In this project funded by the Experimental Physical Chemistry Program, Professor Markus Hoffmann of The College at Brockport, State University of New York, and his students will investigate the ion pairing and aggregation behavior of ionic liquids dissolved in low polarity solvents. Specific focus will be on ionic liquids containing the bis (trifluoromethansulfonyl) amide anion, which have unusually long ion-pairing lifetimes in these low polarity solvents. The research will employ nuclear magnetic resonance (NMR) methods, specifically diffusion ordered spectroscopy (DOSY), to measure molecular diffusion parameters. The NMR data will be augmented by classical viscometric, conductivity and tensiometric measurements. Additional calorimetric studies, done in collaboration with Prof. Dr. Ondruschka and Dr. Stark from the Institute for Technical Chemistry and Environmental Chemistry at the Friedrich-Schiller-University (FSU) in Jena, Germany, are expected to result in a complete thermodynamic description of the ion-pairing and aggregation equilibria.

Ionic liquids have been the subject of increasing interest in science and industry because they have the potential to replace traditional organic solvents that are volatile and hazardous. This research will contribute to our fundamental understanding of ionic liquids, and in particular the connection between their intermolecular (and inter-ionic) interactions and their observed bulk behavior. The participating undergraduate students will benefit immensely from the research experiences with respect to their professional preparation. In this regard, the additional involvement of ACS SEED high school students is a valuable contribution to encourage students from underrepresented groups early in their educational development to pursue careers in the sciences. In an attempt to bring the excitement of research into the chemistry curriculum at The College at Brockport some of the experiments will also be integrated as project-based experiments into the physical chemistry laboratory

Project Report

This research project concerned ionic liquids, which are salts that are liquid below 100oC and are currently heavily investigated with about 7000 journal articles published during the past 12 months. Usually, inorganic salts do not dissolve in organic solvents especially when they are of low polarity (in contrast to water, which is highly polar). Many ionic liquids, however, are very much soluble or completely miscible in organic solvents of low polarity. Because ionic liquids are relatively new compounds such solutions of ionic liquids in nonpolar organic solvents are presently poorly understood. Depending on the nature of salt and solvent as well as experimental conditions such as concentration and temperature, the salt may either completely dissociate or dissolve as individual cation-anion units called ion pairs. Small ion sizes, low solvent polarity, lower temperatures and larger concentrations move the equilibria from freely dissociated ions to ion pairs to aggregate and if ultimately the salt becomes insoluble, to the formation of a second solid or in case of ionic liquids a second liquid phase. Understanding the present speciation and equilibria of such ions in solvents of low polarity is a necessary pre-requisite for developing presently not available theories that can quantitatively describe these unique solutions. In total we have studied 13 different binary systems of ionic liquids and molecular solvents, and most of the results have been published in six peer reviewed journal articles with more dissemination to come in due course. We could show for some cases that the principle structural architecture of the studied ionic liquid stays intact even when large amounts of molecular solvent are added. When we were validating our experimental methods for surface tension measurements against published literature values, we noticed the presence of large disparities in the literature. In a careful study to determine which impurities might have caused the discrepancies we could show that the combination of vacuum grease (from removing water through heating under vacuum) and detergent (from cleaning glassware) would result in similarly flawed data. In a follow-up work we showed that aggregation of ILs with shorter aliphatic side chains occurs gradually over a wide range of concentration in water rather than as an abrupt formation of aggregates at a specific so-called critical aggregation concentration (CAC). One of the most unexpected findings of our research was that we observed for one particular ionic liquid in chloroform the average size of all present ionic liquid species to first increase with concentration, as one would expect since larger concentration favours the formation of ion pairs and eventually aggregates, but then to undergo a maximum at a quite low concentration for some of the binary ionic liquid – molecular solvent system we studied. Such observation was unexpected because the aggregate size should increase and not degrease with ionic liquid concentration. To explain this phenomenon that to the best of our knowledge has not been documented before, we have suggested a new hypothesis that mass transport changes from ion pairs and aggregates moving through the solution by themselves to a hopping mechanism of ion pairs from one aggregate to the next. This hypothesis would possibly also provide a better description of what other researchers termed the re-dissolution of the ionic liquid. The term re-dissolution implies that less aggregates and ion pairs are present at sufficiently high ionic liquid content but with our hypothesis this might not be the case as higher aggregates may very well still be present but do not contribute significantly to the mass transport. Our hypothesis still needs to be carefully scrutinized with much more research, but most recently we could confirm such behaviour for another five binary ionic liquid-molecular solvent systems. The College at Brockport is a primarily undergraduate institution and we carry out our research with undergraduate students only. A total of six undergraduate students worked on the project and received stipends for research in the summer months. Two of them even worked with collaborators in Germany to synthesize several ionic liquids of interest. All of these students are pursuing or are planning to pursue higher education at the graduate level. The grant also provided support for the acquisition of instruments that are now used not only for research but also for teaching undergraduate physical chemistry laboratory as well.

Agency
National Science Foundation (NSF)
Institute
Division of Chemistry (CHE)
Application #
0842960
Program Officer
Colby A. Foss
Project Start
Project End
Budget Start
2009-09-01
Budget End
2013-08-31
Support Year
Fiscal Year
2008
Total Cost
$236,329
Indirect Cost
Name
Suny College at Brockport
Department
Type
DUNS #
City
Brockport
State
NY
Country
United States
Zip Code
14420