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Questions and Answers
What characteristic defines a stable emulsion according to Garrett?
What characteristic defines a stable emulsion according to Garrett?
Which of the following is NOT a symptom of emulsion instability?
Which of the following is NOT a symptom of emulsion instability?
How does flocculation differ from coalescence in emulsions?
How does flocculation differ from coalescence in emulsions?
What role do electrolytes play in emulsion stability?
What role do electrolytes play in emulsion stability?
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Which factor does NOT influence the reversibility of flocculation?
Which factor does NOT influence the reversibility of flocculation?
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What represents a thermodynamically favorable process in emulsions?
What represents a thermodynamically favorable process in emulsions?
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What term describes the irreversible aggregation of emulsion droplets?
What term describes the irreversible aggregation of emulsion droplets?
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What does kinetic stability in emulsions imply?
What does kinetic stability in emulsions imply?
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How can the viscosity of o/w emulsions be increased?
How can the viscosity of o/w emulsions be increased?
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What must be balanced to control the viscosity of emulsions?
What must be balanced to control the viscosity of emulsions?
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What effect does aging have on emulsions?
What effect does aging have on emulsions?
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Which combination is most effective in preventing microbial contamination in emulsions?
Which combination is most effective in preventing microbial contamination in emulsions?
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Which property is expected from a suitable antimicrobial preservative for emulsions?
Which property is expected from a suitable antimicrobial preservative for emulsions?
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What role do clumping or flocculation play in emulsions?
What role do clumping or flocculation play in emulsions?
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Which factors will NOT contribute to emulsion viscosity according to the provided information?
Which factors will NOT contribute to emulsion viscosity according to the provided information?
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According to Stoke's law, how does viscosity influence creaming in emulsions?
According to Stoke's law, how does viscosity influence creaming in emulsions?
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What type of film is characterized by adsorbed surfactant molecules that do not adhere laterally and can move freely?
What type of film is characterized by adsorbed surfactant molecules that do not adhere laterally and can move freely?
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Which type of film is formed when a high concentration of emulsifier leads to a rigid barrier between immiscible phases?
Which type of film is formed when a high concentration of emulsifier leads to a rigid barrier between immiscible phases?
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Which type of surfactant film is associated with molecules having less cohesive interactions, leading to greater expansion?
Which type of surfactant film is associated with molecules having less cohesive interactions, leading to greater expansion?
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What characteristic of interfacial complex condensed films enhances their ability to resist rupture?
What characteristic of interfacial complex condensed films enhances their ability to resist rupture?
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What role do combinations of surfactants play in stabilizing emulsions?
What role do combinations of surfactants play in stabilizing emulsions?
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How does the structure of oleic acid contribute to the formation of expanded films?
How does the structure of oleic acid contribute to the formation of expanded films?
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What is a key feature distinguishing condensed films from gaseous films?
What is a key feature distinguishing condensed films from gaseous films?
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Which surfactants typically produce expanded films due to their structure?
Which surfactants typically produce expanded films due to their structure?
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What is the primary characteristic of surfactants with low HLB values?
What is the primary characteristic of surfactants with low HLB values?
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How is the HLB value of a mixture of two surfactants calculated?
How is the HLB value of a mixture of two surfactants calculated?
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What is the significance of phase inversion temperature (PIT) in emulsion stability?
What is the significance of phase inversion temperature (PIT) in emulsion stability?
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What role do auxiliary emulsifiers play in an emulsion?
What role do auxiliary emulsifiers play in an emulsion?
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Which of the following statements about the HLB value is incorrect?
Which of the following statements about the HLB value is incorrect?
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What happens to surfactants at the phase inversion temperature when emulsions are cooled?
What happens to surfactants at the phase inversion temperature when emulsions are cooled?
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Which of the following is a key requirement for determining the amount of surfactant in an oil-water mixture?
Which of the following is a key requirement for determining the amount of surfactant in an oil-water mixture?
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What is a common misconception regarding the duplication of stable emulsions?
