Molecular-Scale Insights into the Graphite-Water Interface
Dynamic atomic force microscopy (AFM) is a versatile tool for investigating surfaces at the molecular level. When performed in a three-dimensional volume at a solid-liquid interface, information about the solvation structure at the interface can be obtained [1]. Implementing this three-dimensional imaging technique [2] allows for directly mapping the interfacial hydration structure at various surfaces such as calcite [2], dolomite [3] or graphite [4].
In this context, the graphite−water interface constitutes a prime example for studying the water structure at a seemingly hydrophobic surface. Surprisingly, in a large number of atomic force microscopy (AFM) experiments, well-ordered stripes have been observed at the graphite−water interface. The origin of these stripes has been discussed controversially in literature. Proposed origins include contamination, interplanar stacking of graphene layers, formation of methanol−water nanostructures, and adsorption of nitrogen molecules. Especially, the latter interpretation has received considerable attention because of its potential impact on explaining the long-range nature of the hydrophobic interaction. In our experiments, stripes form when we use plastic syringes to insert the water into the AFM instrument. In contrast, when clean glass syringes are used instead, no such stripes form. We, therefore, conclude that contaminations from the plastic syringe constitute the origin of the stripes we observe [5].
At solid surfaces, hydration layers are typically formed. This is also true for graphite. The physical origin of these layers is usually ascribed to (i) confinement of the water at the interface and (ii) attraction of the water to the surface. While confinement is a general property at solid surface, attraction is specific to the given system. In this talk, layer formation by confinement will be discussed in general, i.e., it is explained how layer formation can be explained even in the absence of attraction. With the help of molecular dynamics simulations, however, it is shown that confinement alone does not cause the layer formation in the specific case of graphite surfaces. Thus, attraction between water and graphite is required to explain the layers observed on graphite at ambient conditions [4].
[1] T. Fukuma, Y. Ueda, S. Yoshioka, H. Asakawa, Phys. Rev. Lett. 104 (2010) 016101
[2] H. Söngen, M. Nalbach, H. Adam, A. Kühnle, Rev. Sci. Instrum. 87 (2016) 063704
[3] H. Söngen et al., Langmuir 33 (2017) 125
[4] H. Söngen et al., Phys. Rev. B 100 (2019) 205410
[5] S. Seibert, S. Klassen, A. Latus, R. Bechstein, A. Kühnle, Langmuir 36 (2020) 7789
