Development of United Atom Model for Ionic Liquids and Studying Their Structure and Dynamics on Gold Surfaces Under Various Bias Voltages

Over the past two decades, ionic liquids (ILs) have become very popular solvents in various devices, particularly in electrochemistry. At charged surfaces such as electrodes, IL ions create denser electric double-layer (EDL) structures compared to traditional field-effect transistor (FET) surfaces, leading to differences in the performance of electric double layer transistors (EDLT). However, the relationship between their structure and device function remains unclear. Advanced scanning force microscopy, such as three-dimensional force microscopy (3D-SFM), allows for the visualization of 3D force distribution and is a cutting-edge technology [ 1 ] to reveal the sub-nanoscale 3D distribution of ionic liquids on the charged surface to elucidate the relationship. Our laboratory successfully imaged bias-dependent changes in the interfacial structures of DEME-TFSI sandwiched between Au (111) surfaces at sub-nanoscale resolution by 3D-SFM, and their dynamics at different bias voltages were reproduced using molecular dynamics simulations. Recent research has shown that electrolytes composed of oligomeric molecules, such as IL4-TFSI and IL2-TFSI, outperform monomeric BMI-TFSI and DEME-TFSI in terms of generating high EDL capacitance [2]. Specifically, it has been reported that the bias dependence of charge accumulation in EDLTs varies with the cation species of the ionic liquid, necessitating a comparison of these differences at the molecular scale.<br/><br/>In this research, we developed united-atom models of BMI, IL2, and IL4 cations and simulated the dynamics of BMI-TFSI, IL2-TFSI, and IL4-TFSI sandwiched between Au (111) surfaces at various biasing voltages. The charges on BMI, IL2, and IL4 were calculated by <i>ab initio</i> calculation using Gaussian09 at the MP2 level with a 6-311+G* basis set followed by ESP assignment. The charges on hydrogens were summed into heavy atoms. Partial charges were scaled along with Lennard-Jones parameters of <i>σ</i> to simulate the ionic liquids, particularly to reproduce the viscosity at 300K. A bulk equilibrium system consisting of 1,000 BMI and 1,000 TFSI molecules was prepared by running a 500 ns simulation under successively changing conditions: 10 ns in NPT, 390 ns in NVT, and 100 ns in NVE. This equilibrated system was placed between 5,376 gold atoms arranged in six layers, with three layers at the bottom and three at the top. Charges were uniformly assigned to the gold atoms in the layer closest to the bulk when biased between -1V and +1V, and the system was further equilibrated at different biasing voltages.<br/><br/>To validate our equilibrium state, we calculated viscosity from mean square displacements and used the Poisson-Boltzmann equation to calculate the biased voltages. 3D-SFM imaging by our laboratory of the interfaces between BMI-TFSI and Au (111) electrodes showed multiple layer-like contrasts and their dependence on biased voltages. Additionally, ongoing studies are being conducted on IL2-TFSI and IL4-TFSI. Hence, we are trying to reproduce such voltage-dependent multilayer formation using MD simulations.<br/><br/>References:<br/>1. Fukuma, Takeshi, and Ricardo Garcia. "Atomic-and molecular-resolution mapping of solid-liquid interfaces by 3D atomic force microscopy." <i>ACS nano</i> 12, no. 12 (2018): 11785-11797.<br/><br/>2. Matsumoto, Michio, Sunao Shimizu, Rina Sotoike, Masayoshi Watanabe, Yoshihiro Iwasa, Yoshimitsu Itoh, and Takuzo Aida. "Exceptionally high electric double layer capacitances of oligomeric ionic liquids." <i>Journal of the American Chemical Society</i> 139, no. 45 (2017): 16072-16075.