2015.02.05 Journal Club

System – Charges on air/water interface

-Low pH High pH

Salts (NaCl), acids (HCl), and bases (NaOH)

Lignoceric acid (C23H47COOH)Langmuir monolayer

Interfacial structure model: BIL and DL

Water molecules strongly interacting with charged headgroups and counter cations.

Water molecules reoriented by surface electric field.

(2)

(2)

(3)

z z

:Second-order susceptibility of bonded interface layer

:Second-order electric quadrupole bulk susceptibility

: Third-order bulk susceptibilityk : Phase mismatch in reflection geometry ( k

S

B

B

c

c

cD D SF,z Vis,z IR,zk k k )= - -

Second order susceptibilities from BIL and DL

(2)

(2)

(3)

z z

:Second-order susceptibility of bonded interface layer

:Second-order electric quadrupole bulk susceptibility

: Third-order bulk susceptibilityk : Phase mismatch in reflection geometry ( k

S

B

B

c

c

cD D SF,z Vis,z IR,zk k k )= - -

Experimental Setup – Same as HD-SFVS in Tahara group

Visible beam(narrow band)

IR beam(broad band)

2

2 2 * *

2wher ( ) 1e,

isample LO

i isample LO LO

F

sample LO sam le

S

p

E E e

E E E E e

n d

E E e

f

f f

pfl

D

+ D - D

D =

=

+

-

+

+ +

F.T.2 2

( )

* ( )

* ( )

( ) ( )

( ) ( ) ( ) ( )

( ) ( )

i tref LO

i t i T tLO ref

i T tLO ref

E E e d

I t I e d E E e d

E E e d

ww

w w

w

w w w

w w w w w

w w w

-

- + D -

- D +

+

= =

òò ò

ò*Pulse duration of Ti:Sa Amplifier ~ 100 fs

Strategy – How to deduced c(2)S,DL from c(2)eff

1) Take SF spectra of the LA monolayer/water interface in the pH range far below the half ionization point of carboxyl headgroup(pH < 9)

(2) (2),0~ : Second-order suscepti charge neutbility ral into e f ef r acS Sc c

(2) (2),0~S Sc c at pH < 2.5

Strategy – How to deduced c(2)S,DL from c(2)eff

*Assignment of the OH band

(1) From OH stretch

mode of -COOH

headgroup(H-bonded)

(2) From OH stretch mode

of water moleculesBound to

headgroup

Low pH

High pH

(1)(2)

Strategy – How to deduced c(2)S,DL from c(2)eff(2) Take SF spectra of the COOH (COO-) stretch modes of the LA

monolayer/water interface.

(2) 1434 cm-1

(1) 1410 cm-1(3) 1717 cm-1

Low pH

High pH

(2)

(3)

(1)

Strategy – How to deduced c(2)S,DL from c(2)eff

1COOH COO COO NaX X X- - +×××

+ + =

*From MD simulation ® one Na+ cation prefer to bind to three COO- headgroups

2

: Net surface charge density

(1 ) (1/ 3)

1: Surface number density of carboxyl headgroup ( ~ )20

s COOH s COO Na

s s

N X N X

N NA

s

s- +×××

= - - ×

o

(3) Calculate surface charge density from the fitting result of the low frequency spectra (for the case of pH >9)

Strategy – How to deduced c(2)S,DL from c(2)eff(4) Calculate depth-dependent electric field, E0(z) from PB theory

(for the case of pH > 9)

( ) ( ) ( )(

2

02)

0

0( )

2

(

( ) ( ) for

( ))) )( (

1

e ze zkT

r o r o r

k

r

T

o

o

c e

c e e

c ez e

zzkT k

z e

T

yy

ye e e e e e

yye e

rr +-

+-

--+

Ñ = - = - +-

» <<

+

Positively charged surface

z

Stern layer Diffuse layer

Y(z)

k-1

Bulk

202

( ) ,zor o

c ez e

kTky y k

e e-= =

< Gouy-Chapman model > (PB theory)

