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Groundbreaking Method Unveils Secrets of Water’s Molecular Interactions
Water, while essential for life, possesses an intricate and often enigmatic behavior that is largely influenced by the hydrogen bonds formed between its molecules. These bonds arise when hydrogen atoms from one molecule interact with oxygen atoms of another, a process that involves sharing electronic charges. This phenomenon is fundamental to the unique properties of liquid water. However, the quantum interactions that govern these bonds have predominantly been explored through theoretical models, leaving a gap in experimental understanding.
Now, a team of researchers led by Sylvie Roke from the Laboratory for Fundamental BioPhotonics at EPFL’s School of Engineering has introduced a novel technique known as correlated vibrational spectroscopy (CVS). This method offers a fresh way to observe the behavior of water molecules as they engage in hydrogen bond networks. Unlike traditional techniques, CVS differentiates between molecules that are actively participating in these interactions and those that remain randomly dispersed without bond engagement. Current spectroscopic methods generally assess both groups together, complicating the analysis.
“Existing spectroscopy techniques measure the laser light scattering that occurs due to molecular vibrations across all molecules in a sample. This requires assumptions about the interactions at play,” Roke comments. “CVS, on the other hand, assigns unique vibrational spectra to different molecule types. Each spectrum features distinctive peaks that directly correspond to the movements of water molecules along the hydrogen bonds, enabling us to accurately assess their properties, including shared electronic charges and bond strength,” she adds.
The significance of CVS has been recognized in a recent publication in Science.
New Insights into Molecular Behavior
To effectively differentiate between interacting and non-interacting molecules, the research team utilized femtosecond laser pulses in the near-infrared range to illuminate liquid water. These extremely brief light pulses induce minute oscillations and atomic movements within the water, leading to the emission of visible light. This emitted light carries crucial information regarding the spatial arrangement of the molecules, while the color variations provide clues about their atomic displacements.
“In conventional experiments, the spectrographic detector is set at a 90-degree angle to the incoming laser beam. We discovered that by repositioning the detector and adjusting the type of polarized light used, we could isolate the spectra of interacting molecules,” Roke explains.
The research team further explored how CVS could be employed to dissect the quantum properties of hydrogen bond networks by modifying the pH level of water with hydroxide ions to increase basicity or protons for acidity.
“Hydroxide ions and protons play a critical role in hydrogen bonding, so variations in pH alter the water’s reactivity,” states Mischa Flór, a PhD student and lead author of the study. “With CVS, we’ve quantitatively assessed that hydroxide ions contribute 8% additional charge to hydrogen bond networks, while protons withdraw 4% charge. These precise measurements were previously unattainable through experimental means.” Advanced simulations from collaborators in Italy, the UK, and France supported these findings.
The researchers stress that this method, validated through theoretical analyses, holds promise for application across various materials. Several new projects employing CVS for characterizing different systems are already in progress.
“Quantifying the strength of hydrogen bonds represents a powerful advancement that can provide clarity on the molecular dynamics present in diverse solutions, including those containing electrolytes, sugars, amino acids, DNA, and proteins,” Roke concludes. “Moreover, CVS is not solely limited to water; it has the potential to reveal extensive information about other liquids and processes.”
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