Molecular Structure and Modeling of Water–Air and Ice–Air Interfaces Monitored by Sum-Frequency Generation

Abstract

From a glass of water to glaciers in Antarctica, water–air and ice–air interfaces are abundant on Earth. Molecular-level structure and dynamics at these interfaces are key for understanding many chemical/physical/atmospheric processes including the slipperiness of ice surfaces, the surface tension of water, and evaporation/sublimation of water. Sum-frequency generation (SFG) spectroscopy is a powerful tool to probe the molecular-level structure of these interfaces because SFG can specifically probe the topmost interfacial water molecules separately from the bulk and is sensitive to molecular conformation. Nevertheless, experimental SFG has several limitations. For example, SFG cannot provide information on the depth of the interface and how the orientation of the molecules varies with distance from the surface. By combining the SFG spectroscopy with simulation techniques, one can directly compare the experimental data with the simulated SFG spectra, allowing us to unveil the molecular-level structure of water–air and ice–air interfaces. Here, we present an overview of the different simulation protocols available for SFG spectra calculations. We systematically compare the SFG spectra computed with different approaches, revealing the advantages and disadvantages of the different methods. Furthermore, we account for the findings through combined SFG experiments and simulations and provide future challenges for SFG experiments and simulations at different aqueous interfaces.

Specific Ion–Protein Interactions Influence Bacterial Ice Nucleation

“…We can directly compare the experimental and computational findings by obtaining the squareo ft he integrated 1hcos qi (1 is the density of water), which is approximately proportional to the SFG intensity. [17] The comparison of the SFG intensities and the square of 1hcos qi (calculated SFG intensities) for the INP samples, presented in Figure 4B,r eveals that the simulations reproduce the experimental trend and capturet he effects of the different ions on the water orientation near the INP well. In agreement with the experiments, we observe that weakly hydrated ions are found near the INP surface, rendering the protein more negative and enhancingw ater orientation relative to the active IN planes, while stronglyh ydrated ions show ag radual increasei nt he population when moving away from the INP surfacet ot he bulk water (Figure 4C).…”

“…We analyzed the water orientation ( ) relative to the IN planes of the INP in the presence of the different salts (Figure S4), where is the angle between the water molecule's bisector and the plane normal of the active sites (see Experimental Section). We can directly compare the experimental and computational findings by obtaining the square of the integrated ( is the density of water), which is approximately proportional to the SFG intensity [17] …”

The Bending Mode of Water: A Powerful Probe for Hydrogen Bond Structure of Aqueous Systems

“…In contrast, MD simulations show that the imaginary part of the bending mode SFG spectrum exhibits both positive and negative features. 7 , 11 , 49 Tahara and co-workers attributed this discrepancy of the experimentally measured and simulated Im(χ bend (2) ) to the bulk quadrupole contribution; the simulation had assumed that the SFG signal arises solely from the interface because the interfacial dipole contribution is stronger than the bulk quadrupole contribution (electric dipole approximation), 50 while the experimental data may contain not only the leading contribution of the interfacial dipole but also contributions from the bulk quadrupole. 51 If the bulk quadrupole indeed dominates the bending mode signal, there is no chance to extract the interfacial water contribution from the SFG bending mode signal.…”

Structure and sum-frequency generation spectra of water on neutral hydroxylated silica surfaces

“…The intensity of the bands is controlled by the transition dipole moment magnitude and by the angle θ that both depend on the intermolecular interactions. A very recent review by Tang et al 78 summarizes the experimental and theoretical investigations of this and related systems.…”