New Nanoscopic Water and Proton Transfer - Del Mar Photonics · 2007. 5. 3. · reverse micelles O...

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


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.


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


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


/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


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)


Waterw0 = 60w0 = 60w0 = 40 w0 = 40 w0 = 20w0 = 20w0 = 10w0 = 10w0 = 5w0 = 5w0 = 2w0 = 2


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


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


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


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


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


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.


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


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.


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)


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)


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?


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.


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



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


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.


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.


Reverse Micelles - WaterIvan PileticDavid MoilanenBen SpryProf. Nancy Levinger

SupportDOE

Proton transferBen SpryKsenija Glusac

Nafion – WaterDavid Moilanen


New Nanoscopic Water and Proton Transfer - Del Mar Photonics · 2007. 5. 3. · reverse micelles O...

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