Molecular Structure and Dynamics of Water at the Water–Air Interface Studied with Surface‐Specific Vibrational Spectroscopy

Abstract

Water interfaces provide the platform for many important biological, chemical, and physical processes. The water-air interface is the most common and simple aqueous interface and serves as a model system for water at a hydrophobic surface. Unveiling the microscopic (<1 nm) structure and dynamics of interfacial water at the water-vapor interface is essential for understanding the processes occurring on the water surface. At the water interface the network of very strong intermolecular interactions, hydrogen-bonds, is interrupted and the density of water is reduced. A central question regarding water at interfaces is the extent to which the structure and dynamics of water molecules are influenced by the interruption of the hydrogen-bonded network and thus differ from those of bulk water. Herein, we discuss recent advances in the study of interfacial water at the water-air interface using laser-based surface-specific vibrational spectroscopy.

Trimethylamine-N-oxide: its hydration structure, surface activity, and biological function, viewed by vibrational spectroscopy and molecular dynamics simulations

“…The water-air interface is an ideal platform for this investigation. Here, to probe TMAO near the interface spectroscopically, we consider surface specific vibrational SFG spectroscopy 69,70 at the TMAO aqueous solution-air interface. The SFG signals are generated by overlapping an IR pulse and a visible pulse at the surface.…”

“…Since the SFG optical process is a second-order nonlinear process, the signal from the centrosymmetric bulk vanishes, providing selectively the response from molecules located at the interfaces. 69,70 As such, we can selectively probe the molecular response near the interface.…”

Structure and dynamics of water at water–graphene and water–hexagonal boron-nitride sheet interfaces revealed by ab initio sum-frequency generation spectroscopy

“…Since the second-order nonlinear susceptibility is zero for centrosymmetric media owing to the selection rule of the even-order response, SFG can selectively probe the molecular response of the interfacial molecules. 10,11 The SFG response of the water molecules' O-H (O-D) stretch mode at the water-air 10,12 and water-oil 13 interfaces commonly shows a sharp B3700 (2750) cm À1 peak. Since this peak arises from the dangling, non-hydrogen-bonded, O-H group sticking out of the water-air interface, it constitutes a microscopic measure for the hydrophobicity of the materials.…”

Mobility of nanometer-size solutes in water driven by electric field

“…In addition, the field strengths required to produce sufficient sampling in simulations are significantly higher than experimental fields [23] and can potentially modify the structure of the solution. From the experimental side, it is not clear how to connect the results of surface-sensitive spectroscopies, which directly report on the polarization structure of the interface [24,25], with measured mobilities.…”