Electrochemistry: From soft interfaces to bioanalytics · Molecular dynamic representation! Distribution of all CCD– anions and of Cs+ ions within 10 Å from the interface. The

Electrochemistry: From soft interfaces to

bioanalyticsHubert H. Girault

ENS Lyon Septembre-Octobre 2010

Saturday, September 18, 2010

Molecular Electrocatalysis for Oxygen Reduction by CobaltPorphyrins Adsorbed at Liquid/Liquid Interfaces

Bin Su,†,9 Imren Hatay,†,! Antonın Trojanek,‡ Zdenek Samec,‡ Tony Khoury,§

Claude P. Gros,§ Jean-Michel Barbe,§ Antoine Daina,¶ Pierre-Alain Carrupt,¶ andHubert H. Girault*,†

Laboratoire d’Electrochimie Physique et Analytique, Ecole Polytechnique Federale deLausanne, Station 6, CH-1015 Lausanne, Switzerland, J. HeyroVsky Institute of Physical

Chemistry of ASCR, V.V.i, DolejskoVa 3, 182 23 Prague 8, Czech Republic, Institut deChimie Moleculaire de l’UniVersite de Bourgogne, ICMUB (UMR 5260),

BP 47870, 21078 Dijon Cedex, France, Department of Chemistry, Selcuk UniVersity,42031 Konya, Turkey, Section des Sciences Pharmaceutiques, Quai Ernest-Ansermet 30,

CH-1211 GeneVe 4, Switzerland

Received October 5, 2009; E-mail: [email protected]

Abstract: Molecular electrocatalysis for oxygen reduction at a polarized water/1,2-dichloroethane (DCE)interface was studied, involving aqueous protons, ferrocene (Fc) in DCE and amphiphilic cobalt porphyrincatalysts adsorbed at the interface. The catalyst, (2,8,13,17-tetraethyl-3,7,12,18-tetramethyl-5-p-amino-phenylporphyrin) cobalt(II) (CoAP), functions like conventional cobalt porphyrins, activating O2 viacoordination by the formation of a superoxide structure. Furthermore, due to the hydrophilic nature of theaminophenyl group, CoAP has a strong affinity for the water/DCE interface as evidenced by lipophilicitymapping calculations and surface tension measurements, facilitating the protonation of the CoAP-O2

complex and its reduction by ferrocene. The reaction is electrocatalytic as its rate depends on the appliedGalvani potential difference between the two phases.

Introduction

Molecular oxygen (O2) reduction reaction (ORR) is animportant topic in biological and energy-related chemistry.1,2

However, it is a spin-forbidden process, which is kineticallyslow at ambient temperature unless a catalyst is present. Naturehas made the choice in aerobic organisms by evolving membrane-bound enzymes that contain a porphyrin substructure ascatalysts.1,3-6 Inspired by their role in nature, a number ofmetalloporphyrins have been chemically synthesized.7-9 Their

catalytic activity has been investigated either electrochemicallyusing the modified electrode methodology9-25 or chemicallyusing molecular electron donors, such as ferrocene (Fc) and its

† Ecole Polytechnique Federale de Lausanne.‡ J. Heyrovsky Institute of Physical Chemistry of ASCR.§ Institut de Chimie Moleculaire de l’Universite de Bourgogne.! Selcuk University.¶ Section des Sciences Pharmaceutiques.9 Present Address: Institute of Microanalytical Systems, Department of

Chemistry, Zhejiang University, Hangzhou 310058, China.(1) Boulatov, R. In N4-Macrocyclic Metal Complexes; Zagal, J. H.,

Bedioui, F., Dodelet, J.-P., Eds.; Springer: New York, 2006; pp 1-36.

(2) Shukla, A. K.; Raman, R. K. Ann. ReV. Mater. Res. 2003, 33, 155–168.

(3) Babcock, G. T. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 12971–12973.(4) Babcock, G. T.; Wikstrom, M. K. F. Nature 1992, 356, 301–309.(5) Sono, M.; Roach, M. P.; Coulter, E. D.; Dawson, J. H. Chem. ReV.

1996, 96, 2841–2887.(6) Wikstrom, M. K. F. Nature 1977, 266, 271–273.(7) Collman, J. P.; Boulatov, R.; Sunderland, C. J.; Fu, L. Chem. ReV.

2004, 104, 561–588.(8) Kim, E.; Chufan Eduardo, E.; Kamaraj, K.; Karlin Kenneth, D. Chem.

ReV. 2004, 104, 1077–1133.(9) Zagal, J. H.; Paez, M. A.; Silva, J. F. In N4-Macrocyclic Metal

Complexes; Zagal, J. H., Bedioui, F., Dodelet, J.-P., Eds.; Springer:New York, 2006; pp 41-75.

(10) Anson, F. C.; Shi, C.; Steiger, B. Acc. Chem. Res. 1997, 30, 437–444.

(11) Chang, C. J.; Deng, Y.; Nocera, D. G.; Shi, C.; Anson, F. C.; Chang,C. K. Chem. Commun. 2000, 1355–1356.

(12) Chang, C. J.; Loh, Z. H.; Shi, C.; Anson, F. C.; Nocera, D. G. J. Am.Chem. Soc. 2004, 126, 10013–10020.

(13) Collman, J. P. Acc. Chem. Res. 1977, 10, 265–272.(14) Collman, J. P.; Boulatov, R.; Sunderland, C. J. In The Porphyrin

Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; AcademicPress: San Diego 2003; Vol. 11, pp 1-49.

