Hydrated Ionic Liquids with and without Solute: The Influence of Water Content and Protein Solutes

In this computational study, the network of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate/water mixtures is analyzed in the presence (and absence) of the protein ubiquitin and a zinc finger motif. Thereby, common radial distribution functions are decomposed into contributions from different Voronoi shells, and the mutual orientation of cations, anions, and water in the bulk phase as a function of the water mole fraction is discussed. Single particle translation and the reorientation of the dipolar axis seem to follow hydrodynamic relations. Using the body-fixed frame as an alternative reference system, translation and rotation can be decomposed into contributions along and about the axes of a well-defined orthogonal trihedron, thus elucidating the principal motions of the cations and anions as a function of the water mole fraction. The structural dipolar orientation may be correlated with single particle dynamics and can be characterized by the static collective Kirkwood order parameter.