[BMIm][BARF] imidazolium salt solutions in alkyl carbonate solvents: Structure and interactions

Abstract

Solutions of weakly coordinating ionic liquids (ILs) in alkyl carbonates are gaining growing attention, as the latter are "green" solvents with high solvation power, but the phase behavior and structure of ILs in organic polar solvents are still poorly understood. Here, we study the interactions and nanoscale structure of 1-butyl-3-methylimidazolium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, [BMIm][BARF], in three symmetrical alkyl carbonate solvents with increasing alkyl chain-length. Electrical conductivity and nuclear magnetic resonance measurements showed that [BMIm][BARF] was mostly undissociated in these solvents, especially at lower IL concentration. Small angle X-ray scattering patterns evidenced the presence of rod-like nanostructures in the IL/solvent mixtures. At higher IL concentration, [BMIm][BARF] is increasingly more dissociated in solvents with lower dielectric constant, as confirmed by analysis of the solvents' carbonyl stretching band via Fourier transform infrared spectroscopy. This trend is opposite to that exhibited by BMIm ILs with less bulky counterions. The bulky BARF(-) is weakly coordinating and has no ability to give strong H-bonding, thus short-range anisotropic van der Waals forces are likely key in the interaction of the ion pairs. The slower self-diffusion of the ions in alkyl carbonates with lower dielectric constants might partially hinder close contact needed for self-assembly into local nano-sized structures. Overall, our results shed light on interactions and self-organization in imidazolium salt-alkyl carbonate mixtures, with potential impact in applicative fields spanning from batteries, catalysis and extraction, up to bio-applications (antimicrobial and bioengineering)

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