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Nanoscopic Water and Proton TransferNanoscopic Water and Proton Transfer
Ultrafast Infrared and Visible SpectroscopyUltrafast Infrared and Visible Spectroscopy
Twτ
M.D. FayerSanford University6/26/2006
Nanoscopic Water Chemistry, Biologywater confined on nm length scale Geology, Materials
Nafion fuel cell membrane
waterorganic phase
surfactant
head group counter ion
reverse micelle
w characterizes size
w0 is directly related to the micelle radius
w0
w0 =H2O[ ]
surfactant[ ]
Nanopool diameter can range from < 1 nm to ~30 nm.w0 = 2 1.7 nm (~40 H2O)w0 = 5 2.4 nm (~300 H2O)w0 = 10 4.0 nm (~1000 H2O)w0 = 20 7.0 nm (~5400 H2O)w0 = 40 17 nm (~77,000 H2O)w0 = 60 28 nm (~350,000 H2O)
Aerosol OT (AOT)reverse micelles
OO
O
SO
OO
-ONa+
head group
counter ion
surfactant
Same head group asNafion
M.D. FayerSanford University6/26/2006
How Does Nanoscopic Confinement How Does Nanoscopic Confinement Change the Properties of Water?Change the Properties of Water?
Over what distances are the properties of water changed by confinement?
Properties of Water Properties of Water -- Hydrogen Bond Network and Its EvolutionHydrogen Bond Network and Its Evolution
Structure constantly changing.
Water Water can make up to 4 hydrogen bonds.
OH
H
OH
H
OH
H
OH
H
OH
HO
H
HO
H
H
OH
HO
H
H
OH
HO
H
H
OH
HO
H
H
OH
HO
H
HO
H
H
OH
H
OH
H
OH
H
OH
HO
H
HO
H
H
OH
HO
H
H
OH
HO
H
H
OH
HO
H
H
OH
HO
H
H
Strong bonds become weak.Weak bonds become strong.Bonds form.Bonds break.
M.D. FayerSanford University6/26/2006
IR Spectroscopy of Water Hydroxyl StretchIR Spectroscopy of Water Hydroxyl Stretch ––Direct Probe of Hydrogen Bond Network and Its DynamicsDirect Probe of Hydrogen Bond Network and Its Dynamics
Four Experimental Observables
1. Absorption spectra – FT-IR measurements
2. Vibration population relaxation – Ultra fast IR pump-probe measurements
3. Orientational relaxation – Ultrafast IR pump-probe polarization measurements
4 Spectral diffusion – Ultrafast IR vibrational echo measurements
M.D. FayerSanford University6/26/2006
Linear absorption spectra blue shift as water nanopool gets smalLinear absorption spectra blue shift as water nanopool gets smallerler
d (nm) # H2O water - -w0 = 60 28 350,000w0 = 40 17 77,000w0 = 20 7.0 5400w0 = 10 4.0 1000w0 = 5 2.4 300w0 = 2 1.7 40
hydroxyl stretch (OD of 5% HOD in H2O)
2400 2500 2600 27000.0
0.2
0.4
0.6
0.8
1.0
abso
rban
ce (n
orm
aliz
ed)
frequency (cm-1)
Waterw0 = 60w0 = 40 w0 = 20w0 = 10w0 = 5w0 = 2
2400 2500 2600 27000.0
0.2
0.4
0.6
0.8
1.0
abso
rban
ce (n
orm
aliz
ed)
frequency (cm-1)
Waterw0 = 60w0 = 40 w0 = 20w0 = 10w0 = 5w0 = 2
M.D. FayerSanford University6/26/2006
/2( ) (1 0.8 ( ))
tS t Ae C tτ−= + / 2( ) (1 0.4 ( ))tS t Ae C tτ−⊥ = −
C2(t) is the orientational correlationfunction for a dipole transition (second order Legendre polynomial).
