Theoretical and computational study of static and dynamic properties of homogeneous and inhomogeneous mixtures of ionic liquids

Resumen

Ionic liquids, defined as salts composed solely of ions that can be found in the liquid state under ambient conditions, have been known for about a century. However, although their discovery dates back to 1914 (when Walden described ethylammonium nitrate as an ionic liquid with a melting point of 287:6 K), it was not until the last decades that the interest and activity in the field of ionic liquids has experienced an exponential growth, far surpassing mere scientific or academic curiosity to become potential candidates to various industrial applications (in electrochemical devices, as thermal storage fluids, as lubricants or in the area of separation). Moreover, ionic liquids have many fascinating properties that characterize them as ¿green solvents", such as low vapor pressure, thermal stability up to high temperatures, high electrical and thermal conductivity and very good solvents properties for a wide variety of compounds. Thus, the possibility of customizing ionic liquids for many specific demands, making use of the nearly infinite combinations of suitable cations and anions, makes them to be termed "designer solvents". This extent of tunability makes computational models to be of fundamental importance to systematically explain and predict the properties of ionic liquids. Due to the huge number of ionic liquids that can be produced, and also to the fact that small variations of the ions composing an ionic liquid lead to a substantial alteration of its characteristics, computer simulations can be used with the aim of knowing their properties a priori instead of through trial-and-error tests. Not only is this approach less time consuming but it also allows researchers to reduce a lot the money involved in the process. This can be observed in the increase in the number of computational works published along the last decades, as well as in the more and more numerous efforts devoted to the development of new potentials and more powerful computational tools. In the present thesis we present a computational study of structural and transport properties of ionic liquids. By means of molecular dynamics simulations we investigated the effect of several solutes (water, alcohols and salts of electrochemical interest) on the behaviour of these dense ionic solvents. Despite the increasing interest that ionic liquids have been generating, the solvation mechanism of diverse molecules and ions in these novel compounds is still scarcely comprehended. Additionally, the vast majority of the huge number of studies of ionic liquids and their mixtures reported up to now has been performed using experimental techniques, whereas the computational efforts have remained more little in spite of being an essential tool for getting a detailed knowledge of ionic liquids at the atomic level. By that means, the main goal of this research is to provide a deeper knowledge of how molecular level properties of ionic liquids are affected by the addition of the former solutes to the mixture. The whole set of simulations included in this thesis was carried out using the open source and free software Gromacs 4:5:4 package.