We applied Scanning Probe Microscopy and Density Functional Theory (DFT) to discover the basics of how adsorbates wet insulating substrates, addressing a key question in geochemistry. To allow experiments on insulating samples we added Atomic Force Microscopy (AFM) capability to our existing UHV Scanning Tunneling Microscope (STM). This was accomplished by integrating and debugging a commercial qPlus AFM upgrade. Examining up-to-40-nm-thick water films grown in vacuum we found that the exact nature of the growth spirals forming around dislocations determines what structure of ice, cubic or hexagonal, is formed at low temperature. DFT revealed that wetting of mica is controlled by how exactly a water layer wraps around (hydrates) the K+ ions that protrude from the mica surface. DFT also sheds light on the experimentally observed extreme sensitivity of the mica surface to preparation conditions: K atoms can easily be rinsed off by water flowing past the mica surface.
Density Functional Theory points to a key role of K+ solvation in the low-energy two-dimensional arrangement of water molecules on the basal surface of muscovite. At a coverage of 9 water molecules per 2 surface potassium ions, there is room to accommodate the ions into wetting layers wherein half of them are hydrated by 3 and the other half by 4 water molecules, with no broken H-bonds, or wherein all are hydrated by 4. Relative to the “fully connected network of H-bonded water molecules” that Odelius et al. found to form “a cage around the potassium ions,” the hydrating arrangements are several tens of meV/H2O better bound. Thus, low-temperature wetting on muscovite is not driven towards “ice-like” hexagonal coordination. Instead, solvation forces dominate.
This report summarizes the work completed under the Laboratory Directed Research and Development (LDRD) project 10-0973 of the same title. Understanding the molecular origin of the no-slip boundary condition remains vitally important for understanding molecular transport in biological, environmental and energy-related processes, with broad technological implications. Moreover, the viscoelastic properties of fluids in nanoconfinement or near surfaces are not well-understood. We have critically reviewed progress in this area, evaluated key experimental and theoretical methods, and made unique and important discoveries addressing these and related scientific questions. Thematically, the discoveries include insight into the orientation of water molecules on metal surfaces, the premelting of ice, the nucleation of water and alcohol vapors between surface asperities and the lubricity of these molecules when confined inside nanopores, the influence of water nucleation on adhesion to salts and silicates, and the growth and superplasticity of NaCl nanowires.
The reported theoretical 'average binding of 0.20 eV per C atom, relative to a free graphene sheet and a clean metal slab' was an artifact of faulty evaluation of the energy of the free graphene sheet. Escaping our notice, the error occurred in the electron-density update algorithm, where two of six nearly degenerate eigenvectors were dropped [1]. With the error corrected, the computed binding energy of the graphene layer to Ir(111), is much smaller, just 0.18 eV per moire unit cell, or 0.9 meV per C atom. With a finer, 3 x 3 sample of the 10 x 10 graphene supercell's surface Brillouin zone, it increases to 2 meV/C atom. The cost of having to stretch the graphene sheet by {approx}0.3 linear percent to make it epitaxial on an underlying 9 x 9 Ir(111) supercell is incorporated in these values.
A molecular-scale interpretation of interfacial processes is often downplayed in the analysis of traditional water treatment methods. However, such an approach is critical for the development of enhanced performance in traditional desalination and water treatments. Water confined between surfaces, within channels, or in pores is ubiquitous in technology and nature. Its physical and chemical properties in such environments are unpredictably different from bulk water. As a result, advances in water desalination and purification methods may be accomplished through an improved analysis of water behavior in these challenging environments using state-of-the-art microscopy, spectroscopy, experimental, and computational methods.