Research
Overview
The goal of the Sturlaugson Lab's research is to understand how the structure of an ionic liquid (IL) determine the liquid's macroscopic properties such as melting point and viscosity. ILs are defined as salts with a melting temperature below 100°C, and, due to their large structural variability, it is estimated that there are over a trillion ILs possible. Research in ILs has shown tremendous growth in the past decades, largely due to their enormous variability and the intersection of several desirable traits such as tunable solubility, negligible volatility, good thermal and electrochemical stability, and reasonable conductivity. Current biomedical applications include antimicrobial agents, solvents for biocatalysis, drug delivery systems, and medicinal analytics.5 However, it is not well understood how the chemical structure of an IL determines the bulk properties of the liquid. The specific focus of the research in the Sturlaugson Lab is to investigate how the structure of an IL influences the hydrogen bonding and entropy of the liquid and how those parameters effect the liquid's viscosity and melting point. A better understanding of these structure-property relationships will aid rational ionic liquid design.
In 1,3-dialkylimidazolium ILs, hydrogen bonding between the imidazolium cation and the anion has observed, both in experiment16,17 and in computations14,18 [see left figure above]. When this hydrogen bonding is removed by methylation at the C-2 position, the melting point of the IL rises [see right figure above]. This increase in melting point is abnormal since, typically, a decrease in hydrogen bonding decreases the melting point of a liquid. To explain this anomalous behavior, Fumino et. al. theorized that the hydrogen bonding in dialkylimidazolium ILs leads to a reduction in viscosity and melting point due to a disruption of the optimal Coulombic interactions that would typically lead to crystallization.25 In contrast, Hunt proposed that the reduction in melting point and viscosity for ILs is due to an increase in entropy for the dialkylimidazolium ILs compared to the trialkylimidazoliums.14 In particular, Hunt suggested that in dialkylimidazolium ILs the high conformational flexibility of butyl chain fluidizes the liquid by making crystallization difficult, but that C-2 methylation limits butyl flexibility, thereby raising the melting point. The overarching goal of the Sturlaugon Lab's research is to determine the influence of these two effects--hydrogen bonding and/or entropy--on the melting point and transport properties of ionic liquids, thereby aiding rational IL design.
Synthesis of Novel Ionic Liquids
Synthesis of ionic liquids is a straightforward SN2 reaction, followed by anion exchange for the desired anion.
In the Sturlaugson lab, we are synthesizing a family of novel pyrazolium based ionic liquids with varying degrees of hydrogen bonding capabilities and butyl tail flexibility.
Characterization of Ionic Liquids
Schlenk Line
After synthesis, the solvent must be removed from the ionic liquid. Many ionic liquids are hygroscopic--meaning they absorb water from the air--and any water impurities can drastically change the ionic liquid's melting point and transport properties (such as viscosity). Because of this, care must be taken to produce dry ionic liquids. The Schlenk line is used to thoroughly dry the ionic liquids and prepare them for storage in the nitrogen glovebox.
Nitrogen Glovebox
Samples are stored, prepared for analysis, or sometimes analyzed, in the nitrogen glovebox. The chamber is purged with dry nitrogen gas to prevent water absorption by the ionic liquids. Our viscometer is shown in the middle of the glovebox. Running viscosity measurements in the glovebox keeps the ionic liquid dry during the measurement--a task that would be practically impossible on a regular lab bench.
Karl Fisher Titrator
Karl Fisher titration is used to measure the final water content of the ionic liquid after synthesis. KF titration is a specialized titration that can measure water concentrations down to 1 ppm. After drying, samples for the titrator are prepared in the glovebox and run on this automated Mettler Toledo KF titrator.
NMR Spectroscopy
The NMR spectrum is taken to confirm synthesis of the desired ionic liquid. We often take the spectrum after the initial SN2 reaction and after the anion exchange reaction. We have a 60 MHz Anasazi NMR spectrometer.
Differential Scanning Calorimetry
The melting point of a compound is directly related to its intermolecular attractions and structure. Because the goal of our research is to better understand this relationship, determining the melting point of an ionic liquid is critically important. We measure melting points using our Perkin-Elmer DSC, which has a measurement range of -130 °C to 400 °C.
Density Meter
The ionic liquid's density is measured using an Anton-Paar density meter. The experimentally determined density can be compared to the simulated density from molecular dynamics simulations to help validate the simulation.
