Microemulsions

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Microemulsions
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Microemulsions

• Microemulsions are thermodynamically stable,
  optically transparent, isotropic dispersions of
  aqueous and hydrocarbon liquids stabilized by an
  interfacial film of surfactant molecules
• Microemulsions are monodispersed spherical
  droplets (diameter lt 100 nm) of water in oil or
  oil in water, depending on the nature of the
  surfactant.

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Microemulsions and (Macro) emulsions are Different
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Formation of Microemulsions
DGm free energy change for microemulsion
formation DG1 free energy change due to
increase in total surface area DG2 free energy
change due to interaction between droplets DG3
free energy change due to adsorption of
surfactant at the oil/water
interface from bulk oil or water DS increase
in entropy due to dispersion of oil as droplets
Ruckenstein, E, Chi, J. C., Faraday Trans. II
71 (1975) 1690.
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Stability of MicroemulsionsWhy are
microemulsions thermodynamically stable?
?Gm gt 0 for C D ? emulsion formation
ve
? Gm ? G1 ? G2 ? G3 - T?S
Unstable
0
Stable
-ve
?Gm lt 0 for A B in certain R range
? microemulsion formation in that R range
NOTE Microemulsions form spontaneously only when
IFT is small. (order of 10-3 mN/m)
Ruckenstein, E, Chi, J. C., Faraday Trans. II
71 (1975) 1690.
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Watch the apparent dichotomy !
The Young Laplace Equation predicts an inverse
relation of pressure drop with droplet radius
Further, free energy arguments should predict, a
rapid coalescence of droplets that are lt 100 nm
Then why are microemulsions thermodynamically
stable?
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Microemulsions are formed because ...

• The penalty for the apparent increase in free
  energy is compensated by the lowering of IFT to
  ultra low levels (10-2 10-3 mN/m)
• The work done in lowering IFT is achieved through
  a gain in system entropy ?S due to the creation
  of a large number of sub-micron sized droplets

All this occurs when at molecular levels,
surfactants form the most condensed interfacial
film between oil and water
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Microemulsions form when CPP 1
The most condensed interfacial film between oil
and water is formed when maximum number of
surfactants pack Occurs when surfactants orient
vertically !
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Formulating Microemulsions Bancrofts rules
(CPP lt 1) O/W emulsion
(CPP gt 1) W/O emulsion
Change in variables (T, Salting out electrolyte
etc.)
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Formulating Microemulsions Bancrofts rules
Surfactant
3 phase domain (oil - ?E -water)
(Bicontinuous ?E)
1 phase microemulsion
Water
Oil
W/O or O/W microemulsion
CPP 1
Sometimes, a co-surfactant such as a short chain
alcohol is used in conjunction with the
surfactant to facilitate condensed interfacial
film formation
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But! Watch what we just did !
Surfactant
3 phase domain (oil - ?m -water)
(Bicontinuous ?E)
1 phase microemulsion
Water
Oil
W/O or O/W microemulsion
CPP 1
We just happened to bring OIL AND WATER, 2
IMMISCIBLE LIQUIDS, INTO 1 SINGLE PHASE
See the Potential for Enhanced Oil Recovery?
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Tracking phase change using test tubes
Surfactant
Surfactant
Surfactant
Water
Oil
Water
Oil
Water
Oil
T3
T1
T2
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Some Definitions / Conventions
Surfactant
Surfactant
Surfactant
Water
Oil
Water
Oil
Water
Oil
T3
T1
T2
Winsor I
Winsor II
Winsor III
Lower phase ?E
Middle/ 3 phase ?E
Upper phase ?E
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Manipulating Micro-Emulsions
Variables to play with

• Salinity
• Oil chain length
• Alcohol conc.
• Temperature

Total surfactant brine/oil ratio surfactant/oil
ratio M Wt. of surfactant
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Interfacial Tension with Salinity
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Solubilization

• For middle phase microemulsion,
• 1 mol CaCl2 ? 16-19 moles of NaCl
• and for oil-external microemulsions,
• 1 mol CaCl2 ? 4 moles of NaCl
• Values of optimal salinity
• LiCl gt NaCl gt KCl gt NH4Cl
• At phase inversion, partition coefficient is
  near unity.
• Repulsion forces between micelles decreases due
  to the neutralization of surface charge of
  micelles by counterions.

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Enhanced Oil Recovery by Microemulsion Flooding

• Proper selection of chemicals in formulating the
  surfactant slug.
• Surfactant formulation- surfactant, alcohol and
  brine with or without added oil.
• Correlation between interfacial tension and
  interfacial charge.

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Enhanced Oil Recovery by Microemulsion Flooding
Vol. fraction of middle phase
TRS 10-410 wt.

• Even at very low surfactant concentration,
  microscopic amount of middle phase remains.
• Oil recovery maximum near the optimal salinity of
  the system.

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Enhanced Oil Recovery by Microemulsion Flooding

• For high salinity reservoirs, mixed surfactants
  are promising for enhanced oil recovery.
• Electrophoretic Mobility Maximum mobility
  corresponds to minimum in interfacial tension at
  the crude oil/caustic interface.
• Transient Processes
• At optimal salinity
• Fastest coalescence occurs
• Minimum in pressure jump
• Minimum in apparent viscosity

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• Various phenomena occurring at the optimal
  salinity

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Production of Complex Oxides/ nanoparticles
through microemulsions

• W/O microemulsions provide a novel vehicle for
  synthesis of a micro-particulate oxalate
  precursor which yields very high density sintered
  pellets of YBa2Cu3O7-x.
• Steric barrier by surfactant monolayer restricts
  the growth of precipitated particles and hinders
  intergrain coagulation.

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• Schematic of W/O microemulsion and the reaction
  mechanism

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Microstructure of microemulsion method synthesis

• Microstructures of sintered YBa2Cu3O7-x
• prepared by W/O microemulsion

Microstructures of Pd, Pt nanoparticles
prepared using W/O microemulsion
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Drug Detoxification by Microemulsion
Drug
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Test on Isolated Guinea-pig Heart
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Experiment Detail Measured QRS intervalInfused
Bupivacaine (5 µM) to cause cardiotoxicity (QRS
prolongation 20 msec)
Test on Isolated Guinea-pig Heart
R
Q
S
2) Bupivacaine
1) Control
3) Bupivacaine ME
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Isolated Guinea Pig Heart - Results
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De-Emulsification

• Cotton, wool, glass fibers and teflon to promote
  coalescence
• Addition of acid/base neutralizes the particle
  charge and leads to coagulation
• Application of high voltages
• Heating of emulsion.