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?