Distributed polarizability models for imidazolium-based ionic liquids

Unraveling the Morphology of [CnC1Im]Cl Ionic Liquids Combining Cluster and Aggregation Analyses

A characteristic feature of ionic liquids is their nanosegregation, resulting in the formation of polar and nonpolar domains. The influence of increasing the alkyl side chain on the morphology of ionic liquids has been the subject of many studies. Typically, the polar network (charged part of the cation and anion) constitutes a continuous subphase that partially breaks to allow the formation of a nonpolar domain with the increase of the alkyl chain. As the nonpolar network expands, the number of tails per aggregate increases until the ionic liquid percolates. In this work, we demonstrate how the complementary software packages TRAVIS and AGGREGATES can be employed in conjunction to gain insights into the size and morphology of the [CnC1Im]Cl family, with n ∈ {2, 4, 6, 8, 10, 12}. The combination of the two approaches rounds off the picture of the intricate arrangement and structural features of the alkyl chains.

Using atomic charges to model molecular polarization

We review different models for introducing electric polarization in force fields, with special focus on methods where polarization is modelled at the atomic charge level. While electric polarization has been included in several force fields, the common approach has been to focus on atomic dipole polarizability. Several approaches allow modelling electric polarization by using charge-flow between charge sites instead, but this has been less exploited, despite that atomic charges and charge-flow is expected to be more important than atomic dipoles and dipole polarizability. A number of challenges are required to be solved for charge-flow models to be incorporated into polarizable force fields, for example how to parameterize the models and how to make them computational efficient.

Molecular Dynamics Simulations of Ionic Liquids and Electrolytes Using Polarizable Force Fields

Many applications in chemistry, biology, and energy storage/conversion research rely on molecular simulations to provide fundamental insight into structural and transport properties of materials with high ionic concentrations. Whether the system is comprised entirely of ions, like ionic liquids, or is a mixture of a polar solvent with a salt, e.g., liquid electrolytes for battery applications, the presence of ions in these materials results in strong local electric fields polarizing solvent molecules and large ions. To predict properties of such systems from molecular simulations often requires either explicit or mean-field inclusion of the influence of polarization on electrostatic interactions. In this manuscript, we review the pros and cons of different treatments of polarization ranging from the mean-field approaches to the most popular explicit polarization models in molecular dynamics simulations of ionic materials. For each method, we discuss their advantages and disadvantages and emphasize key assumptions as well as their adjustable parameters. Strategies for the development of polarizable models are presented with a specific focus on extracting atomic polarizabilities. Finally, we compare simulations using polarizable and nonpolarizable models for several classes of ionic systems, discussing the underlying physics that each approach includes or ignores, implications for implementation and computational efficiency, and the accuracy of properties predicted by these methods compared to experiments.

Rheology of phosphonium ionic liquids: a molecular dynamics and experimental study

Comparison between the theoretical and experimental viscosity of an ionic liquid.

Additive polarizabilities in ionic liquids

An extended designed regression analysis of experimental data on density and refractive indices of several classes of ionic liquids yielded statistically averaged atomic volumes and polarizabilities of the constituting atoms. These values can be used to predict the molecular volume and polarizability of an unknown ionic liquid as well as its mass density and refractive index. Our approach does not need information on the molecular structure of the ionic liquid, but it turned out that the discrimination of the hybridization state of the carbons improved the overall result. Our results are not only compared to experimental data but also to quantum-chemical calculations. Furthermore, fractional charges of ionic liquid ions and their relation to polarizability are discussed.