Transcript PowerPoint 演示文稿 - The Ohio State University

Ethylenediamine at Air/Liquid and
Air/Silica Interfaces: Protonation Versus
Hydrogen Bonding Investigated by Sum
Frequency Generation Spectroscopy
Man Xu, Dingfang Liu, Heather C. Allen
Environmental Science Graduate program
The Department of Chemistry
The Ohio State University
Introduction
Ethylenediamine (NH2CH2CH2NH2, EDA)
 Ten conformers
 pKa1=10.71, pKa2=7.56. pH of 25% EDA is 11.9
 Usage: fuel additives, bleach activators, chelating agents,
corrosion inhibitors
 Used as a model system
Silica
• Constitutes 59% of the mass of the earth’s crust
• One of the most common mineral components on earth
• A good model mineral oxide
Amorphous Silica
• Surface is fully hydroxylated
• Isolated silanol OH group: SiOH
Surface silanol groups
Adsorption
The adsorption process on mineral oxide surfaces is an important
factor influencing migration & distribution of contaminants in the soil
environment
Previous studies: IR, NMR, XPS, total internal reflection
fluorescence correlation spectroscopy (TIR-FCS)
Present study: SFG, which provides direct molecular level
information, is employed to investigate the adsorption of EDA onto
SiO2 surfaces
Sum Frequency Generation(SFG) Spectroscopy
An Ideal Tool to Probe the Interface
• Media must lack inversion
symmetry
• Vibrational transition must
be Raman & IR active
SFG spectroscopy is a surface specific technique
The hydroxylated silica/air interfaces lack inversion
symmetry
SFG provides information on the orientation of the
molecules adsorbed to the surface
Experiments
Adsorption of Ethylenediamine at the Air/Silica Interface
Organics
SiO 2
Purged with organic free air
for 5 minutes & 24 hours
SFG
• Reference systems: aq. solutions of EDA, singly &
doubly protonated EDA
Results
SFG Intensity
EDA at
Air/Liquid
Interfaces
60
3700 free
OH
a
Neat Water, ssp
b
0.05 m.f. EDA, ssp
50
40
30
20
10
0
SFG Intensity
40
2860 CH2-SS
30
3300 NH2-SS
2920
CH2-AS
20
3360 NH2-AS
10
SFG Intensity
0
140
c
0.2 m.f. EDA, ssp
120
100
80
60
40
20
0
Exposure time 30s, Average of > 2 spectra
2800 3000 3200 3400 3600 3800
Incident Infrared (cm-1)
80
SFG Intensity
EDA at Air/Silica Interfaces
20
10
2920
CH2-AS
6
3300 NH2-SS
3360 NH2-AS
4
2
c
Silica with EDA (24 hours purging)
40
SFG Intensity
amino groups from EDA
2860 CH2-SS
8
0
between surface silanol OH groups and
Silica with EDA (5 minutes purging)
40
Suppression of the silanol OH peak
adsorption occurs through interaction
b
3750
isolated
silanol
OH
12
SFG Intensity
adsorption of EDA to silica surface
Silica (RH 50%)
60
0
14
SFG responses in C-H & N-H region
a
30
20
10
0
2800 3000 3200 3400 3600 3800
Incident Infrared (cm-1)
Exposure time 30s, Average of > 2 spectra
Possible Adsorption Mechanisms
I)
liquidlike aqueous layer
II)
hydrogen bonding
III) singly protonation
IV)
doubly protonation
pKa1(EDA)=10.71
pKa2(EDA)=7.56
pKa(surface silanol OH)<9.0
????
5 mins purging vs. 24 hrs purging
SFG Intensity (a.u.)
Silica with EDA (5 minutes purging)
10
8
SFG Intensity (a.u.)
5 mins: liquidlike aqueous layer
6
4
24 hrs: liquidlike layer is removed, but
2
0
SFG responses after 24 hrs
purging >> 5 mins purging
Silica with EDA (24 hours purging)
surface adsorbed EDA remains/ SFG
response is from the EDA-silanol complex
40
30
20
SFG spectrum after 24 hrs purging
reveals the adsorption mechanism
10
0
2800 3000 3200 3400 3600 3800
Incident Infrared (cm-1)
C-H stretching peaks are used as
spectral references
40
a
0.05 m.f. EDA
30
10
peak splitting
b
0.07 m.f. Singly Protonated EDA
SFG Intensity
50
peak position (blue shift)
30
20
Silica with EDA (24 hrs purging)
10
25
SFG Intensity
orientation changes (~40° to ~80°)
2872
40
0
(CH2-SS)
After protonation
20
0
Reference Aqueous Solutions
c
0.05 m.f. EDD
2872
2883
20
40
30
15
20
10
10
5
0
2700 2750 2800 2850 2900 2950
Incident Infrared (cm-1)
0
2750 2800 2850 2900 2950 3000
Incident Infrared (cm-1)
SFG Intensity
SFG Intensity
2862
Conclusion
EDA molecules are chemisorbed to the silica surface
through the protonation of one EDA amino group by a surface
silanol OH group
The surface acidity of the silanol OH group (pKa (HOSi)) is
between the two pKa values of EDA (in the range of 7.56 to
10.71) at the air/silica interface
Acknowledgements
•
DOE (DOE-BES Geosciences DE-FG02-04ER15495)
•
EMSI @ The Ohio State University