear deviations from this scaling behavior both in simulation and experimental data.

    @Article{JMolLiq-185-1,
    Title = {Heat transport in liquid water at extreme pressures: A non equilibrium molecular dynamics study },
    Author = {Fernando Bresme and Frank Römer},
    Journal = {Journal of Molecular Liquids },
    Year = {2013},
    Number = {0},
    Pages = {1 - 7},
    Volume = {185},
    Abstract = {We investigate the structure and heat transport of liquid water at high pressures and temperatures, 1–50 kbar and 300–600 K, i.e., in a region of the phase diagram that is challenging for experimental investigations. Using equilibrium and non equilibrium molecular dynamics simulations and the TIP4P/2005 water model, we compute the structure and thermal conductivity of liquid water. At extreme pressures, 20–50 kbar the tetrahedral order characteristic of the hydrogen bonded network is severely disrupted, and the liquid radial distribution function becomes very similar to that of simple liquids. At these extreme conditions the thermal conductivity does not feature an anomalous behavior, and decreases with temperature as observed in a wide range of simple liquids. The dependence of the thermal conductivity with temperature and pressure follows experimental observations, and we find that it can be accurately predicted in terms of the liquid isothermal compressibility, by using a modified Leibfried–Schlömann equation. We also analyze whether the thermal conductivity follows the T scaling behavior characteristic of hard sphere fluids, a behavior that has been suggested following the analysis of high pressure and high temperature experimental data. Upon close inspection we find clear deviations from this scaling behavior both in simulation and experimental data. },
    Doi = {10.1016/j.molliq.2012.09.013},
    ISSN = {0167-7322},
    Keywords = {Computer simulations}
    }

  • F. Römer, Z. Wang, S. Wiegand, and F. Bresme, “Alkali Halide Solutions under Thermal Gradients: Soret Coefficients and Heat Transfer Mechanisms” Journal of Physical Chemistry B 117, 8209-8222 (2013). doi:10.1021/jp403862x
    [BibTeX] [Abstract]

    We report an extensive analysis of the non-equilibrium response of alkali halide aqueous solutions (Na+/K+–Cl–) to thermal gradients using state of the art non-equilibrium molecular dynamics simulations and thermal diffusion forced Rayleigh scattering experiments. The coupling between the thermal gradient and the resulting ionic salt mass flux is quantified through the Soret coefficient. We find the Soret coefficient is of the order of 10–3 K–1 for a wide range of concentrations. These relatively simple solutions feature a very rich behavior. The Soret coefficient decreases with concentration at high temperatures (higher than T ∼ 315 K), whereas it increases at lower temperatures. In agreement with previous experiments, we find evidence for sign inversion in the Soret coefficient of NaCl and KCl solutions. We use an atomistic non-equilibrium molecular dynamics approach to compute the Soret coefficients in a wide range of conditions and to attain further microscopic insight on the heat transport mechanism and the behavior of the Soret coefficient in aqueous solutions. The models employed in this work reproduce the magnitude of the Soret coefficient, and the general dependence of this coefficient with temperature and salt concentration. We use the computer simulations as a microscopic approach to establish a correlation between the sign and magnitude of the Soret coefficients and ionic solvation and hydrogen bond structure of the solutions. Finally, we report an analysis of heat transport in ionic solution by quantifying the solution thermal conductivity as a function of concentration. The simulations accurately reproduce the decrease of the thermal conductivity with increasing salt concentration that is observed in experiments. An explanation of this behavior is provided.

