Diffusion coefficients in binary electrolyte mixtures by dynamic light scattering and molecular dynamics simulations

Abstract

In this work, three binary electrolyte mixtures represented by different molecular solvents and dissolved lithium bis(trifluoromethylsulfonyl)imide ([Li][NTf2]) are characterized by determining their Fick diffusion coefficient D11. For this, dynamic light scattering (DLS) and molecular dynamics (MD) simulations were applied in the temperature range between (293.15 and 398.15) K at a solute mole fraction x[Li][NTf2] of 0.05. DLS is a contactless measurement technique that requires no calibration and allows to investigate multiple thermophysical properties, including D11, in macroscopic thermodynamic equilibrium. To get reliable thermophysical properties for the binary mixtures from MD simulations, force fields (FFs), describing inter- and intramolecular interactions between atoms, for the pure solvents were taken from the literature and reparametrized. The objective for the reparameterization was to accurately predict the static dielectric constant ε, density ρ, and viscosity η of the solvents. Using the optimized FFs, D11 for the binary electrolyte mixtures could be predicted via MD simulations and the results agree with those from DLS within an average absolute relative deviation of 23%. Most data points agree within combined uncertainties, considering an average expanded statistical uncertainty (coverage factor k = 2) of 18% for D11 from MD simulations and an average expanded experimental uncertainty (k = 2) of 10% for D11 from DLS. η and ε of the solvent were found to correlate with D11 and, thus, the diffusive mass transport in electrolyte mixtures. A large η leads to small D11, due to the slower dynamics in the system. Also a small solvent ε leads to small D11, which is due to the formation of ion pairs or clusters, which hinders the free movement.