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Discontinuous hygroscopic growth of a Discontinuous hygroscopic growth of a mixed organic/inorganic aerosol particle mixed organic/inorganic aerosol particle levitated in an levitated in an electrodynamicelectrodynamicbalancebalance
ByBy
Vacharaporn SoonsinVacharaporn Soonsin
Summer School on Organic Aerosols Summer School on Organic Aerosols 26.06.200826.06.2008
SupervisorsSupervisorsProf. Thomas PeterProf. Thomas PeterDr. Ulrich KriegerDr. Ulrich Krieger
Institute for Atmospheric and Climate ScienceInstitute for Atmospheric and Climate ScienceETH Zurich, SwitzerlandETH Zurich, Switzerland
Outlines
• Introduction• Experimental setup• Preliminary experiments• Conclusion• Take home messages
Phases and phase changes of aerosol particles
%RH
T/K
Inorganic salt e.g. NaCl, (NH4)2SO4
DRHERH
%RH
T/K
Mixed organic/inorganic particles
DRHERH
Hygroscopicity cycles of inorganic salts:Textbook knowledge
Hygroscopicity cycles of mixed particles:e.g. Marcolli & Krieger, 2006
Phases and phase changes of aerosol particles
Mixed organic/inorganic particles with liquid-liquid phase separation
%RH
T/K DRHERH2-liquid phases
1-liquid phase
Influence of liquid-liquid phase separation on hygroscopicity cycles
Hygroscopicity cycle from the EDB
0 10 20 30 40 50 60 70 80 90
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
ERH43 %
DRH75 %
mas
s / d
ry m
ass
RH [%]
ERH59%
DRH89 %
C6E2/NaCl/H2O with mole ratio of NaCl/C6E2 = 0.32Particle dry size = 5 µm, Temp. = 288 K
NaCl/H2O
C6E2/NaCl/H2O
C6E2Diethylene glycol monohexly etherCH3(CH2)5(OCH2CH2)2OH
Chemical properties
7.08x10-8
57.2
74.58
0.970
306.44
C16H34O5
CH3(CH2)7(OCH2CH2)4OH
C8E4
[Tetraethylene glycol
monooctyl ether]
1.82x10-3
38.7
57.80
0.933
190.28
C10H22O3
CH3(CH2)5(OCH2CH2)2OH
C6E2
[Diethylene glycol
monohexyl ether]
Properties
Chemical formular
Molecular weight
Density
[g/cm3]
Enthalpy of vaporization
[kJ/mol]
Polar surface area
[Å2]
Vapor pressure
[Torr]
Experimental setup
Scattering pattern of particle
Liquid
Solid
Image of particle
Liquid
Solid
Electrodynamic balance : EDB
Ref: Zardini et. al., Optical Express 2006
Experimental setup
Electrodynamic balance : EDB
Methods to characterize aerosol particle
1. The DC voltage applied to compensate the gravitational force is proportional to and a measure for the mass of the particle.
2. Mie phase functions are used to deduce the radius of the particle and to detect phase changes.
3. Raman spectroscopy is employed to measure the particle’s composition.
4. The radius of a liquid (spherical) particle can be measured withhigh precision using a tunable diode laser or a LED-“white”-light source [Zardini et al., 2006] and analyzing the Mie resonance spectra.
5. Temporal light-scattering fluctuations and spatial asymmetry in the 2D scattering patterns are used to deduce solid-to-liquid partitioning as well as phase changes and morphology of the aerosol particle.
Different pathways through phase diagram
Mie Resonance Modeling
λπr
xtcoefficienMie2==
r1
r2m1
m2
r1 = core radius
r2 = shell radius
m1 = refractive index in core
m2 = refractive index in shell
Ref. Kaiser et. al., Computer in Physics, 1993
Phase transition modeling
Time [a.u.] Time [a.u.]
Preliminary Experiment from EDB of C 8E4/NaCl/H2O system
Low surfactant concentration
Higher concentration
Critical Micelle concentration
Hydrophobic tail
Hydrophilic head
Micelles
Micelle formation
Critical Micelle Concentration (CMC)
Surfactant concentration
EDB Preliminary Experiment of C8E4/NaCl/H2O system
35000 42000 49000 56000 63000 700004.5
5.0
5.5
6.0
6.5
U [
V]
t [s]
600
590
580
570
560
R
H[%
]λλ λλ
[nm
]
929496
98100102104
35000 42000 49000 56000 63000 70000
C8E
4/H
2O
0 20000 40000 60000 80000 100000120
140
160
180
200
220
240
C8E
4/NaCl/H
2O
t [s]
U [
V]
600
590
580
570
560
RH
[%
]λλ λλ
[[ [[ nm
]
92949698
100102104
0 20000 40000 60000 80000 100000
Step size analysis
13.04
13.06
13.08
13.10
13.12
16000 18000 20000 22000 24000 2600095.4
95.6
95.8
96.0
96.2
191
192
193
194
195
19616000 18000 20000 22000 24000 26000
(b)
∆r = 1.9 nm
r [µ
m]
∆r = 6.3 nm
∆t = 4285 s
(c)
t [s]
RH
[%]
U [V
]
(a)
Radius Step Size
0 5 10 15 20 25 300
20
40
60
80
100
occu
renc
e
radius step size [nm]
13 to 15 µm particle
0.0 5.0x106 1.0x107 1.5x107 2.0x1070
20
40
60
80
100
occu
renc
e
number of C8E
4 molecules
# C8E4
Number of C8E4 molecules
StepRadiusRadius
StepSurface
0.04 0.06 0.08 0.10 0.12 0.140
20
40
60
80
100oc
cure
nce
equivalent sphere radius [µm]
equivalent sphere size
Equivalent Sphere Size
Conclusion
• We speculate that the discontinuous, step like, growth is causedby disaggregation of a micelle needed to conserved the monolayerof surfactant molecules on the aqueous aerosol particle surface upon growing.
• We did not observe the discontinuous growth with binary C8E4/water particles.
• We intend to perform further experiments to investigate the influence of NaCl/C8E4 ratio on size distribution of micelles.
Take home messages
• When a liquid-liquid phase separation occurs, the inorganic salt is redistributed according to the solubility. If the organic substance is a surfactant, it will lower the surface tension and micelles may be formed if the concentration of the organic exceeds a certain limit.
• The discontinuous growth is caused by disaggregation of a micelle needed to conserved the monolayer of surfactant molecules on theaqueous aerosol particle surface upon growing.
• The concentration of salt in the particle might have an influence to size distribution of micelles.
Thanks :Thanks :
Prof. Thomas PeterProf. Thomas PeterDr. Ulrich Krieger Dr. Ulrich Krieger Dr. Claudia MarcolliDr. Claudia MarcolliFriends from Atmospheric Chemistry GroupFriends from Atmospheric Chemistry GroupInstitute for Atmospheric and Climate ScienceInstitute for Atmospheric and Climate ScienceETH Zurich, SwitzerlandETH Zurich, Switzerland
Reference: Marcolli, C., Luo, B., &. Peter, T. (2004). J. Phys. Chem. A, 108, 2216-2224.Zardini, A.A., Krieger, U.K., & Marcolli, C. (2006) . Optics Express, 14, 6951-6962.