    @Article{JPCB-117-8209,
    Title = {Alkali Halide Solutions under Thermal Gradients: Soret Coefficients and Heat Transfer Mechanisms},
    Author = {R\"{o}mer, Frank and Wang, Zilin and Wiegand, Simone and Bresme, Fernando},
    Journal = {Journal of Physical Chemistry B},
    Year = {2013},
    Number = {27},
    Pages = {8209-8222},
    Volume = {117},
    Abstract = {We report an extensive analysis of the non-equilibrium response of alkali halide aqueous solutions (Na+/K+–Cl–) to thermal gradients using state of the art non-equilibrium molecular dynamics simulations and thermal diffusion forced Rayleigh scattering experiments. The coupling between the thermal gradient and the resulting ionic salt mass flux is quantified through the Soret coefficient. We find the Soret coefficient is of the order of 10–3 K–1 for a wide range of concentrations. These relatively simple solutions feature a very rich behavior. The Soret coefficient decreases with concentration at high temperatures (higher than T ∼ 315 K), whereas it increases at lower temperatures. In agreement with previous experiments, we find evidence for sign inversion in the Soret coefficient of NaCl and KCl solutions. We use an atomistic non-equilibrium molecular dynamics approach to compute the Soret coefficients in a wide range of conditions and to attain further microscopic insight on the heat transport mechanism and the behavior of the Soret coefficient in aqueous solutions. The models employed in this work reproduce the magnitude of the Soret coefficient, and the general dependence of this coefficient with temperature and salt concentration. We use the computer simulations as a microscopic approach to establish a correlation between the sign and magnitude of the Soret coefficients and ionic solvation and hydrogen bond structure of the solutions. Finally, we report an analysis of heat transport in ionic solution by quantifying the solution thermal conductivity as a function of concentration. The simulations accurately reproduce the decrease of the thermal conductivity with increasing salt concentration that is observed in experiments. An explanation of this behavior is provided.},
    Doi = {10.1021/jp403862x},
    Eprint = {http://pubs.acs.org/doi/pdf/10.1021/jp403862x}
    }

2012

  • F. Römer, B. Fischer, and T. Kraska, “Investigation of The Nucleation and Growth of Methanol Clusters from Supersaturated Vapor by Molecular Dynamics Simulations” Soft Materials 10, 130-152 (2012). doi:10.1080/1539445X.2011.599704
    [BibTeX] [Abstract]

    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 complete system consisted of methanol and argon, which is a carrier gas removing the latent heat of condensation from the system. The growth of the largest cluster in the system, the average cluster size, and the initial cluster size distributions were analyzed. The results were compared to calculations with the classical nucleation theory (CNT) using macroscopic properties obtained from simulations with the same molecular model. The rates calculated with the CNT and the simulation data at high supersaturation differed by two to three orders of magnitude. Simulation results and experimental data taken from the literature were consistently below the CNT values.

    @Article{SoftMat-10-130,
    Title = {Investigation of The Nucleation and Growth of Methanol Clusters from Supersaturated Vapor by Molecular Dynamics Simulations},
    Author = {R\"{o}mer, Frank and Fischer, Bj\"{o}rn and Kraska, Thomas},
    Journal = {Soft Materials},
    Year = {2012},
    Number = {1-3},
    Pages = {130-152},
    Volume = {10},
    Abstract = { 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 complete system consisted of methanol and argon, which is a carrier gas removing the latent heat of condensation from the system. The growth of the largest cluster in the system, the average cluster size, and the initial cluster size distributions were analyzed. The results were compared to calculations with the classical nucleation theory (CNT) using macroscopic properties obtained from simulations with the same molecular model. The rates calculated with the CNT and the simulation data at high supersaturation differed by two to three orders of magnitude. Simulation results and experimental data taken from the literature were consistently below the CNT values. },
    Doi = {10.1080/1539445X.2011.599704},
    Eprint = {http://www.tandfonline.com/doi/pdf/10.1080/1539445X.2011.599704}
    }

  • F. Römer and T. Kraska, “A force field for naproxen” Molecular Simulation 38, 152-160 (2012). doi:10.1080/08927022.2011.608847
    [BibTeX] [Abstract]

    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 initio calculations. The dispersive interaction contribution is taken from the literature. By MD simulation using a naproxen film in slap geometry, the temperature dependence of the density, surface tension and self-diffusion coefficient as well as the melting temperature for the developed potential model are obtained.

