Hydration Structure at the α-Al2O3 (0001) Surface: Insights from Experimental Atomic Force Spectroscopic Data and Atomistic Molecular Dynamics Simulations

Atomic force spectroscopic data obtained in water on the (0001) face of fully hydroxylated alpha-Al2O3 substrate using a silicon tip are presented. The data are obtained by implementing the Brownian force profile reconstruction method (BFPR), originally proposed by Ashby and Lieber [J. Am. Chem. Soc.2004, 126, 16973]. The method allows for an accurate reconstruction of the high stiffness force-distance curve that highlights the hydration structure. The experimental data are interpreted with the aid of massive atomistic molecular dynamics simulations in which one silicon dioxide disc of similar to 2 nm diameter represents the silicon atomic force microscopy (AFM) tip. The umbrella sampling method is employed to obtain the water-mediated surface-disc force profile. It is possible to distinguish two hydration layers confined between tip and surface in both simulations and experiment. Small variations of the disc features yield some differences in the simulated force-distance curve, and the small disc size is responsible for weakening the evidence for the second hydration layer. One dense layer of water molecules is in contact with the alpha-Al2O3 (0001) substrate. This hydration layer yields a pronounced repulsive force when the AFM tip penetrates it, suggesting highly structured interfacial water. The second hydration layer yields much less intense repulsive forces. The position of the peaks with respect to the solid substrate is consistent with recent experimental X-ray reflectivity data reported by Catalano [Geochim. Cosmochim Acta 2011, 75, 2062] and with previous atomistic simulations conducted for a thin film of water supported on sapphire at ambient conditions by Argyris et al. [J. Phys. Chem. C 2011, 115, 2038].1