Surfactant mediated charging and electrostatic
particle interaction in nonpolar dispersions
85th ACS Colloid and Surface Science Symposium
Montreal, June 21, 2011
Qiong Guo, Crystal C. Clemmons, Carlos E. Espinosa, and Sven H. Behrens
Georgia Institute of Technology
School of Chemical & Biomolecular Engineering
www.chbe.gatech.edu/behrens
The problem of introducing ions in a nonpolar liquid
2a
+
-
+
Electrostatic self-energy
of a monovalent ion:
U B  e2 8 0 a
+
-
Bjerrum length:
U B B

kT 2a
Thermal energy scale
B  e2 4 0 kT
(water: 0.7 nm,
hexane: 29 nm)
Ion size
Probability of charging exceedingly low in oils…
BUT: surfactant additives (“charge control agents”) can
• dramatically raise the electric conductivity
• promote particle charging
prevent explosion hazards
used in electrophoretic
displays (Kindle etc.)
due to flow electrification
Case of ionic surfactants
CMC
AOT
(in hexadecane)
S. K. Sainis, J.W. Merrill, E.R. Dufresne, Langmuir 24, 13334 (2008).
Case of ionic surfactants
Disproportionation:
-
(in hexadecane)
S. K. Sainis, J.W. Merrill, E.R. Dufresne, Langmuir 24, 13334 (2008).
Case of ionic surfactants
Dissociation:
-+
+
+
S. K. Sainis, J.W. Merrill, E.R. Dufresne, Langmuir 24, 13334 (2008).
Case of ionic surfactants
CMC
I.
II. Transition
III.
S. K. Sainis, J.W. Merrill, E.R. Dufresne, Langmuir 24, 13334 (2008).
Particle charging with ionic surfactants
Electrostatic surface potential of
PMMA microparticles in
AOT/dodecane
Significant surface charging only
for C > CMC.
Kemp, R., Sanchez, R., Mutch, K. J., Bartlett, P., Langmuir 26, 6967 (2010).
Hypothesized mechanism for particles charging
-
1 M.
-+
-+
-
dissociation of
surface groups
due to acid-base
interaction with
the surfactant3
dissociation
of individually
adsorbed
surfactant
molecules2
+
asymmetric
adsorption
of charged
micelles1
Micelles as
charge acceptors
+
F. Hsu, E. R. Dufresne, D. A. Weitz, Langmuir 21, 4881 (2005); G. S. Roberts, R. Sanchez, R. Kemp, T. Wood, P. Bartlett, Langmuir 24, 6530 (2008).
Kemp, R. Sanchez, K. J. Mutch, P. Bartlett, Langmuir 26 (10), 6967-6976 (2010).
3 S. Poovarodom and J. C. Berg, J. Colloid Interface Sci. 346 (2), 370-377 (2010).
2 R.
Nonionic surfactants
A. S. Dukhin; P. J. Goetz, J. Electroanal. Chem. 588, 44 (2006).
Nonionic surfactants
Span 85: mixture of sorbitan trioleate and tetra-oleate, HLB ~ 1.8
Span 85
A. S. Dukhin; P. J. Goetz, J. Electroanal. Chem. 588, 44 (2006).
Span 85 in hexane
Spherical micelles above a CMC of ~10
mM (DLS diameter and interfacial tension)
CMC
Linear conductivity increase in
hexane above and below the CMC.
Q. Guo, V. Singh, SHB, Langmuir 26, 3203 (2010).
Deliberate „contamination“ with ionizable impurities
Adding large amounts of the most likely ionic impurity does not increase conductivity!
Does this mean, ionizable impurities play no role?
Q. Guo, V. Singh, SHB, Langmuir 26, 3203 (2010).
Hypothesized charging mechanism
C < CMC
2
Ionizable impurity
C > CMC
2
Two steps: 1) inclusion of impurity in surfactant micelle or pre-micellar complex
2) charge disproportionation
rate limiting step!
Zeta potential of supended PMMA sulfate particles
10
0
-10
-20
0.52 m
0.11 m
C. E. Espinosa, Q. Guo, V. Singh, SHB, Langmuir 26, 16941 (2010).
Zeta potential of supended PMMA sulfate particles
Particle charge before and
after solvent replacement:
repeated dilution & centrifugation
Aqueous
dispersion
Particles in
alcohol
C. E. Espinosa, Q. Guo, V. Singh, SHB, Langmuir 26, 16941 (2010).
Particles in
hexane/Span 85
Zeta potential of supended PMMA sulfate particles
Particle charge before and
after solvent replacement:
repeated dilution & centrifugation
Aqueous
dispersion
Particles in
alcohol
Particles in
hexane/Span 85
Particle charge in hexane NOT
due to surface headgroups !!