    @Article{MolSim-38-152,
    Title = {A force field for naproxen},
    Author = {R\"{o}mer, Frank and Kraska, Thomas},
    Journal = {Molecular Simulation},
    Year = {2012},
    Number = {2},
    Pages = {152-160},
    Volume = {38},
    Abstract = { 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 initio calculations. The dispersive interaction contribution is taken from the literature. By MD simulation using a naproxen film in slap geometry, the temperature dependence of the density, surface tension and self-diffusion coefficient as well as the melting temperature for the developed potential model are obtained. },
    Doi = {10.1080/08927022.2011.608847},
    Eprint = {http://www.tandfonline.com/doi/pdf/10.1080/08927022.2011.608847}
    }

  • F. Römer, A. Lervik, and F. Bresme, “Nonequilibrium molecular dynamics simulations of the thermal conductivity of water: A systematic investigation of the SPC/E and TIP4P/2005 models” Journal of Chemical Physics 137, 74503 (2012). doi:10.1063/1.4739855
    [BibTeX] [Abstract] [Download PDF]

    We report an extensive nonequilibrium molecular dynamics investigation of the thermal conductivity of water using two of the most accurate rigid nonpolarizable empirical models available, SPC/E and TIP4P/2005. Our study covers liquid and supercritical states. Both models predict the anomalous increase of the thermal conductivity with temperature and the thermal conductivity maximum, hence confirming their ability to reproduce the complex anomalous behaviour of water. The performance of the models strongly depends on the thermodynamic state investigated, and best agreement with experiment is obtained for states close to the liquid coexistence line and at high densities and temperatures. Considering the simplicity of these two models the overall agreement with experiments is remarkable. Our results show that explicit polarizability and molecular flexibility are not needed to reproduce the anomalous heat conduction of water.

    @Article{JCP-137-074503,
    Title = {Nonequilibrium molecular dynamics simulations of the thermal conductivity of water: A systematic investigation of the SPC/E and TIP4P/2005 models},
    Author = {Frank R\"{o}mer and Anders Lervik and Fernando Bresme},
    Journal = {Journal of Chemical Physics},
    Year = {2012},
    Number = {7},
    Pages = {074503},
    Volume = {137},
    Abstract = {We report an extensive nonequilibrium molecular dynamics investigation of the thermal conductivity of water using two of the most accurate rigid nonpolarizable empirical models available, SPC/E and TIP4P/2005. Our study covers liquid and supercritical states. Both models predict the anomalous increase of the thermal conductivity with temperature and the thermal conductivity maximum, hence confirming their ability to reproduce the complex anomalous behaviour of water. The performance of the models strongly depends on the thermodynamic state investigated, and best agreement with experiment is obtained for states close to the liquid coexistence line and at high densities and temperatures. Considering the simplicity of these two models the overall agreement with experiments is remarkable. Our results show that explicit polarizability and molecular flexibility are not needed to reproduce the anomalous heat conduction of water.},
    Doi = {10.1063/1.4739855},
    Eid = {074503},
    Keywords = {heat conduction; molecular dynamics method; polarisability; thermal conductivity; water},
    Numpages = {8},
    Publisher = {AIP},
    Url = {http://wgserve.de/fr/wp-content/papercite-data/pdf/JChemPhys_137_074503_(2012).pdf}
    }

  • F. Römer, F. Bresme, J. Muscatello, D. Bedeaux, and M. J. Rub’i, “Thermomolecular Orientation of Nonpolar Fluids” Physical Review Letters 108, 105901 (2012). doi:10.1103/PhysRevLett.108.105901
    [BibTeX] [Abstract] [Download PDF]

    We investigate the response of molecular fluids to temperature gradients. Using nonequilibrium molecular dynamics computer simulations we show that nonpolar diatomic fluids adopt a preferred orientation as a response to a temperature gradient. We find that the magnitude of this thermomolecular orientation effect is proportional to the strength of the temperature gradient and the degree of molecular anisotropy, as defined by the different size or mass of the molecular atomic sites. We show that the preferred orientation of the molecules follows the same trends observed in the Soret effect of binary mixtures. We argue this is a general effect that should be observed in a wide range of length scales.