(15) Collman, J. P.; Chang, L. L.; Tyvoll, D. A. Inorg. Chem. 1995, 34,1311–1324.

(16) Collman, J. P.; Denisevich, P.; Konai, Y.; Marrocco, M.; Koval, C.;Anson, F. C. J. Am. Chem. Soc. 1980, 102, 6027–6036.

(17) Collman, J. P.; Fu, L.; Herrmann, P. C.; Zhang, X. Science 1997, 275,949–951.

(18) Collman, J. P.; Hutchison, J. E.; Lopez, M. A.; Tabard, A.; Guilard,R.; Seok, W. K.; Ibers, J. A.; L’Her, M. J. Am. Chem. Soc. 1992,114, 9869–9877.

(19) Collman, J. P.; Marrocco, M.; Denisevich, P.; Koval, C.; Anson, F. C.J. Electroanal. Chem. Interfacial Electrochem. 1979, 101, 117–22.

(20) Deng, Y.; Chang, C. J.; Nocera, D. G. J. Am. Chem. Soc. 2000, 122,410–411.

(21) Durand, R. R., Jr.; Anson, F. C. J. Electroanal. Chem. InterfacialElectrochem. 1982, 134, 273–89.

(22) Shi, C.; Anson, F. C. Inorg. Chem. 1990, 29, 4298–4305.(23) Shi, C.; Steiger, B.; Yuasa, M.; Anson, F. C. Inorg. Chem. 1997, 36,

4294–4295.(24) Collman, J. P.; Wagenknecht, P. S.; Hutchison, J. E. Angew. Chem.,

Int. Ed. 1994, 33, 1620–1639.(25) Le Mest, Y.; Inisan, C.; Laouenan, A.; L’Her, M.; Talarmin, J.; El

Khalifa, M.; Saillard, J. Y. J. Am. Chem. Soc. 1997, 119, 6095–6106.

Published on Web 02/04/2010

10.1021/ja908488s " 2010 American Chemical Society J. AM. CHEM. SOC. 2010, 132, 2655–2662 9 2655

Molecularcatalyst

Electron donors

Protons

O2

H2 O2

ΔφN

N

N

N

NH2

Co

AmphiphilicCoAP porphyrin

J. AM. CHEM. SOC. 2010, 132, 2655–2662Saturday, September 18, 2010

Electrochemistry is great!

The only problem is the electrode...Jean-Michel SavéantAustin 4- 2- 2010

Three periods :

- Mercury electrode - Polarography- Single crystal- Molecular grafting - Diazonium chemistry


Membrane Electrochemistry

Membrane function :Separate reactants and productsProvide an electrochemical driving force


Bio-inspired electrochemistry

ITIES functions :Separate reactants and productsProvide an electrochemical driving force

water

organic

ITIES Highly reproducible

Defect free


Electrochemistry

Study of chemical reactions for which the standard Gibbs energy depends not only on the temperature,

but also on a potential difference.

We can also drive electrochemical reactions without electrodes


Liquid-liquid interfaces

• Interfacial structure

• Polarised liquid-liquid interfaces

• Electrocapillary phenomena

• Charge transfer reactions

• Photocurrent

• Nanoparticle adsorption - Plasmonics

• Artificial photosynthesis


Interfacial structure

• Classical systems : water-nitrobenzene, 1,2-dichloroethane, heptanone, octanol and 2-nitrophenyloctylether (NPOE)

• Dynamic molecular interface with thermal fluctuations

• Relatively sharp at the molecular level but with corrugations caused by thermal fluctuations and capillary waves

• Water molecules tend to arrange themselves so as to maximize the number of hydrogen bonds and to minimize their potential energy

• Hydrogen bond lifetime τw-DCE = 15 ps, τw-NB = 10 ps, τw-

CCl4 = 7 ps, longer than the bulk value of about 5 ps.


Molecular dynamic representation

!Distribution of all CCD– anions and of Cs+ ions within 10 Å from the interface. The surface of the interface is color coded as a function of its z-position

G. Chevrot, R. Schurhammer, and G. Wipff, in J Phys Chem B, Vol. 110, 2006, p. 9488


Surface roughness

Effect of the probe sphere radius on the identification of the interfacial molecules. The upper part of the figure shows the same water configuration with three different probe spheres of the radii Rps = 0.5 Å (left), Rps = 2.0 Å (middle), and Rps =10.0 Å (right). The molecules that are not identified as interfacial ones with a given probe sphere are plotted in lighter colors.

L. B. Partay, G. Hantal, P. Jedlovszky, A. Vincze, and G. Horvai, in J Comput Chem, Vol. 29, 2008, p. 945.


Bivariate representation

Orientational maps of the surface water molecules located at the surface layer (first column), in regions C and B of the surface layer (second and third column), and in region A of the surface layer (fourth column) of the aqueous phase of the system simulated. Lighter shades of grey indicate higher probabilities; the spacing of the contour lines is 10–4. The water orientations corresponding to the observed peaks are also illustrated. X is the surface normal vector pointing to the vapour phase.

L. B. Partay, G. Horvai, and P. Jedlovszky, in Phys Chem Chem Phys, Vol. 10, 2008, p. 4754.


Surface water structure

Illustration of the relation of the preferred water orientations and local curvature of the interface. The insets demonstrate the similarities between the water orientations found to be preferred at positions of locally concave curvature (i.e., at the ‘‘wells’’) of the interface that is flat on the macroscopic scale, and at the surface of small apolar solutes.