Viscometer
The viscosity of a liquid is related to its molecular structure and intermolecular interactions, such as hydrogen bonding. Since that relationship is exactly what we are studying, we measure the viscosity of our ionic liquids using a Brookfield viscometer, typically set up in the glovebox to ensure the sample stays dry.
Molecular Dynamics Dimulations of Ionic Liquids
We are developing the force field parameters necessary to run molecular dynamic simulations on the novel pyrazolium family of ionic liquids. To do this, we are extending the commonly used CL&P ionic liquid force field.28 This requires quantum mechanical calculations to determine the partial charges, equilibrium bond lengths and angles, and the dihedral energy profile. The workflow is shown below.
Snapshot of an MD simulation box
We run our quantum calculations and molecular dynamics simulations on a dedicated workstation computer from Bizon. Specs include:
Processor: 32-Core 3.70 GHz AMD RYZEN Threadripper 3970X 3rd Gen
RAM: 128 GB (DDR4)
Graphics Cards: 4 x NVIDIA RTX 2080 Ti 11 GB
Drives: 1 TB NVME (OS), 2 TB NVME
Linux Ubuntu 22.04
Funding
The SD BRIN program provides paid summer research opportunities for undergraduates at participating SD BRIN institutions. More information, including a list of participating faculty and the application process, can be found at the BRIN website here.
NOTE: The application for summer 2024 is now closed.
The BRIN program is supported by the National Institutes of Health (NIH).
USF Natural Science Research Program
This program is supported by the generous donations of University of Sioux Falls alumni.
Free Software
GROMACS - Molecular dynamics simulations
ORCA - Quantum mechanical calculations
Avogadro - Molecular generation and visualization
Open Babel - Chemical file format convertor
Multiwfn - Electronic wavefunction analysis
Packmol - Molecular dynamics simulation box packing
VMD - Molecular dynamics visualization and analysis
Chimera - Molecular visualization and analysis
TRAVIS - Molecular dynamics analysis
References & Further Reading
Review Papers
Recommended first readings are highlighted in yellow.
Welton, T. Ionic Liquids: A Brief History. Biophys. Rev. 2018, 10 (3), 691–706. https://doi.org/10.1007/s12551-018-0419-2.
Forsyth, S. a.; Pringle, J. M.; MacFarlane, D. R. Ionic Liquids — An Overview. ChemInform 2004, 35 (20), 113–119. https://doi.org/10.1002/chin.200420291.
Singh, S. K.; Savoy, A. W. Ionic Liquids Synthesis and Applications: An Overview. J. Mol. Liq. 2020, 297, 112038. https://doi.org/10.1016/j.molliq.2019.112038.
Kar, M.; Plechkova, N. V.; Seddon, K. R.; Pringle, J. M.; MacFarlane, D. R. Ionic Liquids – Further Progress on the Fundamental Issues. Aust. J. Chem. 2019, 72 (2), 3. https://doi.org/10.1071/CH18541.
Egorova, K. S.; Ananikov, V. P. Fundamental Importance of Ionic Interactions in the Liquid Phase: A Review of Recent Studies of Ionic Liquids in Biomedical and Pharmaceutical Applications. J. Mol. Liq. 2018, 272, 271–300. https://doi.org/10.1016/j.molliq.2018.09.025.
Welton, T. Room-Temperature Ionic Liquids: Solvents for Synthesis and Catalysis. Chem. Rev. 1999, 99 (8), 2071–2084. https://doi.org/10.1021/cr1003248.
Hallett, J. P.; Welton, T. Room-Temperature Ionic Liquids: Solvents for Synthesis and Catalysis. 2. Chem. Rev. 2011, 111 (5), 3508–3576. https://doi.org/10.1021/cr1003248.
Wilkes, J. S. A Short History of Ionic Liquids—from Molten Salts to Neoteric Solvents. Green Chem. 2002, 4 (2), 73–80. https://doi.org/10.1039/b110838g.
Vekariya, R. L. A Review of Ionic Liquids: Applications towards Catalytic Organic Transformations. J. Mol. Liq. 2017, 227, 44–60. https://doi.org/10.1016/j.molliq.2016.11.123.
Synthesis Papers
Burrell, A. K.; Sesto, R. E. Del; Baker, S. N.; McCleskey, T. M.; Baker, G. a. The Large Scale Synthesis of Pure Imidazolium and Pyrrolidinium Ionic Liquids. Green Chem. 2007, 9 (5), 449. https://doi.org/10.1039/b615950h.
