Date of Publication
Dr. Markus M. Hoffmann, Professor, Chemistry
Ionic liquids are salts that are liquid below 100oC. Ionic liquids are of very low volatility and especially those ionic liquids with high conductivity and lower viscosities are used as medium for chemical synthesis and electrochemistry. Normally, salts do not dissolve in solvents of low polarity. However, many ionic liquids are very soluble or completely miscible in low polarity solvents and the physical chemistry of the resulting solutions is not well understood. The goal of our ongoing research is to elucidate the speciation (freely dissolved ions, ion pairs and aggregates) present for ionic liquids dissolved in solvents of low polarity. In prior work in Dr. Hoffmann’s laboratory, it was found that the ionic liquid 1-‐hexyl-‐3-‐methylimidazolium bis(trifluoromethylsulfonyl)amide ([C6mim][NTf2]), which is completely miscible in chloroform (CHCl3), displays an aggregate size maximum at surprisingly dilute concentrations of 0.1 molal (m). In Dr. Hoffmann’s lab, this was interpreted as an apparent re-‐dissolution by a change of mass transport mechanism from ion pairs and aggregates self-‐diffusing as individual species to a “hopping” motion of single ion pairs between aggregates. These results motivated further research to discern if this particular behavior can be observed for other pairs of molecular solvent and ionic liquid solutes. This thesis work presents experimental results for concentration and temperature dependent self-‐diffusion coefficients measured by NMR spectroscopy in conjunction with viscosity measurements to determine the average hydrodynamic radii of the present species. Five systems were investigated: [C6mim][NTf2] dissolved in dichloromethane (CH2Cl2), tetrahydrofuran (THF), and chlorobenzene (C6H5Cl), and two other ionic liquids, 7 1-‐butyl-‐3-‐methylimidazolium bis(trifluoromethylsulfonyl)amide [C4mim][NTf2] and 1-‐ ethyl-‐3-‐methylimidazolium bis(trifluoromethylsulfonyl)amide [C2mim][NTf2] in CH2Cl2. Our findings thus far show that similar behavior to that of [C6mim][NTf2] in CHCl3 occurs in three systems: [C4mim][NTf2] in CH2Cl2, [C2mim][NTf2] in CH2Cl2, and [C6mim][NTf2] in C6H5Cl . Specifically, for the three systems we observe a maximum of the average radius size, and the corresponding concentration varies from system to system, from about 0.10 molal for [C6mim][NTf2] in C6H5Cl to about 0.18 molal for [C2mim][NTf2] in CH2Cl2.The remaining two systems, [C6mim][NTf2] in CH2Cl2 and [C6mim][NTf2] in THF, did not indicate a maxima in the hydrodynamic radius.
Cade, Elise, "Aggregation Properties of Ionic Liquids in Low Polarity Solvents" (2015). Senior Honors Theses. 88.