Influence of Charge Scaling on the Solvation Properties of Ionic Liquid Solutions

Abstract

Scaled-charge force fields (FFs) are widely employed in the simulation of neat ionic liquids (ILs), where the charges on the ions are empirically scaled to approximately account for electronic polarization and/or charge transfer. Such charge scaling has been found to yield significant improvement in liquid-state thermodynamic and dynamic properties (when compared to experiment). However, the mean field approximation inherent in charge scaling becomes suspect when applied to IL mixtures or solutions. In this work, we simulate solutions of IL with various nonpolar and polar gas solutes and compare results of charge-scaled and polarizable FFs to experiment. Our results demonstrate that scaling of the Coulomb interaction inherent in scaled-charge FFs leads to an underestimation of the solute–solvent electrostatic interaction and thus also the enthalpy and free energy of solvation; this effect is particularly pronounced for polar solutes. In some cases, we find that this artificial reduction in the solute–solvent interaction can also alter the apparent phase behavior of the resulting solution. Overall, the totality of our results suggests that explicit polarization (rather than charge scaling) is likely necessary to provide high transferability to both neat IL and IL mixtures and solutions.

Charge transfer and polarisability in ionic liquids: a case study

“…2 identifies Δ q total ≈ 0.2 e , related to a scaling factor of ≈0.8, which is commonly used for ILs. 15,19,20,27,28,30–33 Using a simple linear relationship y = mx + c where x = Δ q total , the slope ( q neutral i − q charged i )/Δ q total (for Δ q total = 1) and intercept q charged i (for Δ q total = 0) leads to eqn (3). q i = Δ q total ( q neutral i − q charged i ) + q charged i Another way of accessing information on the impact of charge transfer is to consider Fukui functions. The Fukui function f + ( r ) is the density difference between the cation and its neutral analogue (with one electron added), eqn (4) ‡…”

“…One straightforward way of recovering some of the charge transfer and polarisation contributions is to adapt the atomic charges to emulate the average effects of charge transfer and polarisation. 25–29,119 The Coulomb potential U Coulomb is defined in eqn (1) where q C is the charge on the cation, q A is the charge on the anion. 18 Typically q C and q A are formally assigned based on the molecular species, i.e.…”

“…Scaling factors around 0.8 are commonly used in MD simulations of ionic liquids. 15,19,20,23,27,28,30–33 The rationale behind the scaling is that the charge transfer and polarisation effects are encapsulated within the empirical dielectric constant ε r . q C and q A are now “effective” charges, taking into account a range of underlying processes.…”

The CL&Pol polarizable force field for the simulation of ionic liquids and eutectic solvents

“…Unfortunately, with scaled charges densities tend to become too low, by up to 5 %, and in mixtures and solutions the balance between solute-solute, solute-solvent and solvent-solvent interactions may be modified, compromising the prediction of solubility, miscibility and phase behaviour. 39 Studying mixtures and solutions involves calculations for molecular or ionic species in different environments and a charge scaling factor optimized for a certain state may not be transferable to different environments. 38 Charge scaling also affects dielectric properties.…”

Confined Ru‐catalysts in a Two‐phase Heptane/Ionic Liquid Solution: Modeling Aspects

“…[16,45] This approach is commonly used to study bulk IL solutions. However, alternative approaches do exist (refer to [47,48] ). In order to resemble typical experimental conditions [19] all simulations were conducted at T = 353 K. The temperature was controlled by an improved velocity-rescaling thermostat [49] using a coupling time constant of 0.1 ps.…”