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Material Science & Engineering (MSE)

   

Molecular Simulation of Ionic Liquids

Molecular Simulation of Ionic Liquids

Ionic liquids, a novel designer medium: Ionic liquids are molten salts, i.e. they consist entirely of ions, with surprisingly low melting points below 100 °C or even below room temperature. We are simulating ionic liquids on an atomic level of detail to predict their macroscopic properties and to understand how the choice of the comprising ions determines the properties of the medium. The strong ion interactions in the liquid are responsible for a spatial long range ordering in the material on one hand, while in contrast the liquid-like dynamics disorders the material, thereby combining solid-like and liquid-like properties in one medium. This leads to a very versatile new material with unique properties that gained recently a lot of attention from chemical research and industry. Ionic liquids typically exhibit an extremely low volatility, a high reusability and are chemically and thermally very stable. They are therefore considered as a green and recyclable material. What makes ionic liquids so attractive for many applications is that many of their properties can be adjusted by combining different sort of cations and anions to generate a cornucopia of materials with different abilities. In fact, ionic liquids are often dubbed designer materials due to their versatility. They have been used successfully e.g. in membranes to separate mixtures of compounds, as filters for gases, as electrolytes in batteries and for the catalysis of various chemical reactions1.

Why simulations of ionic liquids are important: The number of ionic liquids that can be produced is virtually unlimited. However, for a given problem, only specific ionic liquids are suitable. Comparably small modifications of the constituting ions often lead to substantially altered properties of the liquid. Computational models of ionic liquids, with which structures, dynamics and energetics of the liquid can be simulated on an atomic level of detail, are capable of predicting the macroscopic properties of ionic liquids and, most importantly, lead to an understanding of their molecular origin. These insights are helping to design improved versions of ionic liquids.

How to simulate ionic liquids: For the simulation of ionic liquids we are applying molecular dynamic simulations (MD). Newton’s equation of motion is used to evaluate the dynamics of the atoms at a chosen temperature and pressure. The required atomic forces are derived from the potential energy of an empirical classical model, which is tailored to the ionic liquid. In this model, all atomic nuclei in the material are represented explicitly as point-like mass-particles. Although electrons are not treated directly, such as in computationally expensive quantum mechanical methods, their effect is accounted for implicitly through covalent bonds. These bonds are represented in the model as mechanical springs that are linking the mass-particles together. In the model the mass-particles are additionally interacting with each other through individually assigned electrical charges using a Coulomb potential. The empirical parameters of the model are adjusted by using data from experiments as well as data from quantum mechanical calculations with a multi scale approach. With this method we are capable of simulating ionic liquids that consist of up to one hundred thousand atoms on a time scale of several nanoseconds.

Fig. 1. Atomic structure of single simulated ions on the left side (shown are organic guanidinium-based cations and compact nitrate anions) and a simulation setup of 500 ion pairs in an ionic liquid with cations in green and anions in red on the right side.

Recent research on ionic liquids at IHPC: We developed a force field that can be used to simulate a new promising class of ionic liquids that contain guanidinium-based cations2. With this force field and quantum mechanical methods we simulated the liquid phase and observed electron charge transfer between ions and molecular polarization within these ions. We predicted various properties of these liquids, such as ion structures, radial distribution functions of ion-ion distances, the formation of cavities as well as domains of low electrostatic potential, melting temperatures and vaporization enthalpies. We studied self-diffusion of ionic liquids and identified a diffusion mechanism that resembles the diffusion in conventional molecular liquids in contrast to the previously assumed ion vacancy or cavity driven type of diffusion3.

Fig. 2. Our proposed diffusion model describes the diffusion of ions in the ionic liquid as a chain of single diffusive events that involves short displacement lengths of around 2 Å per event.

1 H. Weingärtner; “Understanding Ionic Liquids at the Molecular Level: Facts, Problems, and Controversies”, Angew. Chem. Int. Ed., 2008, 47, 654-670. DOI: 10.1002/anie.200604951.

2 M. Klähn, A. Seduraman, P. Wu; “A Force Field for Guanidinium-Based Ionic Liquids That Utilizes the Electron Charge Distribution of the Actual Liquid: A Molecular Simulation Study”, J. Phys. Chem. B, 2008, J. Phys. Chem. B, 2008, 112, 10989-11004.

3 M. Klähn, A. Seduraman, P. Wu; “A Model for Self-Diffusion of Guanidinium-Based Ionic Liquids: A Molecular Simulation Study”, J. Phys. Chem. B, 2008, in press.

 

 

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This page is last updated at: 26-MAY-2009