A New Discovery of Water Molecule Behavior at Saltwater Surfaces: Spectroscopy Technique Sheds Light on Electrolyte Interfaces

3d illustration of water molecule © artegorov3@gmail - stock.adobe.com

A revolutionary study, led by Yair Litman, Kuo-Yang Chiang, Takakazu Seki, Yuki Nagata, and Mischa Bonn, reveals a novel understanding of water molecule organization at saltwater interfaces. Conducted at the Max Planck Institute for Polymer Research in Germany and the University of Cambridge in the UK, the research addresses a fundamental question in atmospheric and chemical sciences: how do ions behave at the air-water interface? This work, published in Nature Chemistry, employs advanced vibrational sum-frequency generation (VSFG) spectroscopy combined with neural network-assisted molecular dynamics (MD) simulations, providing new insights into the surface structure of common electrolyte solutions (1,2).

What is the Sum-Frequency Signal?

The sum-frequency signal in VSFG occurs when the frequencies of two incident laser beams combine at a material interface to create a new light wave. This new light wave has a frequency equal to the sum of the frequencies of the two incident beams (hence the term sum frequency [SF]). Mathematically, if the frequencies of the IR and visible beams are denoted as: ​

W_SF = W_IR + W_Vis

This signal only arises from regions lacking inversion symmetry, such as interfaces, because in bulk materials, where molecules are symmetrically arranged, the sum-frequency signal cancels out. At interfaces, however, molecular asymmetry allows the generation of this nonlinear signal. As a result, VSFG is highly surface-specific, making it ideal for probing molecular structures at boundaries like the air-water interface (1).

Revealing Stratification at the Water-Air Interface
The team’s research focuses on the behavior of ions at the water-air interface of electrolyte solutions, challenging a long-held assumption that large ions accumulate directly at the surface. Through precise VSFG measurements and ab initio molecular dynamics simulations, the researchers found that most ions do not, in fact, reside at the outermost surface of the water. Instead, they are concentrated just beneath the surface, creating a distinct stratification that separates water molecules into two layers: an ion-depleted top layer and an ion-rich subsurface. This discovery is critical because it revises the understanding of how electric fields at these interfaces influence water reorganization. This discovery gives greater understanding to the concepts and theories of evaporation and transpiration (1).

VSFG Spectroscopy: Probing Water Molecule Behavior
The research relied heavily on VSFG spectroscopy, a technique that probes the vibrational responses of molecules at interfaces. Since atomic ions do not vibrate, their distribution and effects are inferred by studying the vibrational patterns of the surrounding water molecules. The team focused on the 3,000–3,600 cm⁻¹ range of the water spectrum, which is sensitive to how water molecules are oriented and bonded at the surface (1).

Their key breakthrough came from the use of heterodyne-detected VSFG (HD-VSFG), which enabled them to analyze both the phase and intensity of the signals, offering a more precise view of the molecular dynamics than earlier techniques. By observing subtle shifts in the water vibrational spectrum when salts such as NaCl, NaBr, and NaOH were added, the researchers concluded that the electric double-layer (EDL) model, which suggests a uniform layer of ions at the surface, does not fully explain the water reorganization at these interfaces (1).

Simulation and Data Analysis Reveal New Insights
Complementing their experimental findings, the team applied neural network-aided ab initio molecular dynamics (NN-AIMD) simulations. These simulations, which replicate the behavior of water and ions at the atomic level, allowed the researchers to model a wide range of electrolyte solutions, including HCl, NaCl, and MgSO4, among others. The combination of experimental VSFG data and high-level simulations was crucial for disentangling the complex interactions between water and ions, leading to a more accurate depiction of how ions influence surface water molecules (1).

Impact on Environmental and Chemical Models
The discovery of subsurface ion stratification has broad implications. The surface behavior of water in electrolyte solutions plays a key role in atmospheric chemistry, especially in processes like ocean evaporation and aerosol formation. By improving the understanding of ion distributions at these critical interfaces, the research opens new avenues for developing more accurate environmental and chemical models (1).

For example, understanding how water molecules reorganize around ions can shed light on chemical reactivity at the surface of oceans, potentially influencing models of climate change and air quality. Furthermore, the findings have implications for a range of applied sciences, including the development of more efficient batteries, where electrolyte interfaces play a crucial role in performance (1,2).

Conclusion
This study significantly advances the molecular-level understanding of electrolyte interfaces, using advanced spectroscopic techniques to challenge conventional models. The work not only refines the scientific understanding of the air-water interface but also emphasizes the importance of water-ion interactions in shaping molecular structures at the surface. Through their innovative approach combining VSFG and molecular dynamics, the researchers have paved the way for future explorations into the behavior of water and ions at a variety of interfaces, from natural waters to engineered systems (1).

References

(1) Litman, Y.; Chiang, K Y.; Seki, T.; et al. Surface stratification determines the interfacial water structure of simple electrolyte solutions. Nat. Chem. 16, 644–650 (2024). DOI: 10.1038/s41557-023-01416-6

(2) Tahara, T. Working on a dream: bringing up the level of interface spectroscopy to the bulk level. BCSJ 2024, 97 (4), uoae012. DOI: 10.1093/bulcsj/uoae012