L. B. Partay, G. Horvai, and P. Jedlovszky, in Phys Chem Chem Phys, Vol. 10, 2008, p. 4754.







• Photocurrent




Inner Potential - Galvani Potential

The potential is constant in a phase that is neutral by definition

Sphère

ψ =Q / 4πε0R

Surface potential

Outer potential

+++++

Positively chargedsphere

Pote

ntia

l


Electrochemical Potential

%µi = µi + ziFχ + ziFψ = µi + ziFφ

= µio + RT lnai + ziFφ

µi : Chemical contribution All short-range interactions includingelectrostatic interactions

ziFψ : Electrical approach work

ziFχ : Electrical entry work


Gibbs energy of transfer

ΔGtr,i

w→o = %µio − %µi

w

Also, the difference between solvation and hydration energy

At equilibrium ΔGtr,i

w→o = 0

%µi

o ,o + RT ln aio + ziFφ

o = %µio ,w + RT ln ai

w + ziFφw

I w I o

%µi

o ,o + RT ln aio + ziFφ

o = %µio ,w + RT ln ai

w + ziFφw

Nernst equation Δo

wφ = Δowφtr,i

o +RTziF

lnai

o

aiw

⎛

⎝⎜⎜

⎞

⎠⎟⎟


Standard transfer potential

lipophilic ion µio ,w

µio ,o

ΔGtr,io ,w→o

ΔGtr,io ,w→o = µi

o ,o − µio ,wStandard Gibbs energy of

transfer

Δowφi

o =ΔGtr,i

o ,w→o

ziFStandard transfer potential


Salt partition

water solvent

C+ C+

A– A–

At equilibrium, both phases are neutralbut the interface is charged.


Distribution Potential

�

Δowφ = Δo

wφC+o +

RTF ln

aC+o

aC+w

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

�

Δowφ = Δo

wφA–o −

RTF ln

aA–o

aA–w

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

Nernst equation for both the cation and the anion

�

Δowφdis =

ΔowφC+

o / + ΔowφA–

o /

2 =ΔowφC+

o + ΔowφA–

o

2 +RT2F ln

γC+o γ A−

w

γC+w γ A–

o

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

Distribution potential independent of phase ratio


Distribution Potential

C+ C+

A– A– Δowφdis =

ΔowφC+

o + ΔowφA–

o

2

�

ΔDCEeau φTBACl = −360mV

Cl–

-0.5 0

�

Δφ /VTBA+

-0.22

�

ΔDCEeau φNaTPB = 445mV

0 0.5

�

Δφ /VNa+

0.560.33

TPB–


Potential determining Ion

I+ I+

A– X–3 Nernst equations2 electroneutrality conditions

cI+tot

1+ reF(Δo

wφ−ΔowφI+o / )/RT

−cX–tot

1+ re−F(Δo

wφ−Δowφ

X−o / )/RT

−cA–tot

1+ re−F(Δo

wφ−Δowφ

A−o / )/RT

= 0

cI+w = cIA

w cI+o = cIX

o

Δowφ = Δo

wφI+o +

RTFln

aI+o

aI+w

�

�

��

�

�

��= Δo

wφI+o +

RTFln aIX

o

aIAw

�

��

�

��


How to polarise a liquid|liquid interface?

Pioneered by C. Gavach et al., J. Koryta et al., M. Senda et al.

Hydrophilic aqueous electrolytee.g. LiCl

Lipophilic organic electrolytee.g. BATBRef : Junction BATB/BACl


Potential window

TPB-

TBA+

Li+

SO2-4

0 0.1 0.2 0.3 0.4 0.5-0.1-0.2-0.3-0.4

Δowφ / Volts

PPPh

Ph

Ph

Ph

Ph

Ph

N+

4

BCl

-

Lipophilic cation bis(triphenylphosphoranylidene)ammonium

Lipophilic aniontetrakis(pentafluorophenyl)borate

+

–

–

+

Lithium sulfate in water / Tetrabutylammonium tetraphenylborate in 1,2-DCE

Absolute potential scale

FF

F F

F

B-

FF

F

F F F

FF

F

F F F

F

FF

Lipophilic aniontetrakis(pentafluorophenyl)borate

Lipophilic cation bis(triphenylphosphoranylidene)ammonium

P N P+

BA+ TB−


Molecular Electrocatalysis for Oxygen Reduction

!

Ag AgCl

DCE Water

Ag2SO4 Ag

Water Ref

0.01 M LiCl1 mM BACl

5 mM BATB0 or 50 μM CoAP

0 mM Fc

0.01 M LiClHCl pH=2


How to pump protons to the organic phase?

HCl

BATB

H+

TB–

Potentiostatic control

HCl + LiTB HTB

Chemical control

+

–F

F

F F

F

B-

FF

F

F F F

FF

F

F F F

F

FF

TB−


Distribution concentration10 mM HCl + 5 mM LiTB

5 mM BATB

Δowφtr

o / V

H+ Li+ BA+ TB– Cl–

0.55 0.59 –0.6 0.65 –0.53

At equilibrium Δo

wφeq = 0.541V

5.8 mM H+ 4.3 mM Li+ 0.14 mM TB– 10 mM Cl–

5 mM BA+ 4.2 mM H+ 0.7 mM Li+ 9.86 mM TB–


(a) Aqueous phase: 5 mM LiTB + 10 mM HCl.DCE phase: 1 mM Fc + 5 mM BATB (flask 1), 1 mM Fc + 20 M CoAP + 5 mM BATB (flask 2) and 20 M CoAP + 5 mM BATB (flask 3). The upper left inset shows the colors of three DCE solutions before contacting with the water solution

!

