Effects of shear flow on the structure and dynamics of ionic liquids in a metallic nanoconfinement

Molecular dynamics simulations of electrified interfaces including the metal polarisation

Understanding electrified interfaces requires an accurate description of the electric double layer which also takes into account the metal polarisation. Here we present a simple approach to the molecular dynamics simulation of electrified interfaces which combines fixed charges and a core-shell model for the description of the polarisable electron density on the metal electrode. The approach has been applied to the Au(111) surface in contact with a NaCl aqueous electrolyte solution in order to calculate the differential capacitance and to gain a detailed picture of the charging mechanism. Metal polarisation enhances the interfacial capacitance with a difference between the cathode and anode. In particular, we find that the influence of the metal polarisation on the electric double layer depends on the ion's solvation shell structure and, for the investigated interface, is more important at the cathode, where it modifies the sodium ion distribution.

Confined ionic liquids films under shear: The importance of the chemical nature of the solid surface

Ionic liquids have generated interest in applications as lubricants and as additives to conventional lubricants due to their unique physical properties. In these applications, the liquid thin film can be subjected simultaneously to extremely high shear and loads in addition to nanoconfinement effects. Here, we use molecular dynamics simulations with a coarse-grained model to study a nanometric film of an ionic liquid confined between two planar solid surfaces both at equilibrium and at several shear rates. The strength of the interaction between the solid surface and the ions was changed by simulating three different surfaces with enhanced interactions with different ions. The increase in the interaction with either the cation or the anion leads to the formation of a solid-like layer that moves alongside the substrates; however, this layer can exhibit different structures and stability. An increase in the interaction with the high symmetry anion produces a more regular structure that is more resistant to the effects of shear and viscous heating. Two definitions were proposed and used for the calculation of the viscosity: a local definition based on the microscopic characteristics of the liquid and an engineering definition based on the forces measured at the solid surfaces, with the former displaying a correlation with the layered structure induced by the surfaces. Because of the shear thinning behavior of the ionic liquids as well as the temperature rise brought on by viscous heating, both the engineering and the local viscosities decrease as the shear rate increases.

Role of density and electrostatic interactions in the viscosity and non-newtonian behavior of ionic liquids – a molecular dynamics study

Both viscosity and the shear-thinning of ionic liquids are determined mainly by ionic interaction, with density having a secondary effect.