C. E. Espinosa, Q. Guo, V. Singh, SHB, Langmuir 26, 16941 (2010).
Zero field mobility and zeta potential
140
2
0.5
ZETA POTENTIAL  / mV
200
0.4
150
0.4
100 0.3
0.2
50 0.2
0.5 mM
2mM
10 mM
30 mM
-0.2
0
10
1 mM
5 mM
20 mM
50 mM
20
30
0
100
80
Charging
0.6
0.0
120
60
40
CMC
0.1
0.52 m Dia.
0.11 m Dia.
-50
40
50
FIELD STRENGTH / (kV/m)
60
0.0
0
10
20
30
40
CONCENTRATION CSPAN 85 / mM
C. E. Espinosa, Q. Guo, V. Singh, SHB, Langmuir 26, 16941 (2010).
50
20
0
ZETA POTENTIAL  / mV
0.8
-8
-8
ELECTROPHORETIC MOBILITY / (10 m /Vs)
0.52m PMMA
2
ELECTROPHORETIC MOBILITY / (10 m /Vs)
Electrophoretic mobility depends on field strength!
Zero field mobility and zeta potential
140
2
0.5
ZETA POTENTIAL  / mV
200
0.4
150
0.4
100 0.3
0.2
50 0.2
0.5 mM
2mM
10 mM
30 mM
-0.2
0
10
1 mM
5 mM
20 mM
50 mM
20
30
0
100
80
Charging
0.6
0.0
120
60
40
CMC
0.1
0.52 m Dia.
0.11 m Dia.
-50
40
50
FIELD STRENGTH / (kV/m)
60
0.0
0
10
20
30
40
50
CONCENTRATION CSPAN 85 / mM
Particle charging appears
not to require micelles!
C. E. Espinosa, Q. Guo, V. Singh, SHB, Langmuir 26, 16941 (2010).
20
0
ZETA POTENTIAL  / mV
0.8
-8
-8
ELECTROPHORETIC MOBILITY / (10 m /Vs)
0.52m PMMA
2
ELECTROPHORETIC MOBILITY / (10 m /Vs)
Electrophoretic mobility depends on field strength!
Interaction measurements
PMMA coated
glass coverslips
1 m PMMA particles
microscope find 2-d particle locations*
Pair interaction energy
from radial distribution
function and 2-d OrnsteinZernicke integral equation
with hypernetted chain
closure **
*J.C. Crocker and D.G. Grier, J. Colloid Interface Sci. 179, 298 (1996)
**SHB, D.G. Grier, Phy. Rev. E, 88, 050401 (2001)
Insensitivity to plate separation
Interaction measurements
PMMA coated
glass coverslips
1 m PMMA particles
microscope find 2-d particle locations*
Pair interaction energy
from radial distribution
function and 2-d OrnsteinZernicke integral equation
with hypernetted chain
closure **
*J.C. Crocker and D.G. Grier, J. Colloid Interface Sci. 179, 298 (1996)
**SHB, D.G. Grier, Phy. Rev. E, 88, 050401 (2001)
)
Screened Coulomb form confirmed by logarithmic plot
Screened Coulomb Potential
( Z *e) 2 exp(d )
u (r ) 
exp( r )
2
4 0 r 1  d / 2
fits well above and below CMC
Screened Coulomb form confirmed by logarithmic plot
)
Screened Coulomb form confirmed by logarithmic plot
Screened Coulomb Potential
( Z *e) 2 exp(d )
u (r ) 
exp( r )
2
4 0 r 1  d / 2
fits well above and below CMC
Ion size from conductivity and particle interaction
???
Ion size from conductivity and particle interaction
2
dH 
12 2B 
e2
= size of small ion responsible
for conductivity and screening
Compare to DLS result of (2.8 ± 0.2) nm for micelle size
Ion size from conductivity and particle interaction
2
dH 
12 2B 
e2
= size of small ion responsible
for conductivity and screening
Compare to DLS result of (2.8 ± 0.2) nm for micelle size
 consistent with small pre-micellar complex as ionic species
below the CMC.
Conclusions
• Nonionic surfactants can promote charging in nonpolar liquids
• Differences to charging by ionic surfactants:
- linear conductivity increase
- surface charging
- screened Coulomb interaction
even below the CMC!
• Mechanism of surface charging not yet understood:
- micelles not needed
- ionic surfactant group not needed
- particle surface head group possibly irrelevant
- possibly relevant:
(Lewis) acid-base interaction between PMMA and surfactant
Acknowledgements
Virendra
Singh
Carlos
Espinosa
Qiong
Guo
Sven
Behrens
Adriana
San Miguel
The Camille & Henry Dreyfus Foundation