Simulation Studies of Ionic Liquids

Ionic liquids (IL) offer a variety of physical properties that make them attractive replacements for traditional organic solvents. Because of their non-volatility, ILs have gained increased attention as ``Green Chemistry`` solvents in the past decade. Most of them are moisture-stable thus offering the opportunity to use IL/water mixtures as novel solvents. By varying the molar ratio of IL vs. water, the role of ILs as solvents/co-solvents can be studied systematically. Due to strong electrostatic interactions, pure ILs may be seen as an ionic network. Therefore, IL/water mixtures are characterized by the co-existence and competition of a hydrogen-bond and an ionic network. First experimental results for these fascinating binary networks exist, but systematic computer simulation studies have not been reported yet. Therefore, we plan intensive investigations of ILs as solvents/co-solvents by molecular dynamics simulation. Collective properties, such as viscosity, conductivity and dielectric constant provide a quantitative measure of the cooperativity of the underlying ionic/hydrogen bond networks. For example, the tight coupling of ion pairs results in a low static conductivity in pure ILs. In mixtures with water, however, the competition with and integration into the hydrogen bond network weakens ionic pairs and thus enhances the conductivity as well as it reduces the viscosity. In addition, the higher polarity of the additional water component leads to a dielectric increment. The potential of ILs (pure or in mixtures with water) as novel, benign solvents offers interesting applications in biomolecular solvation. This part of the project will be the most challenging one, because now three networks, the IL ionic network, the water hydrogen bond network and the internal network of the biomolecular solute co-exist and compete. The simulation and interpretation of such a complex triple network goes beyond current routine computational studies. We will follow a stepwise approach involving smaller biomolecular solutes. In detail, we will analyze three major aspects: (i) The structure and stability of the hydrogen bond network of the biomolecular solute, (ii) the structure of the solvent/co-solvent in the vicinity of the solute (iii) the dynamics of the solvent/co-solvent molecules, in particular their retardation as compared to the bulk. Altogether, the computational analysis of the collective behavior of ILs requires extremely good statistics. Thus, the simulation period to be covered has to be approximately 20 times longer than that of state of the art simulations.

In the past decades, ionic liquids (IL) attracted enormous industrial and scientific interest. In contrast to conventional volatile solvents, ionic liquids are composed of molecules possessing a molecular dipole and a net charge. The Coulomb interactions between these charges reduce the vapor pressure and broaden the temperature range of the liquid phase. In this project, the structure and dynamics due to the complex interplay of dipoles and charges were analyzed for various ILs and decomposed into two regions: The interactions in the immediate proximity of the ions are determined by the anisotropy of the molecules. The dipoles of neighboring cations are in parallel alignment, whereas anions prefer an antiparallel orientation. With increasing interaction range, long-ranged electrostatic forces become more important because of the charged nature of the ionic liquid molecules. In mixtures with water, a strong anion-water network emerges expelling the cations which may form hydrophobic clusters or accumulate on the surface of solvated biomolecules. This project was the first to report the computation of a complete dielectric spectrum. Therefore, a computer-adapted dielectric theory was developed in order to evaluate all important contributions to the dielectric spectrum on the basis of simulation data. It turned out that the translation of the ions contributed in a significant manner to the generalized dielectric constant. ILs are commonly used as catalysts in biochemical reactions and for the extraction of biomaterials. The effect of the ions on surface dynamics and the stability of secondary structure during these processes were analyzed in computational studies of biomolecular solvation in IL/water mixtures with varying ionic strength. In addition to the theoretical results on the interactions in ionic liquids, several program packages have been developed and improved in this project: The program code GEPETTO allows for common structural analysis, e.g. radial distribution functions, with optimized algorithms of Voronoi tessellation. This way, the project enabled the decomposition of structural and dynamical effects into shell specific contributions. The program package GENDICON calculates the complete frequency-dependent dielectric spectrum on the basis of rotational and translational contributions of the molecular species. As a result, the thorough interpretation of experimental dielectric spectra can be achieved.