Measuring the Thickness of Water’s Airy Interface

The air-water interface is one of the most prevalent boundaries in nature and plays a key role in many natural and industrial processes. Yet despite decades of research, scientists have never directly measured one of the interface’s most fundamental properties – how thick the layer of anisotropic water molecules is at the boundary between air and the water bulk. Now, a new study using an innovative approach reveals this thickness to be surprisingly short, with implications for our understanding of interfacial water structure and the interpretation of nonlinear optical experiments probing interfaces.

Water molecules behave differently at interfaces compared to in the bulk liquid. At the air-water boundary, they exhibit preferential orientations, weaker hydrogen bonding, and slower dynamics. This distinct interfacial structure arises from the asymmetric hydrogen bonding environment water molecules experience near the phase boundary with air. However, despite extensive experimental and computational investigations, the length scale over which the orientation of water molecules remains anisotropic moving away from the interface – known as the thickness of the structural anisotropy – has never been directly measured.

Past work has determined related properties like the thickness of anisotropy in density and dielectric constant to be around 3-5 angstroms (Å). But molecular orientations and hydrogen bonding connectivity decay over a length scale that depends on the orientation correlations beyond single molecular layers. Molecular dynamics simulations predict this structural anisotropy thickness to be surprisingly short at only around 6Å, suggesting orientational correlations extend over just 1-2 molecular layers below the interface. However, as simulations can be sensitive to forcefield details, independent experimental validation was crucial.

The research group of Dr. Martin Thämer at the Fritz Haber Institute in Germany has now directly measured this important structural parameter using a novel approach combining phase-resolved vibrational sum and difference frequency generation (SFG/DFG) spectroscopy with isotopic dilution. SFG/DFG spectroscopy is highly sensitive to molecular orientations and hydrogen bonding, making it well-suited for probing interfacial structure. And the researchers’ technique enables depth profiling of interfacial signals on the sub-nanometer scale.

In their experiments, the team selectively measured the signature OD stretches of deuterated water (D2O) molecules at the air-water boundary. By simultaneously obtaining phase-resolved SFG and DFG response spectra across the OD vibration band, they could separate signals originating from different depths within the interface. The key insight is that dipolar responses sourcing from increasing depths experience opposite phase shifts for SFG versus DFG due to their distinct coherence properties.

By considering half of each phase-resolved response containing the same resonant phase, the researchers isolated the resonant phase contribution from any depth-induced propagation phase. Then, by measuring both isotopologues H2O and D2O, they could further decompose the overall response into purely resonant and non-resonant components with different spatial origins. From the phase and amplitude differences between SFG and DFG, they were finally able to extract the anisotropic decay length.

The phase difference between SFG and DFG signals from the vibrationally resonant response yielded a decay length of 7.7 ± 1.0 Å. Furthermore, depth-dependent second-order spectra calculated from ab initio molecular dynamics simulations of the air-water interface were found to be in excellent agreement, predicting structural anisotropy over 6Å. This places the experimental measurement and simulations within a single molecular layer of each other.

Strikingly, the researchers obtained a shorter decay length of 3.1 ± 0.9 Å from the non-resonant response. They attributed this to the non-resonant signals containing a large contribution from the isotropic bulk, arising due to its sensitivity to electric quadrupolar sources rather than just interfacial dipoles. This suggests the non-resonant response probes a shallower structural anisotropy than the resonant contribution.

The consensus from experiment and simulation of a remarkably short anisotropic layer of just 6-8Å was unexpected. It indicates orientational correlations of water molecules extend over, at most, the first three molecular layers below the interface – making them even shorter-ranged than in the isotropic water bulk where correlations span several coordination shells. This ultra-thin anisotropic zone highlights the important role of both reduced hydrogen bonding connectivity and entropic gains in dictating interfacial structure.

The researchers’ findings place fundamental restrictions on the interpretation of optical techniques studying aqueous interfaces. In particular, the non-resonant response was shown to not be a selective probe of anisotropic environments and contain large isotropic bulk signals. This calls into question the analysis of nonlinear experiments solely utilizing non-resonant or intensity-based measurements. Meanwhile, the vibrationally resonant response provided direct access to the structural anisotropy with quantitative agreement between experiment and simulation.

The work clearly demonstrates the power of the researchers’ depth-resolved approach for characterizing nanoscale interfacial structure. By probing multiple pathways, they could disentangle resonant from non-resonant contributions and extract precision structural information not accessible before. The technique holds promise for examining a variety of environmentally, biologically and technologically important aqueous interfaces where molecular orientations and hydrogen bonding patterns differ from the bulk. It also highlights the need for careful interpretation of non-resonant nonlinear optical data from such interfaces.

Overall, the study represents a milestone in directly measuring one of water’s most fundamental interfacial properties that simulations had previously only predicted. By validating simulations’ view of the interface’s strikingly thin anisotropic layer, it provides powerful confirmation of our molecular-level understanding. The approach also sets a new standard for interfacing nonlinear vibrational spectroscopy with atomistic modeling to fully leverage their combined insights into soft matter and biological interfaces on the nanoscale.

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Reference(s)

1. https://doi.org/10.48550/arXiv.2404.12247

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