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Queen Mary

University of
London

Neutron Reflectivity study of interfaces

A.
Zarbakhsh

a.zarbakhsh@qmul.ac.uk

COST Action D43

Colloid and Interface Science for Nanotechnology


Sofia 2011


Neutron Reflectivity


application of neutron
reflectometery

in resolving these buried
interfaces


Fluid
-
Fluid interface and why and how ?


Application of neutron
reflectometery

in resolving these buried
interfaces


Typical results

Concluding remarks and future work.

Conformation of
alkylated

azacrown

ether at;

Air
-

water interface,

Oil
-

water interface.

& the role of fatty acid,


Structure studies of lipids at the oil
-
water interface


The Neutron

l

m
n

v

m
n

= 1.674 x 10
-
27

kg

Wave
-
particle duality

de Broglie (1924)

l

=

h

m
n
v

Thus:

10
-
10

m

E

~

k
B
T

l

~

Neutron
Reflectivity

Neutrons scattered by nucleus

isotopic substitution
-

labelling

b
H

=
-
3.74 x 10
-
15

m

b
D

= +6.67 x 10
-
15

m

complex sample environment

repetitive measurements

nuclear reactors or spallation sources

Reactor Source

The
Spallation

Neutron

Reflectivity

Theory

A newer version of well known phenomenon


Refraction of light

Dense (n)

Less dense

(air n =1 )

i
r
n
sin
sin

i

r

A =
Nb
/(2

)



C = N

A
/(4

)


1.

Principle of Optics

2.

Quantum approach

C
i
A
n
l
l



2
1
Where N is the atomic number density of medium

B is the bound atom coherent scattering length.

The term

A
is the absorption cross
-
section

For polymeric species and solvents of low relative molecular mass Nb

can be replaced by the scattering length density of the polymer segment or solvent molecule,

.


In case of neutron for most materials n <<1

Neutron
Reflectivity

In French, Snell's Law is called "la
loi

de Descartes"

The neutron

Reflectivity

q < q
crit

q  q
crit

q > q
crit

q

I(
q
)

q
crit


Reflection

Reflection

&

Refraction

Neutron
Reflectivity

The Neutron

Reflectivity

Structural information



d

Simplest Case

r

to the surface

Lateral structure give rise to Off
-

Specular

Complex Case



d

Neutron
Reflectivity

The Reflectivity

Data Analysis

Ideal world



d



d

Proposed a model and compare with the data

Optical matrix method


The transmission and reflection from one layer to the
next is described as a matrix multiplication product.

Neutron
Reflectivity

Scattering length density A
2

Critical angle

Contrast variation

The neutron Reflectivity

Applications

Surface Chemistry

Surfacatnts, Polymers, Protein, Lipids

at air

water

solid

liquid and

liquid

liquid interfaces

Solid surfaces

Thin films, multilayers, polymers

Magnetic
sample

Magnetic multilayers, superconductors

and thin films

Neutron
Reflectivity

The Liquid/Liquid Interface Why and How?

Why


Transport properties of cell membrane

Stabilisation of emulsion

Mixed surfactants and cold water cleaning

Proteins (and surfactants)
at
Oil / Water: Important
in biological & food systems

Proteins stabilise a diverse range of colloids including:
blood, ice
-
cream
,
milk

How


Sum Frequency Spectroscopy

x
-
ray
reflectivity

Neutron
reflectivity

Limitation:

the maintenance of a uniform condensed oil layer is


a non
-
trivial and very time
-
consuming task. the technique is restricted


to the study of volatile oils limited contrast can be used.

thick oil

Oil
-
water

water

Oil
-
water

few micron oil
-

water

Log Reflectivity

Q x 10

2

/

Å

-

1

0

2

4

6

1.50

1.00

2.00

2.50

Log Reflectivity

Q x 10

2

/

Å

-

1

0

2

4

6

1.50

1.00

2.00

2.50

Log Reflectivity

-
2

/

Å

-

1

0

2

4

6

1.50

1.00

2.00

2.50

Q x 10


2

/ Å
-
1

Method:
Relies on balancing the condensation and drainage


rates of the oil film



Thermostatted plate

water

Neutron beam

oil

Thermostatted vapour
-
tight vessel

Thermostat 2

Thermostat 1

Condensation Method

Spin
-
Freeze method

Hydrophobe

silicon surface: Chemically modify using


trichlorosilane

from chloroform Spread oil using a


spinner. Freeze rapidly, assemble cell and introduce aqueous phase.



