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.

Mid-synthesis of a pyrazolium ionic liquid.

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:

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.

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

References & Further Reading

Review Papers

Recommended first readings are highlighted in yellow.

Synthesis Papers

Hydrogen Bonding & Entropy Papers

Molecular Dynamics Simulation Papers