monochromator
detectorpolarizers -computer controlled
pumphorizontal sample
probe - 45°
⊥
||
Ultrafast IR Wavelength Selected Polarized IR PumpUltrafast IR Wavelength Selected Polarized IR Pump--Probe ExperimentsProbe ExperimentsPopulation Relaxation and Orientational Relaxation
λ/2
2
( )( )
( ) 0.4 ( )( )
2 ( )r
S tS t
tS t
C tS t
⊥
⊥
−+
= =orientational anisotropy
C2(0) = 1; therefore r(0) = 0.4
)( )) (( 2S tP tt S ⊥+=population relaxation
M.D. FayerSanford University6/26/2006
Vibrational Population Relaxation Slows as Water Nanopool Gets SVibrational Population Relaxation Slows as Water Nanopool Gets Smaller.maller.
d (nm) # H2O water - -w0 = 60 28 350,000w0 = 40 17 77,000w0 = 20 7.0 5400w0 = 10 4.0 1000w0 = 5 2.4 300w0 = 2 1.7 40
w0 = 2 5.2 psw0 = 5 4.4 psw0 = 10 2.7 psw0 = 20 2.1 psw0 = 40 1.7 psw0 = 60 1.8 pswater 1.7 ps
P(t)
0 5 10 15 20 250.0
0.2
0.4
0.6
0.8
1.0
1.2
P(t)
(pop
ulat
ion
rela
xatio
n)
t (ps)
hydroxyl stretch (OD of 5% HOD in H2O)
Waterw0 = 60w0 = 60w0 = 40 w0 = 40 w0 = 20w0 = 20w0 = 10w0 = 10w0 = 5w0 = 5w0 = 2w0 = 2
M.D. FayerSanford University6/26/2006
0.0
0.1
0.2
0.3
0.4
r(t)
(ani
sotro
pic
pum
p-pr
obe
sign
al) Water
w0 = 60w0 = 40 w0 = 20
0 1 2 3 4 5t (ps)
Orientational Anisotropy RelaxationOrientational Anisotropy Relaxation
exponential decays
d (nm) # H2O slowest componentwater - - 2.7 psw0 = 60 28 350,000 2.7 psw0 = 40 17 77,000 2.8 psw0 = 20 7.0 5400 3.0 psw0 = 10 4.0 1000 18 psw0 = 5 2.4 300 30 psw0 = 2 1.7 40 50 ps
hydroxyl stretch (OD of 5% HOD in H2O)
0 1 2 3 4 50.0
0.1
0.2
0.3
r(t)
(ani
sotro
pic
pum
p-pr
obe
sign
al)
t (ps)
Relaxation slows with decreasing size.nonexponential decays
w0 = 5
w0 = 2
w0 = 10
water
M.D. FayerSanford University6/26/2006
Ultrafast 2D Infrared Vibrational Echo Spectroscopy
sample
k1
k2 k3
kecho
end view
kecho = k2+k3-k1k1
k2k3
k1
k2k3
sample
monochromator
loca
l osc
illat
or
beam combiner
MCT array
vib. echo
1 2
vib. echo
τ ≤τ
emission
3Twτ – coherence periodsTw – populations period
M.D. FayerSanford University6/26/2006
frequencyω(0)
A particular hydroxylstretch (strong) at t = 0.
absorption spectrum
Spectral Diffusion
ω(t)
Measure spectral diffusion hydrogen bond network evolution
frequency at t = 0 ω(0)frequency at t ω(t)
Frequency-Frequency Correlation Function FFCFProbability that ω(t) is same as ω(0) averaged over all starting frequencies.
( ) (0)tδω δω
0.0
0.2
0.4
0.6
0.8
1.0
t
δωδω
(t)(0
) FFCF 0Freq randomized.Structure randomized.
The FFCF is the connection between microscopic water models and experimental observables. M.D. FayerSanford University
6/26/2006
Detailed 2D IR vib. echoes and simulations (Skinner) of bulk water have determinedthe nature of the hydrogen bond network motions that occur on different time scales.
Short times – very local H-bond length changes. Long times – global H-bond network randomization.
