Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session J27: Focus Session: Confined and Biological Water III |
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Sponsoring Units: DCP Chair: Sol M. Gruner, Cornell University Room: D137 |
Tuesday, March 16, 2010 11:15AM - 11:51AM |
J27.00001: Molecular Studies of Bulk Water and Hydration Water at Interfaces Invited Speaker: My talk describes an experimental and theory/simulation study of water studied as bulk, solution, and under spatial confinement. Using x-ray scattering, quasi-elastic neutron scattering, and/or atomistic and coarse-grained chemical models combined with simulation, we address fundamental questions about the origin of waters thermodynamic and dynamic anomalies, the microscopic features of the potential energy landscape that define the origin of these anomalies, and how these are altered at model biological interfaces. [Preview Abstract] |
Tuesday, March 16, 2010 11:51AM - 12:03PM |
J27.00002: {\it Ab initio} liquid water from PBE0 hybrid functional simulations Zhaofeng Li, Xifan Wu, Roberto Car For reasons of computational efficiency, so far most {\it ab initio} molecular dynamics simulations of liquid water have been based on semi-local density functional approximations, such as PBE and BLYP. These approaches yield a liquid structure that, albeit qualitatively correct, is overstructured compared to experiment, even after nuclear quantum effects have been taken into account.\footnote{J. A. Morrone and R. Car, Phys. Rev. Lett. \textbf{101}, 017801(2008)} A major cause of this inaccuracy is the delocalization error associated to semi-local density functional approximations, which, as a consequence, overestimate slightly the hydrogen bond strength in the liquid. In this work we adopt the PBE0 hybrid functional approximation, which, by mixing a fraction of exact (Hartree-Fock) exchange, reduces significantly the delocalization error of semi-local functionals. Our approach is based on a numerically efficient order-N implementation of exact exchange.\footnote{X. Wu, A. Selloni, and R. Car, Phys. Rev. B \textbf{79}, 085102(2009)} We find that PBE0 systematically improves the agreement of the simulated liquid with experiment. Our conclusion is substantiated by the calculated radial distribution functions, H-bond statistics, and molecular dipole distribution. [Preview Abstract] |
Tuesday, March 16, 2010 12:03PM - 12:15PM |
J27.00003: Temperature Effect on Water Sorption: from Nanoconfined Water in SWNTs to the Hydration Water of Proteins Hai-Jing Wang, Xue-Kui Xi, Alfred Kleinhammes, Yue Wu The properties of water under nanoconfinement and at interfaces of biomolecules play a pivotal role in a variety of important phenomena such as the protein folding and functions. Water inside single-walled carbon nanotubes (SWNTs) can provide an ideal system for investigating nanoconfined interfacial water on hydrophobic surfaces, provided that the SWNTs can be opened without introducing excess defects. Here, we report a hydrophobic-hydrophilic transition upon cooling from 22\r{ }C to 8\r{ }C via the observation of water adsorption isotherms in SWNTs measured by nuclear magnetic resonance. A considerable slowdown in molecular reorientation of such adsorbed water was detected. The thermodynamics of protein hydration was also studied by water sorption. The temperature effect on the isotherms could reveal the mechanism of protein hydration. These observations demonstrate that the structure and dynamics of interfacial water could depend sensitively on temperature, which could lead to intriguing temperature dependences involving interfacial water. [Preview Abstract] |
Tuesday, March 16, 2010 12:15PM - 12:27PM |
J27.00004: Vibrational Spectroscopy and Unique Structures of Water Molecules Confined in Molecular Films and Membrane Hong-fei Wang Development of nonlinear spectroscopy, such as Sum Frequency Generation Vibrational Spectroscopy (SFG-VS), enables in-situ measurement of the structural details of molecular films and membrane. Vibrational spectral signatures of various water species have thus been observed and identified. These observation and knowledge have provided detail pictures of the water mediated specific structural changes in molecular films and membrane. Here the study on the accommodation and penetration of water molecules into molecular monolayer films is to be presented. These results not only demonstrated that surface nonlinear spectroscopic methods are effective tools for interrogating spectroscopy of confined and biological water, they also provided unique model molecular systems in understanding and predicting their behaviors. [Preview Abstract] |
Tuesday, March 16, 2010 12:27PM - 12:39PM |
