High-resolution imaging of solvation structures with amplitude-modulation atomic force microscopy

At the interface with solids, liquid molecules tend to adopt a particular structural arrangement, which depends on the local interplay of forces between the solid and the liquid. As a result, this interfacial liquid does not behave like bulk liquid, often exhibiting higher densification and ordering (solvation structure) that strongly depends on the local properties of the solid surface. Being a link between a solid and its surroundings, the interfacial liquid largely determines how a particular solid will interact with its environment. This is particularly relevant for nanomedicine where synthetic nano-objects are injected in a liquid environment for therapeutic purposes. Experimentally, little is known about the local structure and properties of interfacial liquid due to the lack of techniques offering sufficient resolution over inhomogeneous surfaces. Recently we demonstrated how amplitude-modulation atomic force microscopy (AM-AFM) could be used to probe and quantify solid-liquid interfaces locally and with sub-nanometer resolution. Here we apply this technique to compare the interface formed by selected solids with water, dimethylsulfoxide and 1-hexanol. The results highlight differences in the solvation structure formed by the different liquids over a same solid. Importantly, our results show that AM-AFM images reflect the structure of the interfacial liquid, which is not necessarily following the atomic structure of the interfaced solid in a trivial manner. These findings have important implications for the interpretation of AM-AFM images in liquid.