FcBATB

FcCoAPBATB

CoAPBATB

FcCoAPBATBNo O2


(b) The isolated top aqueous solutions with added excess NaI;

(c) Further addition of starch to the flasks shown in (b)

(d) A 4-hour-two-phase reaction in the glovebox with the same solution composition as the flask 2 in (a).







• Photocurrent




Interfacial tension

γ γ

dW = γ dAWork to enlarge a surface

Work to reduce a volume dW = − p dV

Superhydrophobic surface


Electrocapillarity

-1.0 -0.5 0 0.5

KOH

NaClKCNS

NaBr

KI

420

380

340

300

(E - Epzc(NaF) ) / V

Tens

ion

Inte

rfaci

ale

/ m

N.m

-1

+ + + _ _ _

Positive charge in solution No adsorption

Negative charge in solutionAdsorption - Interaction


Lippmann’s equations

Interfacial charge σM = �

∂γ∂E

��

��T , p,µi

Differential capacitance

Cd =∂σM

∂E= �

∂2γ∂E2

�

��

�

��T , p,µi

Gabriel Lippmann1845-1923

Prix Nobel de PhysiquePhotographie couleurElectrocardiographe

ΓCd2+(Hg) dµCd + ΓMg2+

(H2O) dµMgCl2 + ΓH+(H2O) dµHCl + σM dE = − dγ

Gibbs adsorption equation : Hg-Cd/MgCl2 + HCl


Gouy-Chapman theory

Spatial ionic distribution in solution next to a charged wall

Louis Georges Gouy1854-1926

David Leonard Chapman1869-1958

0.20

0.15

0.10

0.05

0.00

c /

M

20151050

x / nm

c = 0.01 M

φ(0) = 100 mV

Cation

Anion

Next to a positively charged wall, there is an excess of anions and a depletion of cations. Overall, the ionic strength increase


Polarised liquid-liquid interfaces

Two back-to-back diffuse layers with some interpenetration

Δowφ

few nanometers

+

+

+

+

–

–

–

–


X-ray reflectivity

23

free energy profile for ion transfer across the interface when this profile is described by a simple

analytic form or by a potential of mean force from molecular dynamics simulations.

Figure 4: (Left) Cross-sectional view of stainless steel sample cell. W: mylar windows; T: thermistors to measure temperature. The kinematics of surface X-ray reflectivity is also indicated: kin is the incoming X-ray wave vector, kscat is the scattered (reflected) wave vector, ! is the angle of incidence and reflection [68] (Middle) X-ray reflectivity, R(Qz), as a function of wave vector transfer Qz from the interface between 0.01 M TBATPB in nitrobenzene and a TBABr in water at five concentrations (0.01, 0.04, 0.05, 0.057, and 0.08 M, bottom to top) at a room temperature of 24 ± 0.5°C. Solid lines are predictions using the potential of mean force from MD simulations. Dashed lines are predicted by the Gouy-Chapman model. No parameters have been adjusted in these two models. Data for different concentrations are offset by factors of 10 (R = 1 at Qz = 0). Error bars are indicated by horizontal lines through the square data points and are usually much smaller than the size of the squares. The points at Qz = 0 are measured from transmission through the bulk aqueous phase. (Inset) The kinematics of x-ray reflectivity: kin, incoming x-ray wave vector; kscat, scattered wave vector; and !, angles of incidence and reflection. (Right) Ion distributions at the interface between a 0.08 M TBABr solution in water and a 0.01 M TBATPB solution in nitrobenzene. Solid lines, TBA+; short-long dashed line, Br–; short dashed line, TPB–. (A) Gouy-Chapman theory. (B) Calculation from MD simulation of the potential of mean force[70]. Reprinted with permission.

When varying the temperature of the system, the authors showed clearly that the measured

interfacial width differs from the predictions of capillary wave theory with a progressively

smaller deviation as the temperature is raised. It was therefore concluded that both molecular

layering and dipole ordering parallel to the interface must take place. Either layering or a

bending rigidity, that can result from dipole ordering, could explain these measurements [69]. It

can be regretted that capacitance measurements were not carried out directly on the systems

Science, 311 (2006) 216

Dashed lines arepredicted by the Gouy-Chapman model.

Ion distributions at the interface between a 0.08 M TBABr solution in water and a 0.01 M TBATPB solution in nitrobenzene.Solid lines, TBA+; short-long dashed line, Br–; short dashed line, TPB–. (A) Gouy-Chapman theory. (B) Calculation from MD simulation of the potential of mean force


Lipophilicity map

!

�

Pi =aio

aiw= exp ziF Δo

wφ − Δowφi

o( ) / RT[ ]

Ion distribution

�

Pi =aio

aiw= Pi

o exp ziFΔowφ /RT[ ]

�

lnPio = −

µio ,o − µi

o ,w

RT = −ΔGtr,i

o ,w→o

RT = −ziFRT Δo

wφio

ADME-Tox (Absorption, Distribution, Metabolism, Excretion - Toxicology)


Interfacial acid-base equilibria

AH

AH

A– + H+

A– + H+

Kao =

aA–o aH+

o

aAHo = Ka

w PA–PH+

PAHo = Ka

w PA–o PH+

o

PAHo


pH titration of CoAP

! !

