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C: Surfaces, Interfaces, Porous Materials, and Catalysis
Phase, Structure, and Dynamics of the Hydration Layer Probed by Atomic Force Microscopy Liyi Bai, Zhengqing Zhang, and Joonkyung Jang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b04736 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019
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The Journal of Physical Chemistry
Phase, Structure, and Dynamics of the Hydration Layer Probed by Atomic Force Microscopy
Liyi Bai, Zhengqing Zhang, and Joonkyung Jang* Department of Nanoenergy Engineering, Pusan National University, Busan 46241, Republic of Korea 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 along the 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.
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INTRODUCTION In an aqueous solution or under an ambient condition, a structurally-ordered layer of water ubiquitously grows on a solid surface.1-3 Such a hydration layer significantly affects the wetting of a surface4-7 and the freezing process involving a heterogeneous nucleation.8 Interestingly, a hydration layer develops not only on a hydrophilic but on a hydrophobic surface.9, 10 Atomic force microscopy (AFM) is widely used to probe the molecular structure of a hydration layer.11 With the frequency-modulated AFM, one can measure the force on the tip with tens of pN resolution and the tip–surface distance can be resolved as small as 1 Å.12-14 As an AFM tip approaches a surface within 1 nm, the force on the tip typically oscillates with varying the distance between the tip and surface.15-17 If the period of oscillation in the force–distance curve matches the molecular diameter of water, the oscillation is ascribed to the transition between the molecular layers.18 Unfortunately, no exact relationship exists between the force measured by AFM and the structure of the hydration layer. A theory which relates the forcedistance curve to the molecular density profile has been developed but the scope of theory is limited to a hard-sphere liquid probed by a single molecular probe.19, 20 Probing the hydration layer on a hydrophobic surface by means of AFM is further complicated by the possible phase transition of water: if confined between two close hydrophobic objects, a liquid water evaporates to a vapor. This drying transition gives rise to a hydrophobic force which is stronger and longer-ranged than the van der Waals force between the tip and surface. The previous studies on the drying transition focused on, for simplicity of analysis, the case where two parallel plates are immersed in a liquid water.21-24 Less is known about the hydrophobic force relevant to the AFM experiment where a sharp nanoscale tip interacts with a surface.
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AFM primarily probes the molecular packing along the direction normal to a surface (e.g. force vs. distance from the surface). However, for water is an associative liquid forming the hydrogen bond network, its structure shows a preferential orientation at an interface.25 Therefore, a full description of the structure of the hydration layer requires the molecular orientation as well. Also of interest are the dynamic properties such as the molecular diffusion and reorientation dynamics. These dynamic properties of the hydration layer are related to the transport of water in the microfluidics for example. In principle, the molecular orientation and dynamics of water can be explored by using the spectroscopies such as the sum-frequency generation spectroscopy, Xray and neutron scatting, and two-dimensional infrared spectroscopy.26-29 These advanced spectroscopic experiments are more challenging to execute and difficult to interpret than their bulk phase counterparts, because the bulk phase water, dominating in number, easily wipe out the signals from the interfacial water. Given the experimental hurdles in probing the phase, structure and dynamics of a hydration layer as probed in AFM, a molecular simulation emerges as a viable option. Herein, using allatom molecular dynamics (MD) simulation, we study the hydration layer formed between an AFM tip and plate both of which are made of hydrophobic carbon. In the context of an AFM experiment performed in a liquid water, we investigate the drying transition and the hydrophobic force measured by an AFM tip. We derive the scaling behavior of the hydrophobic force with varying the size of tip. We investigate how the molecular packing and orientation of the hydration layer are influenced by the presence of the tip. The molecular diffusion and reorientation dynamics in the hydration layer are investigated and compared with those in the bulk water. Molecular Dynamics Simulation Methods
