In saltwater solutions, water molecules rapidly move around salt ions at a rate of more than a trillion times per second, according to experiments and simulations conducted by scientists at New York University and the Sorbonne.
“There’s more to saline solutions than meets the eye,” said Alexej Jerschow, a professor in NYU’s Department of Chemistry and one of the senior authors of the study. “This became evident when we measured and modeled the very fast dynamics of sodium chloride ions and surrounding water molecules.”
The findings, published in Nature Communicationsit will allow researchers to build more reliable models for predicting ion dynamics, which can be used for a variety of scientific endeavors, from improving rechargeable batteries to magnetic resonance imaging.
Ions are ubiquitous and essential for life. Many ions, such as sodium and potassium, are widespread in the human body and determine cell viability, nerve signaling and the structural integrity of tissues. How ions interact with solvents also plays a critical role; for example, rechargeable batteries rely on the movement of ions through electrolyte solutions.
Ions in a water-based solution are normally surrounded by four to six water molecules, but it is not well understood how far these molecules move as a unit and how much motion water molecules experience. Previously used models were inadequate to capture the combined motion between water and ions.
To study the movement of salt and water molecules, the researchers used nuclear magnetic resonance (NMR) spectroscopy, a versatile tool that is routinely used to determine the structure of molecules, and combined the experimental data with detailed computer simulations that can model the dynamics around salt ions at the atomic scale.
Testing salt water at a wide range of concentrations and temperatures, and combining experimental data and computer simulations, the researchers observed that water molecules move around sodium and chloride ions at an extremely rapid rate – over a trillion times per second. Furthermore, it was previously assumed that the ions move together with the surrounding solvent molecules as a unit, but the experiment has shown that this is not the case; instead, the water molecules move much faster than the water-ion complex.
“We found excellent agreement between experiments and simulations, which allows us to build reliable models for ion dynamics,” said Jerschow.
“We are now turning to more complex electrolytes and what happens near solid surfaces, and combining experiments with simulations will again be essential to make progress,” said Benjamin Rotenberg of the Sorbonne Université and the Center national de la recherche scientifique (CNRS) of the France, and the other senior author of the study.
“We anticipate that this work can provide information in many areas – from medicine to energy storage – which is based on a good understanding of ion dynamics in solution,” added Jerschow.
The research was supported by the National Institutes of Health (R01EB026456), European Research Council (863473) and National Science Foundation (CHE2108205).