Towards open boundary molecular dynamics simulation of ionic liquids

Significance Statement

Ionic liquids are becoming popular owing to a wide range of characteristics and amazing groundbreaking developments in biology, materials science, chemistry, and physics. The possibility of adequately designing these systems at the molecular level has attracted numerous research work aiming at understanding them from a perspective of optimizing and manipulating their chemical design in a bid to obtain large scale properties. Molecular simulation allows for a molecular based understanding of ionic liquid large scale properties, therefore, presenting a powerful tool for development and use of ionic liquids.

Advanced resolution method allows for obtaining full atomistic details of a selected region and coupling it with a region with generic molecular model, coarse-grained, without atomistic details, that functions as a reservoir of energy as well as particles. This indicates that grand canonical-like advanced resolution can be implemented as a tool for identifying the essential atomistic degree of freedom needed to get a certain property.

The study aimed at evaluating the feasibility of implementing the grand canonical-like advanced resolution to ionic liquids. However, the challenging part would be the presence of explicit ions and the ability to build a coarse-grained procedure for a physically valid system-reservoir coupling. Therefore, Dr. Christian Krekeler and Professor Luigi Delle Site at Freie Universität of Berlin proposed two different methods for developing an accurate coarse-grained model. One method focused on a coarse-grained model which did not carry charges but reproduced the ion-ion radial distribution functions of a full atomistic simulation. In the other approach, the ions in the coarse grained model carried also charges. Their work is now published in Physical Chemistry Chemical Physics.

Simulations performed in the study was using the package GROMACS. Researchers adopted 1,3-dimethylimidazolium chloride as the test system on the basis that it’s complex enough to test the robustness of the proposed simulation method and conversely simpler, therefore, allowing for a large number of numerical analyses in a simple way without expensive computational efforts.

The researchers set up two systems. The first one with 350 ion pairs was applied to derive two coarse grained potentials which were then transferred to a larger system with over 1000 ion pairs. Simulation temperature was set at 400K and a 2fs time step. The electrostatic interactions were computed through particle mesh Ewald method. They prove that the grand-canonical simulation can be applied to ionic liquids, despite the challenging conceptual and computational aspects of the system.

The grand canonical-like advanced resolution allows for the treatment of atomistic spherical regions embedded in a large reservoir of structureless molecules, therefore, the dependence of dynamic and structural properties as a function of size of the atomistic region can be systematically analyzed. By analyzing the results obtained from various simulations defined by different size atomistic regions, it will be possible to establish the connection between time and length scales of given properties. For instance, one could calculate the hydrogen bond-hydrogen bond autocorrelation function expressed as a function of size of the atomistic region.

Comparison between the results obtained with various ionic liquids would be necessary in clarifying the effect of specific molecular chemical structure of these ions. This study paves way for a direct check and an in-depth understanding of the hypothesis of rattling of ions in long-living ion cages.

Towards open boundary molecular dynamics simulation of ionic liquids (Advances in Engineering)

The cartoon (taken as a snapshot from a real simulation) illustrates a  system of Ionic Liquids where molecules are treated at atomistic and coarse-grained resolution according to their position in space. The different regions are open, that is molecules are free to diffuse from one to another region; thus when  a molecules crosses the atomistic (the coarse-grained) region than it takes a hybrid resolution which slowly transforms the molecular resolution from atomistic to coarse-grained and vice versa.

About The Author

Dr.Christian Krekeler

2004  Diploma (Master) in Chemistry, Georg-August University Göttingen
2008 PhD in Chemistry, Max-Planck-Institute for Polymer Research, Mainz
2008 – 2009  Postdoc, Max-Planck-Institute for Polymer Research, Mainz
2009 – 2011 Postdoc at the Frankfurt Institute for Advanced Studies (FIAS), Goethe University,   Frankfurt am Main and TU Darmstadt, Clemens Schöpf Institute for Organic Chemistry and Biochemistry
2012-2016 Postdoctoral Researcher , TU Braunschweig/Innovation Lab GmbH, Heidelberg
since June 2016 Researcher Associate within the ECAM project, FU Berlin

About The Author

Prof. Dr. Luigi Delle Site

1995 Laurea (Master) degree in Mathematical Physics, University “la Sapienza” Rome.
1999 PhD in Physics, The Queen’s University of Belfast.
1999-2003 Postdoc in the Theory group at the Max-Planck-Institute for Polymer Research, Mainz
2003-2011 Group Leader of the “Multiscale Modeling Project” , Max-Planck-Institute for Polymer Research, Mainz.
2011 Awarded the Heisenberg Fellowship of the Deutsche Forschungsgemeinschaft (DFG, German Science Foundation).
2011-2015 Heisenberg Fellow at the Institute for Mathematics of the Freie Universität of Berlin
2012-2015 Director of the CECAM-DE-MMS node of Berlin.
Since 2015 Guest Professor in ” Theoretical and Mathematical Physics in Molecular Simulation”, at the Institute for Mathematics of the Freie Universität of Berlin.

Reference

Christian Krekeler and Luigi Delle Site. Towards open boundary molecular dynamics simulation of ionic liquids. Phys. Chem. Chem. Phys., 2017, 19, 4701—4709.

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