ωτ (cm-1)
ωm
(cm
-1)
2700
2500
23002300 2500 2700 2700
ωτ (cm-1)2500
Tw = 0.1 ps Tw = 1.6 ps
Spectrally Resolved Vibrational EchoesSpectrally Resolved Vibrational Echoes
Detailed fitting of FFCF to fullset of decay curves and linearspectrum for each size shows:
fastest components, very localmotions, change somewhat,
major change in slowest component,global hydrogen bond networkrearrangement slows dramatically as water nanopool becomes smaller.
0.0 0.5 1.0 1.5 2.0
0
20
40
60
80
100
120
peak
shift
(fs)
Tw (ps)
waterw0 = 60w0 = 40w0 = 20w0 = 10w0 = 5w0 = 2
waterw0 = 60w0 = 40w0 = 20w0 = 10w0 = 5w0 = 2
1.5 ps1.6 ps1.6 ps2.5 ps3.8 ps6 ps15 ps
slowest
hydroxyl stretch (OD of 5% HOD in H2O)
The vibrational echo peak shift is determined by thefrequency-frequency correlation function.As Tw increases, there is more spectral diffusion thatmakes the vibrational echo decay faster, andreduces the peak shift.
-200 -100 0 100 200 300
0.0
0.2
0.4
0.6
0.8
1.0N
orm
aliz
ed In
tens
ity
t (fs)
w0 = 10Tw = 300 fs
Peak shift
M.D. FayerSanford University6/26/2006
Orientational relaxation and spectral diffusion are intimately related to H-bond network evolution.Core and shell H-bond networks are continuous.Dynamics in the core and shell are coupled through the H-bond network.
OHH
OH
H
OH
H
OH
H
O H
H
OH
H
OHH
OH
H
OH
H
OH
H
OH
H
OH
H
OH
HO
H
H
OH
H
OH
H
cartoon
shell
core Complete orientational relaxationandcomplete spectral diffusionrequire randomization of theH-bond network.Core-shell dynamics coupled.
Core-shell exchange time very slow.
J. Phys. Chem. A 110, 4585-4599 (2006). M.D. FayerSanford University6/26/2006
Proton Transfer in Nanoscopic Water Proton Transfer in Nanoscopic Water –– Photoacid DynamicsPhotoacid Dynamics
405 nm510 nm
Ka*
Ka
Photoexcitation - Ka increases 107
protonated
deprotonated
proton comes off
remains excited
ground state recombinationon long time scale
SO
OH
Na+Na+
Na+
O
O
O
OOO-
O-
O-
S
S
HPTS
HPTS in reverse micelle
O
O
O
O
O
O
O
O
O
OOO
OO
O
O
O
O
O
O
O
OO
O
O
O
O
O
O
O
OO
O
OO O
O O
O
OO
O
O
OO
OO
O
O
O
O
O
O
O
O
O
O
OOO
OO
O
O
O
O
O
O
O
OO
O
O
O
O
O
O
O
OO
O
OO O
O O
O
OO
O
O
OO
OO
O
How does nanoscopic water dynamicsHow does nanoscopic water dynamicsinfluence proton transfer?influence proton transfer?
Sulfonates and Na+ counter ions same as head groups and counter ions of AOT.A variety of experiments (anisotropy decays,spectra) demonstrate thatHPTS in water nanopool, not associated withAOT head groups or buried in surfactant.
M.D. FayerSanford University6/26/2006
First, what happens in bulk water.First, what happens in bulk water.
0 100 200 300-1.0
-0.5
0.0
0.5
1.0
chan
ge in
abs
orba
nce
t (ps)
MPTS – methyl rather than OH.No proton transfer.
HPTS
Pump – 405 nm; probe 540 nm
400 450 500 550 600
-1.0
-0.5
0.0
0.5
1.0
1.5
p
ump-
prob
e si
gnal
wavelength (nm)
100 fs600 fs2 ps4 ps
300 ps-500 fs
HPTS – pump 405 nm; continuum probedeprotonatedexcited stateabsorption
deprotonatedstimulatedemission
protonated excited state absorption
Time LineTime LineStoke’s Shift
~1 psProton Transfer – Non-Local – Migration
~100 psInitial Proton Transfer – Local
~5 ps
Detailed MCD and polarization studies show that none of the dynamics involves shifting of theelectronic excited states of the protonated form following optical excitation. M.D. Fayer
Sanford University6/26/2006
Model and FittingModel and Fitting
Formation of charge-transfer complex. Four coordinate hydrogen bond complex becomes three coordinate hydronium ion.Establish equilibrium.
