Abstract
When we speak of nanometric water films on surfaces we are speaking about a truly ubiquitous phenomenon in nature. All surfaces exposed to ambient conditions are covered by a thin film of water that affects or mediates surface chemistry, general physical-chemical processes on surfaces, and even solid–solid interactions. We have investigated this phenomenon for over a decade by exploiting dynamic atomic force microscopy and have (1) described how these layers affect apparent height measurements, (2) analyzed the excitation of subharmonics, (3) investigated its effects on surface functionality over time (“aging”), (4) monitored and quantified the time-dependent wettability of several relevant surfaces such as highly oriented pyrolytic graphite and monolayer systems, and (5) developed high-resolution and highly stable modes of imaging. Here, we discuss these findings to elucidate the present and future of the field. We further provide a brief but general discussion of solvation and hydration layers in vacuum, liquid, and air that center around current controversies and discuss open possibilities in the field.
Highlights
We discuss the presence of nanometric layers and water films on surfaces in ambient conditions with a focus on the advances by our team in relation to (1) apparent height measurements in dynamic atomic force microscopy, (2) the excitation of subharmonics, (3) the monitoring of the formation and aging of these films, and (4) the possibilities that the presence of these films offer to imaging with atomic force microscopy (AFM)
While our discussion is based on the films, or molecular adsorption, encountered in air or ambient conditions, we provide a brief but general discussion of solvation and hydration layers and, more generally, the adsorption of molecules on surfaces as they age in a given
If we maintain that wetting is the capacity of a droplet to spread on a surface and that the surface and droplet will repel each other if they cannot weakly bond or attract each other via surface forces, it becomes clear that the structuring or arrangement of water molecules over a surface will be a thermodynamic process resulting in passivation.[3,4]
Summary
The accumulation of molecules such as hydrocarbons and water on surfaces leads to the formation of a thin nanometric interface that mediates and even regulates solid−medium interactions.[1−3] This interface results in the so-called set of interfacial properties,[4,5] that is, electronic, thermal, physicochemical, and general adhesion, and effectively modifies or establishes the set of surface properties that constitute or give place to the final material structure with the accompanying set of properties.[6,7] far from being an artificial and uncommon system, this nanometric interface regulates the solid−medium interactions that are common in nature[8] and most of those that are encountered as everyday phenomena.[6,9] We purposefully speak of a solid−medium interface to emphasize that the solid’s surface might be exposed to vacuum, air, or liquid environments acting as the medium, but the presence of water molecules in the proximity of the surface will play a key role in the restructuring and reorientation of hydrogen bonds,[3] dangling bonds, and OH (hydroxyl) groups.[7] Still, rather than considering this interface part of the constitutive final structure of a solid and its surface, it is common in surface science to refer to it as “contamination”.10 This terminology emphasizes the fact that (1) the atomic-molecular structure of a pristine solid surface abruptly terminates at the surface and that (2) surface properties will change as molecules adhere to it. Environment, that is, vacuum, liquid, and air, that centers around current controversies and discuss open possibilities in the field
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