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.

中文翻译：

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水作为造影剂使用二次谐波散射和成像量化表面化学和物理：一个观点

发生在不同材料界面的分子水平相互作用决定了它们的局部电学、化学和机械性能。在固体界面的情况下，该信息传统上是通过需要超高真空条件的实验技术获得的。然而，这些方法不适用于研究水界面的表面化学。最近，出现了一种使用界面水作为造影剂来探测此类界面的方法。这种方法基于带电界面附近的水分子产生的二次谐波，可用于散射和显微镜几何学。在这篇《观点》中，我们更详细地解释了这种方法，并为一组不同的应用提供了示例和比较：胶体科学和固态物理学，通过二氧化硅-水表面化学和生物物理学来说明，通过膜-水-离子通道相互作用来说明。这两种不同的应用表明，通过跟踪界面水的结构，可以提取和量化重要的化学参数，例如表面电位值、双电层结构和局部解离常数，这些参数在许多不同的情况下都很有用。