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Scientists discover new way to observe the microscopic structure of water
It is the first time the phenomenon of Intermolecular Radiative Decay (IRD) has been observed in liquids; the technique allows probing the ion layer in solution
The technique involves X-rays that can directly probe the electronic and configurational structure of the solvation layer, the first layer of water molecules surrounding an ion in solution – Photo: Pexels
Deepening knowledge about the interaction between water and dissolved ions—such as sodium and magnesium – is the objective of a study led by researchers from Uppsala University in Sweden, with the participation of USP. The team developed a novel X-ray technique capable of analyzing how water organizes itself around dissolved ions, particularly the first solvation shell. The technique enables the investigation of important processes ranging from basic physics and environmental chemistry to industrial applications that depend on solution chemistry.
The phenomenon represents the first observation of a process called Intermolecular Radiative Decay (IRD) in liquids. In this case, when an ion in water loses electrons after being struck by X-rays, another electron from a nearby water molecule fills the vacancy in the ion’s inner shell, and the released energy is emitted as an X-ray photon. The emitted photon carries a distinct fingerprint of the ion’s immediate environment, allowing researchers to probe the solvation shell from the inside.
Solvation shells play crucial roles in intermolecular interactions, reactivity, stability, and other properties of solutions. “The solvation shell determines how ions behave in water, influencing everything from biological function to corrosion and battery chemistry”, said lead author Johan Söderström. “Our discovery shows that X-rays can now be used to directly reveal the electronic structure of this critical interfacial region”.
The study, published in Nature Communications, used synchrotron radiation at the MAX IV Laboratory in Lund, Sweden, where the team investigated aqueous solutions of sodium and magnesium ions. By analyzing the emitted X-ray photons, they identified distinct spectral signatures originating from neighboring water molecules – evidence of the newly discovered IRD process.
Other methods already exist to study specific properties of solvated systems. However, many of them rely on electron detection, which makes them limited in liquid environments, as electrons hardly escape from the solution. IRD changes this scenario: it is based on the production of X-ray photons that easily traverse the liquid. Professor Lucas Cornetta, from USP’s Institute of Physics (IF), told Jornal da USP that “IRD inaugurates an alternative way to probe these specific characteristics in the context of intermolecular interactions in liquid environments”.
Lucas Cornetta – Photo: ResearchGate
The research is the result of a theoretical–experimental collaboration and includes Cornetta’s participation on the theoretical front of the work. He is one of those responsible for the theoretical and computational modeling of the IRD signal, encompassing simulations to characterize the structures of metal ions in solution at the molecular level, as well as calculations of non-local X-ray emission cross-sections based on quantum chemistry methods.
“In addition to deepening the understanding of the phenomenology of intermolecular interactions in solution, elucidating non-local decay mechanisms opens the way for a new characterization technique, capable of complementing already established methods such as photoelectron spectroscopy”, said the professor.
Experimental results
The theoretical modeling confirmed that Intermolecular Radiative Decay arises from orbital hybridization (regions where electrons are most likely to be found) between the ion and the surrounding water molecules. In other words, the subtle hybridization between the ion’s and water’s orbitals is what enables and gives intensity to the phenomenon. According to the researchers, the process is sensitive only to the first solvation shell, making IRD an exceptionally selective probe of local chemical environments in liquids.
“This is the radiative cousin of the well-known Intermolecular Coulombic Decay”, said senior author Olle Björneholm. “But unlike electron-based methods, IRD emits X-rays that can escape from deep within the liquid, allowing us to explore the properties of the solution as a whole”.
In addition to sodium and magnesium, the researchers also suggest that IRD may be detectable in other systems, including transition metal ions and anions, indicating that the phenomenon is general – and potentially transformative for the study of aqueous and biological chemistry. This discovery paves the way for element- and site-specific studies of solvation structure, chemical bonding, and ultrafast dynamics in liquids, using advanced synchrotron and free-electron laser sources, as well as investigations into how the molecular environment affects chemical effects on reactions, biological processes, and material properties in solution, offering an “inside-out” view of solvation.
The article Non-local X-ray intermolecular radiative decay probes solvation shell of ions in water is available at this link.
*Written with information provided by the Press Office of the Institute of Physics.
**Intern under the supervision of Moisés Dorado
English version: Nexus Traduções, edited by Denis Pacheco
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A reprodução de matérias e fotografias é livre mediante a citação do Jornal da USP e do autor. No caso dos arquivos de áudio, deverão constar dos créditos a Rádio USP e, em sendo explicitados, os autores. Para uso de arquivos de vídeo, esses créditos deverão mencionar a TV USP e, caso estejam explicitados, os autores. Fotos devem ser creditadas como USP Imagens e o nome do fotógrafo.