Water as a contrast agent to quantify surface chemistry and physics using second harmonic scattering and imaging: A perspective

Molecular level interactions that take place at the interface of different materials determine their local electrical, chemical, and mechanical properties. In the case of solid interfaces, this information has traditionally been obtained with experimental techniques that require ultra-high vacuum conditions. However, these methods are not suitable for studying surface chemistry of aqueous interfaces. Recently, an approach emerged for probing such interfaces using interfacial water as a contrast agent. This approach is based on second harmonic generation from water molecules next to a charged interface and can be utilized in both scattering and microscopy geometries. In this Perspective, we explain this approach in more detail and provide examples and comparisons for a diverse set of applications: colloid science and solid state physics, illustrated by silica-water surface chemistry, and biophysics, illustrated by membrane-water-ion channel interactions. Those two diverse applications show that by following the structure of interfacial water, it is possible to extract and quantify important chemical parameters such as surface potential values, structure of the electric double layer, and local dissociation constants that are useful in many different contexts. 2022 Author(s).