How scientists track molecules to improve chemical separations

Iowa State University’s Department of Chemistry and the U.S. Department of Energy’s Ames National Laboratory are studying how solvents behave at the smallest scales to support the development of more energy-efficient methods of separating chemicals. These processes are involved in refining petroleum, producing medicine, and manufacturing everyday materials, but they currently consume large amounts of energy.

Professor of Chemistry and Ames Laboratory Chemical and Biological Sciences Division Director Emily Smith and graduate student Jemima Opare-Addo (Ph.D. ’25 analytical chemistry) are using imaging techniques to study ionic liquids – a class of solvents made of charged molecules, or ions. You can think of these as salts that stay liquid at room temperature.

Ionic liquids are known for their potential as “green” alternatives in separation processes, and play an important role in pharmaceutical, energy, and manufacturing industries. Smith and Opare-Addo’s work focuses on how their nanoscale structure influences separation efficiency.

“Ionic liquids can form different structures – these nanostructures, as we’ve been calling them, have different properties,” said Smith. “We hypothesize that the way the molecules interact with these nanostructures is different. When one molecule goes into a structure, and another molecule enters another structure, this impacts how the molecules are separated from each other.”

Nanoscale is a minuscule spatial scale that is much smaller than what we can see with our eyes.

“Our goal is to understand the structure of the solvent itself,” Opare-Addo said. “How are ionic liquids organized at the nanoscale? That information could help others develop more effective separation methods.”

Exploring nanoscale structures

Despite its liquid state, ionic liquids do not behave like water. Instead, they may form structures that are only a few nanometers across, smaller than the width of a human hair. These petite structures can affect how other molecules move through them, which is important for these processes.
To study the structures, Opare-Addo uses a technique called single-molecule imaging because traditional light-based methods have limitations and cannot detect objects this small.

By introducing a fluorescent molecule into the ionic liquid and observing how it moves, they can track how molecules behave in different domains within the solvent.

The team tracks the motion of individual molecules within the liquid. Each molecule’s path provides clues about the environment it’s moving through, much like how watching a person walk across sand versus concrete reveals differences about the surface.

By analyzing each path, the researchers can calculate local diffusion coefficients, which are measures of how fast molecules move and provide insight into the solvent’s structure.

Data collection is conducted in dark rooms to enhance sensitivity and minimize light interference. This task requires clean conditions; researchers wear protective gear and use strong chemical solutions to clean each glass slide, as a fingerprint or a speck of dust could distort the data.

“Any background signal could interfere with our ability to see the molecules clearly,” Opare-Addo added.

Once the data is collected – often a full day of imaging for a single data point – it undergoes careful computer analysis. The researchers extract movement patterns from hundreds of molecules, combining them into a statistically reliable picture of how the solvent behaves.

By comparing the diffusion coefficients of molecules in different regions of the solvent, the researcher can detect differences in structure.

“If a molecule moves faster in one region of the liquid than in another, it tells us that the liquid has structural differences at the nanoscale level,” Opare-Addo said.

Linking nanoscale behavior to industrial solutions

This work is part of a larger U.S. Department of Energy-supported initiative to reduce the energy footprint of chemical separations.

“We’re uncovering how nanoscale structure of these green solvents influences solvent behavior,” Opare-Addo noted. “Our collaborators can then use this to improve solvent design for real-world applications.”

Other members of the team at Ames National Laboratory and Iowa State University use this information to improve how solvents perform in actual separation processes, evaluating how nanostructures relate to performance.

“Energy can be thought of as money. If a process uses a lot of energy, it uses a lot of money,” Smith said. “Part of our work through the Ames National Laboratory is to do research that helps the nation develop ways to reduce the energy need for everything that has been touched by chemical separations.”