Analysis on Ionic Liquid System under an Electric Field by Molecular Dynamics Simulation

Abstract

Ionic liquids have unique characteristics including low volatility, non-flammability and high conductivity, and are expected to find diverse applications such as electrolyte materials. When ionic liquids are confined to a nanostructure, however, it behaves differently from the bulk. A number of researches have emphasized the importance of the physical phenomena far from the bulk, such as electric double layer (EDL) overlapping or co-ion exclusion in nano-scale system. Existing continuum models, however, have limitations to investigate those far from bulk. Thus, the atomistic simulations (e.g., molecular dynamics or Monte Carlo simulations) have been received considerable attention as alternative analysis methods. Among them, molecular dynamics (MD) simulations are the preferable tools because it take account time into calculation. They can predict not only static but also dynamic properties in a nanoconfinement. In this thesis, MD simulations are used in order to investigate the electrostatic and dynamic properties of ionic liquids confined to a nanometer scale system. It is expected to provide the basic understanding for the microscopic characteristics of ionic liquids under an external electric field. At first, the confinement effect on the electrostatic characteristics are investigated. Both MD and continuum (Bazant-Storey-Kornyshev) methods are used to analyze the pressure acting on the wall and EDL structure of primitive ions in nanoslits. The slit width varies from 1 to 20 times of the ion size, in which the BSK model may not be compatible. However, the comparison with the MD model shows that the continuum model can successfully predict the averaged charge density and decay of the pressure. The continuum model also shows the exact location of the 1st and 2nd layer of ions from the confinement wall when considering both steric and correlation effects. However, the continuum model does not account for the oscillatory behaviors of the ion density and the pressure due to the typical characteristics of ionic discreteness. Also, it slightly overestimates the counter-ion concentrations compared with the MD analysis. In conclusion, the continuum model can be used to estimate the overall tendency of the pressure acting on the wall, although the discreteness of ions cannot be predicted exactly. It can provide useful information in designing nano-porous structures for various electrochemical applications. Then the unique negative pressure have been focused on, which occurs when the nanopore size becomes extremely small. When the interaction between a charged pore and its counter-ions is predominantly electrostatic, the internal electrolyte can exert a force that contracts the pore’s wall. As a result, the total pressure becomes negative. Predicting how the size and shape of ions in ionic liquids influences the negative pressure is difficult. Here, molecular dynamics simulations are used to perform this analysis. Negative pressure is observed when the slit width is less than 1 nm in scale, due to competition between the contact and electrostatic components of the total pressure. The negative pressure is determined to have order of 103 atm, and is 20 to 40 times higher than the bulk osmotic pressure of each ionic liquid. Ion shape and size are found to affect the contact component, but these factors barely affect the negative pressure when the electrostatic component is dominant. It is because the ions re-arrange their positions in the slit. This study is expected to help to explain the unfamiliar concepts that can occur in realistic cases, and how characteristics of ionic liquids affect the pressure acting on the pore walls. On the other hand, the dynamics of ionic liquids under an external electric field are also covered in this thesis. A unique behavior of cation depletion of an ionic liquid droplet is studied via MD simulations. 200 ion pairs of 2 combinations of ionic liquids (EMIM-NTf2 and EMIM-ES) are analyzed to clearly explain mechanism of the phenomenon. Shape deformation due to electric stress and ion distributions inside the droplet are calculated to validate the simulation. The intermolecular force between a single pair of ions can be significantly different due to the nature of the structure and charge distribution of the ions. Together with an analytical interpretation of the conducting droplet in an electric field, we show that the MD simulation successfully explains the mechanism of selective ion depletion of an ionic liquid droplet in an electric field and the retreating motion of the droplet observed by experiments.