Using the now-decommissioned Summit supercomputer, researchers at the Department of Energy's Oak Ridge National Laboratory ran the largest and most accurate molecular dynamics simulations yet of the interface between water and air during a chemical reaction. The simulations have uncovered how water controls such chemical reactions by dynamically coupling with the molecules involved in the process.
This new understanding of water's role could help researchers develop methods to accelerate chemical reactions at the interface, potentially increasing their efficiency and productivity for industrial processes. Specifically, the team from ORNL's Chemical Sciences Division investigated a bimolecular nucleophilic substitution reaction, known as SN2. SN2 is one of the most common mechanisms in chemical, physical, biological and atmospheric chemistry. For example, SN2 reactions are vital in drug synthesis and were once used in the production of ibuprofen.
"This is the first paper that answers the question - 'What is the dynamic role of the air-water interface in modulating the reaction rate of chemical reactions?'" said Vyacheslav Bryantsev, leader of ORNL's Chemical Separations group and co-author of the study, which was published in the Journal of the American Chemical Society . "We confirm in this study that the overall reaction rate at the air-water interface becomes faster compared to the reaction rate in the main environment of water alone."
The team's simulations indicate that chemical reactions involving water and air could be sped up by drawing the interacting molecules out of the water's bulk environment (meaning, deep into the water, away from the interface) and closer to its surface, where air and water interact. This results in a reduction of water's dynamic coupling with those molecules, allowing the chemical reaction to proceed with less interference.
It is expected that water should influence the reactions rate since it mediates the reaction-however, to what extent and how water controls the reaction were unknown.
"We found that the more the water molecules couple, the more they hinder the reactions. If we can reduce that dynamic coupling, we'll have a faster reaction rate," said Santanu Roy, a scientist in ORNL's Carbon and Composites group and co-author of the study. "Our study suggests that if we can control that coupling by changing the environment at the interface - how water affects the reactions - then we should be able to control the reaction rate."
Our theories would not have been possible to validate or investigate if we didn't have leadership computing power.
Using the open-source CP2K code, the ORNL team modeled the reaction trajectories of the molecules on the Summit supercomputer. They then conducted a kinetic analysis of these paths to form an energy profile of the process.
"Our theories would not have been possible to validate or investigate if we didn't have leadership computing power," Roy said. "We needed to run thousands of trajectories - for every point in that energy profile. We had to run a lot of simulations at the electronic level, which takes a lot of time, and we had to run all of those in parallel. Without Summit, it's really impossible to do."
Based on previous experimental work that showed that positively charged surfactant molecules will attract negatively charged amino acids, the researchers simulated such a surfactant to draw more amino acids into the interface and confirmed an increased reaction rate of 10% to 15%. The ORNL team's study showed that as a gas reacts with amino acids, it goes through repeated dynamic coupling cycles with the water molecules, slowing down the chemical reaction before finally resolving into a new product.
"The challenge here was to actually understand the role of water and how it controls the reaction rates and their pathways - the mechanism. To do that, we really had to understand the reaction path. This is where Summit came in, and it helped us a lot."
Summit was managed by the Oak Ridge Leadership Computing Facility, a DOE Office of Science user facility located at ORNL. The OLCF offers leadership-class computing resources to researchers from government, academia, and industry who have many of the largest computing problems in science. This project was supported by the SummitPLUS program, which allocated computing time on Summit in its last, extended year of service in 2024.
UT-Battelle manages ORNL for DOE's Office of Science, the single largest supporter of basic research in the physical sciences in the United States. DOE's Office of Science is working to address some of the most pressing challenges of our time. For more information, visit energy.gov/science . - Coury Turczyn
