Non-equilibrium molecular dynamics of ionic liquids

Abstract

Many low temperature molten salts, the so-called ionic liquids, are viscous liquids with complex phase diagrams, as glass transition or crystallization takes place depending on the rate of change of temperature and pressure. In the Laboratório de Espectroscopia Molecular, LEM/IQ-USP, we have been studying Thermodynamics, structure, and dynamics of ionic liquids by calorimetry, X-ray and Raman spectroscopies, and also by computer simulation by equilibrium molecular dynamics. In this research project, we will broaden the computational approach by non-equilibrium molecular dynamics (NEMD). One motivation for NEMD simulations comes from the fact that ionic liquid can be found in mesophases, for which a deep understanding of its nature on the basis of a microscopic view of structure and dynamics is still lacking. NEMD simulations of the systems under different shearing rates will allow for a molecular basis of the rheology of ionic liquids. Furthermore, a promising application of ionic liquids is their usage as lubricants, so that within the context of tribology it is demanding the knowledge of structural and dynamics properties of the fluid under different situations such as under load on the surfaces in relative motion. In this project we will calculate viscosity of ionic liquids under shearing and load in order to relate the apparent viscosity to the structural changes resulting from the non-equilibrium condition. The computer simulations will be also used for the direct calculation of the time correlation function of structural relaxation of ionic liquids in non-equilibrium condition. Models commonly used in computer simulations of ionic liquids, i.e. based on a pair-wise potential energy function, give high viscosity, low ionic diffusion and conductivity of the simulated system in comparison to experimental data. Thus, in the project we will also perform NEMD simulations with polarizable models in order to assess the polarization effects on the viscoelasticity of ionic liquids.