ISBN 9783868537628

978-3-86853-762-8, Reihe Thermodynamik

Jan Fischer
Molecular Modeling of Complex Systems for Applications in Thermodynamics

241 Seiten, Dissertation Technische Universität Dortmund (2010), Softcover, A5

Zusammenfassung / Abstract

Within this thesis several methods were developed to enable the determination of thermodynamic properties of complex systems, using atomistic or mesocale molecular dynamics simulation. Results for a number of associating components (alcohols, ethers, glycols) in aqueous solution are in good accordance with experiments. These successful first results demonstrate the capacity of the method for complex substances, in particular because force fields fitted to pure component properties were used without introducing an adjustable binary parameter. The inclusion of such a parameter or the refinement of evaluation methods could further improve the accuracy in the small concentration regime.

To examine the transferability of potential models, both with explicit and implicit solvent, over concentration and chain length coarse-grained potentials were fitted to atomistic structures of small poly(oxyethylene) oligomers in aqueous solution. The force field for the atomistic reference simulations was partially reparameterized to improve the conformer representation by fitting the dihedral potential terms to ab initio data of the dimer dimethoxyethane.

It turned out that concentration dependent changes in the solution structure can be reproduced fairly well, but not when potentials are transfered to a larger/smaller molecule. The explicit-solvent potentials perform slightly better in diluted solutions. The transfer over chain length is most successful using the implicit, decamer-fitted potentials (including a dihedral potential fit). These are able to reproduce local and global structures from atomistic simulations as well as experimental gyration radii for 150-times larger molecules (up to 60,000 g/mol) well. The other implicit-solvent potentials, which do not include explicit dihedral interactions, reproduce qualitatively the scaling of gyration radii as well, but the absolute values are either overestimated (decamer potential) or underestimated (trimer potential). The trimer potentials produced chain collapse in some of the long-chain simulations, but this could be prevented by making the potential less attractive. The trimer thus is obviously too small a molecule to reliably derive potentials for a polymer.

Finally, several alternatives were deduced and tested for integrating a pressure and energy correction into the potential iteration procedure. Potentials were generated for some simple molecules, which could successfully reproduce potential energy and pressure of the reference simulation besides the structure. For water, only one of either energy or pressure could be reproduced together with the correct structure. These potentials were subsequently tested to derive chemical potentials. A comparison with atomistic overlapping distributions and chemical potentials revealed considerable deviations. This suggests that the fitting of energy, pressure and radial distribution functions is not yet a sufficient condition to reproduce other properties like the chemical potential.