J27.00005: Dynamic Shear Modulus of Nanometric Water Wire Manhee Lee, Bongsu Kim, Wonho Jhe Water has been one of the perfect Newtonian viscous fluid, which is exactly described by navier-stokes equation. However, as its volume is decreased to the nanoscale, there arise various interesting phenomena. For example, the effective shear viscosity of 10nm thick water confined between mica crystals is very different from that of 3-dimensional bulk water. While some researchers have measured very high viscoelasticity of the confined liquid, other researchers have reported the bulk fluidic nature of water confined between mica surfaces at $<$3.5 nm interfacial separation. Although these 2-D thin films of water have been extensively studied for the past several years, it is still difficult to investigate novel features of a 1-D wire like configuration of water molecules at the nanoscale. Here, we present an experimental method for the formation and manipulation of the nanometric water wire and its mechanical properties including dynamic shear modulus, viscoelasticity, and dissipation energy. [Preview Abstract] |
Tuesday, March 16, 2010 12:39PM - 12:51PM |
J27.00006: Measurements of the Tensile Strength of Nano-scale Water Columns Corey Stambaugh, Manhee Lee, Wonho Jhe When an AFM tip is approached to a flat surface a nano-scale water capillary spontaneously forms between tip and surface. Under such conditions water, which is normally a viscous material, has been shown to possess visco-elastic properties. In past experiments, the AFM was operated in the amplitude modulation mode. Changes in the oscillation amplitude were related to the gradient of the interaction force which resulted from the nano-scale water capillary. In this experiment, the movable top plate of a micro-mechanical force sensor is used as the flat surface providing a direct measurement of the waters interaction force. By adjusting the distance between the cantilever and the micro-mechanical force sensor the nano-scale water column is stretched. The response of the AFM and of the micro-mechanical force sensor provides a simultaneous measurement of the elasticity and force of the nano-scale water column. The measurements are made under a variety of environmental conditions and the results are compared to theory. [Preview Abstract] |
Tuesday, March 16, 2010 12:51PM - 1:03PM |
J27.00007: Infrared Spectra of Water and Simple Negative Ions from Ab-initio Simulations Cui Zhang, Davide Donadio, Giulia Galli, Ivan Duchemin, Francois Gygi We report an analysis of the much debated infrared spectra (IR) of water, as obtained by ab-initio molecular dynamics simulations with the PBE and PBE0 energy functionals. We compare power spectra of bond-bond correlations, those of Wannier function centers and IR spectra. Such comparison allows us to identify signatures, in IR spectra, arising from ionic vibrations and signatures stemming from the liquid electronic structure. We then analyze the changes the latter undergo in the presence of simple negative ions such as chloride. Finally, we discuss differences between results obtained at the PBE and PBE0 level of theory. Our results contribute a fundamental step toward the understanding of structural and spectroscopic properties of water at interfaces and complex solutions. Work supported by DOE/SciDACDE-FC02-06ER25794 and NSF/OCI-0749217. [Preview Abstract] |
Tuesday, March 16, 2010 1:03PM - 1:15PM |
J27.00008: A van der Waals DFT Approach to Modeling Water Brian Kolb, Timo Thonhauser We present density functional theory calculations for small water clusters and bulk water including van der Waals interactions via the non-local functional vdW-DF [1]. Historically, standard functionals such as LDA or GGA have been unable to accurately predict vibrational frequencies of small water clusters or the freezing point of bulk water, which has been partly attributed to the lack of van der Waals interactions [2]. We have implemented vdW-DF in the PWscf package, using an efficient convolution approach [3]. Our results for the vibrational frequencies of small water clusters show that vdW-DF gives a significant improvement compared to LDA or GGA. While the discrepancy between experiment and LDA/GGA is as much as 28\% for certain modes, vdW-DF reduces this error to only about 6\%. We also present results for the vibrational spectrum and Raman spectrum of periodic ice, again showing the advantages of vdW-DF. In addition, we show preliminary results for bulk water from our vdW-DF MD simulations.\\[4pt] [1] Thonhauser et al., Phys. Rev. B {\bf 76}, 125112 (2007).\newline [2] H. Sit and N. Marzari, J. Chem. Phys. {\bf 122}, 204510 (2005). \newline [3] G. Roman-Perez and J. Soler, Phys. Rev. Lett. {\bf 103}, 096102 (2009). [Preview Abstract] |
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