Transcript WATER Ira Waluyo Nilsson Group Stanford Synchrotron Radiation Lightsource SASS Talk 10/14/09 Ban DHMO! www.dhmo.org “DHMO is a colorless and odorless chemical compound…Its basis is the highly.

WATER
Ira Waluyo
Nilsson Group
Stanford Synchrotron Radiation Lightsource
SASS Talk 10/14/09
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?
“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 Water
Water denser than ice
Density of the liquid higher than the solid
Normal liquid (ethanol, gasoline,etc)
Solid more dense than liquid
density
Normal liquid
ddd
ddd
ddd
ddd
Sssssssssssssssssssssss
-50 -25 0 25 50 75 100
s
Temperature/
°C
At the bottom of the glass is 4 °C water
Some Anomalies of Water
High Boiling Point
Temperature °C
100
H2O
H2Po
50
Room
Temp
H2Te
0
H2Se
-50
H2S
-100
SnH4
GeH4
SiH4
-150
CH4
-200
0
50
100
150
200
Molecular mass
Water should be a gas at room temperature
Why not?
250
Water and the Hydrogen Bond
H2O
O-H chemical bonds
Lone pairs
d+
d3Å
d-
2Å
d+
1Å
electrostatic interaction
tetrahedral coordination
Seems simple so far…what’s the fuss about?
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
X-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 XAS
Single donor
(asymmetrically
distorted H-bonds)
Double donor
(tetrahedral-like
H-bonds)
ambient
60-75%
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. Science 2004, 304, 995
25-40%
Some people were not very happy
about this….
X-ray Emission Spectroscopy
Increasing hydrogen bonding
XES: probes occupied states
Hypothetical Water
Homogeneous
1b1
Gas
Ice
Tokushima et al. Chem. Phys. Lett. 2008, 460, 387
X-ray Emission Spectroscopy
XES: probes occupied states
Increasing hydrogen bonding
Gas
1b1
Experimental Result
Real Water
Two peaks = two components
Ice
Tetrahedral:
20-30%
Distorted:
70-80%
Tokushima et al. Chem. Phys. Lett. 2008, 460, 387
Small Angle X-ray Scattering
Probe for density variations in liquids on the
nanometer scale
scattered
k’
Q

incident k
Small Angle X-ray Scattering of Liquid Water
Experimental Water
Hypothetical Water
Homogeneous
10-20 Ångstrom in Size
Minimum gives us size
Enhancement showing heterogeneity
Huang et al., PNAS 2009, 106, 36
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
What happens when the H-bond network is
disrupted?
e.g. temperature increase, addition of salt
Temperature Dependence
XAS
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)
XES
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 Addition
Näslund et al., J. Chem. Phys. A 2005, 109, 5995
KCl and AlCl3 have opposite effects
KCl: similar to temperature increase
AlCl3 shifts spectrum to higher energy
More distorted H-bonded species
More strongly H-bonded species
K+: “structure-breaker”
Al3+: “structure-maker”
Conclusion
Water is more complicated that it seems
Current textbook picture is wrong
Continuum Models
“Infinite Network of disordered
tetrahedral
water.”
Mixture models
“Small number of different species
with well defined bond
angles/lengths.”
MD simulations!
~3.5 HB/molecule
Controversial but supported by experiments and simulations