Session T41: Focus Session: The Role of Water in Energy Production and Utilization I

Deciphering the morphology of ice films on metal surfaces

Although extensive research has been aimed at the structure of ice films [1], questions regarding basic processes that govern film evolution remain. Recently we discovered how ice films as many as 30 molecular layers thick can be imaged with STM [2]. The observed morphology yields new insights about water-solid interactions and how they affect the structure of ice films. This talk gives an overview of this progress for crystalline ice films on Pt(111) [2-5]. STM reveals a first molecular water layer very different from bulk ice: besides the usual hexagons it also contains pentagons and heptagons [3]. Slightly thicker films ($\sim $1nm, at T$>$120K) are comprised of $\sim $3nm-high crystallites, surrounded by the one-molecule-thick wetting layer. These crystals dewet by nucleating layers on their top facets [4]. Measurements of the nucleation rate as a function of crystal height provide estimates of the energy of the ice-Pt interface. For T$>$115K surface diffusion is fast enough that surface smoothing and 2D-island ripening is observable [5]. By quantifying the T-dependent ripening of island arrays we determined the activation energy for surface self-diffusion. The shape of these 2D islands varies strongly with film thickness. We attribute this to a transition from polarized ice at the substrate towards proton disorder at larger film thicknesses. Despite fast surface diffusion ice multilayers are often far from equilibrium. For example, ice grows between $\sim $120 and $\sim $160 K in its cubic variant rather than in its equilibrium hexagonal form. We found this to be a consequence of the mismatch in the atomic Pt-step height and the ice-bilayer separation and propose a mechanism of cubic-ice formation via growth spirals around screw dislocations [2]. \\[4pt] [1] A. Hodgson and S. Haq, Surf. Sci. Rep. 64, 381 (2009). \\[0pt] [2] K. Th\"{u}rmer and N. C. Bartelt, Phys. Rev. B 77, 195425 (2008). \\[0pt] [3] S. Nie, P. J. Feibelman, N. C. Bartelt and K. Th\"{u}rmer, Phys. Rev. Lett. 105, 026102 (2010). \\[0pt] [4] K. Th\"{u}rmer and N. C. Bartelt, Phys. Rev. Lett. 100, 186101 (2008). \\[0pt] [5] S. Nie, N. C. Bartelt, and K. Th\"{u}rmer, Phys. Rev. Lett. 102, 136101 (2009).   

{\it A Priori} Method for First Principles Study of Aqueous Electrochemistry: Application to Biofuels and Catalysis

We present a novel description of water which will allow the first {\it a priori} studies of catalysis of biofuels in aqueous electrochemical environments. Our method offers a computationally efficient alternative to the thermal sampling required by molecular dynamics yet provides a more realistic description of bulk water than including explicit frozen water or traditional continuum solvation models. Into Joint Density Functional Theory (JDFT), which joins an electron density-functional for the solute with classical density-functional theories for liquid water\footnote{R. Sundararaman et al, unpublished, to be presented at the APS March Meeting (2011)} into a single variational principle for the free energy of the combined system, we introduce the innovation of an {\it a priori} form of the coupling functional between the quantum-mechanical system and liquid water based on a local density approximation to the Hohenberg-Kohn density-only functional. Without any fits to solvation data whatsoever, this new method predicts solvation energies of small organic molecules well compared to state-of-the art empirical quantum-chemical cavity approaches. The site interaction potentials produced closely resemble the widely used TIP3P site potentials for water without requiring any empirical parameters.   

\emph{A priori} classical density functionals of water: toward first principles exploration of aqueous based energy systems

The microscopic structure of inhomogeneous water plays a critical role in the properties of a wide variety of important energy systems including fuel cells and photoelectrochemical cells. Joint density functional theory has proven to be an efficient tool for the quantum-mechanical modeling of systems such as Pt electrodes in the presence of water, but requires theories for water which go beyond semi-empirical continuum solvation models, and accurate models for the coupling between water and electronic systems {\footnote{K. L. Weaver et al, to be presented at APS March Meeting 2011}}. Toward this end, we present a new density-functional description of liquid water capable of predicting interatomic correlation functions, the linear and nonlinear dielectric response, and solvation energies without empirical fit parameters. The functional itself is built upon the site-potential representation of the ideal gas, a hard sphere reference fluid for the repulsive correlations, and an equation of state that reproduces the bulk properties of water over the entire extent of its liquid phase. Hydrogen bonding, the local tetrahedral structure and orientational correlations are captured \textit{a priori} by a density-functional reformulation of the Kirkwood model for the dielectric constant.   

