Abstract
Our quantitative understanding of water adsorption onto salt surfaces under ambient conditions is presently quite poor owing to the difficulties in directly characterizing this interfacial layer under these conditions. Here we determine the thickness of the interfacial layer on NaCl at different relative humidities (RH) based on a novel application of atomic force spectroscopy and capillary condensation theory. In particular, we take advantage of the microsecond-timescale of the capillary condensation process to directly resolve the magnitude of its contribution in the tip-sample interaction, from which the interfacial water thickness is determined. Further, to correlate this thickness with salt dissolution, we also measure surface conductance under similar conditions. We find that below 30% RH, there is essentially only the deposition of water molecules onto this surface, typical of conventional adsorption onto solid surfaces. However, above 30% RH, adsorption is simultaneous with the dissolution of ions, unlike conventional adsorption, leading to a rapid increase of surface conductance. Thus, water adsorption on NaCl is an unconventional biphasic process in which the interfacial layer not only exhibits quantitative differences in thickness but also qualitative differences in composition.
Highlights
While electrostatic or van der Waals interactions proceed essentially at rates set by the speed of light, capillary condensation occurs at a much slower rate, since it is a thermodynamic process associated with a free energy barrier[30,31,32]
The adsorption of water on NaCl(100) under ambient conditions has long remained of great interest, both from a theoretical perspective as an important initial event during salt dissolution and from the more practical view of the significant role it is expected to play in many atmospheric reactions
We introduce a novel method by which the thickness of the interfacial water layer can be reproducibly measured under ambient conditions using AFM and capillary condensation theory
Summary
We combine atomic force spectroscopy with the well-established theory of capillary condensation to provide a novel means of determining the thickness of the aqueous interfacial layer on NaCl(001) over a wide range of relative humidities. Our results reveal that water adsorption on this surface is an unconventional, biphasic process, not explainable with existing theory, changing significantly in both thickness and composition at different humidities. In practical terms, these results indicate that atmospheric surface reactions within this layer that depend on interactions with water molecules will be profoundly dependent on the ambient relative humidity
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