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WATER

WATER. Ira Waluyo Nilsson Group Stanford Synchrotron Radiation Lightsource SASS Talk 10/14/09. Ban DHMO!. www.dhmo.org.

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WATER

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  1. WATER Ira Waluyo Nilsson Group Stanford Synchrotron Radiation Lightsource SASS Talk 10/14/09

  2. Ban DHMO! www.dhmo.org “DHMO is a colorless and odorless chemical compound…Its basis is the highly reactive hydroxyl radical, a species shown to mutate DNA, denature proteins, disrupt cell membranes, and chemically alter critical neurotransmitters.” • Some dangers of DHMO • Death due to accidental inhalation of DHMO, even in small quantities. • Prolonged exposure to solid DHMO causes severe tissue damage. • Gaseous DHMO can cause severe burns. • Contributes to soil erosion. • Often associated with killer cyclones in the U.S. Midwest and elsewhere, and in hurricanes including deadly storms in Florida, New Orleans and other areas of the southeastern U.S

  3. What is DHMO? “Dihydrogen monoxide” H2Oa.k.a water It is everywhere Covers 2/3 of Earth’s surface Comprises 50-65% of human body No water = no life But it exhibits strange properties and it’s liquid structure is still a mystery….

  4. Some Anomalies of Water density dddddddddddd Ssssssssssssssssssssssss -50 -25 0 25 50 75 100 Temperature/ °C Water denser than ice Density of the liquid higher than the solid Normal liquid (ethanol, gasoline,etc) Solid more dense than liquid Normal liquid At the bottom of the glass is 4 °C water

  5. Some Anomalies of Water Temperature °C 100 H2O H2Po 50 Room Temp H2Te 0 H2Se -50 H2S SnH4 -100 GeH4 SiH4 -150 CH4 -200 0 50 100 150 200 250 Molecular mass High Boiling Point Water should be a gas at room temperature Why not?

  6. Water and the Hydrogen Bond O-H chemical bonds Lone pairs d + d - d- d+ 2 Å 1 Å H2O 3 Å electrostatic interaction tetrahedral coordination Seems simple so far…what’s the fuss about?

  7. The Controversy: Mixture vs. Continuum Model Ice Tetrahedral structure Old debate prior to 1980 Two extreme models for water Mixture models “Small number of different species with well defined bond angles/lengths.” Continuum Models “Infinite Network of disordered tetrahedral water.” MD simulations! ~3.5 HB/molecule Röntgen 1892 Mostly accepted picture

  8. X-ray Absorption Spectroscopy XAS: probes unoccupied states Franck-Condon Principle Electronic excitation time scale much faster than nuclear motion Atoms can be considered frozen during excitation XAS represents a snapshot structure Dipole selection rule O1s  O2p

  9. Water Structure from XAS ambient 60-75% 25-40% Single donor (asymmetrically distorted H-bonds) Double donor (tetrahedral-like H-bonds) Some people were not very happy about this…. Pre-edge: sensitive to distorted/broken H-bond Post-edge: sensitive to intact H-bond Cavalleri et al. Chem. Phys. Lett.2002, 364, 363 Wernet et al. Science2004, 304, 995

  10. X-ray Emission Spectroscopy Increasing hydrogen bonding Gas 1b1 Hypothetical Water Homogeneous Ice XES: probes occupied states Tokushima et al. Chem. Phys. Lett. 2008, 460, 387

  11. X-ray Emission Spectroscopy Increasing hydrogen bonding Gas 1b1 Experimental Result Real Water Two peaks = two components Ice XES: probes occupied states Tetrahedral: 20-30% Distorted: 70-80% Tokushima et al. Chem. Phys. Lett. 2008, 460, 387

  12. Small Angle X-ray Scattering Probe for density variations in liquids on the nanometer scale

  13. Small Angle X-ray Scattering of Liquid Water Experimental Water Hypothetical Water Homogeneous Minimum gives us size 10-20 Ångstrom in Size Enhancement showing heterogeneity Huang et al., PNAS 2009, 106, 36

  14. Mixture Model LDL HDL • The two components in liquid water: • High density liquid water (HDL) – disordered • Low density liquid water (LDL) – ordered In pure liquid water, LDL and HDL structures interconvert continuously Huang et al., PNAS 2009, 106, 36

  15. What happens when the H-bond network is disrupted? e.g. temperature increase, addition of salt

  16. Temperature Dependence XAS XES Increased pre-edge and main edge, decreased post-edge Double donor (LDL) converted to single donor (HDL)  H-bond breaking Existing HDL thermally excited (becomes more gas-like) Ratio of 1b1” to 1b1’ peak increase Consistent with XAS (LDL converted to HDL) 1b1” peak shifts closer to gas phase Also consistent with XAS (HDL thermally excited, more gas-like) Huang et al., PNAS 2009, 106, 36

  17. Salt Addition Näslund et al., J. Chem. Phys. A 2005, 109, 5995 KCl and AlCl3 have opposite effects KCl: similar to temperature increase More distorted H-bonded species K+: “structure-breaker” AlCl3 shifts spectrum to higher energy More strongly H-bonded species Al3+: “structure-maker”

  18. Conclusion Mixture models “Small number of different species with well defined bond angles/lengths.” Water is more complicated that it seems Current textbook picture is wrong Continuum Models “Infinite Network of disordered tetrahedral water.” MD simulations! ~3.5 HB/molecule WRONG! Controversial but supported by experiments and simulations

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