    @Article{PRL-108-105901,
    Title = {Thermomolecular Orientation of Nonpolar Fluids},
    Author = {R\"omer, Frank and Bresme, Fernando and Muscatello, Jordan and Bedeaux, Dick and Rub\'\i, J. Miguel},
    Journal = {Physical Review Letters},
    Year = {2012},
    Month = {Mar},
    Pages = {105901},
    Volume = {108},
    Abstract = {We investigate the response of molecular fluids to temperature gradients. Using nonequilibrium molecular dynamics computer simulations we show that nonpolar diatomic fluids adopt a preferred orientation as a response to a temperature gradient. We find that the magnitude of this thermomolecular orientation effect is proportional to the strength of the temperature gradient and the degree of molecular anisotropy, as defined by the different size or mass of the molecular atomic sites. We show that the preferred orientation of the molecules follows the same trends observed in the Soret effect of binary mixtures. We argue this is a general effect that should be observed in a wide range of length scales.},
    Doi = {10.1103/PhysRevLett.108.105901},
    Issue = {10},
    Numpages = {4},
    Publisher = {American Physical Society},
    Url = {http://wgserve.de/fr/wp-content/papercite-data/pdf/PhysRevLett_108_105901_(2012).pdf}
    }

  • F. Römer and F. Bresme, “Heat conduction and thermomolecular orientation in diatomic fluids: a non-equilibrium molecular dynamics study” Molecular Simulation 38, 1198-1208 (2012). doi:10.1080/08927022.2012.709631
    [BibTeX] [Abstract]

    We investigate heat transfer in fluids consisting of diatomic molecules in a wide range of thermodynamic conditions, from densities and temperatures characteristic of the liquid state to supercritical conditions. The interactions are modelled using a two-centre Lennard-Jones model, which enable us to quantify the impact that the incorporation of dispersion interactions has on the recently reported thermo-molecular orientation effect [F. Römer, F. Bresme, J. Muscatello, D. Bedeaux, and J.M. Rubí, Phys. Rev. Lett. 108 (2012), p. 105901]. The temperature gradient imposes a preferred orientation on the molecules. The orientation is stronger in the liquid state and for heteronuclear molecules featuring a large asymmetry in the diameters of the two atoms. We also analyse the microscopic mechanism of heat transport. The transport mechanism is dominated by collisional terms, hence following the general trend observed in liquids. The larger site in the molecule transports a larger amount of energy, the latter being proportional to the site exposed area. We also show that the molecular anisotropy has a large impact on the reduced thermal conductivity of the fluid, being larger for homonuclear molecules. The treatment of the molecules intramolecular bonds, rigid or flexible, does not have a significant impact on the thermal conductivity of the fluid.

    @Article{MolSim-38-1198,
    Title = {Heat conduction and thermomolecular orientation in diatomic fluids: a non-equilibrium molecular dynamics study},
    Author = {Römer, Frank and Bresme, Fernando},
    Journal = {Molecular Simulation},
    Year = {2012},
    Number = {14-15},
    Pages = {1198-1208},
    Volume = {38},
    Abstract = { We investigate heat transfer in fluids consisting of diatomic molecules in a wide range of thermodynamic conditions, from densities and temperatures characteristic of the liquid state to supercritical conditions. The interactions are modelled using a two-centre Lennard-Jones model, which enable us to quantify the impact that the incorporation of dispersion interactions has on the recently reported thermo-molecular orientation effect [F. Römer, F. Bresme, J. Muscatello, D. Bedeaux, and J.M. Rubí, Phys. Rev. Lett. 108 (2012), p. 105901]. The temperature gradient imposes a preferred orientation on the molecules. The orientation is stronger in the liquid state and for heteronuclear molecules featuring a large asymmetry in the diameters of the two atoms. We also analyse the microscopic mechanism of heat transport. The transport mechanism is dominated by collisional terms, hence following the general trend observed in liquids. The larger site in the molecule transports a larger amount of energy, the latter being proportional to the site exposed area. We also show that the molecular anisotropy has a large impact on the reduced thermal conductivity of the fluid, being larger for homonuclear molecules. The treatment of the molecules intramolecular bonds, rigid or flexible, does not have a significant impact on the thermal conductivity of the fluid. },
    Doi = {10.1080/08927022.2012.709631},
    Eprint = {http://www.tandfonline.com/doi/pdf/10.1080/08927022.2012.709631}
    }

