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