First, the Soret band at 423 nm increases at the cost

of the original one at 397 nm, as well as the

appearance of a new band at 356 nm, with increasing

the concentration of TFA. This is assigned to be the

proton of CoAP-O2

Spectra of titrating 25 M CoAP with TFA

When the TFA concentration is higher than 5 mM, both the second Soret band and the band at 356 nm start decreasing due to the protonation of the peripheral amine. As the TFA concentration is above 20 mM, the second Soret band resides at 420 nm, which does not shift but decrease in intensity with the TFA concentration.


Techniques

• Pendant drop

• Sessile drop

• Quasi electric light scattering


Pendant drop

!

!

2π xγ sinθ = π x2γ 1

rin+

1rout

⎛

⎝⎜

⎞

⎠⎟ +Vρmg

β =ρmgb

2

γ

At the inflexion plane: γ =

Vρmg2π x sinθ


Sessile drop

!

!!


Quasi elastic light scattering

Lamb's equation

�

fo =12π

γ k3

ρw + ρoSaturday, September 18, 2010

Na4ZnTPPC adsorption

inte

nsity

/ a.

u.

8000600040002000

frequency / Hz

2nd order spot

Pure water/DCE

0.4 mM ZnTPPCaq/DCE


Potential controlled adsorption of ZnTPPC4–

25x10-6

20

15

10

5

0

Cdl

/ F

cm-2

0.30.20.10.0-0.1-0.2 Δφ / V

DCE/water 0.00 mM0.01 mM0.05 mM0.10 mM

Li2SO4 | BTPPATPFB

Electrocapillary curvesCapacitance dataSaturday, September 18, 2010

!


−dγ T,P = σdΔowφ + ΓH2OdµH2O + Γ

Li+dµLiCl + Γ

H+ + ΓCoAP−O2 H+( )dµHCl

+ ΓCoAP + ΓCoAP-O2+ Γ

CoAP-O2 H+( )dµCoAP + ΓCoAP-O2+ Γ

CoAP-O2 H+( )dµO2

+ ΓDCEdµDCE + ΓBA+ dµBATB

!


ΓCoAP + Γ Co-O2( )AP + ΓCo-O2H( )AP⎡⎣ ⎤⎦

+ = −1RT

∂γ∂ lncCoAP

⎛⎝⎜

⎞⎠⎟ T ,P,Δowφ ,µi ≠µCoAP


! !


Liquid lenses


Electrowetting with electrolytes

Courtesy of A. Kucernak, Imperial College, London







• Photocurrent




Charge Transfer Reactions

IonTransfer

Assisted IonTransfer

ElectronTransfer


Nernst equation - Ion distribution

Ion distribution is potential dependent

�

izi

�

izi

�

Δowφ = φw −φ o = Δo

wφio +

RTziF

ln aio

aiw⎛

⎝ ⎜ ⎞

⎠ ⎟

ΔGtr,io ,w→o = ziFΔo

wφio


0.2 0.3 0.4 0.5 0.6

0

2

4

6

8

10

-2

-4

-6

I / A

x 1

0-5

Δ owφ / V

Ion transfer voltammetry

Tetramethylammonium transferwater|1,2-dichloroethane interface

+

Electrochemical methods can measure the Gibbs energy of

transfer

Δowφi

o /

Diffusion

Diffusion

Transfer

+

E / V


Δowφ

pH

ΔowφBH+

o

pKaw

pKa,effw

logPBBHw+

BHo+

Bo

Distribution diagramof a lipophilic base

Cation transferpH independent

BH+

BH+

pH dependentproton pump

B

H+

BH+


H2TPP Proton transfer

CVs of blank and in the presence of 0.05mM H2TPP at 50mV/s

20

10

0

-10

j / µ

A c

m-2

0.40.20.0-0.2Δo

wφ / V

pH = 1 Blank 50µM H2TPP

Ag AgCl5 mM BTPPATPFPB

0.05 mM H2TPP0.01 M LiCl

(pH 1)

DCE Water

0.01 M LiCl1 mM BTPPACl AgCl AgCell:


Scan rate dependence

ip = 0.4463nFcnFvDRT

Randles-Sevcik equation:

slope=1.0624 ± 0.0334

CVs at various scan rates in the presence of 0.05mM H2TPP

The first anodic peak current as a function of square root of scan rate.

DH2 TPP = 6.25 ×10−6 cm2 s-1

20

10

0

-10

j / µ

A c

m-2

0.40.20.0-0.2Δo

wφ / V

pH = 150µM H2TPPScan rate (mV/s): 9,16,25,36,49,64

10

8

6

4

2

0

j p / µA

cm

-2

86420v1/2 / mV1/2s-1/2



(pH 1)

DCE Water



ip = 0.4463nFcnFvDRT

Randles-Sevcik equation:

slope=0.1037 ± 0.00282

v = 25 mV/s

CVs at various H2TPP concentrations at 25mV/s

The first anodic peak current as a function of H2TPP concentration.