Hydrophobic silicon surface



Spread hexadecane



Spin (2000 rpm for 20 s)



Cool block to
T
< 18 ºC




Assemble cell



Warm block to
T
>18 ºC



Measure
reflectivity

Thickness of oil film is determined by mass of oil added and spinning speed

“A new approach to measuring neutron reflection from a liquid/liquid interface”

A, Zarbakhsh, J. Bowers and
J.R.P.Webster
. M. Sci.
Technol
, 10, 738
-
743

1999
.



Use a
supermirror

to change

Q



Film
stable/reproducible



l

l
l
N
i
Nb
n
4
2
1
)
(
2



2
1
2
1
2
1
tot
1
)
1
(
R
AR
R
AR
R
R












q



oil
oil
sin
2
exp
d
A

Experimental & analysis

Oxide


d
4


d
1

d
2

Si


(CH
3
)
3
-
layer

q  q
0

q
oil


Oil

d
n


R
2

n

interface

layers


Aqueous subphase


Oil


d
oil
=

d
3


q
oil


R
1

l
Å
0
1
2
3
4
5
6
7

/10
-5
Å
-1
0
2
4
6
8
10
T = exp(
-

l
)

Sensitivity to Thickness of Oil
l
(Å)
3
4
5
6
7
Reflectivity
1e-3
1e-2
1e-1
1e+0
fit - oil layer 3

exp data
oil layer 1

oil layer 2

oil layer 4

oil layer 6

q
=0.29º
Interfacial width at bare hexadecane
-
water interface



The

interfacial width,




0

including the intrinsic


CW

capillary
-
wave contribution


2
CW
2
0
2






CW


0



oil

water

NR


0

㴠㘮〠


ㄮ1Å

X删


0

= 6.0


〮0Å

Expected

0

㴠=⸵㔠Å





min
max
2
CW
ln
)
2
/(
Q
Q
kT





q

l

sin
2
min


Q


a
Q

2
max

interfacial tension varies as



 
济

1

= 54.5


0.076(
T
/K

298)


The reflectivity can be written

Ln
(R/R
F
) =
-
Q
z
Q
T

2

0.002
0.004
0.006
0.008
log(
R
/
R
F
)
-22
-20
-18
-16
Temperature / K
290
300
310
320
330
340
350
2
4
6
8
10
12
Total interfacial width / Å
293.4 K
305.1 K
316.7 K
327.7 K
339.3 K
344.5 K
Q
Z
Q
T
/ Å
-2
“Width of the hexadecane

water interface: A discrepancy resolved”

Ali Zarbakhsh*, James Bowers, and John Webster.

Langmuir
,

21

(25), 11596
-
11598,
2005
.


R = C
16
H
33

Macrocyclic

ligands

at the air
-
water and the oil
-
water interface



Cationic transport carriers in different extraction base techniques



The Langmuir films of these
azacrown

ether have potential in chemical


sensing and molecular electronics applications



They can act as a good representative of biological transport systems

Stabilization of
alkylated

azacrown

ether by fatty acid at the air
-
water interface.

Zarbakhsh A,
Campana

M, Webster JR, Wojciechowski K

Langmuir 26(19):15383
-
15387 05 Oct 2010


Buffle

et al. used the
alkylated

azacrown

ethers in conjunction with fatty acids for transporting heavy metal ions (Cu(II),
Cd
(II),
Pb
(II))
against their concentration gradient in so called Permeation Liquid Membrane (PLM) devices. A typical PLM device consists of
a
hydrophobic membrane separating two aqueous compartments between which the transport of metal ions takes place. The process
involves thus two extraction steps
-

one at each aqueous
-
membrane interface. The membrane could be either unsupported (bulk
organic phase), or supported in the pores of a thin inert polymer support. In either case the membrane consists of a solution

of

the
carrier (e.g. a mixture of
azacrown

ether and fatty acid) in a
nonpolar

solvent.