2011

  • J. Muscatello, F. Römer, J. Sala, and F. Bresme, “Water under temperature gradients: polarization effects and microscopic mechanisms of heat transfer” Phys. Chem. Chem. Phys. 13, 19970-19978 (2011). doi:10.1039/C1CP21895F
    [BibTeX] [Abstract] [Download PDF]

    We report non-equilibrium molecular dynamics simulations (NEMD) of water under temperature gradients using a modified version of the central force model (MCFM). This model is very accurate in predicting the equation of state of water for a wide range of pressures and temperatures. We investigate the polarization response of water to thermal gradients{,} an effect that has been recently predicted using Non-Equilibrium Thermodynamics (NET) theory and computer simulations{,} as a function of the thermal gradient strength. We find that the polarization of the liquid varies linearly with the gradient strength{,} which indicates that the ratio of phenomenological coefficients regulating the coupling between the polarization response and the heat flux is independent of the gradient strength investigated. This notion supports the NET theoretical predictions. The coupling effect leading to the liquid polarization is fairly strong{,} leading to polarization fields of [similar]103-6 V m-1 for gradients of [similar]105-8 K m-1{,} hence confirming earlier estimates. Finally we employ our NEMD approach to investigate the microscopic mechanism of heat transfer in water. The image emerging from the computation and analysis of the internal energy fluxes is that the transfer of energy is dominated by intermolecular interactions. For the MCFM model{,} we find that the contribution from hydrogen and oxygen is different{,} with the hydrogen contribution being larger than that of oxygen.

    @Article{PCCP-13-19970,
    Title = {Water under temperature gradients: polarization effects and microscopic mechanisms of heat transfer},
    Author = {Muscatello, Jordan and R\"{o}mer, Frank and Sala, Jonas and Bresme, Fernando},
    Journal = {Phys. Chem. Chem. Phys.},
    Year = {2011},
    Pages = {19970-19978},
    Volume = {13},
    Abstract = {We report non-equilibrium molecular dynamics simulations (NEMD) of water under temperature gradients using a modified version of the central force model (MCFM). This model is very accurate in predicting the equation of state of water for a wide range of pressures and temperatures. We investigate the polarization response of water to thermal gradients{,} an effect that has been recently predicted using Non-Equilibrium Thermodynamics (NET) theory and computer simulations{,} as a function of the thermal gradient strength. We find that the polarization of the liquid varies linearly with the gradient strength{,} which indicates that the ratio of phenomenological coefficients regulating the coupling between the polarization response and the heat flux is independent of the gradient strength investigated. This notion supports the NET theoretical predictions. The coupling effect leading to the liquid polarization is fairly strong{,} leading to polarization fields of [similar]103-6 V m-1 for gradients of [similar]105-8 K m-1{,} hence confirming earlier estimates. Finally we employ our NEMD approach to investigate the microscopic mechanism of heat transfer in water. The image emerging from the computation and analysis of the internal energy fluxes is that the transfer of energy is dominated by intermolecular interactions. For the MCFM model{,} we find that the contribution from hydrogen and oxygen is different{,} with the hydrogen contribution being larger than that of oxygen.},
    Doi = {10.1039/C1CP21895F},
    Issue = {44},
    Publisher = {The Royal Society of Chemistry},
    Url = {http://wgserve.de/fr/wp-content/papercite-data/pdf/PCCP_13_19970_(2011).pdf}
    }

2010

  • F. Römer and T. Kraska, “Molecular dynamics simulation of the formation of pharmaceutical particles by rapid expansion of a supercritical solution” Journal of Supercritical Fluids 55, 769-777 (2010). doi:10.1016/j.supflu.2010.08.010
    [BibTeX] [Abstract]

    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 box. Comparison with an accurate reference equation of state for the pure solvent carbon dioxide shows that the simulation system follows an adiabatic expansion path. The expansion of a solution of naproxen in supercritical carbon dioxide leads to a highly supersaturated system that starts to form particles. The nucleation and growth kinetics of this particle formation process is investigated and the effect on the particle structure is analyzed.

