Thorsten Merker

Molecular model for carbon dioxide optimized to vapor-liquid equilibria

A molecular model for carbon dioxide is presented, and the parameters of the Lennard-Jones sites, the bond length, and the quadrupole moment are optimized to experimental vapor-liquid equilibrium data. The resulting molecular model shows mean unsigned deviations to the experiment over the whole temperature range from triple point to critical point of 0.4% in saturated liquid density, 1.8% in vapor pressure, and 8.1% in enthalpy of vaporization. The molecular model is assessed by comparing predicted thermophysical properties with experimental data and a reference equation of state for a large part of the fluid region. The average deviations for density and residual enthalpy are 4.5% and 1.7%, respectively. The model is also capable to predict the radial distribution function, the second virial coefficient, and transport properties, the average deviations of the latter are 12%.

Engineering Molecular Models: Efficient Parameterization Procedure and Cyclohexanol as Case Study

Molecular models for applications in engineering were parameterized using a strategy based on quantum mechanical (QM) ab initio calculations and thermodynamic data. A new procedure for adjusting such molecular models to thermodynamic data via reduced units is introduced. As a case study, it was applied for developing a new molecular model of cyclohexanol. Compared to experimental data, the resulting molecular model for cyclohexanol showed mean unsigned errors of 0.2% in saturated liquid density and 3% in vapor pressure over the whole temperature range from triple point to critical point. The model was used to predict the second virial coefficient and the transport properties, the average deviations from experimental data were 0.1 l/mol and 25%, respectively.

Comment on “An optimized potential for carbon dioxide” [J. Chem. Phys. 122, 214507 (2005)]

A molecular model for carbon dioxide is assessed regarding vapor-liquid equilibrium properties. Large deviations, being above 15%, are found for vapor pressure and saturated vapor density in the entire temperature range.

Flexible or rigid molecular models?- A study on vapour-liquid equilibrium properties of ammonia

The influence of the intramolecular degrees of freedom on the vapour–liquid equilibrium properties of ammonia is studied for vapour pressure, saturated densities and enthalpy of vaporization. Molecular force fields with and without intramolecular degrees of freedom, keeping all other parameters unchanged, show significantly different phase envelopes. For ammonia, the angle potential is particularly important, because the hydrogen sites are more aligned in the liquid than in the vapour, leading to a significantly enhanced molecular dipole moment in the condensed phase. Based on a rigid force field for ammonia from prior work of our group [Eckl et al., Mol. Phys. 106, 1039 (2008)], a new accurate force field with intramolecular degrees of freedom is developed.

Molecular Modeling of Hydrogen Bonding Fluids: New Cyclohexanol Model and Transport Properties of Short Monohydric Alcohols

Currently, molecular modeling and simulation gains importance for the prediction ofthermophysical properties of pure fluids and mixtures, both in research and industry.This is due to several reasons: Firstly, the predictive power of molecular models al-lows forresults with technicallyrelevantaccuracyoverwide rangeofstate pointsthatis superior to classical methods. Secondly, a given molecular model provides accessto the full variety of thermophysical properties, such as thermal, caloric, transportor phase equilibrium data. Finally, through the advent of cheaply available powerfulcomputing infrastructure, reasonable execution times for molecular simulations canbe achieved which are of particular importance for industrial applications. Molecu-lar modeling and simulation are based on statistical thermodynamics which directlylinks the intermolecular interactions to the macroscopic thermophysical properties.That sound physical background also supports the increasing acceptance comparedto classical phenomenological modeling.Modeling thermophysical properties of hydrogen bonding systems remains achallenge. Phenomenologicalmodels often fail to describe the interplay between theenergetics of hydrogen bonding and its structural effects. Molecular force field mod-els, however, are much better suited for solving that task as they explicitly considerthis interplay. Most of the presently available molecular models use crude assump-tions for the description of hydrogen bonding which can, for instance, be simplymodeled by point charges eccentrically superimposed to Lennard-Jones (LJ) sites.One benefit of this simple modeling approach for hydrogen bon ding is the compa-rably small number of adjustable model parameters. Furthermore, the approach iscompatible with numerous LJ based models from the literature and it can success-fully be applied to mixtures. This simple modeling approach emerged to be fruitfulin many ways, although many of the molecular models proposed in the literaturelack in the quantitatively sound description of thermophysical properties. The aim

Molecular Modeling of Hydrogen Bonding Fluids: Phase Behavior of Industrial Fluids

Six new rigid models for Hydrogen chloride, Phosgene, Toluene, Benzene, Chlorobenzene and Ortho-Dichlorobenzene, that are based on quantum chemical calculations, are presented. Only the parameters of the dispersive and repulsive interactions are fitted to macroscopic thermodynamic properties to achieve an optimal agreement with experimental vapor-liquid equilibrium data.

Molecular Modeling of Hydrogen Bonding Fluids: Transport Properties and Vapor-Liquid Coexistence

Predictions of the transport properties self-diffusion coefficient and shear viscosity are presented for a recently developed molecular ammonia model. These data show mean unsigned deviations to the experiment over a temperature range from 200 to 500 K of 8 % for the self-diffusion coefficient and 12% for the shear viscosity. Furthermore, the vapor-liquid equilibria of the ternary system carbon dioxide + cyclohexanol + cyclohexane and its binary subsystems are investigated. The modified Lorentz-Berthelot combination rule with one state-independent binary interaction parameter was used for the pairwise unlike dispersive interactions. Per binary subsystem, the parameter was adjusted to a single experimental vapor pressure. The binary subsystems are in good agreement with experimental data throughout the entire composition range. For the ternary system, the vapor pressure is underpredicted by about 12 %.

Molecular Modeling of Hydrogen Bonding Fluids: Vapor-Liquid Coexistence and Interfacial Properties

A major challenge for molecular modeling consists in optimizing the unlike interaction potentials. A broad study on fluid mixtures [1] recently showed that among the variety of combination rules that were proposed in the past, none is clearly superior. In many cases, all are suboptimal when accurate predictions of properties like the mixture vapor pressure are needed. The well known Lorentz-Berthelot rule performs quite well and can be used as a starting point. If more accurate results are required, it is often advisable to adjust the dispersive interaction energy parameter which leads to very favorable results [1,2,3,4,5].