Abstract
We perform molecular dynamics simulations to study the structure and dynamics of the ionic liquid [Omim][TFSI] in a broad temperature range. A particular focus is the progressing nanoscale segregation into polar and nonpolar regions upon cooling. As this analysis requires simulations of large systems for long times, we use the iterative Boltzmann inversion method to develop a new coarse-grained (CG) model from a successful all-atom (AA) model. We show that the properties are similar for both levels of description at room temperature, while the CG model shows stronger nanoscale segregation and faster diffusion dynamics than its AA counterpart at low temperatures. Exploiting these features of the CG model, we find that the characteristic length scale of the structural inhomogeneity nearly doubles to ∼3 nm when the temperature is decreased to about 200 K. Moreover, we observe that the nanoscale segregation is characterized by a bicontinuous morphology. In worm-like nonpolar regions, the ends of the octyl rests of the cations preferentially aggregate in the centers, while the other parts of the alkyl chains tend to be aligned parallel on a next-neighbor level and point outward, allowing for an integration of the imidazolium head groups of the cations into polar regions together with the anions, resembling to some degree the molecular arrangement in cylindrical micelles.
Highlights
Ionic liquids (ILs) are very interesting materials from the viewpoints of both fundamental and applied science
We will show that tail-end aggregation is key to the nanoscale segregation of the AA and CG models and, large fluctuations may be expected for the E–E interactions
Some deviations occur for the E–E radial distribution functions (RDFs), which shows a higher first neighbor peak for the CG model than for the AA model, implying that the former is slightly overstructured relating to the arrangement of neighboring tail ends, which will become more evident in the following analysis of nanoscale segregation
Summary
Ionic liquids (ILs) are very interesting materials from the viewpoints of both fundamental and applied science. Because its position and height depend systematically on the size of the cation, this prepeak was often taken as evidence for the emergence of cation clusters [6, 7, 9], and other interpretations were put forward [8] This structural inhomogeneity of ILs comes along with complex and heterogeneous dynamics, in particular, at reduced temperatures [11,12,13,14]. The high chemical accuracy comes along with big computational efforts and, interferes with the calculation of long trajectories for large systems, as required for studies of nanoheterogeneous structures and dynamics, in particular, at reduced temperatures To overcome this computational obstacle, it is desirable to develop coarse-grained (CG) variants from successful AA models. Because ILs are prototypical glass-forming liquids, this knowledge is useful to improve our understanding of the complex structuredynamics relations in the supercooled state in general
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