An MD Study of the Applicability of the Walden Rule and the Nernst–Einstein Model for Ionic Liquids

Abstract

Six different ionic liquids containing imidazolium, pyrrolidinium and ammonium cations paired with bis[(trifluoromethyl)sulfonyl]imide and bis[(perfluoroethyl)sulfonyl]imide anions were studied using molecular dynamics simulations. Ionic conductivities, shear viscosities and self-diffusivities were computed and compared to see what relationships exist between these three transport properties. The simulations suggest that these ionic liquids follow closely the Walden rule, which relates ionic conductivity to fluidity. The computed ionic conductivities agree well with those predicted from the Nernst-Einstein model, suggesting that long-time correlation among ion velocities is small, or that the "ionicity" of these ionic liquids is high.

Bridging Database Analysis with Microrheology to Reveal Super-Hydrodynamic Conductivity Scaling Regimes in Ionic Liquids

“…One possible explanation for the surprising temperature dependence of the Nernst-Einstein ratio presented in Figure 4 is a pronounced decoupling of the conductivity and viscosity arising from ioncorrelations and non-classical ion-transport mechanisms, as discussed in prior works. 4,64 This agrees with a developing body of work that suggests alternative meanings to the free ion fraction interpretation based on ionic correlations, the presence of charge transfer events, or proton transfer in protic ionic liquids, 52,[65][66][67][68] which may contribute to different temperature dependencies of conductivity and viscosity.…”

“…Our database analysis supports earlier suggestions that deviations from Nernst-Einstein are best explained by ionic correlations, 52,65,67,69 which can be accounted for using diffusivities determined via velocity correlation coefficients. 35,[67][68][69][70][71] Since individual ion trajectories are required, this approach is limited to molecular dynamics studies, though the concept translates to experimental studies since some superconcentrated electrolytes deviate from the Nernst-Einstein model, as the model does not account for ionic correlations. Accordingly, deviations may indicate differences in ionic liquid transport mechanisms.…”

Structure and dynamics of CO2and N2in a tetracyanoborate based ionic liquid

“…1) to be consistent with the generalized AMBER force field (GAFF), 20 following the established procedure that has been successfully applied in many studies. [21][22][23] Briefly, the total energy is expressed in terms of bond stretching, angle bending, dihedral torsion, as well as van der Waals and electrostatic interactions. Atom type, bond, angle and improper terms were taken directly from the GAFF, while partial charges were computed from the ab initio calculations on individual ions using the restrained electrostatic potential (RESP) method.…”

Ionic Liquids: A Simple Model to Predict Ion Conductivity Based on DFT Derived Physical Parameters

“…Indeed, there are many different standard modeling approaches applied to ILs from ab initio , DFT, MD, to ab initio molecular dynamics (AIMD) (Del Pópolo et al, 2005; Tsuzuki et al, 2005; Borodin, 2009; Kirchner, 2009; Maginn, 2009; Angenendt and Johansson, 2010; Johansson et al, 2010; Ueno et al, 2010; Liu and Maginn, 2012; Tsuzuki, 2012), and more analytical methods (Abbott, 2004, 2005; Matsuda et al, 2007; Slattery et al, 2007; Tochigi and Yamamoto, 2007; Preiss et al, 2010; Eiden et al, 2011). All aim at predicting/estimating melting points (Slattery et al, 2007; Preiss et al, 2010), viscosities (Abbott, 2004; Matsuda et al, 2007; Slattery et al, 2007; Tochigi and Yamamoto, 2007; Ueno et al, 2010; Eiden et al, 2011), and not the least the ionic conductivities (Abbott, 2005; Del Pópolo et al, 2005; Tsuzuki et al, 2005; Matsuda et al, 2007; Slattery et al, 2007; Tochigi and Yamamoto, 2007; Borodin, 2009; Johansson et al, 2010; Ueno et al, 2010; Eiden et al, 2011; Liu and Maginn, 2012; Tsuzuki, 2012). While in general highly successful for their purposes, they often, however, have either limited accuracy, are time-consuming or require ion-specific or empirical parameters.…”