Earle, M. J.; Gordon, C. M.; Plechkova, N. V; Seddon, K. R.; Welton, T. Decolorization of Ionic Liquids for Spectroscopy. Anal. Chem. 2007, 79 (2), 758–764. https://doi.org/10.1021/ac061481t.
Giernoth, R.; Bankmann, D. Transition-Metal-Free Synthesis of Perdeuterated Imidazolium Ionic Liquids by Alkylation and H/D Exchange. European J. Org. Chem. 2008, 2008 (17), 2881–2886. https://doi.org/10.1002/ejoc.200700784.
Hydrogen Bonding & Entropy Papers
Fumino, K.; Wulf, A.; Ludwig, R. Strong, Localized, and Directional Hydrogen Bonds Fluidize Ionic Liquids. Angew. Chem. Int. Ed. Engl. 2008, 47 (45), 8731–8734. https://doi.org/10.1002/anie.200803446.
Hunt, P. A. Why Does a Reduction in Hydrogen Bonding Lead to an Increase in Viscosity for the 1-Butyl-2,3-Dimethyl-Imidazolium-Based Ionic Liquids? J. Phys. Chem. B 2007, 111 (18), 4844–4853. https://doi.org/10.1021/jp067182p.
Zhang, Y.; Maginn, E. J. The Effect of C2 Substitution on Melting Point and Liquid Phase Dynamics of Imidazolium Based-Ionic Liquids: Insights from Molecular Dynamics Simulations. Phys. Chem. Chem. Phys. 2012, 14 (35), 12157. https://doi.org/10.1039/c2cp41964e.
Abdul-Sada, A. K.; Greenway, A. M.; Hitchcock, P. B.; Mohammed, T. J.; Seddon, K. R.; Zora, J. A. Upon the Structure of Room Temperature Halogenoaluminate Ionic Liquids. J. Chem. Soc. Chem. Commun. 1986, No. 24, 1753. https://doi.org/10.1039/c39860001753.
Avent, A. G.; Chaloner, P. A.; Day, M. P.; Seddon, K. R.; Welton, T. Evidence for Hydrogen Bonding in Solutions of 1-Ethyl-3-Methylimidazolium Halides, and Its Implications for Room-Temperature Halogenoaluminate(III) Ionic Liquids. J. Chem. Soc. Dalt. Trans. 1994, No. 23, 3405. https://doi.org/10.1039/dt9940003405.
Dommert, F.; Schmidt, J.; Qiao, B.; Zhao, Y.; Krekeler, C.; Delle Site, L.; Berger, R.; Holm, C. A Comparative Study of Two Classical Force Fields on Statics and Dynamics of [EMIM][BF4] Investigated via Molecular Dynamics Simulations. J. Chem. Phys. 2008, 129 (22), 224501. https://doi.org/10.1063/1.3030948.
Fumino, K.; Wittler, K.; Ludwig, R. The Anion Dependence of the Interaction Strength between Ions in Imidazolium-Based Ionic Liquids Probed by Far-Infrared Spectroscopy. J. Phys. Chem. B 2012. https://doi.org/10.1021/jp306173t.
Peppel, T.; Roth, C.; Fumino, K.; Paschek, D.; Köckerling, M.; Ludwig, R. The Influence of Hydrogen-Bond Defects on the Properties of Ionic Liquids. Angew. Chemie Int. Ed. 2011, 50 (29), 6661–6665. https://doi.org/10.1002/anie.201100199.
Dong, K.; Zhang, S. Hydrogen Bonds: A Structural Insight into Ionic Liquids. Chem. - A Eur. J. 2012, 18 (10), 2748–2761. https://doi.org/10.1002/chem.201101645.
McDaniel, J. G.; Yethiraj, A. Understanding the Properties of Ionic Liquids: Electrostatics, Structure Factors, and Their Sum Rules. J. Phys. Chem. B 2019, 123 (16), 3499–3512. https://doi.org/10.1021/acs.jpcb.9b00963.
Fumino, K.; Wulf, A.; Verevkin, S. P.; Heintz, A.; Ludwig, R. Estimating Enthalpies of Vaporization of Imidazolium-Based Ionic Liquids from Far-Infrared Measurements. ChemPhysChem 2010, 11 (8), 1623–1626. https://doi.org/10.1002/cphc.201000140.
