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WATER. Ira Waluyo Nilsson Group Stanford Synchrotron Radiation Lightsource SASS Talk 10/14/09. Ban DHMO!. www.dhmo.org. - PowerPoint PPT Presentation
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WATER
Ira Waluyo
Nilsson Group
Stanford Synchrotron Radiation Lightsource
SASS Talk 10/14/09
Ban DHMO!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
What is DHMO?What is DHMO?
“Dihydrogen monoxide”
H2O a.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….
Some Anomalies of WaterSome Anomalies of Water
Water denser than ice
Density of the liquid higher than the solid
Normal liquid (ethanol, gasoline,etc)Solid more dense than liquid
At the bottom of the glass is 4 °C water
Ssssssssssssssssssssssss
25 50 75 1000-25-50
Temperature/ °C
dddddddddddd
density Normal liquid
Some Anomalies of WaterSome Anomalies of Water
Temperature °C
Molecular mass
50 100 150 200 250
100
50
-50
-100
-150
-200
0
0
RoomTemp
H2O
SnH4
GeH4
SiH4
CH4
H2S
H2Po
H2Te
H2Se
Water should be a gas at room temperatureWhy not?
High Boiling Point
Water and the Hydrogen BondWater and the Hydrogen Bond
H2O
3 Å
O-H chemical bonds
Lone pairs
electrostatic interaction
- +
2 Å 1 Å
tetrahedral coordination
Seems simple so far…what’s the fuss about?
Mixture models“Small number of different species
with well defined bond angles/lengths.”
Continuum Models“Infinite Network of disordered tetrahedralwater.”
MD simulations!
~3.5 HB/molecule
Ice Tetrahedral structure
Two extreme models for water
Röntgen 1892 Mostly accepted picture
Old debate prior to 1980
The Controversy: Mixture vs. Continuum The Controversy: Mixture vs. Continuum ModelModel
X-ray Absorption SpectroscopyX-ray Absorption Spectroscopy
XAS: probes unoccupied states
Dipole selection rule
O1s O2p
Franck-Condon Principle
Electronic excitation time scale much faster than nuclear motion
Atoms can be considered frozen during excitation
XAS represents a snapshot structure
Water Structure from XASWater Structure from XAS
Pre-edge: sensitive to distorted/broken H-bond
Post-edge: sensitive to intact H-bondCavalleri et al. Chem. Phys. Lett. 2002, 364, 363
Wernet et al. Science 2004, 304, 995
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….
X-ray Emission SpectroscopyX-ray Emission Spectroscopy
XES: probes occupied states
Hypothetical WaterHomogeneous
Gas
Ice
1b1
Increasing hydrogen bonding
Tokushima et al. Chem. Phys. Lett. 2008, 460, 387
X-ray Emission SpectroscopyX-ray Emission Spectroscopy
XES: probes occupied states
Tetrahedral: 20-30%
Distorted: 70-80%
Experimental ResultReal Water
Two peaks = two components
Gas
Ice
1b1
Increasing hydrogen bonding
Tokushima et al. Chem. Phys. Lett. 2008, 460, 387
Small Angle X-ray ScatteringSmall Angle X-ray Scattering
k
incident
scatteredk’
Qk
incident
scatteredk’ k
incident
scatteredk’
Q
Probe for density variations in liquids on the nanometer scale
Small Angle X-ray Scattering of Liquid WaterSmall Angle X-ray Scattering of Liquid Water
Enhancement showing heterogeneity10-20 Ångstrom in Size
Hypothetical WaterHomogeneous
Experimental Water
Minimum gives us size
Huang et al., PNAS 2009, 106, 36
Mixture ModelMixture Model
The two components in liquid water:
• High density liquid water (HDL) – disordered
• Low density liquid water (LDL) – ordered
HDL
LDL
In pure liquid water, LDL and HDL structures interconvert continuously
Huang et al., PNAS 2009, 106, 36
What happens when the H-bond network is disrupted?
e.g. temperature increase, addition of salt
Temperature DependenceTemperature Dependence
XAS XES
Increased pre-edge and main edge, decreased post-edgeDouble donor (LDL) converted to single donor (HDL) H-bond breakingExisting 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
Salt AdditionSalt 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”
ConclusionConclusion
Water is more complicated that it seems
Current textbook picture is wrong
Continuum Models“Infinite Network of disordered tetrahedralwater.”
MD simulations!
~3.5 HB/molecule
Mixture models“Small number of different species
with well defined bond angles/lengths.”
WRONG
!Controversial but supported by experiments and simulations