5.5 Pickering emulsions

Pickering emulsions are a unique type of emulsion stabilized by solid particles instead of surfactants. These emulsions offer enhanced stability and robustness, making them valuable in various industries from food to pharmaceuticals.

Understanding Pickering emulsions involves exploring particle characteristics, preparation methods, and applications. Key factors include particle size, shape, and wettability, which influence emulsion properties and stability. This knowledge is crucial for developing innovative, stable formulations.

Definition of Pickering emulsions

• Pickering emulsions are a type of emulsion stabilized by solid particles that adsorb at the oil-water interface
• Differ from traditional emulsions stabilized by surfactants or polymers
• Named after S.U. Pickering who first described the phenomenon in 1907

Key characteristics of Pickering emulsions

Stabilization by solid particles

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• Solid particles adsorb at the oil-water interface creating a mechanical barrier that prevents droplet coalescence
• Particle adsorption is irreversible due to high energy of attachment leading to enhanced emulsion stability
• Stabilization mechanism depends on particle wettability, size, shape, and concentration

Particle size and shape effects

• Smaller particles (nanometer to micrometer range) are more effective stabilizers due to higher surface area and coverage
• Anisotropic particles (rods, ellipsoids) can provide better stabilization compared to spherical particles
• Particle size distribution influences emulsion droplet size and polydispersity

Contact angle and wettability

• Contact angle of particles at the oil-water interface determines their position and stabilizing efficiency
• Hydrophilic particles (contact angle < 90°) stabilize oil-in-water emulsions while hydrophobic particles (contact angle > 90°) stabilize water-in-oil emulsions
• Particles with intermediate hydrophobicity (contact angle close to 90°) are most effective stabilizers

Comparison of Pickering vs surfactant-stabilized emulsions

Stability and robustness

• Pickering emulsions exhibit higher stability against coalescence and Ostwald ripening compared to surfactant-stabilized emulsions
• Solid particles provide a more robust interfacial layer resistant to environmental stresses (pH, temperature, ionic strength)
• Pickering emulsions can be stable for months to years while surfactant-stabilized emulsions often break down within days to weeks

Interfacial structure and properties

• Solid particles form a densely packed layer at the oil-water interface with unique viscoelastic properties
• Particle-laden interfaces have higher interfacial elasticity and viscosity compared to surfactant-stabilized interfaces
• Interfacial rheology of Pickering emulsions can be tuned by varying particle properties and concentration

Preparation methods for Pickering emulsions

High-energy emulsification techniques

• Include rotor-stator homogenizers, high-pressure homogenizers, and ultrasonic emulsification
• Provide intense disruptive forces to break up droplets and disperse particles leading to smaller droplet sizes
• Require optimization of operating parameters (energy input, time) and formulation (particle concentration, oil-to-water ratio)

Low-energy emulsification techniques

• Exploit physicochemical properties of the system to spontaneously form emulsions with minimal energy input
• Examples include phase inversion temperature (PIT) method and emulsion inversion point (EIP) method
• Suitable for sensitive ingredients and scalable production but limited control over droplet size and distribution

Factors affecting emulsion formation

• Particle hydrophobicity, size, and concentration influence emulsion type (O/W or W/O), droplet size, and stability
• Oil phase composition (polarity, viscosity) and volume fraction determine ease of emulsification and final emulsion properties
• Aqueous phase pH, ionic strength, and presence of co-stabilizers (surfactants, polymers) can modulate particle interactions and emulsion stability

Particles used in Pickering emulsions

Inorganic particles

• Include silica, clay minerals (montmorillonite, laponite), metal oxides (titanium dioxide, iron oxide), and carbon-based materials (graphene oxide, carbon nanotubes)
• Offer high mechanical strength, thermal stability, and chemical resistance
• Can be synthesized with controlled size, shape, and surface chemistry

Organic particles

• Include cellulose nanocrystals, starch granules, chitosan, and protein-based particles (zein, whey protein)
• Derived from renewable sources and offer biocompatibility and biodegradability
• Sensitive to environmental conditions (pH, temperature) and may require chemical modification for improved stability

Surface modification of particles

• Particle surface chemistry can be tailored through chemical or physical methods to optimize wettability and interfacial activity
• Examples include silanization of silica particles, grafting of polymers (PEG, PMMA), and adsorption of surfactants or polyelectrolytes
• Allows fine-tuning of particle hydrophobicity, charge, and steric stabilization for specific applications

Characterization techniques for Pickering emulsions

Microscopy methods

• Optical microscopy provides qualitative information on emulsion microstructure, droplet size, and stability
• Confocal laser scanning microscopy (CLSM) enables 3D visualization of particle distribution and droplet packing
• Cryogenic scanning electron microscopy (cryo-SEM) allows high-resolution imaging of particle-stabilized interfaces in their native state

Scattering techniques

• Dynamic light scattering (DLS) measures droplet size distribution and zeta potential in dilute emulsions
• Small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) probe interfacial structure and particle organization at the nanoscale
• Diffusing wave spectroscopy (DWS) monitors emulsion stability and viscoelastic properties in concentrated systems

Rheological measurements

• Steady-shear and oscillatory rheology characterize flow behavior and viscoelastic properties of Pickering emulsions
• Interfacial shear rheology probes the mechanical properties of particle-laden interfaces and correlates with emulsion stability
• Microrheology techniques (particle tracking, diffusing wave spectroscopy) measure local viscoelastic properties and heterogeneity in emulsions

Applications of Pickering emulsions

Food and beverage industry

• Used in the formulation of low-fat spreads, sauces, dressings, and dairy products
• Particles can replace or reduce the amount of synthetic emulsifiers and stabilizers
• Provide enhanced stability, texture, and sensory properties

Pharmaceuticals and drug delivery

• Serve as carriers for controlled release and targeted delivery of drugs, vitamins, and bioactive compounds
• Protect sensitive ingredients from degradation and improve bioavailability
• Examples include Pickering emulsion-based gels, creams, and injectable formulations

Cosmetics and personal care products

• Employed in skin care, hair care, and sunscreen products
• Offer improved sensory properties, long-term stability, and water resistance
• Particles can provide additional benefits such as UV protection, antioxidant activity, and skin conditioning

Oil and gas industry

• Used in enhanced oil recovery (EOR) processes to improve oil displacement and recovery efficiency
• Particles can stabilize oil-in-water emulsions and modify rock wettability
• Potential for CO2 sequestration and reduction of environmental impact

Challenges and future perspectives in Pickering emulsion research

Scalability and industrial production

• Need for cost-effective and large-scale production methods for particles and emulsions
• Optimization of processing parameters and formulation for consistent quality and performance
• Integration with existing industrial infrastructure and processes

Environmental and safety considerations

• Development of eco-friendly and biocompatible particles from renewable sources
• Assessment of particle toxicity, biodegradability, and environmental fate
• Compliance with regulatory guidelines and safety standards for specific applications

Novel particle development

• Design of stimuli-responsive particles for triggered release and dynamic emulsion behavior
• Exploration of hybrid particles combining inorganic and organic components for synergistic effects
• Development of multifunctional particles with additional properties (antimicrobial, antioxidant, optical)