DH2 TPP = 5.95 ×10−6 cm2 s-1-20

0

20

j / µ

A c

m-2

0.40.20.0-0.2Δo

wφ / V

pH = 120,50,100,200µM

20

10

0j p

/ µA

cm

-2

2001000cH2TPP / µM

Concentration dependence



(pH 1)

DCE Water



pH dependence

CVs in the presence of 0.05mM H2TPP at various pH (25mV/s)

15

10

5

0

-5

-10

j / µ

A c

m-2

0.40.20.0-0.2Δo

wφ / V

50µM H2TPP pH1 pH2 pH3

10

5

0

-5

j / µ

A c

m-2

0.40.20.0-0.2Δo

wφ / V

50µM H2TPP pH5 pH7



(pH = x)

DCE Water



Acidity ConstantsAcidity constants:

Transfer potential dependence on pH.

H4TPPH2+ Ka1⎯ →⎯ H3TPPH

+ + H+

H3TPPHKa2⎯ →⎯ H2TPPH + H+

Ka1=3.83×106

Ka2=1.31×1010

0.6

0.4

0.2

0.0Δ owφ 1o &

Δowφ 2p

c / V

6420pH

Δowφ1

o

Δowφ2

o

Δo

wφ o = Δowφ

H+o −

RTF

ln Kao +

2.303RTF

pH

Δowφ = Δo

wφH+o +

RTF

lna

H+o

aH+w

⎛

⎝⎜⎜

⎞

⎠⎟⎟

Ka1 =aH3TPPH+aH+aH4TPPH2+

Ka2 =aH2TPPaH+aH3TPPH+

Equation de Nernst

Apparent standard transfer potential


Ion partition diagram

H2TPPo

H4TPP2+ w

H4TPP2+ o

H3TPP

+ o


Micro-ITIES

!

!Optical microscopic image of a micropipette (Scale bar = 10 μm). Background-subtracted CV of simple TEA+ transfer at a 2.1 μm radius pipette. The open circles represent simulated CVs [110].

!

Video micrographs of a 15.5 m-radius micropipette filled with an aqueous KCl solution and immersed in a DCE solution of DB18C6. (B) No external pressure was applied to the pipette, and the micro-ITIES is flat. The insets show corresponding steady-state voltammograms of facilitated transfer of potassium [239]

Ion transferIngress-egress

Assisted ion transfer

ISS = 3.35π zFDCr


Nano-ITIES

!

ISS = π zFDcr sinθ

!

Steady-state egress current

4μm

100 nm

1.2 nm

Angew. Chem. Int. Ed. 2009, 48, 8010 –8013


Redox potential on the SHE scale

ESHE

Working

Electrode

Platinum

Electrode

H2

Acid ª 1M SolutionOx

Red

salt bridge


Nernst Equation

�

ESHE = EO/Ro[ ]SHE +

RTnF ln

aOaR

⎛ ⎝ ⎜

⎞ ⎠ ⎟

�

EO/Ro[ ]SHE Standard redox potential

with respect to the standard hydrogen electrode

a = γ c

c oActivity : Effective concentration normalised by

the standard concentration of 1 mol·L–1

RT lnγ Activity coefficient : Mesure of the solute-solute interaction energy. γ→1 for diluted solutions


Nernst Equation

Formal redox potential (Apparent standard)with respect to the standard hydrogen electrode

ESHE = EO/R

o /⎡⎣ ⎤⎦SHE +RTnFln cO

cR

⎛⎝⎜

⎞⎠⎟

EO/R

o /⎡⎣ ⎤⎦SHE


Ionisation energy

Reduced state

Oxidised state+

electron at rest in vacuum

Gas Phase

Ionisationenergy


Absolute redox potential

Reduced statein water

Oxidised state in gas phase+

electron at rest in vacuumIonisation

energy

Reduced statein the gas phase

Oxidised state in water+

electron at rest in vacuum

−ΔGhyd(R) ΔGhyd(O)

F EO/R[ ]abs

F EO/R[ ]abs = ΔGhyd(O) − ΔGhyd(R) + EI


ET at ITIES

O1 + R2 R1 + O2

O1

R2

R1

O2

Water

Organic

Hydrophilic

Lipophilic


Bulk ET reactionO1 + R2 R1 + O2

The standard Gibbs energy of the reaction

Ce4+ + Fe2+ Ce3+ + Fe3+

Eox/red

o⎡⎣ ⎤⎦SHEFe3+ /Fe

2+

Ce4+ /C

e3+

H+ /H

2

0 0.77 1.44Volt

ΔGro

ΔGr

o = nF EO2 /R2o⎡⎣ ⎤⎦SHE − EO1 /R1

o⎡⎣ ⎤⎦SHE( )


Non-aqueous redox potential scale

Standard redox potential in the organic phasevs a standard hydrogen electrode

oxo + n/2 H

2 n H

+ + red

o

EO/R

o⎡⎣ ⎤⎦SHE

o= µO

o ,o − µR o ,o − nµ

H+ o ,w + n

2 µH2 o⎡

⎣⎤⎦ / F

Difficult to assemble an aqueous reference electrode for an organic solution. Difficult to determine the liquid junction potential.


