On the Trail of Dissolution: Only 30 Water Molecules Separate Salt

Whether in oceans, soil, or living cells, salt (sodium chloride, NaCl) dissolves all around us. Yet until now, Physics lacked a precise understanding of exactly what happens step by step when the molecules come into contact. A research team led by Prof. Dr. Marcel Mudrich from the Institute of Physics at the University of Kassel has now tracked the process molecule by molecule for the first time.

For their analyses, the researchers used a special method: They embedded the particles in tiny helium nanodroplets near absolute zero and examined the process using ultra-high-resolution electron spectroscopy. Complementary computer simulations visualized the resulting microstructures. This allowed the researchers to isolate how individual water molecules weaken the bond between sodium and chlorine until, eventually, a complete hydrate shell forms around both ions.

Water molecules envelop ions at different rates

The measurements reveal clear differences between the two partners: While a chloride ion is completely surrounded by as few as 17 water molecules, a sodium ion requires up to 34 molecules. Only once a threshold of about 30 water molecules is reached do the ions remain permanently separated. “The water envelops the ions step by step, like a veil that gradually dissolves the salt,” says Mudrich. “Using a new measurement method, we were able to demonstrate for the first time when the ions become completely separated and are surrounded by water.”

Basic Knowledge Improves Models of Nature and Chemistry

Since this is purely basic research, the focus is not on immediate practical applications but on gaining a fundamental understanding of a key natural process. Many chemical and biological reactions take place in an aqueous environment. In a normal liquid, however, it is nearly impossible to isolate the individual intermediate steps.

“Our method makes it possible, so to speak, to selectively add individual water molecules and observe in real time how the electronic structure changes,” explains Mudrich. “In the long term, this knowledge will help us better understand chemical and biological processes—such as those in atmospheric or marine chemistry—at the molecular level and model them more realistically in computer simulations.”

About the study: https://onlinelibrary.wiley.com/doi/10.1002/sstr.202500747 

Contact
Prof. Dr. Marcel Mudrich
Institute of Physics, University of Kassel
Email: mudrich[at]physik.uni-kassel[dot]de

What does this mean in a nutshell?

• Molecular threshold discovered: A research team from Kassel, in collaboration with international partners, has demonstrated for the first time with atomic precision that salt only permanently dissolves into its ions (sodium and chloride) when there are at least 30 water molecules present.
• Foundation for more accurate models: The new measurement method provides a better understanding of chemical and biological processes in aqueous environments, which, for example, makes computer models in atmospheric and marine chemistry more precise.