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We emulated an AFM experiment by placing a hemispherical tip above a plate in liquid water (Figure 1). Both the tip and plate were made of carbon and carved out from the face-centered cubic (FCC) lattice with the lattice spacing of 3.567 Å. This fictitious FCC lattice was used to create atomically smooth surfaces of the tip and plate. The radius of the tip, R, was varied as 9, 11, 15 and 17 Å by fixing the lateral dimension of the plate to 34 Å × 34 Å. We varied the vertical distance between the bottom of the tip and plate D from 2 to 10 Å with an interval of 1 Å. The number of water molecules was around 8000, varying with D. We employed the extended simple point charge (SPC/E)
30
model of water which has proven
to capture the experimental features such as the surface tension, compressibility, and vaporliquid equation of state under the ambient conditions.31, 32 In this model, each water molecule had partial charges of ― 0.8476 and +0.4238 on oxygen and hydrogen atoms, respectively. The 𝑞𝑖𝑞𝑗
partial charges interacted via the Coulomb potential, 𝑈𝐶(𝑟𝑖𝑗) = 4𝜋𝜀0𝑟𝑖𝑗, where 𝑞𝑖 is the partial charge on atom 𝑖, 𝜀0 is the vacuum permittivity, and 𝑟𝑖𝑗 is the distance between two atoms 𝑖 and 𝑗. The long-ranged Coulomb interaction was taken into account by using the Ewald sum method.33 Oxygen atoms additionally interacted with each other through the Lennard-Jones (LJ) potential,21 𝜎𝑖𝑗 12
𝑈𝐿𝐽 (𝑟𝑖𝑗) = 4𝜀𝑖𝑗[( 𝑟𝑖𝑗 )
𝜎𝑖𝑗 6
― ( 𝑟𝑖𝑗 ) ], where 𝜀𝑖𝑗 and 𝜎𝑖𝑗 are the energy and length parameters set to
3.166 Å and 0.6502 kJ mol ―1, respectively.34 The pair interaction between oxygen and carbon atoms was described by the LJ potential with 𝜎𝑖𝑗and 𝜀𝑖𝑗 of 3.190 Å and 0.4389 kJ mol ―1, respectively.35 The contact angle of water on the present plate was reported to be 107.74 °,35 confirming the present plate and tip were hydrophobic. We equilibrated each simulation system by running a 200 ps-long MD simulation in the canonical ensemble (NVT) at 300 K. We then ran a 2-ns long isothermal-isobaric (NPT) simulation at 300 K and 1 atm by applying the Nose-Hoover thermostat and barostat.36, 37 Water 4|Page ACS Paragon Plus Environment
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molecules were held rigid by using the SHAKE algorithm.21 The cut-off distance of the LJ interaction was 10 Å. The equation of motion was propagated by using the velocity Verlet algorithm38 with a time step of 2 fs. All the simulation methods were implemented by using the Large-Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) MD package.39 Results and Discussion A. Phase Behavior and Force-Distance Curve for the Hydration Layer Figure 1 illustrates the drying transition of water confined between the tip and plate for tips with various radii Rs. With closely approaching the tip toward the plate (D10 Å and became repulsive for D7 Å. Therefore, the third molecular layer, induced by the tip, was not as welldeveloped as the first and second layers due to the plate. The sharp curvature of the tip could not give an ordered molecular packing along the direction normal to the plate. Figure 4b illustrates how ρ(z) was affected by reducing D from 10 to 4 Å (for the tip with R = 11 Å). As D decreased from 10 to 9 Å, the third minor peak of ρ(z) disappeared and the second peak broadened. This signifies that the second and third molecular layers merged due to an increased confinement by approaching the tip toward the plate. With further reducing D to 8 Å, the second peak shrank in width back to its original value, indicating that the third layer vanished and the well-defined second layer re-emerged. Up to this point, the first peak was nearly unchanged in its position, height, and width. With decreasing D from 8 to 7 Å, the second peak significantly shifted to a lower value in position and height, and reduced in width, signaling that 8|Page ACS Paragon Plus Environment
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the second layer faded away. The first peak also reduced in height slightly. As D reduced from 7 to 6 Å, the first peak drastically reduced in height and the second peak vanished, due to the drying transition. With further decreasing D, the density peak shortened further in height but not as drastically as in the change found for the drying transition (D from 7 to 6 Å). The drying transition (D
