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David BartonBaldwin High School
Pittsburgh, PAScience and Engineering Apprenticeship Program
Naval Research LaboratoryWashington, DC
Mentor: Dr. David A. Kidwell
10 August 2007
Effects of Counterions on Reverse Micelles
www.bioeng.cstm.kyushu-u.ac.jp
Background
• Reverse micelles form when water
is added to a surfactant in an
organic solvent
• The water core has slightly altered
properties compared to bulk water
because the water bonds with the
surfactant headgroup
S u r f a c t a n t
R e v e r s e M i c e l l e ( w a t e r i n o i l )
Jeff Owrutsky, 2007
The water structure of RMs are still widely unknown. Determining the
properties of the water core will help us understand what applications they
can be used for…
Infrared Spectroscopy
Fourier Transform Infrared (FTIR) Spectroscopy
Reverse Micelle Probes
• Research has shown that in different
surfactants, as more water is added to
the reverse micelles, the ion probe in
water shifts in frequency
• Traditionally, water is less polar in
RMs than bulk water.
– Hydrogen bonding is limited due to
counterion associations with the water
– Anomalous blue shift is seen with AOT
– More polar??
w0
Sando et al.
Azide
Ion-Pairing in Bulk DMSO
M(OCN) FTIR Spectra
Li(OCN)Na(OCN)
K(OCN)
OCN-
• We tested ion-pairs in
bulk solvent, using
dimethyl sulfoxide
(DMSO) as a solvent
to find the frequencies
at which these ion-
pairs vibrate using
FTIR
Ion-Pair Equilibrium ConstantsM+ + OCN- M(OCN)
Frequency of Cation vs. Binding Constant
0
5
10
15
20
25
30
35
40
2140 2145 2150 2155 2160 2165 2170 2175
Frequency (cm-1)
Bin
din
g C
on
stan
t (M
-1)
• Binding constant is found to
be proportional to absorbance
frequency and charge-density
of the cation
• J. Rannou and M. Chabanel
performed the same
calculations
– JR and MC values were
linear with current results.
K
Na
Li
214521572169Frequency
16.446.5110.1JR and MC Keq
9.031.078.8Average Keq
K+Na+Li+ M+
Reverse Micelles
• We created reverse
micelles with the isocyanate
ion probe using different
molar water concentrations,
w0 ( = [H2O] / [surf.] ), to see
how this affected the
spectral shift of the probe
compared to other w0 values
OCN-
Surfactant (AOT)
• Hypothesis: Ion-exchanging sodium for other counterions (Li+, K+, NH4
+) may cause this unusual shift to lessen or reverse
AOT
-Performed by a liquid-liquid extraction.
Changing w0 in Li(AOT)
0
0.2
0.4
0.6
0.8
1
1.2
2000 2050 2100 2150 2200 2250 2300
Wavenumbers (cm-1)
Ab
so
rba
nc
e (
u)
w = 1
w = 2
w = 4
w = 6
w = 8
w = 10
w = 12
w = 14
w = 18
w = 22
Bulk Water
Initial Water Concentration
• Initial w0 was calculated using standard addition by integrating the water O-H peaks in the spectra for each known added quantity
Water Concentration vs. Water's Integrated Absorbance
y = 35.087x + 128.48
R2 = 0.9993
0
100
200
300
400
500
600
0 2 4 6 8 10 12
w0
Inte
gra
ted
Ab
so
rba
nc
e o
f W
ate
r R
eg
ion
initial w0 = 3.7
56 um spacers NH4(AOT) Background: Isooctane
Generated Data
Molar Water Concentration vs. Frequency
2165
2170
2175
2180
2185
2190
2195
2200
0 5 10 15 20
w0
vm
ax (c
m-1
)
Na(AOT) - 1
Li(AOT) - 2
NH4(AOT)
K(AOT)
Linear (BulkWater)
w0
Azide
Cyanate
Conclusions
• At low w0, there is less hydrogen-
bonding in water because of the strong
attraction between the AOT headgroup
and the polar water molecule
• Using NH4(AOT) and K(AOT) the water
in the reverse micelles becomes more
bulk-like in bonding, as observed by the
isocyanate ion probe
Levinger, Science 2002
Acknowledgements
• Dr. David A. Kidwell, Mentor
• Dr. Jeff C. Owrutsky
• Dr. Michael B. Pomfret
• Science and Engineering Apprenticeship Program
Questions?