(709i) Topological Data Analysis and Molecular Simulations for Identifying Nanostructure Transitions in Ionic Liquids

Ionic liquids (ILs) are room temperature salts that have garnered attention as versatile electrolytes and designer solvents owing to their highly modifiable properties based on their cation and anion constituents. Unlike conventional solvents, ILs can self-assemble to exhibit nanostructures that can significantly impact IL properties such as polarity, hydrophobicity, and heterogeneity characteristics [1, 2]. These changes occur due to alterations in ion arrangements, inducing different molecular domains [3]. Variations in ion structuring can lead to incompatible molecular arrangements and subsequent segregation into different phases on a small scale [4], [5], [6]. However, the molecular-scale structure of neat ILs remains inadequately characterized and difficult to predict, hindering their rational application. Molecular simulations offer a viable approach to model the dynamic formation of IL structure but quantifying structural properties remains challenging the dynamic nature of IL structure, particularly in relation to its underlying molecular architecture.

In this work, we demonstrate a novel post-processing method that uses topological data analysis to convert molecular simulation data into a density field resulting in a Euler characteristic signature curve that identifies an IL structural transition. Atomistic molecular dynamics (MD) simulations were employed to quantify structure-dependent variations in 1-alkyl-n-imidazolium tetrafluoroborate (n-Mim BF4) pure ionic liquids as the dataset. For this specific imidazolium cation series, increasing the alkyl chain length increases the solvophobic interactions between the nonpolar alkyl chains and overpowers the electrostatic interactions of the cation and anion which in turn create a different nanostructure of ions. Literature has shown evidence that in the imidazolium cation family, the length of the alkyl chain at C=4 regime indicates significant change in nanostructure is observed computationally [7], [8], [9]. Nanostructure separation within the imidazolium cationic family is crucial, as it allows for precise tuning of ionic liquid properties based on variations in ionic structures. For each datapoint, we converted the MD trajectory into a density field resulting in a Euler characteristic curve where each n-Mim BF4 (n = 2, 3, 4, 5, 6, 8, 10, and 12) obtained their own “signature”.

We demonstrate that the Euler characteristic curve identifies revealed a nanostructure transition in the IL imidazolium homologous series at n = 4 with less computational resources. At low alkyl chain lengths, we observe the ions dispersed as a homogenous nanostructure, but as the length of the alkyl side chain length increases, we observe a structural evolution where the nanostructure clusters heterogeneously with polar and non-polar regimes agglomerated. Investigating these effects of varying alkyl chain lengths on specific nanostructure transitions will contribute valuable insights to the literature on imidazolium cationic families and other IL systems.

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