+-
hν
+-
hν
k1
k-1
+- -+
+- -+
k2
k-2
Charge separation. A more global rearrangement of hydrogen bond network is required to separate the anion and hydronium.
0 100 200 300 400 500-1.2
-0.8
-0.4
0.0
0.4
0.8
1.2
pum
p-pr
obe
sign
al
t (ps)
425 nm – protonated decrease
550 nm – deprotonated increase
k1 = 0.03 ps-1k-1 = 0.08 ps-1k2 = 0.05 ps-1 k-2 = 0
Solve differential equations for above model. Fit data.Solve differential equations for above model. Fit data.Fits include: Stoke’s shift, formation of charge-transfer complex, charge separation.
P. Leiderman, L. Genosar, and D. Huppert, J. Phys. Chem. A., 109, 5965 (2005).
400 450 500 550 600 650-0.2
-0.1
0.0
0.1
0.2
0.3
pum
p-pr
obe
sign
al
wavelength (nm)
100 fs600 fs
2 ps4 ps
59 ps
90 ps180 ps300 ps
28 ps
exps. and fits
M.D. FayerSanford University6/26/2006
On to Nanoscopic Water On to Nanoscopic Water –– AOT Reverse MicellesAOT Reverse Micelles
w0 =
water and w0 = 20, 40
3
4
5
7
deprotonatedprotonated
fluorescence – steady state
10
400 450 500 550 600 6500.0
0.2
0.4
0.6
0.8
1.0
flu
ores
cenc
e (a
rb. u
nits
)
wavelength (nm)
Because the excited state lifetimes are so long, the steady state fluorescencespectra reflect the end points of the photoinduced proton transfer but priorto recombination on the ground statesurface.
0 100 200 300 400 500-1.0
-0.5
0.0
0.5
1.0
chan
ge in
abs
orba
nce
(arb
. uni
ts)
t (ps)
transient absorbance – 540 nm
waterw0 = 40, 20, 10
w0 = 3w0 = 4w0 = 5w0 = 7
The proton transfer dynamics have a well defined size dependence similar tothe dynamics of nanoscopic water in AOT.Nanoscopic water dynamics first deviates frombulk at w0 = 10 rather than 7.The largest sizes (d = 17 nm, 7 nm, and 4 nm)show no size dependence and are almost thesame as bulk water. The difference is the longtime asymptotic value.
M.D. FayerSanford University6/26/2006
The proton transfer dynamics in nanoscopic water are fit with the same model used for bulk water. The same quality of fits are obtainedover all times and wavelengths.
+-
hν
+-
hνk1
k-1
+- -+
+- -+
k2
k-2
0 100 200 300 400 500-1.0
-0.5
0.0
0.5
1.0
chan
ge in
abs
orba
nce
(arb
. uni
ts)
t (ps)
transient absorbance – 540 nm
waterw0 = 40, 20, 10
w0 = 3w0 = 4w0 = 5w0 = 7
0 100 200 300 400 500-1.0
-0.5
0.0
0.5
1.0
chan
ge in
abs
orba
nce
(arb
. uni
ts)
t (ps)
transient absorbance – 540 nm
waterw0 = 40, 20, 10
w0 = 3w0 = 4w0 = 5w0 = 7
sample k1 k-1 k2 k-2 Keq
water 0.033 0.079 0.048 0 ∞w0 = 40, 20, 10 0.024 0.041 0.039 0.0004 57w0 = 7 0.020 0.031 0.015 0.002 5.2w0 = 5 0.017 0.029 0.009 0.003 1.6w0 = 4 0.021 0.046 0.009 0.005 0.8w0 = 3 0.013 0.039 0.011 0.007 0.5
1 2
1 2
Keqk k
k k− −=
k1 – initial forward proton transfer kinetics slow as water nanopool becomes smaller
− becomes smaller as nanopool gets smaller.Less generation of “free” protons. Longtime off-set increases.