Source

(aqueous

medium

+

metal

species

(M))

Membrane

(organic

medium

+

carriers

(
C
))

Strip

(aqueous

medium

+

Complexing

agent

(
S
))

M
M + C
C
M + s
M
M + C
C
M + s
Source
Membrane
Strip
M
M + C
C
M + s
M
M + C
C
M + s
Source
Membrane
Strip





0
5
10
15
20
25
30
35
-8
-7
-6
-5
-4
-3
-2
log C
22DD
[M]

[mN/m]
0.0
0.1
0.2
0.3
0.4
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
0.01
0.1
1
10
100
1000
10000
100000
reflectivity
q
z
[Ang.
-1
]
ACE-10
ACE-16
ACE-24
PAH
ACE-10/PAH
X
-
ray reflectivity : toluene
-
water interface


Surface tension isotherm can not be understood

using a simple Langmuir or
Frumkin

model


ACE10

(a)
A possible expanded model

(b)
Reoriented model

X
-
ray data could not distinguish between these

Possible models.

Oil

Water

Oil

Water

(a)

(b)

Air

water interface experiment

R = C
16
D
33

Two contrasts

1. Air


null reflecting water

2. Air


D
2
O

Q / Å
-1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Reflectivity
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
1

/ mN m
-1
11
15
20
25
30
Reflectivity data for contrast 1 (air
-
null reflecting water) interface for a solution of d
-
ACE
-
16 measured at 1.5

.
The solid lines correspond to a single layer model with film thickness 21.5
±

0.5 Å.

25.7Å

21.5
±

0.5 Å

90
100
110
120

/ mN m
-1
10
15
20
25
30
35
Area per molecule measured / A
2

90
100
110
120
Area per molecule / A
2
calculated
20
40
60
80
100
120
140
(a)
(b)
Area per molecule measured / A
2

Analyses of reflectivity data measured for contrast 1: (a) π
vs

area per molecule for d
-
ACE
-
16 estimated from
the area of the trough (b) the area per estimated from the area of the trough as a function of the area per
molecule obtained from the fits to the neutron profiles.

Reflectivity data for contrast 2 (air
-
D
2
O) interface for a solution of d
-
ACE
-
16 measured at 1.5

. The solid
lines correspond to a single layer model with film thickness 21.5
±

0.5 Å.

Q / Å
-1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Reflectivity
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2

/ mN m
-1
11
15
20
25
30
z / Å
-10
-5
0
5
10
15
20
25
30
Nb / A
-2
0
1e-6
2e-6
3e-6
4e-6
5e-6
6e-6
7e-6

/ mN m
-1
11
15
20
25
30
Air
Water (D
2
O)
Scattering length density profiles used to model the contrast 2 (for d
-
ACE
-
16 at the air
-
D
2
O)

Oil

water interface experiment

R = C
16
D
33

Two contrasts

1. Si contrast matched oil


Si contrast matched water

2. Si contrast matched oil

D
2
O

d

q

f

R
tot

R
1

R
2

silicon

oil

water

1

2

silicon

oil

water

Oxide


d
1

= 8 Å Nb = 3.40 x 10

6
Å

2


d
1

= 5 Å Nb = 0.50 x 10

6
Å

2


Si


(CH
3
)
3
-
layer

q  q
0

q
oil


Oil

R
1


Nb = 2.07 x 10

6
Å

2

Interlayer roughness, each of
2.2
Å

R
1

Part of reflectivity

Reflectivity data measured at 1.4

, for contrast 3 (Si
-

hexadecane scattering length density matched to
the Si
-

aqueous solution with scattering length density matched to Si), for a series of spread amounts for
the d
-
ACE
-
16. The solid lines correspond to a single layer model with film thickness 29.0
±

2.0 Å.


l
/ Å
2
3
4
5
6
7
Reflectivity
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
1.5e-6 mol m
-2
1.80e-6mol m
-2
2.25e-6 mol m
-2
3.00e-6 mol m
-2
4.51e-6 mol m
-2
Spread amount / mol m
-2
1
2
3
4
5

/ mol m
-2
0
1
2
(x 10
-6
)
(x 10
-6
)
The fitted adsorbed amount deduced from contrast 3 for the d
-
ACE
-
16 at the oil
-
water interface is plotted as

a function of spread amount. The dash line shows the idealised line.

