Ordered water structure at hydrophobic graphite interfaces observed by 4D, ultrafast electron crystallography

Abstract

Interfacial water has unique properties in various functions. Here, using 4-dimensional (4D), ultrafast electron crystallography with atomic-scale spatial and temporal resolution, we report study of structure and dynamics of interfacial water assembly on a hydrophobic surface. Structurally, vertically stacked bilayers on highly oriented pyrolytic graphite surface were determined to be ordered, contrary to the expectation that the strong hydrogen bonding of water on hydrophobic surfaces would dominate with suppressed interfacial order. Because of its terrace morphology, graphite plays the role of a template. The dynamics is also surprising. After the excitation of graphite by an ultrafast infrared pulse, the interfacial ice structure undergoes nonequilibrium ''phase transformation'' identified in the hydrogen-bond network through the observation of structural isosbestic point. We provide the time scales involved, the nature of ice-graphite structural dynamics, and relevance to properties related to confined water.hydrophilic and hydrophobic interactions ͉ ice-substrate structure and dynamics ͉ nonequilibrium phase transition W ater at interfaces is fundamental to the understanding of various phenomena, such as wetting, molecular recognition and macromolecular folding. When compared with bulk phases (1, 2), the nano-scale interface is believed to have a unique function in nanotribology (3, 4), chemical reactivity (4-6) and biological structure and dynamics (7-10). From the structural point of view, considering the energetics, the determining factor at interfaces is the delicate balance of hydrogen bonding among water molecules and the comparable interactions with a substance, defining the 2 extremes of hydrophobic and hydrophilic behavior. However, the time scales of structural dynamics are important for defining the microscopic mechanisms of relaxations and the role of substrate structure and morphology (11). For water ice on a hydrophilic substrate, the ordered layers are evidenced by their diffraction (Bragg spots), and this long-range order is lost when the ice assembly becomes at a distance from the substrate (12). On hydrophobic surfaces, the expected picture is that randomly oriented crystallites form with no interfacial long-range order, because of the stronger intermolecular interactions when compared with those of water-substrate.In the current study, we report the determination of structure and dynamical behavior of water assembly on highly oriented pyrolytic graphite (HOPG), a hydrophobic substrate. Using ultrafast electron crystallography (UEC) (11) that has been described in detail in refs. 12 and 13, provides, through diffraction, the position of atomic planes and the temporal change of the structure. Electron crystallography (14), because of the large electron scattering cross section, is ideal for these surface and interface probings. Here, it is shown that the layered structure of HOPG serves as a substrate and promotes the crystalline order in the ice thin film along the surface normal direction...

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