    @Article{JSupcritFl-55-769,
    Title = {Molecular dynamics simulation of the formation of pharmaceutical particles by rapid expansion of a supercritical solution},
    Author = {Frank R\"{o}mer and Thomas Kraska},
    Journal = {Journal of Supercritical Fluids},
    Year = {2010},
    Note = {<ce:title>100th year Anniversary of van der Waals' Nobel Lecture</ce:title> },
    Number = {2},
    Pages = {769 - 777},
    Volume = {55},
    Abstract = {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 box. Comparison with an accurate reference equation of state for the pure solvent carbon dioxide shows that the simulation system follows an adiabatic expansion path. The expansion of a solution of naproxen in supercritical carbon dioxide leads to a highly supersaturated system that starts to form particles. The nucleation and growth kinetics of this particle formation process is investigated and the effect on the particle structure is analyzed. },
    Doi = {10.1016/j.supflu.2010.08.010},
    ISSN = {0896-8446},
    Keywords = {Molecular dynamics simulation}
    }

2009

  • S. Braun, F. Römer, and T. Kraska, “Influence of the carrier gas molar mass on the particle formation in a vapor phase” Journal of Chemical Physics 131, 64308 (2009). doi:10.1063/1.3204780
    [BibTeX] [Abstract] [Download PDF]

    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 of carrier gas atoms by few gas atoms with effective parameters. In this context the efficiency of the heat exchange with the carrier gas depending on its molar mass is analyzed. As a result one finds for varying molar masses and unchanged interaction parameters a competition between the efficiency and the number of the collisions. For too small molar masses the energy exchange per collision is too small while for too high masses the carrier gas atoms become very slow, decreasing the number of collisions.

    @Article{JCP-131-064308,
    Title = {Influence of the carrier gas molar mass on the particle formation in a vapor phase},
    Author = {S. Braun and F. R\"{o}mer and T. Kraska},
    Journal = {Journal of Chemical Physics},
    Year = {2009},
    Number = {6},
    Pages = {064308},
    Volume = {131},
    Abstract = {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 of carrier gas atoms by few gas atoms with effective parameters. In this context the efficiency of the heat exchange with the carrier gas depending on its molar mass is analyzed. As a result one finds for varying molar masses and unchanged interaction parameters a competition between the efficiency and the number of the collisions. For too small molar masses the energy exchange per collision is too small while for too high masses the carrier gas atoms become very slow, decreasing the number of collisions.},
    Doi = {10.1063/1.3204780},
    Eid = {064308},
    Keywords = {heat transfer; nanofabrication; nanoparticles; vapour phase epitaxial growth},
    Numpages = {6},
    Publisher = {AIP},
    Url = {http://wgserve.de/fr/wp-content/papercite-data/pdf/JChemPhys_131_064308_(2009).pdf}
    }

  • T. Kraska, F. Römer, and A. R. Imre, “The Relation of Interface Properties and Bulk Phase Stability: Molecular Dynamics Simulations of Carbon Dioxide” Journal of Physical Chemistry B 113, 4688-4697 (2009). doi:10.1021/jp808789p
    [BibTeX] [Abstract]

    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 Waals loop in the two-phase region of the phase diagram. By application of the thermodynamic stability criteria, the location of the spinodal can be determined. The spinodal determined in this way is called interface spinodal here. Furthermore, the simulation provides equation of state properties in the complete metastable region of the phase diagram. The performance of different correlation equations for the density and the pressure tensor profiles with respect to the estimation of the spinodal is compared. It has been found that the interface spinodal coincides with the thermodynamic mean field spinodal within some reasonable deviation. Finally the influence of the size of the simulation box on the spinodal properties is investigated showing that the temperature-density spinodal data are independent of the interface thickness. Additional simulations using a Lennard-Jones fluid confirm these results over a range of 1.5 orders of magnitude for the systems size. A further result is that interface systems require a very long simulation time in order to obtain reliable results.

