Pickering Emulsions

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Pickering Emulsions
Interfacial Phenomena and Nanostructured
Materials April 17th, 2007 Raghavendra
Devendra Purushottam Dixit Terrence
Dobrowsky Bryan Grabias
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Pickering Emulsions

• Emulsions
• One liquid dispersed in another liquid as
  droplets
• Consist of three phases
• Liquid A, Liquid B, Emulsifying Agent
• Typically surfactant, The water, oil and soap
  scenario.
• Pickering Emulsions
• Use solid particles to stabilize emulsions

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Significance of Pickering Emulsions

• Already used in
• Crude oil recovery, oil separation, cosmetic
  preparation and waste water treatment
• Convenient model system for solid particles at
  liquid-liquid interfaces
• Utilization of self assembly
• Generate well-defined three-phase system for
  evaluation of particle interactions
• Easily changeable interfacial properties
• Visualization of 2D particle diffusion

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Self Assembly

• Widely used as a catch phrase
• Not considered here as synonymous with formation
• Here we consider pre-existing components with
  direct relationship to design
• Fabrication of nano microstructured components
• Filling the gap for sizes manipulated by
  chemistry and manufacturing

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Discussing the Topic

• Equilibrium state related to surface tension and
  system geometry
• System properties modified easily
• Utilizing system stability
• Simple assumptions for easily modeling transient
  equilibrium particle interactions
• New uses for the system through greater
  understanding of governing equations

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Pickering Particle Stabilizers

• Particle Criteria
• Size
• Solid Concentration
• Phase Volume Fraction

• Examples
• Alumina w/o
• Botonite o/w
• Fat crystals w/o
• Magnesium oxide w/o
• Magnesium trisilicate w/o
• Titanium dioxide (coated) o/w, w/o
• Silica o/w
• Tin oxide o/w

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Criteria for Particles at Interface

• Partial dual wettability
• , hydrophilic , o/w emulsions
• , hydrophobic, w/o emulsions
• , maximum stabilization

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Adsorption Energetics

• Not perturbed by thermal activation
• Irreversible adsorption

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Stabilized Particles

• Partitioning of particles between phases

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Kinetics and Interactions

• Van der Waals
• Solvation
• Electrostatic
• Undulations to Aggregation
• Electrodipping

• Aggregation
• Attraction/Repulsion

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Stability of Pickering Emulsions

• Pickering emulsions are thought to be stable than
  normal emulsions
• Particles present at the liquid liquid interface
  tend to minimize the interface interaction by
  volume exclusion
• Particles have mobility on the interface but the
  motion normal to the interface is highly
  suppressed A typical case of 2-D diffusion
• What is the magnitude of this stability with
  respect to kT units

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Thermodynamic Modeling

• Assumptions
• The formation of pickering emulsion is a
  multi-scale process
• Rate phenomena are fast compared to
  reorganization and stabilization
• Additional assumptions required for modeling
• Molecular interactions are small ranged compared
  to particle size
• All interactions are clubbed in three surface
  tension parameters
• Problems with surface tension formulation

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Single Particle Potential Well
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Consequences and limitations

• Absolute values of free energy barriers are
  larger than thermal fluctuation
• Analysis is not valid for small particle
• Molecular forces become important
• Analysis not valid for dense system
• Effects of particle particle interactions
• Curvature effects
• Curvature plays a role in droplet stability and
  area estimation

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Effect of Curvature and Inter-droplet Repulsion

• Work of interfacial deformation

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Effect of curvature contd.

• Closed packed and sparsely packed droplets will
  repel each other
• Visualization of the contact angle

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Thermodynamic Criterion

• Work done for 1-2 system
• Work done for 2-1 system
• Where

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Thermodynamic Criterion (contd.)
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Particle Interactions
Repulsion
Attraction

• Ordering of particles at surface is summation of
  repulsive electrostatic interactions and
  attractive forces
• Controversy exists regarding the observance of
  long-range attraction between like-charged
  particles

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Pickering Emulsions Current Trends

• Interest in Pickering emulsions has recently been
  spurred by the potential to create novel
  materials. . .

. . . like semipermeable capsules for drug
delivery, food additives, or biomedical
applications
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Current Trends Colloidosomes

• Can create capsules with a wide array of
  interesting properties by tailoring particle
  chemistry
• Hydrogel particles swell with water at lower
  temperatures
• Reversible over a modest temperature range

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Current Trends Advantages over other
Encapsulation Technologies

• Lack of toxic solvents/surfactants enhances
  biocompatibility
• Very flexible technology
• Size, permeability, and mechanical strength
  can be modified easily
• Allows a variety of material options
• Ease of synthesis
• Geometry not limited to spherical particles

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Conclusions

• Excellent model system
• Energetics of particle adsorption well understood
• Particle interactions at the phase boundary still
  being explored
• Relevant in many industries (cosmetics, oil, etc)
• Finding application in controlled release
  technologies and in the creation of novel
  materials

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