Quasicrystal and ice phases tiled with pentagons in confined water

Bulk water is known to form a wealth of ice polymorphs and two distinct amorphous phases. Less is known of the structures that confined and interfacial water can adopt, and whether there is a correspondence between the structures and phase diagrams of water in bulk and in confinement. In this talk I will present a molecular simulations study of the phase behavior of a water bilayer confined between two non-hydrogen bonding walls and demonstrate that a water bilayer also presents rich polymorphism, including an ice crystal fully tiled by pentagons and a quasicrystal, the first ever reported for water. The water quasicrystal and the ice polymorph tiled with pentagons are not templated by the confining surfaces. This indicates that these novel phases are intrinsically favored in bilayer water and suggests that they may be formed, without confinement, on surfaces. \\[4pt] [1] J.C. Johnston, N. Kastelowitz and V. Molinero, ``Liquid to quasicrystal transition in bilayer water,'' J. Chem. Phys. 133, 154516 (2010)\\[0pt] [2] N. Kastelowitz, J. C. Johnston and V. Molinero, ``The anomalously high melting temperature of bilayer ice,'' J. Chem. Phys. 132, 124511 (2010).   

The role of interfacial water in ``nano''

To understand the role of interfacial water on nanostructred surfaces is important for materials science and biology. The talk will describe some of our recent progress in predicting and understanding the effects of nanopatterning on topologically or chemically heterogeneous surfaces on wetting via \emph{in silico} experiments.   

Ordering of confined water between metallic surfaces

It has been pointed out (PCCP 2010, Poissier et al.) that the hydrogen bonding type interaction occuring at water/metal interface makes the two type of interfacial water orderings (hydrophobic or hydrophilic overlayers) very close in energy. The most stable, hydrophobic, overlayer has very small net dipole moment perpendicular to the surface, while the least stable (in vacuum) hydrophilic interface has a large ($\approx 1.8 D$) net dipole moment. First principles molecular dynamics simulations of liquid water confined between two Pd surfaces have been performed and structural and electronic water properties have been studied in detail. We show that water confinement in this situation results in a spontaneous symmetry breaking of the system, inducing an electric field across the liquid water slab. We discuss the origin of this spontaneous polarization and show its dependence with the confinement distance along the direction perpendicular to the planes of the surfaces.   

Exploring hydration at the nanoscale

It is widely appreciated that water molecules contribute a critical element to the forces governing chemical processes in an aqueous environment, and the purported differences in water structure induced by the presence of confining surfaces are correspondingly likely to play a role in interfacial chemistry. The development of a detailed understanding of the organization of fluid water at the interface with real materials is therefore of great interest. In this presentation, results obtained from fully atomistic computer simulations of water in the presence of confining interfaces will be discussed, with the goal of elucidating the molecular level influence of surface character on water structure and energetics. Further, we emphasize the extension of studies to temperatures and pressures well outside the conventional realm of the ambient solvent thermodynamic state. The interface examples to be considered in this presentation emphasize systematic studies designed to elucidate guiding principles. These include extended and nanoscale hydrophobic and hydrophilic crystalline surfaces and interfaces with systematically patterned hydrophobicity.   

Energy transport during sessile-water-droplet evaporation

Energy transport mechanisms for a steadily evaporating water droplet maintained on Cu or Au(111) surfaces are compared. In the absence of buoyancy-driven convection, thermal conduction and thermocapillary convection are the active modes of energy transport. The dominant mode varies along the liquid-vapor interface. Although thermal conduction is the dominant mode in regions far from the contact line, thermocapillary convection is by far the larger mode of energy transport near the three-phase contact line. The latter region is where most of the droplet evaporation occurs. Evaporation experiments on both Cu and Au(111) suggest that the thermocapillary convection provides more than 92\% of the total energy required for the evaporation.