Phase, Structure, and Dynamics of the Hydration Layer Probed by Atomic Force Microscopy

Abstract

Using the molecular dynamics method we simulate the hydration layer probed by an atomic force microscope (AFM) tip. We investigate how the AFM tip affects the phase, structure, and dynamics of the intrinsic hydration layer formed on a hydrophobic carbon plate. Without the AFM tip the molecular packing and orientation of the hydration layer are ordered up to the second molecular layer. With approaching the tip, the hydration layer is perturbed and eventually evaporates. The force−distance curve in AFM lacks an oscillation typically found for the hydration layer formed on a hydrophilic surface. The molecular diffusions parallel and perpendicular to the plate are, respectively, enhanced and restricted by the tip. The molecular reorientation is significantly slowed down by the tip and plate, which disrupt the hydrogen-bond network present in the bulk water.

Molecular Insights on the Wetting Behavior of a Surface Corrugated with Nanoscale Domed Pillars

“…As H of the domed pillars increased from 12.5 to 16.1 Å, however, ρ decreased because the confinement of water by the solid surfaces was less effective as the bulk water on top of the pillars significantly contributed in this case. Similarly, the prior work reported that layering of water molecules near a solid surface vanished at distances >1 nm from the surface …”

“…Similarly, the prior work reported that layering of water molecules near a solid surface vanished at distances >1 nm from the surface. 57 We examined how the l−v interface penetrated down into the interpillar gap in the transition from the CB (Φ = 0) to the WZ (Φ = 1) state. Figure 3a,b illustrates the density profiles for various Φs for the surfaces patterned with the domed and rectangular pillars of H = 1.97 nm, respectively.…”

Molecular Features of Hydration Layers: Insights from Simulation, Microscopy, and Spectroscopy

“…The orientations of OH bonds were checked by varying the tip−surface distance D (Figure 9, left). 31 When the tip was 8 Å above the surface (Figure 9, left), three distinct orientations, called the bulk (b), tangential (t), and dangling (d) orientations, were found in the first molecular layer (at θ ≈ 20°, 100°, and 154°in Figure 9). In the b orientation (Figure 9, right), an OH group is pointing away from the surface and toward the bulk water.…”

“…Therefore, a classical MD simulation employing a rigid body model of water with point charges is commonly used. 31 Nevertheless, such a classical MD simulation can be computationally demanding because the tip, surface, and liquid water need to be considered altogether. Simulating a VSFG experiment requires the consideration of the intramolecular vibrations that are quantum mechanical in nature.…”

“…The molecular vibrations of water are largely irrelevant to a typical AFM imaging, which mainly probes the molecular stacking. Therefore, a classical MD simulation employing a rigid body model of water with point charges is commonly used . Nevertheless, such a classical MD simulation can be computationally demanding because the tip, surface, and liquid water need to be considered altogether.…”