( ) 1 for highly charged surface

( (0) 100 mV)

e zkTy

y

³

>

2 222 *0 0

20 0

20

0

2( ) sinh sinh (1)

2e ( )where, y= , and

y y

r B r B

B r B

c e c ed y e e y yk T k Tdz

c ezk T k T

ke e e e

y ke e

-= - = = ×××

=

Strategy – How to deduced c(2)S,DL from c(2)eff

*Explicit solution of PB equation for 1:1 electrolyte solution surface

22 *

22 2 sinh (2)dy d y dy ydz dzdz

k× = × × × ×

2 ' 2 2 ' 2 ' 2 *1' ' '

21

( ) ( ) 2 sinh 2 sinh 2 cosh (3)

2

d dy dy dydz ydz ydy y Cdzdz dz dz

C

k k k

k

= = × = = + ×××

= -

ò ò ò

From M.S. dissertation of W. M. Sung

Strategy – How to deduced c(2)S,DL from c(2)eff

*2cosh 2 2 sinh( ) (4)2

for negatively charged surface- for positively charged surface

dy yydz

k k= ± - = ± ×××

+

From M.S. dissertation of W. M. Sung

0

0

' ' *2'

/ 2

2 0/ 2

1 2 2ln(tanh ) 2 2 (5)4sinh

21 e (0)ln( ), =1

y

yB

ydy dz z Cy

eC yk Te

k k

y

= Þ = + ×××

-=

+

ò ò

Strategy – How to deduced c(2)S,DL from c(2)eff

From M.S. dissertation of W. M. Sung

0

0

/ 4 / 4 / 2*

2/ 4 / 4 / 2

/ 2

2 0/ 2

1ln( ) ln( ) (6)1

1 e (0)ln( ), =1

y y y

y y y

y

yB

e e e z Ce e e

eC yk Te

k

y

-

-

- -= = - + ×××

+ +-

=+

Strategy – How to deduced c(2)S,DL from c(2)eff

*Grahame equation for charge neutrality

0 000

(7)r e rz

ddzdzys e e r e e

¥

=

= - = - ×××ò

00 08 sinh( ) (8)

2r BB

eC k Tk Tys e e= × ×××

From M.S. dissertation of W. M. Sung

Once surface charge density, s and bulk concentration, C are determined, depth profile of surface electric field is uniquely determined by PB equation (pH > 9)

Strategy – How to deduced c(2)S,DL from c(2)eff

At neutral and acidic pH (pH <9)

(2) 1434 cm-1

(1) 1410 cm-1(3) 1717 cm-1

COO- and COO-Na+ modes cannot be measured accurately…

Strategy – How to deduced c(2)S,DL from c(2)eff

By introducing surface pH, pHs

known

Known: 5.6

e (0)

e (0)

10 10 10

[ ] (0) [ ]

log ([ ] (0)) log ([ ] ) log ( )

B

B

k Ts b

k Ts b

H H e

H H e

y

y

-+ +

-+ +

=

- = - -

0 0(0)8 sinh( ) (8)

2r BB

eC k Tk Tyes e= × ×××

Strategy – How to deduced c(2)S,DL from c(2)eff

(2) (2),0~ : Second-order suscepti charge neutbility ral into e f ef r acS Sc c

known

Converge to whendeprotonation fraction of COOH is low.

(2),0Sc

Deduced c(2)S,DL spectra

Deduced c(2)S spectra

c(3)B does not sensitively depend on interface property

In summary,

We have demonstrated a scheme using PS-SFVS to separately deduce the

vibrational spectra of the BIL and the diffuse layer of a charged water interface.

For any water interface with a given surface charge density σ, it is now possible to

find the spectrum of the diffuse layer and, in turn, the spectrum of the BIL from

measurement. Even if σ is not known, one can still carry out a measurement with

several different phase mismatches Δkz, and deduce both σ and the spectrum of

the BIL, which are intimately related to the microscopic structure of BIL. Such work

offers new opportunities to explore various charged water interfaces at a deeper

molecular level, providing a base for the understanding and theoretical modeling of

such interfaces.