Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session H27: Focus Session: Confined and Biological Water II |
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Sponsoring Units: DCP Chair: Teresa Head?Gordon, University of California, Berkeley Room: D137 |
Tuesday, March 16, 2010 8:00AM - 8:36AM |
H27.00001: Water confined at the nanoscale: insight from first principles calculations Invited Speaker: We will discuss structural, vibrational and electronic properties of water confined between non polar surfaces, as obtained from a series of ab-initio simulations. In particular we consider graphene, carbon nanotubes and hydrogenated diamond surfaces as confining media and we present a discussion of so called hydrophobic interactions, taking into account the details of the interface electronic properties. [Preview Abstract] |
Tuesday, March 16, 2010 8:36AM - 8:48AM |
H27.00002: Dynanimcal Properties of Water in Nanoconfinement Michael Chandross, Christian D. Lorenz, J. Matthew D. Lane, Gary S. Grest We report the results of large-scale Molecular Dynamics (MD) simulations of water confined to sub-nanometer thicknesses. We vary the amount of water and the applied pressure to examine the effects on the structure and dynamics of the confined water. Calculations of two dimensional diffusion constants indicate that the water remains liquid-like in all cases. The water is subjected to shear to measure the viscosity and microscopic friction. We find that while the viscosity increases by as much as a factor of six for low coverage and high loads, there is still no evidence of ice-like layers being formed. Friction coefficients can only be calculated at high shear velocities due to the low viscosity of the water and are found to decrease with increasing amounts of water, similar to experimental results. [Preview Abstract] |
Tuesday, March 16, 2010 8:48AM - 9:00AM |
H27.00003: The importance of van der Waals dispersion forces in ice B. Santra, A. Michaelides, M. Scheffler For decades it is known that the condensed phases of water are held together through a combination of hydrogen bonds (HBs) and van der Waals (vdW) dispersion forces. Whilst HBs have received widespread attention, relatively little is known about the importance of vdW forces. Partly this is down to the now most popular electronic structure theory (density-functional theory (DFT)) providing an inadequate description of vdW forces when standard exchange-correlation functionals are used. However, improved semi-empirical vdW corrections to DFT [1] and the non-local vdW functional [2] mean that it is now possible to shed light on this issue. Here, we report simulations on a range of ambient and high pressure phases of ice to understand the delicate interplay of HBs and vdW. We find that the proportion of the lattice energy coming from vdW forces monotonously increases as the density of the ice phases increases, and as a consequence vdW plays a crucial role in determining the relative stabilities of the high density phases of ice. In addition, our calculations show that the effective volume of the water molecules is reduced when vdW is accounted for and provide general insights into the importance of vdW forces in other condensed phases of water. [1] Tkatchenko \emph{et al.}, PRL \textbf{102}, 073005 (2009), [2] Dion \emph{et al.}, PRL \textbf{92}, 246401 (2004). [Preview Abstract] |
Tuesday, March 16, 2010 9:00AM - 9:12AM |
H27.00004: Density Functional Theory study of the equilibrium density of water at normal conditions Jue Wang, Guillermo Roman-Perez, Jose M. Soler, Emilio Artacho, Marivi Fernandez-Serra {\it Ab initio} molecular dynamics of liquid water with the use of density functional theory (DFT) currently underperform experimental equilibrium density 1g/cm{$^3$} under room temperature. At constant density, not much is known about the equilibrium density of commonly used GGA functionals in liquid water simulations. We present a DFT-based AIMD study of liquid water at different densities and analyze the structure and diffusivity of water with different exchange and correlation functionals. We show that all current GGA functionals fail to reproduce experimental density, however, the explicit description of long range correlations through a Van der Walls density functional (DRSLL)\footnote{M. Dion, H. Rydberg, E. Schr\"{o}der, D. C. Langreth, and B. I. Lundqvist, \textit{Phys. Rev. Lett. } \textbf{92}, 246401 (2004)} can potentially transform our current understanding of the structure of liquid water. Our results shows that this new functional improves density, with only 2\% error to experiment. But it underperforms GGA functionals in terms of structure. [Preview Abstract] |
Tuesday, March 16, 2010 9:12AM - 9:24AM |
H27.00005: Fluctuations of Water near Hydrophobic, Hydrophilic and Patterned Surfaces and in Nanoscopic Confinement Amish Patel, David Chandler, Sumanth Jamadagni, Hari Acharya, Shekhar Garde We study water density fluctuations in probe volumes of different shapes and sizes, in the bulk, near various hydrophobic and hydrophilic surfaces as well as in nanoscopic confinement between these surfaces. Specifically, we report the probability of finding $N$ water molecules in large ($>$1 nm$^3$) probe volumes. An examination of these large length-scale fluctuations of water near a hydrophobic surface reveals that the probability of density depletion is significantly larger than that in bulk, akin to density fluctuations near a water-vapor interface. In contrast, fluctuations near a hydrophilic surface are very similar to that in the bulk. We further investigate the effect of surface heterogeneity on fluctuations. We also examine the effect of confinement on density fluctuations as a function of the nature of the confining surfaces as well as the distance between them. [Preview Abstract] |
Tuesday, March 16, 2010 9:24AM - 9:36AM |
