GUDRUN/EPSR data analysis of HCl in water (1:17)

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GUDRUN/EPSR data analysis of HCl in water (1:17)

Valeria Conti Nibali, Dario Corradini, Nancy Leone

Tutors: Silvia Imberti, Rosaria Mancinelli

School of neutron scattering F. P. RicciSanta Margherita di Pula, 2-10-2008

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SANDALS @ ISIS

HCl:H2O

DCl:D2O

HCl:H2O / DCl:D2O (50/50)

Neutron diffraction on hydrogen chloride (HCl) in water (1:17)

Incident Wavelength: 0.05 to 4.5 Å

Q-range: 0.1 to 50 Å

Moderator:Liquid methane at 110K

Incident Flight Path: 11m

Final Flight Paths: 0.75m to 4.0m

3 measurements different interaction of neutrons with H or D ( isotopic contrast)

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GUDRUN CORRECTION

Corrections applied to raw data:

Dead timeBackground scatteringAttenuationMultiple scatteringNormalizing to the incident beam monitorVanadium calibrationInelastic scattering ( for samples)

Corrected after EPSR run with reference potential

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1) Calibration of the containers thickness

GUDRUN CORRECTION

HCl:H2O nominal thickness 0.1 cm gives 96.1 % of expected level; thickness 0.095 cm gives 100.9 % of expected level.

DCl:D2O the container thickness is very different from the nominal value. We found that the value 0.0875 cm gives 101.8 % of expected level.

HCl:H2O / DCl:D2O (50/50) good agreement with nominal

thickness

Flat top-loading TiZr canfor the sample changer

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2) Working on the samples

GUDRUN CORRECTION

0

1

2

3

4

5

6

7

0 5 10 15 20 25 30 35 40 45 50

Q

'SLS34739.mdcs01''SLS34747.mdcs01'

HCl:H2O

Diff

ere

nti

al C

ross

Sect

ion

Q (Å^-1)

Two measures taken at different times gave 96% and 99.7% of expected value probably due to slight beam instability. We have considered the average of these two runs (98.2% of expected value)

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0

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0 5 10 15 20 25 30 35 40 45 50

'SLS34747.MDCS01''SLS34740.MDCS01''SLS34743.MDCS01'

We observe:

1)Large Q level due to incoherent scattering (density)

2)Inelasticity effects at very small Q observed in hydrogenated samples

3)Peaks structure due to coherent scattering

GUDRUN CORRECTION

2) Working on the samples

HCl:H2O

DCl:D2O

HCl:H2O / DCl:D2O

Q (Å^-1)

Diff

ere

nti

al C

ross

Sect

ion

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GUDRUN CORRECTION

3) Incoherent level subtraction

• We subtracted the incoherent level by means of the Top Hat subtraction• We notice that while the D-sample oscillates around 0, the H sample has a rise at low Q. • This is due to inelasticity effect and it is the most difficult correction on diffraction data and it will be done later.

0

0.5

1

1.5

2

2.5

0 5 10 15 20 25

'SLS34747.mdcs01''SLS34740.mdcs01''SLS34743.mdcs01'

HCl:H2O

DCl:D2O

HCl:H2O / DCl:D2O

Q (Å^-1)

Diff

ere

nti

al C

ross

Sect

ion

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-0.5

0

0.5

1

1.5

-2 0 2 4 6 8 10 12 14

'SLS34747.mgor01''SLS34740.mgor01''SLS34743.mgor01'

Fourier transform of the data gave the total g(r) of the three systems. We note that at 0.98 Å the O-H peak is negative and the O-D is positive because the neutron scattering length of D is positive and for H is negative.

HCl:H2O

DCl:D2O

HCl:H2O / DCl:D2O

OH in HCl:H2O

OD in

DCl:D2O

G(r

)

r (Å)

GUDRUN CORRECTION

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Building the EPSR simulation

-15-10

-5 0

5 10 -15

-10

-5

0

5

10-10

-5

0

5

10

z [Å]

x [Å]

y [Å]

z [Å]

We built the simulation box containing 30 H+, 30 Cl-, 510 H20.

We employed SPC/E model for water molecules and we ran a simulation with this reference potential:

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0

0.5

1

1.5

2

0 5 10 15 20

F(Q

)

Q [1/Å]

C:\EPSR17\20080617_EPSR17\run\HCl\exNAFAVA\solution

SLS34747

SLS34740

SLS34743

Comparison between experimental data and EPSR run with reference potential only

Building the EPSR simulation

HCl:H2O

DCl:D2O

HCl:H2O / DCl:D2O

Inelasticity effects

Using GUDRUN inelasticity effects were corrected by means of a stretched exponential function.

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0 5 10 15 20 25

'SLS34743.mdcs01''solution.EPSR.u01' us 1:6

HCl:H2O / DCl:D2O

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EPSR simulation: results

Once inelasticity effects were corrected, the potential refinement has been switched on. After thermalization configurations have been accumulated.

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

2 4 6 8 10 12 14 16 18

F(Q

)

Q [1/Å]


SLS34747

SLS34740

SLS34743

Probably inelasticity effects were over-corrected

HCl:H2O

DCl:D2O

HCl:H2O / DCl:D2O

Fit to experimental data

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0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 2 4 6 8 10 12

g(r

)

r [Å]


OW-OW

OW-HW

HW-HW

EPSR simulation: results (1)Water-water RDFs

O-H+-Owater first shell

water second shell

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O-H+-O structure Water first shell Water second shell

EPSR simulation: results (2)O-O spatial density function (SDF)

2.0-2.5 Å

H+

probableH3

+O-H2O structure

2.0-3.3 Å 2.0-4.8 Å

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0

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10

15

20

0 1 2 3 4 5 6

g(r

)

r [Å]


OW-H

HW-H

EPSR simulation: results (3)H+-water RDFs

H+

1.1 Å 1.3 Å

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0

0.5

1

1.5

2

2.5

3

3.5

2 4 6 8 10

g(r

)

r [Å]


Cl-OW

Cl-HW

EPSR simulation: results (4)Cl--water RDFs

Cl- Cl-

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0

2

4

6

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10

12

14

1 2 3 4 5

g(r

)

r [Å]


Cl-Cl

Cl-H

H-H

0

0.5

1

1.5

2

2.5

1.5 2 2.5 3 3.5 4 4.5 5 5.5

g(r

) and N

(r)

r [A]


Cl-H

EPSR simulation: results (5)Ion-ion RDFs

Cl-H+

Cl- - H+ RDF and coordination number

Solvent separated ions pairing

Large anion-cation peak

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Conclusions

How does HCl modify the structure of water?

The double peak structure in the H+-O RDF and the peak in the O-O RDF at 2.4 Å could be interpreted as:

1) O-H+-O structure (delocalized proton)

or as

2) H3+O-H2O structure

H+

Cl-H+

Presence of solvent separated ions pairing

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FAIL SAFE

Dario Don’t touch

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-10-5

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z [Å]

x [Å]

y [Å]

z [Å]

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"sls34747.mdcs01""solution.EPSR.u01"

Amplitude=1; decay constant=1; stretch constant=1

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"sls34747.mgor01""solution.EPSR.x01"

GUDRUN/EPSR data analysis of HCl in water (1:17)

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Transcript of GUDRUN/EPSR data analysis of HCl in water (1:17)