Effects of Electric Charge, Dipole Moment, and Molecular Size on the Viscosity of Ionic Liquids*

The origin of the viscosity of ionic liquids is often unclear and controversial because our understanding of the local dynamics and collective long-range dynamics remains limited. To address this issue, we developed our Stockmayer-fluid molecular dynamics simulation method for ionic liquids that accounts for the electric polarization at the molecular level. Our coarse-grained simulation models consist of minimal sets of model parameters, yet they provide good agreement with the observed viscosity and glass transition temperature of ethylammonium nitrate. We highlight the effects of electric charge, dipole moment, and molecular size on viscosity and glass transition temperature, and discuss the versatility of our simulation method. We show that the rotational dynamics of the ions is critical to account for the experimental observations. In contrast, the effect of the electric charges on the viscosity and glass transition temperature appears relatively weaker.