Structure and Orientation of Interfacial Water Determine Atomic Force Microscopy Results: Insights from Molecular Dynamics Simulations

Massive all-atom molecular dynamics simulations were employed to study hydration forces near α-Al2O3 (0001) surfaces as sampled during a hypothetical AFM force spectroscopy experiment conducted using a (28,0) single-walled carbon nanotube as the tip at ambient conditions. The results provide the force acting on the carbon nanotube tip, as well as detailed properties of interfacial water, as a function of the nanotube-surface distance. As the tip approaches the solid substrate, interfacial water undergoes conformational and structural changes. These changes are responsible for the features observed in the force profiles, including the range at which forces can be measured (up to two hydration shells), the intensity of the forces experienced by the AFM tip, and their oscillatory character. Our detailed analysis shows that heterogeneous surface chemical composition results in appreciably different force profiles. This observation may explain the variability of AFM data sampling hydration forces even on atomically smooth substrates. In addition, our results suggest that sufficiently accurate AFM force spectroscopy could be used to study how hydration forces depend on surface heterogeneous properties and on the orientation and local density of interfacial water, which could aid our understanding of interfacial phenomena and lead to significant scientific breakthroughs.