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The self-diffusion coefficients of each component in mixtures of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([Bmim][N(SO CF ) ]) and acetonitrile were determined. The results suggest that the hydrodynamic boundary conditions change from "stick" to "slip" as the solvent composition transitions from "ionic liquid dissolved in acetonitrile" (χ <0.4) to "acetonitrile dissolved in ionic liquid" (χ >0.4). At higher χ , the acetonitrile species are affected by "cage" and "jump" events, as the acetonitrile molecules reside nearer to the charged centre on the ions than in the "non-polar" regions. The self-diffusion coefficients of hexan-1-amine, dipropylamine, 1-hexanol and dipropylether in mixtures of [Bmim][N(SO CF ) ] and acetonitrile were determined. In general, the nitrogen-containing solutes were found to diffuse slower than the oxygen-containing solutes; this indicates that there are greater ionic liquid-N interactions than ionic liquid-O interactions. This work demonstrates that the self-diffusion coefficients of species can provide valuable information about solvent-solvent and solvent-solute interactions in mixtures containing an ionic liquid.

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Self-diffusion measurements made by steady or pulsed field gradient spin−echo NMR are not absolute and the magnetic field gradients employed must normally be determined by calibration with liquids with known selfdiffusion coefficients. The primary calibrant is water, with selfdiffusion coefficient values having been extrapolated from the tracer diffusion of HDO and of HTO in ordinary water by Mills, 1 with a relative standard uncertainty of 0.2 %. This and other liquids presently used for calibration all have low viscosities. Current work on ionic liquids, which are generally quite viscous, suggests there may be problems with the pulsed field gradient (PGSE) techniques usually employed as results dependent on the time interval between gradient pulses have been reported by Hayamizu et al. 2 In this work, self-diffusion coefficients, obtained by a steady gradient (SG) technique, are reported for the viscous molecular liquids squalane, ethylhexyl benzoate, and bis(ethylhexyl) phthalate (DEHP), and it is suggested that these substances may be suitable secondary reference materials for the calibration of spin−echo NMR apparatus when self-diffusion in viscous liquids is to be measured. New PGSE measurements for squalane and DEHP are in good agreement with the SG results. We also report on systematic errors found in the secondary calibration data of Holz et al. 3 for cyclohexane, n-dodecane, dimethyl sulfoxide, and pentan-1-ol (though not for 1,4-dioxane) and suggest toluene in their place as a more convenient low-viscosity calibrant that is also suitable for low temperature work.

The Inside Back Cover picture shows the pulse sequence used to determine the diffusion coefficients of components of ionic liquid mixtures. Comparison with viscosity shows the change in the nature of solvation and that interactions with nitrogen centres are more dramatic than with oxygen centres. More information can be found in the Full Paper by J. Harper and co‐workers on page 3853 in Issue 23, 2016 (DOI: 10.1002/cphc.201600927).