Dresden 2011 – wissenschaftliches Programm

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 6: Charged Soft Matter

CPP 6.6: Vortrag

Montag, 14. März 2011, 15:15–15:30, ZEU 160

Optimization of ionic force fields based on ion-pair thermodynamic properties — •Maria Fyta, Immanuel Kalcher, Joachim Dzubiella, and Roland Netz — Physics Department, Technical University of Munich, 85748 Garching

We investigate strategies to optimize ionic force fields based on single-ion and ion-pair properties simultaneously. To that end, we present results using Molecular Dynamics simulations for five different salt solutions, namely CsCl, KCl, NaI, KF, and CsI, at finite ion concentration. The force fields of these ions are systematically varied under the constraint that the single-ion solvation free energy matches the experimental value, which reduces the two-dimensional $\{\sigma, \varepsilon\}$ parameter space of the Lennard Jones interaction to a one dimensional curve. From the finite-concentration simulations, the pair-potential is extracted and the osmotic coefficient is calculated, which is compared to experimental data. A strong dependence of the osmotic coefficient on the force field is evident, which is remarkable as the single-ion solvation free energy and the ion-water structure remain invariant under the parameter variation. Optimization of the force field is achieved for the cations Cs$^+$ and $K^+$, while for the anions I$^-$ and F$^-$ the experimental osmotic coefficient cannot be reached. We suggest, thus, that additional parameters might have to be introduced into the modeling, for example by modified mixing rules and present data along these lines.