09_microemulsions (1)

8/2/2019 09_microemulsions (1)

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Microemulsions


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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 < 100 nm) of water in oil or oil in water,

depending on the nature of the surfactant.

Microemulsions


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Microemulsions and (Macro) emulsions are Different

Microemulsion Macroemulsion

Thermodynamically Stable Kinetically Stable

Droplet size 10 - 100 nm (transparent) 1-10 m (opaque)

High surface area: 200 m2/g Low surface area: 15 m2/g

Ultra Low O/W IFT (10-210-3 mN/m) O/W IFT 1-10 mN/m

W/O , O/W and Bicontinuous types W/O or O/W types

Forms at CPP = 1 Forms at CPP > or < 1


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Formation of Microemulsions*

2

Oil

1 2 3mG G G G T S

Gm = free energy change for microemulsion formation

G1 = free energy change due to increase in total surface area

G2=

free energy change due to interaction between dropletsG3= free energy change due to adsorption of surfactant at the

oil/water interface from bulk oil or water

S = increase in entropy due to dispersion of oil as droplets

* Ruckenstein, E, Chi, J. C., Faraday Trans. II71 (1975) 1690.


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Stability of MicroemulsionsWhy are microemulsions thermodynamically stable?*

* Ruckenstein, E, Chi, J. C., Faraday Trans. II71 (1975) 1690.

NOTE: Microemulsions form spontaneously only

when IFT is small. (order of 10-3 mN/m)

A

B

D

CR*

Gm*

Gm

R

Gm* < 0 for A & B in certain R range

microemulsion formation in that R range

Gm = G1 + G2 + G3 - TSUnstable

Stable

Gm > 0 for C & D emulsion formation

0

+ve

-ve


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Watch the apparent dichotomy !

21 R

1

R

1

P

The Young Laplace Equation predicts an inverse relation of

pressure drop with droplet radius

2G V

R

Further, free energy arguments should predict, a rapid

coalescence of droplets that are < 100 nm

Then why are microemulsions thermodynamically stable?


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The penalty for the apparent increase in free energy is

compensated by the lowering of IFT to ultra lowlevels (10-210-3 mN/m)

The work done in lowering IFT is achieved through again in system entropy S due to the creation of a

large number of sub-micron sized droplets

Microemulsions are formed because ...

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 packOccurs

when surfactants orient vertically !


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Formulating MicroemulsionsBancrofts rules

Surfactant

WaterOil

Surfactant

WaterOil

(CPP < 1)O/W emulsion

(CPP > 1)W/O emulsion

Change in variables (T, Salting out electrolyte etc.)


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Formulating MicroemulsionsBancrofts rules

Surfactant

WaterOil

3 phase domain

(oil - E -water)(Bicontinuous E)

1 phasemicroemulsion

CPP = 1

W/O or O/Wmicroemulsion

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

WaterOil

3 phase domain

(oil - m -water)(Bicontinuous E)

1 phasemicroemulsion

CPP = 1

W/O or O/Wmicroemulsion

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

WaterOil

Surfactant

WaterOil

Surfactant

WaterOil

T1 T2 T3


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Some Definitions / Conventions

Surfactant

WaterOil

Surfactant

WaterOil

Surfactant

WaterOil

T1 T2 T3Winsor I Winsor III Winsor II

Lower phase E Middle/ 3 phase E Upper phase E


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SalinityOil chain length

Alcohol conc.

Temperature

Variables to play with

Total surfactantbrine/oil ratio

surfactant/oil ratio

M Wt. of surfactant

Manipulating Micro-Emulsions


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Interfacial Tension with Salinity


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For middle phase microemulsion,

1 mol CaCl2 16-19 moles of NaCland for oil-external microemulsions,

1 mol CaCl2 4 moles of NaCl

Values of optimal salinity :

LiCl > NaCl > KCl > NH4Cl

At phase inversion, partition coefficient is near unity.

Repulsion forces between micelles decreases due to theneutralization of surface charge of micelles bycounterions.

Solubilization


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Enhanced Oil Recovery by Microemulsion Flooding

Proper selection of chemicals in formulating the

surfactant slug.

Surfactant formulation- surfactant, alcohol and brinewith or without added oil.

Correlation between interfacial tension and interfacial

charge.


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Even at very low surfactant concentration, microscopic amount of

middle phase remains.

Oil recovery maximum near the optimal salinity of the system.

V

ol.fractionofmiddlephas

e

TRS 10-410 wt. %



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For high salinity reservoirs, mixed surfactants are promisingfor 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


i i i i i


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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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Microstructures of sintered

YBa2Cu3O7-x

prepared by W/O

microemulsion

Microstructures of Pd, Pt nanoparticlesprepared using W/O microemulsion

Microstructure of microemulsion method synthesis


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Pluronic

Fatty Acid

Oil

Water

Drug Detoxification by Microemulsion

Drug


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Test on Isolated Guinea-pig Heart


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1) Control 2) Bupivacaine 3) Bupivacaine + ME

Q S

R

Experiment Detail:Measured QRS interval

Infused Bupivacaine (5 M) to cause cardiotoxicity

(QRS prolongation ~ 20 msec)

Test on Isolated Guinea-pig Heart


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Isolated Guinea Pig Heart - Results

Type Commercial

EmulsionME-3 ME-6

Oilconcentration

1% 1% 0.01%

Particle size(nm)

~ 430 ~110 ~ 25

Removaltoxicity

21% 90% 72%


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De-Emulsification

Cotton, wool, glass fibers and teflon to promotecoalescence

Addition of acid/base neutralizes the particle charge

and leads to coagulation

Application of high voltages

Heating of emulsion.

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09_microemulsions (1)