Ferrocene scale

Ferrocene

Decamethylferrocene

EDCMFc+ /DCMFc

o⎡⎣ ⎤⎦Fc+ /Fc

o= − 0.6 V

Fe

Fe

15

10

5

I / n

A

1.00.80.60.40.20.0-0.2E/ V


Ferrocene on SHE scale

EFc+ /Fc

o⎡⎣ ⎤⎦SHE

o= µ

Fc+ o ,o − µ

Fc o ,o( ) − µ

H+ o ,w – 1

2 µH2 o( )⎡

⎣⎢⎤⎦⎥ / F

Standard redox potential in the organic phase

Standard redox potential in the aqueous phase

EFc+ /Fc

o⎡⎣ ⎤⎦SHE

w= µ

Fc+ o ,w − µ

Fc o ,w( ) − µ

H+ o ,w – 1

2 µH2 o( )⎡

⎣⎢⎤⎦⎥ / F

Scale matching

EFc+ /Fc

o⎡⎣ ⎤⎦SHEo

= EFc+ /Fco⎡⎣ ⎤⎦SHE

w+ΔG

tr,Fc+o ,w→o − ΔG

tr,Fco ,w→o

F


Water / 1,2-dichloroethane

EDCMFc+ /DCMFc

o⎡⎣ ⎤⎦SHE

o= EDCMFc+ /DCMFc

o⎡⎣ ⎤⎦Fc+ /Fc

o+ EFc+ /Fc

o⎡⎣ ⎤⎦SHE

o= 0.04 V

Decamethylferrocene

ΔG

tr,Fc+o ,water→DCE = 0.5 ± 0.5 kJ·mol–1

ΔGtr,Fco ,water→DCE = − 24.5 ± 0.5 kJ·mol–1

Scale matching

EFc+ /Fc

o /⎡⎣ ⎤⎦SHEwater

= 0.380V EFc+ /Fc

o⎡⎣ ⎤⎦SHEDCE

= 0.64 ± 0.05V

EFc+ /Fc


= EFc+ /Fco⎡⎣ ⎤⎦SHE

water+ΔG

tr,Fc+o ,water→DCE − ΔG

tr,Fco ,water→DCE

F


Biphasic ET reaction

ΔGr

o = nF EO2 /R2o⎡⎣ ⎤⎦SHE

o− EO1 /R1

o⎡⎣ ⎤⎦SHEw⎡

⎣⎢⎤⎦⎥

Ferrocene scale 0 Volt

Fc+/FcDMFc+/DMFc

–0.6 Eox/red

o⎡⎣ ⎤⎦FcDCE

0.64SHE potential scale 0.04 Eox/red


SHE potential scale Eox/red

o⎡⎣ ⎤⎦SHEFe3+ /Fe

2+

H+ /H

2

0 0.77Volt

Fe(C

N)63– /Fe

(CN)6

4–

0.36


Nernst equationO1 R1

O2R2

Aqueous phase

Organic phase %µR1w + %µO2

o = %µO1w + %µR2

o

At equilibrium

Nernst equation ΔowφET = Δo

wφETo +

RTnFln

aR1w aO2

o

aO1w aR2

o

⎛

⎝⎜⎜

⎞

⎠⎟⎟

Standard Galvani potential differencefor Heterogeneous Electron Transfer (HET)

Δo

wφETo = µR1

o ,w + µO2o ,o − µO1

o ,w − µR2 o ,o⎡

⎣⎤⎦ / nF


Potential control of the reaction

ΔowφET > Δo

wφETo

Nernst equation ΔowφET = Δo

wφETo +

RTnFln

aR1w aO2

o

aO1w aR2

o

⎛

⎝⎜⎜

⎞

⎠⎟⎟

O1 + R2 R1 + O2

ΔowφET < Δo

wφETo O1 + R2 R1 + O2


Ferrocene oxidation by Ferricyanide

0 Volt

Fc+/FcDMFc+/DMFc

–0.6

0.640.04

Fe3+ /Fe

2+

H+ /H

2

0 0.77Volt

Fe(C

N)63– /Fe

(CN)6

4–

0.36

FeIII

Fc

FeII

Fc+

-80

-60-40

-200

2040

0.40.20.0-0.2

106 i

/ A

TPA+

ET

Δowφ / V

Ag AgCl10 mM BATB

0.4 mM Fc

1.5 M Li2SO4

0.1 M Fe(CN)63–

0.01 M Fe(CN)64–

0.01 M LiCl1 mM BACl Ag2SO4 AgCell:


Unique potential scale

O2 + e– → O2•– E o⎡

⎣⎤⎦SHE

w= –0.330 V E o⎡

⎣⎤⎦SHE

o≈ −0.81 V

O2 + 2H+ + 2e– → H2O2 E o⎡⎣

⎤⎦SHE

w= 0.695 V E o⎡

⎣⎤⎦SHE

o= 1.12V

O2 + 4H+ + 4e– → 2H2O E o⎡⎣

⎤⎦SHE

w= 1.229 V E o⎡

⎣⎤⎦SHE

o= 1.73V

H2O2 + 2H+ + 2e– → 2H2O E o⎡⎣

⎤⎦SHE

w= 1.763 V E o⎡

⎣⎤⎦SHE

o= 2.31V

E o⎡⎣

⎤⎦SHE

w

E o⎡⎣

⎤⎦SHE

o

O2+ e

– →O 2•–

O2+ 2H

+ + 2e– →

H 2O 2

O2+ 4H

+ + 4e– →

2H2O

H2O 2+ 2H

+ + 2e– →

2H2O

H+ +e

– →1

2H 2

Fc+

+e– →

Fc0 1 2


Proton coupled electron transfer

Water

1,2-dichloroethane

Private communication:Prof. I. Benjamin

O2 or CO2

Protons

Donors


CoAP + O2 + Fc

! !

