(197w) Molecular Dynamics Simulations and Graph-Theoretic Analyses of Nanostructure Formation in Ionic Liquids

Ionic liquids (ILs), such as 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF₄), have been scrutinized as viable electrolytes for electrochemical reactions due to their highly tunable cation/anion properties, intrinsic conductivity, wide electrochemical stability window, low volatility, and strong electrostatic interactions [1-3]. However, the molecular structure of ILs in bulk and dilute solutions remain poorly understood and prevents the rational design of ILs [4-6]. Molecular simulations can address this problem by modeling the formation of IL nanostructures that is difficult to observe via experiments. Using both the concentration of IL-solvent mixtures knobs, we performed atomistic molecular dynamics (MD) simulations to quantify concentration-dependent changes in IL structure. From MD trajectories, we determined how interactions between the IL and solvent impact structure through graph-theoretic representations of IL nanostructure. We show that EMIM-BF₄ exhibits variations in the tendency to form both charged and neutral molecular-scale clusters as a function of concentration ranging from the dilute to neat regimes. Via these graph-theoretic methods, we investigated quantitative parameters of IL structure (such as length of nonpolar alkyl group) that influence IL structural clustering such as: size of clusters, number of clusters, and number of neutral clusters. These quantitative metrics will permit understanding of how IL structure impacts electrolyte properties which is necessary for fine-tuning electrochemical systems.

[1] K. Shimizu, C. E. S. Bernardes, and J. N. Canongia Lopes, "Structure and Aggregation in the 1-Alkyl-3-Methylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquid Homologous Series," The Journal of Physical Chemistry B, vol. 118, no. 2, pp. 567-576, 2014.

[2] K. Motobayashi, Y. Maeno, and K. Ikeda, "In Situ Spectroscopic Characterization of an Intermediate of CO2 Electroreduction on a Au Electrode in Room-Temperature Ionic Liquids," The Journal of Physical Chemistry C, vol. 126, no. 29, pp. 11981-11986, 2022.

[3] B. A. Rosen et al., "In Situ Spectroscopic Examination of a Low Overpotential Pathway for Carbon Dioxide Conversion to Carbon Monoxide," The Journal of Physical Chemistry C, vol. 116, no. 29, pp. 15307-15312, 2012.

[4] B. A. Rosen et al., "Ionic Liquid-Mediated Selective Conversion of CO2 to CO at Low Overpotentials," (in English), Science, Article vol. 334, no. 6056, pp. 643-644, Nov 2011, doi: 10.1126/science.1209786.

[5] M. A. Gebbie et al., "Long range electrostatic forces in ionic liquids," Chemical Communications, vol. 53, no. 7, pp. 1214-1224, 2017.

[6] B. Liu, W. Guo, and M. A. Gebbie, "Tuning Ionic Screening To Accelerate Electrochemical CO2 Reduction in Ionic Liquid Electrolytes," ACS Catalysis, vol. 12, no. 15, pp. 9706-9716, 2022.

[7] A. M. Smith, A. A. Lee, and S. Perkin, "The Electrostatic Screening Length in Concentrated Electrolytes Increases with Concentration," The Journal of Physical Chemistry Letters, vol. 7, no. 12, pp. 2157-2163, 2016.