Molecular-scale Hydrophilicity Induced by Solute: Molecular-thick Charged Pancakes of Aqueous Salt Solution on Hydrophobic Carbon-based Surfaces

We directly observed molecular-thick aqueous salt-solution pancakes on a hydrophobic graphite surface under ambient conditions employing atomic force microscopy. This observation indicates the unexpected molecular-scale hydrophilicity of the salt solution on graphite surfaces, which is different from the macroscopic wetting property of a droplet standing on the graphite surface. Interestingly, the pancakes spontaneously displayed strong positively charged behavior. Theoretical studies showed that the formation of such positively charged pancakes is attributed to cation-π interactions between Na(+) ions in the aqueous solution and aromatic rings on the graphite surface, promoting the adsorption of water molecules together with cations onto the graphite surface; i.e., Na(+) ions as a medium adsorbed to the graphite surface through cation-π interactions on one side while at the same time bonding to water molecules through hydration interaction on the other side at a molecular scale. These findings suggest that actual interactions regarding carbon-based graphitic surfaces including those of graphene, carbon nanotubes, and biochar may be significantly different from existing theory and they provide new insight into the control of surface wettability, interactions and related physical, chemical and biological processes.