    @Article{JPCB-113-4688,
    Title = {The Relation of Interface Properties and Bulk Phase Stability: Molecular Dynamics Simulations of Carbon Dioxide},
    Author = {Kraska, T. and R\"{o}mer, F. and Imre, A. R.},
    Journal = {Journal of Physical Chemistry B},
    Year = {2009},
    Note = {PMID: 19275205},
    Number = {14},
    Pages = {4688-4697},
    Volume = {113},
    Abstract = { 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 Waals loop in the two-phase region of the phase diagram. By application of the thermodynamic stability criteria, the location of the spinodal can be determined. The spinodal determined in this way is called interface spinodal here. Furthermore, the simulation provides equation of state properties in the complete metastable region of the phase diagram. The performance of different correlation equations for the density and the pressure tensor profiles with respect to the estimation of the spinodal is compared. It has been found that the interface spinodal coincides with the thermodynamic mean field spinodal within some reasonable deviation. Finally the influence of the size of the simulation box on the spinodal properties is investigated showing that the temperature-density spinodal data are independent of the interface thickness. Additional simulations using a Lennard-Jones fluid confirm these results over a range of 1.5 orders of magnitude for the systems size. A further result is that interface systems require a very long simulation time in order to obtain reliable results. },
    Doi = {10.1021/jp808789p},
    Eprint = {http://pubs.acs.org/doi/pdf/10.1021/jp808789p}
    }

  • F. Römer, S. Braun, and T. Kraska, “Development of an EAM potential for zinc and its application to the growth of nanoparticles” Phys. Chem. Chem. Phys. 11, 4039-4050 (2009). doi:10.1039/B820278H
    [BibTeX] [Abstract] [Download PDF]

    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 integral over the electron distribution function rather than fitting it to experimental data. In addition{,} the dimer potential is included in the parameterisation because we focus on the formation of nanoparticles from the vapour phase. With this potential model{,} the growth of zinc clusters consisting of 125 to 1000 atoms is investigated{,} which takes place at elevated temperatures in a liquid-like cluster state. The growing clusters are embedded in an argon carrier gas atmosphere which regulates the cluster temperature. The average thermal expansion of the clusters and the different lattice constants are analysed. For the determination of the cluster structure{,} the common-neighbour analysis method is extended to hexagonal close packed surface structures. During growth{,} small clusters with less than approximately 60 atoms develop transient icosahedral structure before transforming into hexagonal close-packed structure. The surface of the clusters exhibits a transformation from planes with high surface energy to the most stable ones. Besides ambiguous surface structures the final clusters are almost completely in an hexagonal close packed structure.

    @Article{PCCP-11-4039,
    Title = {Development of an EAM potential for zinc and its application to the growth of nanoparticles},
    Author = {R\"{o}mer, F. and Braun, S. and Kraska, T.},
    Journal = {Phys. Chem. Chem. Phys.},
    Year = {2009},
    Pages = {4039-4050},
    Volume = {11},
    Abstract = {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 integral over the electron distribution function rather than fitting it to experimental data. In addition{,} the dimer potential is included in the parameterisation because we focus on the formation of nanoparticles from the vapour phase. With this potential model{,} the growth of zinc clusters consisting of 125 to 1000 atoms is investigated{,} which takes place at elevated temperatures in a liquid-like cluster state. The growing clusters are embedded in an argon carrier gas atmosphere which regulates the cluster temperature. The average thermal expansion of the clusters and the different lattice constants are analysed. For the determination of the cluster structure{,} the common-neighbour analysis method is extended to hexagonal close packed surface structures. During growth{,} small clusters with less than approximately 60 atoms develop transient icosahedral structure before transforming into hexagonal close-packed structure. The surface of the clusters exhibits a transformation from planes with high surface energy to the most stable ones. Besides ambiguous surface structures the final clusters are almost completely in an hexagonal close packed structure.},
    Doi = {10.1039/B820278H},
    Issue = {20},
    Publisher = {The Royal Society of Chemistry},
    Url = {http://wgserve.de/fr/wp-content/papercite-data/pdf/PCCP_11_4039_(2009).pdf}
    }