Ag AgCl

DCE Water

Ag2SO4 Ag

Water Ref


5 mM BATB25 μM CoAP

5 mM Fc

0.01 M LiClHCl pH=2

No CoAP

No O2


Influence of pH

!Saturday, September 18, 2010

CoAP Shake flask

!(a) Photographs of two-phase reactions.Aqueous phase: 5 mM LiTB + 10 mM HCl.DCE phase: 1 mM Fc + 5 mM BATB (flask 1), 1 mM Fc + 20 M CoAP + 5 mM BATB (flask 2) and 20 M CoAP + 5 mM BATB (flask 3). The upper left inset shows the colors of three DCE solutions before contacting with the water solution(b) The isolated top aqueous solutions with added excess NaI; (c) Further addition of starch to the flasks shown in (b)(d) A 4-hour-two-phase reaction in the glovebox with the same solution composition as the flask 2 in (a).

FcBATB

FcCoAPBATB

CoAPBATB

FcCoAPBATBNo O2


DCE

Water

CoAP cycle - Ferrocene

Fc+ ET2 Fc

CoII

H+CoIII

OH+

O

H2O2

CoIII

H+

O2

CoIII

O•

OTautomery

Fc+

ET1

Fc

CoIII

OHO

Tautomery


Cobalt porphine

N

N N

N

Co

!

•Catalyst for oxygen reduction

reported by F. Anson


Voltammetry

10

5

0

-5

j /

µA

cm

-2

0.40.20.0-0.2

!o

w" / V

Blank

CoP

Fc

CoP+Fc

a

Cl -

H +

/Li +

Fc +

PCET

!

Ag AgCl

DCE Water

AgCl Ag

Water Ref


5 mM BATB50 μM CoP

5 mM Fc

0.01 M LiClHCl pH=2


Oxygen concentration dependence

Ag AgCl

DCE Water

AgCl Ag

Water Ref


5 mM BATB50 μM CoP

5 mM Fc

0.01 M LiClHCl pH=2

20

10

0

j /

µA

cm

-2

0.40.20.0-0.2

!o

w" / V

b Air-Saturated N2 -Atmosphere

O2-Saturated

!


pH dependence

Ag AgCl

DCE Water

AgCl Ag

Water Ref


5 mM BATB50 μM CoP

5 mM Fc

0.01 M LiClHCl pH=1, 2, 3

10

5

0

-5

j / µ

A c

m-2

0.40.20.0-0.2

!o

w" / V

Fc+

Fc+

PCET

!


CoP concentration dependence

Ag AgCl

DCE Water

AgCl Ag

Water Ref


5 mM BATBx μM CoP5 mM Fc

0.01 M LiClHCl pH=2

10

5

0

-5

j /

µA

cm

-2

0.40.20.0-0.2

!o

w" / V

a 0

10

25

50

75 µM

Fc+

Fc+

PCET

!

4

2

0

j 0.3

5V

/ µ

A c

m-2

806040200

ccoP

/ µM

b

!


Fc concentration dependence

Ag AgCl

DCE Water

AgCl Ag

Water Ref


5 mM BATB50 μM CoP

x mM Fc

0.01 M LiClHCl pH=2

10

0

j /

µA

cm

-2

0.40.20.0-0.2

!o

w" / V

a

0.2

0.5

1.0

5.0 mM

Fc+

PCET

!

8

6

4

2

0

j 0.3

2V

/ µ

A c

m-2

20151050

cFc

/ mM

b

!


H2O2 production

!

2.0

1.5

1.0

0.5

0.0

Ab

s

700600500400

! / nm

a

CoP385

(Co-O2)P

402

Fc, 439

Fc+, 620

!

1

23

1.5

1.0

0.5

0.0

Abs

500400300

! / nm

I3

-

b286

352

!


Donor dependence

Ag AgCl

DCE Water

AgCl Ag

Water Ref


5 mM BATB50 μM CoP

5 mM FcA, Fc, DFc

0.01 M LiClHCl pH=2

10

0

-10

j /

µA

cm

-2

0.40.20.0-0.2

!o

w" / V

PCETDFc+/Fc

+

DFc+/Fc

+

a

!

-2.0

-1.5

-1.0

log

(j,

A m

–2)

0.40.30.20.1

!o

w" / V

b

!


Mechanism

!


Free base porphyrin

•No adsorption at ITIES!


H2FAP catalysis - O2 reduction

0.8

0.6

0.4

0.2

0.0

A

150010005000

time / s

H2FAP + HTB + Fc

H2FAP + TFA + Fc

Fc only

!Absorbance at λ = 620 nm to monitor ferrocenium concentration.Initial conditions:0 or 50 μM H2FAP, 2.12mM HTB as shown, 0.1 M TFA as shown and 4.94 mM ferrocene.


Conclusion

•Voltammetry at ITIES is useful tool to study charge transfer reactions

•Surface tension measurements provide a direct access to the molecular composition of the interface

•“Electrochemistry in a bottle”

•Molecular electrocatalysis offers new routes for interfacial reactivity


Questions :

• Ion transfer : Transport or reaction?

• Electron transfer : Potential dependent kinetics ?

• Please explain : Phase transfer Catalysis







• Photocurrent




Documents

Electrochemistry: From soft interfaces to bioanalytics · Molecular dynamic representation! Distribution of all CCD– anions and of Cs+ ions within 10 Å from the interface. The