Water molecules do not jump alone
What role does the network of hydrogen bonds in water molecules play in water dynamics? New research supported in part by the EU-funded HyBOP project has provided an answer to this question. Published in the journal ‘Nature Communications’, the study sheds light on how hydrogen-bond network reorganisation is linked to collective reorientational dynamics in liquid water. “The hydrogen-bond network is not fixed. It changes continuously due to thermal fluctuations and other factors that break and form individual hydrogen bonds,” explains study first author Dr Adu Offei-Danso of The Abdus Salam International Centre for Theoretical Physics (ICTP), Italy, in a recent article posted on the ICTP website. He adds: “These fluctuations of the hydrogen-bond network are crucial for various physical, chemical, and biological processes happening in liquid water, and that’s why it is so important to understand their microscopic mechanism.”
Isolated, or not?
More than 10 years ago, researchers discovered that water molecule rotations – believed to play an important role in hydrogen-bond dynamics – do not happen in small diffusive steps but typically involve sudden big jumps. “These large-angular jumps were thought to be isolated events,” remarks study co-author Dr Ali Hassanali, also of ICTP. “When these sudden and quick rotations happen, there is a breakage and forming of hydrogen bonds with neighbouring molecules. So, there is an alteration in the local hydrogen-bond network.” However, what happens to the other water molecules in the network when these jumps occur? Do they remain unaffected, or are they active participants in a more collective process? “This is precisely what we wanted to find out,” states Dr Hassanali. The scientists performed computer simulations of liquid water, adopted statistical physics approaches to understand collective behaviour, and used data science techniques to circumvent or reduce human intervention. “To show whether there is a collective process at all in water reorientation dynamics, we first needed to identify an observable that is able to capture the abrupt angular motion of water molecules,” notes Dr Offei-Danso. The team used this observable to develop a tool that enabled them to see a large number of angular jumps happening simultaneously in the system. “What’s more, we discovered that they occur in a highly orchestrated and coordinated fashion,” observes Dr Offei-Danso. Study co-author and ICTP senior research fellow Dr Uriel Morzan describes the strong interplay the team observed between water molecule angular rotation and changes in topology and density in the hydrogen-bond network: “The picture that emerges from our results is that once a large jump occurs, a cascade of hydrogen-bond fluctuations takes place.” These fluctuations then trigger a subsequent wave of small and large angular jumps. “Our analysis shows collective reorientation and network rearrangements that involve several larger groups of nearby molecules spread throughout the system,” concludes co-author Dr Alex Rodriguez, who is an assistant professor at the University of Trieste. The study lays the foundations for further research into the role of cooperative dynamics in various physical, chemical and biological processes. The 5-year HyBOP (Hydrogen Bond Networks as Optical Probes) project ends in 2027. For more information, please see: HyBOP project
Keywords
HyBOP, water, molecule, hydrogen bond, hydrogen-bond network, angular motion