  • F. Römer and T. Kraska, “Molecular Dynamics Simulation of Naphthalene Particle Formation by Rapid Expansion of a Supercritical Solution” Journal of Physical Chemistry C 113, 19028-19038 (2009). doi:10.1021/jp906478z
    [BibTeX] [Abstract]

    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 equation of state. During the expansion the solubility decreases and naphthalene particles precipitate. The heat of formation is compensated by the Joule−Thomson effect of the expanding solvent. Therefore, no influence of a molecular dynamics simulation thermostat is involved. Here the method is proposed and analyzed in extensive studies. Results are presented for the formation of naphthalene particles using a realistic model for the molecular interaction. In addition, a simplified potential model is employed to analyze finite size effects.

    @Article{JPCC-113-19028,
    Title = {Molecular Dynamics Simulation of Naphthalene Particle Formation by Rapid Expansion of a Supercritical Solution},
    Author = {R\"{o}mer, F. and Kraska, T.},
    Journal = {Journal of Physical Chemistry C},
    Year = {2009},
    Number = {44},
    Pages = {19028-19038},
    Volume = {113},
    Abstract = { 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 equation of state. During the expansion the solubility decreases and naphthalene particles precipitate. The heat of formation is compensated by the Joule−Thomson effect of the expanding solvent. Therefore, no influence of a molecular dynamics simulation thermostat is involved. Here the method is proposed and analyzed in extensive studies. Results are presented for the formation of naphthalene particles using a realistic model for the molecular interaction. In addition, a simplified potential model is employed to analyze finite size effects. },
    Doi = {10.1021/jp906478z},
    Eprint = {http://pubs.acs.org/doi/pdf/10.1021/jp906478z}
    }

2007

  • F. Römer and T. Kraska, “Homogeneous nucleation and growth in supersaturated zinc vapor investigated by molecular dynamics simulation” Journal of Chemical Physics 127, 234509 (2007). doi:10.1063/1.2805063
    [BibTeX] [Abstract] [Download PDF]

    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 parametrization of the embedded atom method for zinc is employed for the interaction potential model. The simulation data are analyzed with respect to the nucleation rates and the critical cluster sizes by two different methods, namely, the threshold method of Yasuoka and Matsumoto [J. Chem. Phys. 109, 8451 (1998)] and the mean first passage time method for nucleation by Wedekind et al. [J. Chem. Phys. 126, 134103 (2007)] . The nucleation rates obtained by these methods differ approximately by one order of magnitude. Classical nucleation theory fails to describe the simulation data as well as the experimental data. The size of the critical cluster obtained by the mean first passage time method is significantly larger than that obtained from the nucleation theorem.

    @Article{JCP-127-234509,
    Title = {Homogeneous nucleation and growth in supersaturated zinc vapor investigated by molecular dynamics simulation},
    Author = {F. R\"{o}mer and T. Kraska},
    Journal = {Journal of Chemical Physics},
    Year = {2007},
    Number = {23},
    Pages = {234509},
    Volume = {127},
    Abstract = {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 parametrization of the embedded atom method for zinc is employed for the interaction potential model. The simulation data are analyzed with respect to the nucleation rates and the critical cluster sizes by two different methods, namely, the threshold method of Yasuoka and Matsumoto [J. Chem. Phys. 109, 8451 (1998)] and the mean first passage time method for nucleation by Wedekind et al. [J. Chem. Phys. 126, 134103 (2007)] . The nucleation rates obtained by these methods differ approximately by one order of magnitude. Classical nucleation theory fails to describe the simulation data as well as the experimental data. The size of the critical cluster obtained by the mean first passage time method is significantly larger than that obtained from the nucleation theorem.},
    Doi = {10.1063/1.2805063},
    Eid = {234509},
    Keywords = {argon; crystal growth from vapour; latent heat; metal clusters; molecular dynamics method; nucleation; zinc},
    Numpages = {10},
    Publisher = {AIP},
    Url = {http://wgserve.de/fr/wp-content/papercite-data/pdf/JChemPhys_127_234509_(2007).pdf}
    }