Dong, K.; Zhang, S.; Wang, J. Understanding the Hydrogen Bonds in Ionic Liquids and Their Roles in Properties and Reactions. Chem. Commun. 2016, 52 (41), 6744–6764. https://doi.org/10.1039/C5CC10120D.
Fumino, K.; Peppel, T.; Geppert-Rybczyńska, M.; Zaitsau, D. H.; Lehmann, J. K.; Verevkin, S. P.; Köckerling, M.; Ludwig, R. The Influence of Hydrogen Bonding on the Physical Properties of Ionic Liquids. Phys. Chem. Chem. Phys. 2011, 13 (31), 14064–14075. https://doi.org/10.1039/c1cp20732f.
Hunt, P. A.; Ashworth, C. R.; Matthews, R. P. Hydrogen Bonding in Ionic Liquids. Chem. Soc. Rev. 2015, 44 (5), 1257–1288. https://doi.org/10.1039/c4cs00278d.
Watanabe, H.; Doi, H.; Saito, S.; Matsugami, M.; Fujii, K.; Kanzaki, R.; Kameda, Y.; Umebayashi, Y. Hydrogen Bond in Imidazolium Based Protic and Aprotic Ionic Liquids. J. Mol. Liq. 2016, 217, 35–42. https://doi.org/10.1016/J.MOLLIQ.2015.08.005.
Molecular Dynamics Simulation Papers
Canongia Lopes, J. N.; Pádua, A. A. H. CL&P: A Generic and Systematic Force Field for Ionic Liquids Modeling. Theor. Chem. Acc. 2012, 131 (3), 1–11. https://doi.org/10.1007/s00214-012-1129-7.
Canongia Lopes, J. N.; Deschamps, J.; Pádua, A. A. H. H. Modeling Ionic Liquids Using a Systematic All-Atom Force Field. J. Phys. Chem. B 2004, 108 (30), 2038–2047. https://doi.org/10.1021/jp0476996.
Canongia Lopes, J. N.; Pádua, A. A. H. Molecular Force Field for Ionic Liquids Composed of Triflate or Bistriflylimide Anions. J. Phys. Chem. B 2004, 108 (43), 16893–16898. https://doi.org/10.1021/jp0476545.
Canongia Lopes, J. N.; Pádua, A. A. H. Molecular Force Field for Ionic Liquids III: Imidazolium, Pyridinium, and Phosphonium Cations; Chloride, Bromide, and Dicyanamide Anions. J. Phys. Chem. B 2006, 110 (39), 19586–19592. https://doi.org/10.1021/jp063901o.
Canongia Lopes, J. N.; Pádua, A. A. H.; Shimizu, K. Molecular Force Field for Ionic Liquids IV: Trialkylimidazolium and Alkoxycarbonyl-Imidazolium Cations; Alkylsulfonate and Alkylsulfate Anions. J. Phys. Chem. B 2008, 112 (16), 5039–5046. https://doi.org/10.1021/jp800281e.
Shimizu, K.; Almantariotis, D.; Costa Gomes, M. F.; Pádua, A. A. H.; Canongia Lopes, J. N. Molecular Force Field for Ionic Liquids V: Hydroxyethylimidazolium, Dimethoxy-2methylimidazolium, and Fluoroalkylimidazolium Cations and Bis(Fluorosulfonyl)Amide, Perfluoroalkanesulfonylamide, and Fluoroalkylfluorophosphate Anions. J. Phys. Chem. B 2010, 114 (10), 3592–3600. https://doi.org/10.1021/jp9120468.
Gajula, M.; Kumar, A.; Ijaq, J. Protocol for Molecular Dynamics Simulations of Proteins. BIO-PROTOCOL 2016, 6 (23), 1–11. https://doi.org/10.21769/BioProtoc.2051.
Zhao, M.; Wu, B.; Lall-Ramnarine, S. I.; Ramdihal, J. D.; Papacostas, K. A.; Fernandez, E. D.; Sumner, R. A.; Margulis, C. J.; Wishart, J. F.; Castner, E. W. Structural Analysis of Ionic Liquids with Symmetric and Asymmetric Fluorinated Anions. J. Chem. Phys. 2019, 151 (7), 074504. https://doi.org/10.1063/1.5111643.