Reflectivity data for the contrast 4 (Si
-

hexadecane scattering length density matched to the Si
-

D
2
O) for a series of
d
-
ACE
-
16 spread amounts measured at 1.4

. The solid lines correspond to a two
-
layer model (17 Å at the
hexadecane side of the interface and 38 Å at solution side of the interface.


l
/ Å
1
2
3
4
5
6
7
Reflectivity
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
1
1.50e-7 mol m
-2
2.25e-6 mol m
-2
6.01e-6 mol m
-2
1.35e-5 mol m
-2
3.00e-6 mol m
-2
z / A
-20
0
20
40
60
80
Nb / A
-2
1e-6
2e-6
3e-6
4e-6
5e-6
6e-6
7e-6
1.50e-7 mol m
-2
2.25e-6mol m
-2
3.00e-6 mol m
-2
6.01e-6 mol m
-2
1.35e-5 mol m
-2
Oil
Water
S
cattering length density profiles used to model the contrast 4
data



/ mN m
-1
5
10
15
20
25
30
35
40
45
Adsorbed amount (mol m
-2
x 10
6
)
1.2
1.4
1.6
1.8
2.0
2.2
2.4


/ mN m
-1
5
10
15
20
25
30
35
40
45
Adsorbed amount / mol m
-2
x 10
6
1.5
2.0
2.5
3.0
3.5
5
10
15
20
25
30
35
40
1.5
2.0
2.5
(b)

/ mN m
-1
1.5
2.0
2.5
3.0
3.5
(a)

Adsorbed amount (NR) / mol m
-2
x 10
6
1.4
1.6
1.8
2.0
2.2
Adsorbed amount (trough) / mol m
-2
x 10
6
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Wavelength / A
2
3
4
5
6
7
Reflectivity
2.0e-5
4.0e-5
6.0e-5
8.0e-5
1.0e-4
1.2e-4
1.4e-4
1.6e-4
1.8e-4
2.0e-4
Spread amount mg m
-2
0.35
0.26
0.20
0.06
bare interface
2
3
4
5
6
7
2.0e-5
4.0e-5
6.0e-5
8.0e-5
1.0e-4
1.2e-4
1.4e-4
1.6e-4
1.8e-4
2.0e-4
(a)
(b)
Wavelength /

2
3
4
5
6
7
Reflectivity
10
-7
10
-6
10
-5
10
-4
10
-3
10
-2
10
-1
spread amount mg m
-2
0.26
0.13
0.06
Structural study of lipids at the oil
-
water interface

15

l, volume fraction for all components
d / Å
0
20
40
60
80
Volume fraction
0.0
0.2
0.4
0.6
0.8
1.0
60

l (above) and 15

l (below),
volume fraction for all components
0.0
0.2
0.4
0.6
0.8
1.0
(a)
(b)
Volume fraction profiles 0.26 & 0.06 mg m
-
2


mg m
-2
0.10
0.15
0.20
0.25
0.30
0.35
0.40
Area per molecule A
2
60
80
100
120
140
160
0
20
40
60
80
Nb / Å
-2
1x10
-6
2x10
-6
3x10
-6
4x10
-6
5x10
-6
6x10
-6
7x10
-6
3.73 x 10
-6
mol m
-2
7.47 x 10
-6
mol m
-2
d / Å
d / Å
0
20
40
60
80
1x10
-6
2x10
-6
3x10
-6
4x10
-6
5x10
-6
6x10
-6
1.87 x 10
-6
mol m
-2
Nb / Å
-2
Oil phase (
Nb

= 2.07 x 10
-
6

Å
-
2
)

Aqueous phase D
2
0 (
Nb

= 6.35
x 10
-
6

Å
-
2
)

Increasing spread amount

d1

d2

d3

d2

d1

Final Remarks



Application and refinement of a method for structural studies at a


liquid
-
liquid interface using neutron reflectometery.




Absorption isotherms and molecular conformations have been deduced.




We have recently extend these studies to lipids conformations


and interactions at Liquid


Liquid interfaces.




We have also working on application of series of isotopic substitutions


to resolve the distribution of oil and water at these important interfaces.

Partial structural study at the oil
-
water interface

a) Possible number density profiles of water (
nw
) and oil (no). The oil number density is per
methylene

unit. b) Corresponding scattering length density profile. The width of the
scattering length density profile is ~5Å which agrees with theory and our previous neutron
experiment.

Contrast


hexadecane

water

1


4e
-
6 Å
-
2




D2O

2


4e
-
6 Å
-
2



H2O

3


4e
-
6 Å
-
2



3e
-
6 Å
-
2


4


CMSi



D2O

5


CMSi




H2O

6


CMSi



3e
-
6 Å
-
2