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Dynamics of Methanol in Ionic Liquids: Validity of the Stokes–Einstein and Stokes–Einstein–Debye Relations
Author(s) -
Herold Elisabeth,
Strauch Matthias,
Michalik Dirk,
Appelhagen Andreas,
Ludwig Ralf
Publication year - 2014
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.201402141
Subject(s) - methanol , chemistry , molecular dynamics , debye , ionic liquid , relaxation (psychology) , thermodynamics , analytical chemistry (journal) , computational chemistry , physics , organic chemistry , psychology , social psychology , catalysis
The validity of Stokes–Einstein (SE) and Stokes–Einstein–Debye (SED) relations for methanol in the physical environment of the ionic liquid (IL) 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide is studied by means of nuclear magnetic resonance (NMR) relaxation time experiments, viscosity measurements and molecular dynamics (MD) simulations. The reorientational correlation times of the hydroxyl groups of pure methanol and of methanol in the IL/methanol mixtures were determined. For that purpose an approach for estimating NMR deuteron quadrupole coupling constants, presented by Wendt and Farrar ( Mol. Phys. 1998 , 95 , 1077–1081), was confirmed. The self‐diffusion coefficients of methanol were taken from the MD simulations. The viscosities of all systems were then measured and the SE and SED relations validated. For pure methanol both relations are valid, whereas they become increasingly invalid with increasing IL concentration, as indicated by effective volumes and radii that are too low. The deviation from the SE and SED relations could be related to dynamical heterogeneities described by the non‐Gaussian parameter α ( t ) obtained from MD simulations. For pure methanol, α ( t ) is close to zero in accord with the validity of both relations. With increasing IL concentration the dynamical heterogeneities of methanol increase strongly. The times t * at the maximum of α ( t ) increase linearly with the relative number of methanol monomers in the mixtures. Thus, the dynamical heterogeneities are largest for single methanol molecules fully embedded in the IL environment. In their own environment methanol molecules are highly mobile, whereas in the IL‐rich region the mobility is strongly reduced leading to the non‐validity of SE and SED relations.