(534i) Evidence for Self-Assembly of Tetra-Alkyl-Phosphonium 2-Cyano-Pyrrolide Ionic Liquids + Triethanolamine, Glycerol and Ethylene Glycol Systems

We use multiple characterization methods including viscosity, conductivity, density and surface tension measurements to investigate the anomalous transport behavior of multiple tetra-alkyl-phosphonium-2-cyano-pyrolide ILs in glycerol, triethanolamine, and ethylene glycol solvents. The viscosity behavior as a function of IL concentration of IL + glycerol and triethanolamine systems demonstrate both increasing and decreasing regimes, which cannot be explained by traditional mixing laws. By characterizing the conductivity and surface tensions of these systems we find significant evidence of microstructure formation driven by the amphiphilic nature of the tetra-alkyl-phosphonium cation [1], thus accounting for the anonymous viscosity behavior. While micelle formation is well known for IL+ water systems, the observation of these microstructures in non-aqueous systems is significant. The conductivity profiles as a function of IL concentration shift drastically at the critical micelle concentration cmc, at which point the ILs undergo solvophobic self-assembly, and this phenomenon is observed several IL/solvent permutations. The cmc is observed to occur at more dilute IL concentrations for ILs containing longer alkyl chain lengths as well as solvents with higher dielectric constant (i.e glycerol v. triethanolamine). We further characterize these systems using Small Angle X-ray Scattering (SAXS) allowing us a more complete understanding of the microstructures present in these systems.

[1] I. Mukherjee, S. Mukherjee, B. Naskar, S. Ghosh, and S. P. Moulik, “Amphiphilic behavior of two phosphonium based ionic liquids,” J. Colloid Interface Sci., vol. 395, no. 1, pp. 135–144, 2013.