Resolving microscopic structure at water-vapor interfaces with unprecedented quality

Many of water’s unusual properties, such as its density maximum near 4°C, can be explained by the intermixture of two different local structures in water — a high-density (HD) local environment and a low-density (LD) local environment — that coexist transiently in its structure. Previous simulation approaches assumed a uniform local environment, which explained bulk behavior well but not behavior at the water-vapor interface.

By using a topological order parameter that works even when the local environment is highly inhomogeneous, Jedlovszky et al. provided the first clear evidence that water’s surface layers have a hidden LD/HD stratification, an observation that could explain its peculiar interfacial properties.

The team ran molecular dynamics simulations of water slabs across a broad temperature range, using an algorithm that identified surface molecules while accounting for ripples that can obfuscate structural details at the interface. To classify each molecule as HD-like or LD-like, the team then applied a topological order parameter that was based on the distance to each molecule’s fourth hydrogen-bonded neighbor.

Their simulations showed that water’s surface consistently stabilizes an outer layer of LD-like water across a wide range of temperatures, with a distinct HD-like layer just beneath it at temperatures below that of water’s density maximum.

“By comparing real and artificial interfaces, we showed that this layering is a genuine feature of the liquid-vapor boundary, not an artifact of the order parameter,” author Marcello Sega said. “Altogether, this demonstrates that water’s surface has a subtle but robust structural stratification tied to its unique hydrogen-bond network.”

A key next step for the researchers is to understand how this layering affects the dynamics and properties of water at surfaces.

Source: “Hydrogen bond topology reveals layering of LDL-like and HDL-like water at its liquid/vapor interface,” by Pal Jedlovszky, Christoph Dellago, and Marcello Sega, Journal of Chemical Physics (2025). The article can be accessed at https://doi.org/10.1063/5.0272802 .