Anne Kaintz - Solute Diffusion in Ionic Liquids, NMR Measurements and Comparisons to Conventional Solvents

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      Publication Details (including relevant citation   information):

      Anne Kaintz, Alan Benesi, Gary Baker, Mark Maroncelli

      Submitted

      Abstract:

      Diffusion coefficients of a variety of dilute solutes in the   series of 1-alkyl-

      1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imides   ([Prn1][Tf2N], n=3, 4, 6, 8, and 10),

      tetracedecyltrihexylphosphonium   bis(trifluoromethanesulfonyl)imides, and assorted imidazolium

      ionic liquids are measured using pulsed field gradient 1H NMR.   These data, combined with

      available literature data, are used to try to uncover the solute   and solvent characteristics most

      important in determining solute diffusion rates. Discussion is   framed in terms of departures from

      simple hydrodynamic predictions for translational friction using   the ratio [Zobs/ZSE], where Zobs is

      the observed friction, determined from the measured diffusion   coefficient D via [Zobs = k T/D],

      and [ZSE = 6 pi eta R] is the Stokes friction on a sphere of   radius R (determined from the solute van der Waals volume) and   eta is the solvent viscosity. In the case of neutral solutes, the   primary

      determinant of whether hydrodynamic predictions are accurate is   the relative size of solute and

      solvent molecules. A single correlation, albeit with considerable   scatter, is found between

      Zobs/ZSE and the ratio of solute to solvent van der Waals   volumes, [Zobs/ZSE = 1+a(Vu/Vv)^-p]

      with a = 1.93 and p = 1.88. In the case of small solutes, the   observed friction is over 100-fold

      smaller than predictions of hydrodynamic models. The dipole   moment of the solute has little

      obvious effect on the friction whereas solute charge has a marked   effect. For monovalent solutes

      of size comparable to or smaller than the solvent ions, the   observed friction is comparable to or

      even greater than what is predicted by hydrodynamics. These   general trends are shown to be

      quite similar to what is observed for solute diffusion in   conventional solvents.

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