structure and dynamics of two-dimensional confined water
The structure and dynamics of water under a geometric confinement is of great significance due to its importance in water flow, surface chemistry and environmental sciences. The physical properties of water at an interface or in a nanopore are often different than its bulk counterpart and can strongly depend on the fine details of the confinement. A systematic understanding of the influence of the confinement on this rich behavior was, until recently poor, because of experimental limitations to access interfacial water structures. The discovery that graphene is stable in its two dimensional form has opened new research possibilities and it has proved to be an instrumental tool for the investigation of confined water structures. Its remarkable mechanical and electronic properties combined with scanning probe techniques allow to directly visualize and measure water structures that are confined between graphene and a variety of supporting substrates. Information regarding the influence of the interface structure and wettability, environmental humidity, temperature, pressure and the presence of foreign species on the structure and dynamics of confined water can be now experimentally accessed in situ and real time with scanning probe microscopies.
In chapter 3, we studied the growth of fractal nanostructures in a 2D system, intercalated between mica and graphene. Based on our scanning tunneling spectroscopy data, we provided evidence that these fractals are 2D ice. They grow while they are in material contact with the atmosphere at 20_C (without significant thermal contact to the ambient) and at low relative humidity. The growth is studied in situ, in real time and space at the nanoscale. We found that the growing 2D ice nanocrystals assume a fractal shape, which is conventionally attributed to DLA. However, DLA requires a low mass density mother phase, in contrast to the actual currently present high mass density mother phase. Latent heat effects and consequent transport of heat and molecules are found to be key ingredients for understanding the evolution of the ice flakes. We conclude that not the local availability of water molecules (DLA), but rather them having the locally required orientation is the key factor for incorporation into the 2D ice nanocrystal. In combination with the transport of latent heat, we attribute the evolution of fractal 2D ice nanocrystals to local temperature dependent rotation limited aggregation. The ice growth occurs under extreme supersaturation, i.e., the conditions closely resemble the natural ones for the growth of complex 2D snow (ice) flakes and we consider our findings crucial for solving the ‘perennial’ snow (ice) flake enigma.
At low humidity, epitaxial one-layer thick ice fractals form. The growth of the ice fractal is initiated by the heat extracted from the system by evaporation, into the 3D ambient, of the second layer of water intercalated between mica and graphene under low humidity conditions. In chapter 4, the fractal shape dependence on the graphene cover and the evaporation rate of the water molecules from the interface is investigated. We found that the thickness of the fractals’ fingers scale as the square root of the ratio of the bending energy of graphene plus the surface energy of the intercalated ice and the product of the velocity of the fractal front and a term related to the hindrance of the water ad-molecules. Ice fractals formed under a thick graphene flake and upon a low evaporation rate are thick with few side branches, whereas fractals grown upon high growth rate under single-layer graphene are thin and very ramified. We attribute the coarsening of fractals to the extra degree of freedom of the surrounding water molecules, enabled by the non-complete adaptation of the ice crystal’s morphology by the graphene cover.
The exact structure of the graphene-ice-mica interface has not been discussed yet. Potassium (K+) ions are known to cover the mica surface in order to preserve charge balance. The distribution and exact lateral organization of K+ on the air-cleaved mica is yet unknown. In chapter 5, we showed, by the use of graphene as an ultra-thin protective coating and scanning probe microscopies, that single K+ ions form ordered structures that are covered by an ice layer. The K+ ions prefer to minimize the number of nearest neighbor ions by forming row-like structures as well as small domains. This trend is a result of repulsive ionic forces between adjacent ions, weakened due to screening by the surrounding water molecules. Using high resolution conductive atomic force microscopy maps, the local conductance of the graphene is measured, revealing a direct correlation between the K+ distribution and the structure of the ice layer. Our results shed light on the local distribution of ions on the air-cleaved mica, solving a long-standing enigma. They also provide a detailed understanding of charge transfer from the ionic domains toward grapheme.
The classic regelation experiment of Thomson in the 1850’s deals with cutting an ice cube, followed by refreezing. The cutting was attributed to pressure induced melting, but has been challenged continuously and only lately consensus emerges by understanding that compression shortens the O:H non-bond and lengthens the H-O bond simultaneously. This H-O elongation leads to energy loss and lowers the melting point. The hot debate survived well over 150 years, mainly due to a poorly defined heat exchange with the environment in the experiment. In chapter 6, we achieved thermal isolation from the environment and studied the fully reversible ice - quasi-liquid water transition for water confined between graphene and muscovite mica. We observe a transition from 2D ice into a quasi-liquid phase by applying a pressure exerted by an atomic force microscopy tip. At room temperature the critical pressure amounts to about 6 GPa. The transition is completely reversible: refreezing occurs when the applied force is lifted. The critical pressure to melt the 2D ice increases with temperature and we measured the phase coexistence line between 293 and 333 K. From a Clausius-Clapeyron analysis we determine the latent heat of fusion of two-dimensional ice at 0.15 eV/molecule, being twice as large as for bulk ice.
In chapter 7, the effect of confinement on the structure and dynamics of alcohol–water mixtures between graphene and mica, has been studied in situ and in real time at the molecular level by AFM at room temperature. AFM images reveal that the adsorbed molecules are segregated into faceted alcohol-rich islands on top of an ice layer on mica, surrounded by a preexisting multilayer water-rich film. These faceted islands are in direct contact with the graphene surface, revealing a preferred adsorption site. Moreover, alcohol adsorption at low relative humidity reveals a strong preference of the alcohol molecules for the ordered ice interface. The growth dynamics of the alcohol islands are governed by supersaturation, temperature, the free energy of attachment of molecules to the island edge and two-dimensional diffusion. The measured diffusion coefficients display a size dependence on the molecular size of the alcohols, and are about 6 orders of magnitude smaller than the bulk diffusion coefficients, demonstrating the effect of confinement on the behavior of the alcohols. These experimental results provide new insights into the behavior of multicomponent fluids in confined geometries, which is of paramount importance in nanofluidics and biology.
Chapter 8 describes the structure and nature of water confined between hydrophobic MoS2 and graphene at room temperature. We found that water forms two-dimensional crystalline ice layers. In contrast to the hexagonal ice ‘bilayers’ of bulk ice, this 2D crystalline ice phase consists of two planar hexagonal layers. Additional water condensation leads to either lateral expansion of the ice layers or to the formation of three-dimensional water droplets on top or at the edges of the two-layer ice, indicating that water does not wet these planar ice films. The hydrophobic character arises from the lack of dangling bonds on either surface of the ice film because of their nontetrahedral bonding geometry. The unusual geometry of these ice films is of great importance in biological systems with water in direct contact with hydrophobic surfaces. The observed phase behavior, phase transitions and dynamics of the confined water structures underline the complexity of the governing physical mechanisms. It is clear that the behavior of the water molecules will heavily depend on the confinement characteristics as well as temperature and pressure. For instance, the fact that the ice structure is different in the case of water confined between graphene and MoS2 compared to ice between graphene and mica, indicates the significance of the interface in defining the geometry of the ice phase. It is evident that a general picture of the state of water under confinement cannot be drawn and the confinement details and conditions should be considered in future investigations.
