Ionic liquids show great industrial potential because they can be designed for different functions by selecting specific compositional cations or anions. While synthesizing ionic liquids is generally straightforward, the system’s dynamics are not entirely predictable. This is because the ionic coordination and alkyl-tails on the cation establish polar and non-polar nano-regimes, respectively, producing a heterogeneous structure. Fluorescent dye molecules exhibit biphasic diffusion in ionic liquids through these heterogeneous structures, revealing preference for either the ionic liquid’s polar or non-polar regimes. We explored how biphasic diffusion changes with alkyl-tail length using fluorescence correlation spectroscopy (FCS) to measure the diffusion of two fluorescent dyes that have a polar or non-polar regime preference. FCS measures the diffusion of the dye molecules by monitoring fluorescence intensity fluctuations as they move through a small confocal volume defined by the focus of the laser beam. We used fluorescent dyes rhodamine 6G (R6G) and 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) in four ammonium-based ionic liquid systems, with R6G being found primarily in the polar regime, while DCM is found primarily in the nonpolar regime because. Probing the individual regimes is essential because these regimes do not contribute equally to the ionic liquid’s bulk properties. The fundamental understanding from our experiments allows other scientists to design appropriate ionic liquids for specific industrial applications.

Measuring molecular diffusion in ammonium-based ionic liquids using fluorescence correlation spectroscopy

2019 Spring National Meeting

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