Abstract
The investigation of solid-liquid interfaces is pivotal for understanding processes like wetting, corrosion, and mineral dissolution and growth. 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. Although many groups have reported on the observation of stripes at this interface, fundamental properties and, in particular, the origin of the stripes are still under debate. 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 this study, we demonstrate that these stripes readily form when using standard plastic syringes to insert the water into the AFM instrument. In contrast, when clean glass syringes are used instead, no such stripes form even though nitrogen was present. We, therefore, conclude that contaminations from the plastic syringe rather than nitrogen constitute the origin of the stripes we observe. We provide high-resolution AFM data that reveal detailed structural insights into the arrangement of the stripes. The rich variability of our data suggests that the stripes might be composed of several different chemical species. Still, we cannot rule out that the stripes observed in the literature might originate from other sources; our study offers a rather straightforward explanation for the origin of the stripes. In the view of these results, we propose to carefully reconsider former assignments.
Highlights
The graphite−water interface constitutes a prime example for studying the water structure at a seemingly simple model surface
Besides the fundamental interest in elucidating the water structure at a surface that is generally believed to be hydrophobic,[1] the graphite/graphene surface serves as a model system for many application-related studies.[2−4] In the past, an ever-growing number of research groups have reported atomic force microscopy (AFM) experiments on the formation of well-ordered stripes on the graphite/graphene surface.[5−18] Despite their ubiquitous appearance, the origin and many structural details of these stripes are still under debate
Some groups have ascribed the stripes to airborne contaminations[11−14] or to the formation of methanol−water nanostructures,[18] others have explained the origin of the stripes by intercalation and different interplanar stackings[12] or the assembly of dissolved nitrogen molecules at the graphite/graphene−water interface.[5−9,17] The interpretation of the stripes being composed of nitrogen has received considerable attention because of its potential impact for explaining the long-range nature of the hydrophobic interaction.[17,20,21]
Summary
The graphite−water interface constitutes a prime example for studying the water structure at a seemingly simple model surface. We present an AFM study of stripe-like adsorbates at the graphite−water interface (in the following referred to as stripes) We demonstrate that these stripes are present when standard plastic syringes are used to place the water droplet into the instrument. Conclude that the stripes observed in our experiments are composed of species that originate from the plastic syringes. The structural features reported here, including the domain orientation, stripe widths, internal structure and solvation structure, exhibit rich variability This variability, which is in excellent agreement with structural investigations of the stripes reported in the literature, suggests that the contamination is composed of different chemical species. As it is sometimes suggested that AFM imaging conditions improve in the presence of ions, we performed some experiments using sodium chloride solutions. The color scale ranges from dark (low value) to bright (high value)
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