Smallest reverse micelles, w0 = 4 and 3, d < 2 nm, don’tfall all trends. Head group water shell, 0.4 nm thick.HPTS contacting interface?
M.D. FayerSanford University6/26/2006
Nafion fuel cell membraneNafion fuel cell membraneTeflon with side chains, sulfonate head groups.
--[(CF2-CF2)n-CF-CF2]m--
(O-CF2-CF-O-CF2-CF2-SO3-M+
CF3
--[(CF2-CF2)n-CF-CF2]m--
(O-CF2-CF-O-CF2-CF2-SO3-M+
CF3Nanoscopic water channels, proton transport.
hydroxyl stretch (OD of 5% HOD in H2O)
2400 2500 2600 2700
0.0
0.2
0.4
0.6
0.8
1.0
abso
rban
ce (n
orm
aliz
ed)
frequency (cm-1)
bulk waterλ = 9λ = 7.5λ = 5λ = 3λ = 1
hydroxyl stretch spectra
λ – number of water molecules per sulfonate head group
5 10 15 20 25 30 35 400.00.10.20.30.40.50.60.70.80.91.0
P(t)
(nor
mal
ized
)
t (ps)
bulk waterλ = 7.5
λ = 5λ = 3
λ = 1
vibrational lifetimes
1.7 ps3.8 ps5.9 ps6.9 ps9.5 ps
Na+ counter ions
M.D. FayerSanford University6/26/2006
hydroxyl stretch (OD of 5% HOD in H2O)
Orientational Anisotropy Decays and Simulations of Water in NafiOrientational Anisotropy Decays and Simulations of Water in Nafion on NanochannelsNanochannels
1 2 3 4 5 6 7 8 9 100.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
r(
t) - a
niso
tropy
t (ps)
Experimental Data
λ = 1λ = 3λ = 5
λ = 7.5
1 2 3 4 5 6 7 8 9 100.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
r(t)
- ani
sotro
py
t (ps)
Simulations – Voth and Co-workers
λ = 1λ = 3λ = 5
λ = 7.5
Decays are non-exponential – comparison of slowest components that leads to complete randomization.
λ = 1λ = 3λ = 5λ = 7.5water
sample experiment simulation
81 ps47 ps21 ps18 ps2.6 ps
73 ps33 ps24 ps13 Ps
−
slowest component
M.D. FayerSanford University6/26/2006
Proton Transfer in Nanoscopic Water Channels Proton Transfer in Nanoscopic Water Channels –– HPTS in NafionHPTS in Nafion
0 100 200 300 400-0.4
0.0
0.4
0.8
pum
p-pr
obe
sign
al
t (ps)
λ = 2
λ = 4
λ = 8
Like the HPTS experiments in AOT reverse micelles,as the water channels contain less water, proton transferdynamics slow.Nafion becomes a proton transport membrane for λ > 5.
M.D. FayerSanford University6/26/2006
Key Points
Water dynamics slow dramatically in reverse micelles with small water nanopools, particularly H-bond network global structural rearrangement.
Photoacid HPTS experiments in bulk water occur on three time scales, Stoke’s shift,local charge transfer complex formation, and proton migration.Excellent agreement between data and model calculations.
Photoacid HPTS proton transfer dynamics unaffected in large reverse micelles,but slow dramatically in small water nanopools. Equilibrium constantshift, less dissociation
Water dynamics slow substantially in water nanochannels of Nafion membranesin a manner akin to reverse micelles.Simulations do a good job of reproducing orientational relaxation.
HPTS photoacid experiments in Nafion water channels show the influence ofchannel size (amount of water per head group) on proton transfer dynamics.Proton transfer dynamics slow significantly as water content is reduced.
M.D. FayerSanford University6/26/2006
Reverse Micelles - WaterIvan PileticDavid MoilanenBen SpryProf. Nancy Levinger
SupportDOE
Proton transferBen SpryKsenija Glusac
Nafion – WaterDavid Moilanen
M.D. FayerSanford University6/26/2006