H27.00006: Nanoconfined water under electric field Alenka Luzar, D. Bratko, C.D. Daub We study the effect of electric field on interfacial tension of nanoconfined water [1,2] using molecular simulations. Our analysis and simulations confirm that classical electrostriction characterizes usual electrowetting behavior in nanoscale hydrophobic channels and nanoporous materials [3]. We suggest a new mechanism to orient nanoparticles by an applied electric field even when the particles carry no charges or dipoles of their own. Coupling to the field can be accomplished trough solvent-mediated interaction between the electric field and a nanoparticle [4]. For nanoscale particles in water, we find the response to the applied field to be sufficiently fast to make this mechanism relevant for biological processes, design of novel nanostructures and sensors, and development of nanoengineering methods [5]. [1]C. D. Daub, D. Bratko, K. Leung and A. Luzar, J. Phys. Chem. C 111, 505 (2007). [2] D. Bratko, C. D. Daub, K. Leung and A. Luzar, J. Am. Chem. Soc. 129, 2504 (2007) [3] D. Bratko, C. D. Daub and A. Luzar, Phys. Chem. Chem. Phys. 10, 6807 (2008). [4] D. Bratko, C. D. Daub and A. Luzar, Faraday Discussions 141, 55 (2009). [5] C. D. Daub, D. Bratko, T. Ali and A. Luzar, Phys. Rev. Lett. 103, 207801 (2009). [Preview Abstract] |
Tuesday, March 16, 2010 9:36AM - 10:12AM |
H27.00007: Water and hydrophobic effects at interfaces and in confinement Invited Speaker: |
Tuesday, March 16, 2010 10:12AM - 10:24AM |
H27.00008: The role of hydrogen bonding in water-metal interactions Adrien Poissier, Sriram Ganeshan, Marivi Fernandez-Serra The hydrogen bond interaction between water molecules adsorbed on a Pd-$\langle 111 \rangle$ surface, a well known nucleator of two dimensional bilayers of ice at low temperatures, is studied using density functional theory calculations. The role of the exchange and correlation potential in the characterization of both the hydrogen bond and the water-metal interaction is analyzed in detail. We conclude that the choice of this potential is critical in determining the cohesive energy of water-metal complexes. The crucial factor nonetheless is not the description of the metal screening, even if this screening represents an important ingredient for the water-metal interaction. The different characterization of the hydrogen bonds between water molecules and the {\it pseudo hydrogen bonds} established between the water and the surface is at the heart of the large disparity we observe in our calculations. These results put in evidence the urgent need for an accurate characterization of the hydrogen bond interaction with density functional theory. [Preview Abstract] |
Tuesday, March 16, 2010 10:24AM - 10:36AM |
H27.00009: Suspension Bridge Induced by Electrostrictive Effect in Dielectric Liquids Chuanshan Tian, Y. Ron Shen It has been demonstrated that upon application of a high voltage between two beakers of water, a suspending water bridge could form that connects the two beakers with a length up to a few centimeters. (see e.g. \textit{J Phys D} \textbf{40}, 6112(2007)). The mechanism of this surprising phenomenon is not yet well understood. We show theoretically and experimentally that the bridge is the result of balance of actions of electrostriction with gravitation force and surface tension. The same phenomenon can be observed also in other dielectric liquids such as methanol, ethanol, isopropanol, and glycerol. In all cases, the experimentally measured bridge length and its dependence on the electric field and the dielectric constant and surface tension of liquid agree well with predictions from our theoretical model. [Preview Abstract] |
Tuesday, March 16, 2010 10:36AM - 10:48AM |
H27.00010: First principles study of the Ga(10$\bar{1}$0)/water interface Jue Wang, Xiao Shen, Marivi Fernandez-Serra Pure GaN is a water-splitting photocatalyst which works in UV light. A first-principles study shows monolayer of water molecules on the (10$\bar{1}$0) surface of wurtzite GaN have negligible barrier to be dissociated into OH$^-$ which bind to Ga$^+$ and H$^+$ which bind to N$^-$.\footnote{X.Shen {\it et al.}, J. Phys. Chem. C {\bf 113}, 3365 (2009)} We present an \textit{ab initio} molecule dynamics study of bulk water molecules on top of the (10$\bar{1}$0) surface of GaN. The dissociation of water molecules happens very fast within the first ps of simulation. We find that OH$^{-}\cdot\cdot\cdot$ OH$^-$ Hydrogen bond are formed between two OH$^{-}$ on the surface. We propose two different types of OH$^{-}$ based on the different H-bond structure between them and the bulk water molecules. We also calculate the vibrational spectrum and the interface electronic structure of the semiconductor/aqueous system. [Preview Abstract] |
Tuesday, March 16, 2010 10:48AM - 11:00AM |
H27.00011: dynamics of water at the 100 and 111 surfaces of platinum electrodes Adam Willard, David Chandler, Paul Madden The results of molecular dynamics simulations of water in a model electrochemical cell is described. The model consists of water contained between two platinum electrodes that are polarizable and capable of being held at constant potential relative to the vacuum. The structure and fluctuations of water molecules adjacent to both the 100 and the 111 surfaces of a solid platinum electrode are considered. It is shown that the orientational distributions as well as the relaxation dynamics of the electrode-adsorbed water molecules depend significantly on electrode geometry. This dependence reflects competition between hydrogen bonding and non-hydrogen bonding interactions of electrode adsorbed water molecules, and whether electrode lattice geometry is commensurate with two-dimensional hydrogen bonding patterns of water. Furthermore the dynamics of adsorbed water molecules are spatially and temporally heterogeneous leading to slow reorganization of the hydrogen bond patterns adjacent to the metal surfaces. [Preview Abstract] |
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