Coarse-grained model of a nanoscale-segregated ionic liquid for simulations of low-temperature structure and dynamics

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.

Understanding and Modeling Polymers: The Challenge of Multiple Scales

“…From the point of view of time bridging, both the speedup and the telescoping are beneficial, as they allow one to access later time scales in CG simulations and study processes on different time scales simultaneously in one simulation. One of the earliest and still widely and successfully used approaches to dynamic coarse-graining has been to simply take advantage of this effect, determine the speedup factor of the process of interest, and use this to map the CG dynamics on real dynamics, − taking into account that the speedup factor might be different for different processes and/or components .…”

Editorial: Multiscale simulation methods for soft matter systems

“…The articles in this special issue illustrate the progress that has been made in the field of multiscale modelling of soft matter in recent years, as well as the many remaining challenges and open questions. They also demonstrate the potential of multiscale simulations for obtaining insights into complex systems and processes, such as nanoscale-segregated ionic liquids [17], high performance rubber materials [28], emergent properties of active or responsive colloids [11,23], or controlled nanoparticle growth [29,30].…”

“…However, in studies of kinetic pathways, it is important that at least the order of time scales remains the same. Kloth et al [17] and Rudzinski et al [18] have examined this for ionic liquids and found that accurate structural coarse-graining also improves the dynamic properties of CG models in regular molecular dynamics. Li et al [20] propose a method to adjust relative time scales in extremely CG Brownian polymer models by adjusting internal friction parameters.…”

The correlation between the micelle morphology of surface-active ionic liquids with self-assembly and thermodynamic characteristics: coarse-grained MD simulation and experiment

“…MD simulations have been extensively used to investigate the structural and physicochemical characteristics of ionic liquids and surfactants in recent years. [22][23][24][25][26][27][28][29] Some in silico studies were focused on the aggregation properties of ILs in different solutions and interfaces in terms of the structure, shape, and size of micelles. 22,23,[30][31][32] To the best of our knowledge, only a few reports are available on the effective factors that influence the nature of SAILs' self-assembly and their morphology.…”