1. DIFFUSION COEFFICIENTS OF IONIC LIQUIDS IN VARIOUS SOLVENTS
S. SARRAUTE, M.F. COSTA GOMES, A. A. H. PADUA
Laboratoire de Thermodynamique et Interactions Moléculaires
Université Blaise Pascal/CNRS, F Aubière, FRANCE
Introduction
Diffusion coefficients of ionic liquids are important whenever mass transport is involved, for example in chemical reactions, in catalytic and extraction processes and to assess the environmental fate of these compounds.
In the present work we have studied the diffusion coefficients in water, acetonitrile, methanol, tetrahydrofuran, dimethyl sulfoxide and 1,4-dioxane between 10 and 60°C, of several ionic liquids constituted by 1-alkyl-3-methylimidazolium cations (CnC1Im, 2≤n≤8) with different anions: chloride, Cl-, tetrafluoroborate, BF4-, and bis(trifluoromethyllsulfonyl)imide, Ntf2-.
Experimental
1-butyl-3-methylimidazolium chloride (C4C1ImCl) and 1-butyl-3-methylimidazolium tetrafluoroborate (C4C1ImBF4) were supplied by Fluka. Their purities are greater than 95% and 97%, respectively. Dialkylimidazolium bis(trifluoromethylsulfonyl)imide (C2C1ImNtf2, C4C1ImNtf2, C6C1ImNtf2, C8C1ImNtf2) used are pure at 99%. C2C1ImNtf2 and C4C1ImNtf2 were prepared by the group of Prof. Wasserscheid (University of Erlanger-Nürnberg, Germany) and C6C1ImNtf2, and C8C1ImNtf2 were prepared at the School of Chemistry and Chemical Engineering at Queen’s University of Belfast.
Diffusion coefficients were measured using the Taylor dispersion method [1, 2]. The dispersed pulse of solute is detected by a conductimetric detector (Waters, model 432) and a refractive index detector (Waters, model 2414) at the extremity of the diffusion tube.
Figure 1:Taylor dispersion-diffusion measurement apparatus
(P) HPLC pump; (V) six-way valve; (B) Thermostatic bath
(R) Conductivity and Refractive Index Detectors; (C) Computer
Results and Discussion
Effect of the anion on diffusion coefficients
Effect of the size of cation alkyl-side chain on diffusion coefficients
(a)
(b)
Figure 1 Diffusion coefficients in water of different ionic liquids based on the C4C1im+ cation:  Ntf2–, this work2; , BF4–, this work2; , Cl–, this work2; , hexafluorophosphate3 PF6–; , tetrafluoroborate3, BF4–
Figure 2 Diffusion coeffients in water (a) and in tetrahydrofuran (b)
of different ionic liquids based on Ntf2- anion:
O C8C1im+ ,  C6C1im+ ,  C4C1im+ ,  C2C1im+
diffusion coefficients decrease with increasing cation size or cation size
Effect of the solvent on diffusion coefficients
diffusion coefficients decrease in the following order:
acetonitrile, tetrahydrofuran, methanol, dimethyl sulfoxide, water and 1,4-dioxane
Figure 3 Diffusion coefficients of C2C1imNtf2 in
O water, O acetonitrile,  tetrahydrofuran,  methanol,  1,4-dioxane and  dimethylsulfoxide
Modelling diffusion coefficients
Table 1 Parameters for modified Wike-Chang equation used to smooth the raw data and the standard deviation of the fit
We have tested the validity of the frequently used Wilke-Chang equation [4] to predict the diffusion coefficient obtained in this work:
M1 and 1 are the molecular weight and the viscosity cinematic of the solvent, V2 is the molar volume of the solute at its normal boiling point (here taken as its molar volume at the experimental temperature).
Solvents
C/10-7 s
H2O
CH3CN
CH3OH
1.79
1.22
1.73
0.038
0.031
0.044
C4H8O
C2H6OS
C4H8O2
1.25
1.69
1.17
0.027
0.028
0.021
Conclusions
Diffusivity decreases with increasing anion size or cation size
Diffusion coefficients decrease in the following order: acetonitrile, tetrahydrofuran, methanol, dimethyl sulfoxide, water and 1,4-dioxane
Diffusion coefficients vary according to Arrhenius law for all the studied solvents
Correlation using a modified Wilke-Chang equation give predicted diffusion coefficient within 5 %.
References
[1] E.L. Cussler, « Diffusion MassTransfert in Fluid Systems », 2nd edition, Cambridge University Press, Cambridge UK, 1997.
[2] S. Sarraute, M. F. Costa Gomes, A. A. H. Padua, J. Chem. Eng. Data, 2009, DOI: /je800817b.
[3] W.C. Su et al, Fluid Phase Equilibria, 2007, 252,
[4] C. R. Wilke, P. Chang, AICHE J. 1955, 1,