What is a common misconception regarding the duplication of stable emulsions?
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What determines the type of emulsion formed based on the viscosity of each phase?
What determines the type of emulsion formed based on the viscosity of each phase?
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Which type of emulsifier is expected to predominantly form oil-in-water emulsions?
Which type of emulsifier is expected to predominantly form oil-in-water emulsions?
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Micro emulsions are often referred to as which of the following?
Micro emulsions are often referred to as which of the following?
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What type of micro emulsion is characterized by water molecules in the polar central portion of a surfactant micelle?
What type of micro emulsion is characterized by water molecules in the polar central portion of a surfactant micelle?
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Which component is NOT typically included in the formulation of emulsions?
Which component is NOT typically included in the formulation of emulsions?
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What is the role of auxiliary emulsifiers in emulsion formulations?
What is the role of auxiliary emulsifiers in emulsion formulations?
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Which surfactant combination is known to aid in forming a w/o micro emulsion?
Which surfactant combination is known to aid in forming a w/o micro emulsion?
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What is the effect of adding oil to a surfactant solution in micellar form?
What is the effect of adding oil to a surfactant solution in micellar form?
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What is the primary structure formed by mixed emulsifiers interacting with water according to recent findings?
What is the primary structure formed by mixed emulsifiers interacting with water according to recent findings?
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How do hydrophilic colloids stabilize emulsions?
How do hydrophilic colloids stabilize emulsions?
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What happens to the contact angle when finely divided solids are preferentially wetted by water?
What happens to the contact angle when finely divided solids are preferentially wetted by water?
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In an o/w emulsion stabilized by sodium soap, how are the surfactant molecules oriented?
In an o/w emulsion stabilized by sodium soap, how are the surfactant molecules oriented?
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What role does the electrical double layer play in emulsion stability?
What role does the electrical double layer play in emulsion stability?
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Which statement is true regarding the stabilization of emulsions by colloidal materials?
Which statement is true regarding the stabilization of emulsions by colloidal materials?
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What type of emulsion is formed when solid particles are preferentially wetted by oil?
What type of emulsion is formed when solid particles are preferentially wetted by oil?
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What does the interaction of emulsifiers with water indicate about emulsions?
What does the interaction of emulsifiers with water indicate about emulsions?
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Study Notes
Emulsions (Liquid-Liquid system)
- Emulsions are two-phase systems
- Two immiscible liquids
- One liquid is dispersed throughout the other in fine droplets
- Typically stabilized by an emulsifying agent
- Heterogeneous system
- Consist of at least one immiscible liquid dispersed in another liquid
- Droplets have a diameter generally exceeding 0.1 µm
- Thermodynamically unstable system
- Contains at least two immiscible liquid phases
- One phase dispersed as globules in the other
- Stabilized by a third substance (emulsifying agent)
- Exhibit an acceptable shelf life near room temperature
- Internal phase (disperse phase or discontinuous phase)
- Surrounded by an external continuous phase
- Occupies no more than 74% of total emulsion volume
- Emulsifier added to increase droplet longevity in immiscible liquids
- It's a stabilizer of the internal phase droplets (globules)
- Composed of hydrophilic (oleophobic) and hydrophobic (oleophilic) portions
- Often called amphiphilic (water and oil loving)
- Common emulsions involve water and oil or a lipid
Types of Emulsions
- Simple emulsions (macro emulsions)
- Diameter greater than 0.1 µm
- Oil-in-water (O/W)
- Water-in-oil (W/O)
- Multiple emulsions
- Oil-in-water-in-oil (O/W/O)
- Water-in-oil-in-water (W/O/W)
- Micro/Nano emulsions
- Thermodynamically stable
- Optically transparent
- Mixtures of a biphasic oil-water system
- Stabilized with surfactants
- Size range 0.01-0.1 µm
Introduction
- Usually only one phase persists in droplet form for a prolonged period (internal phase)
- Internal phase is surrounded by external continuous phase
- Internal phase can occupy no more than 74% of emulsion volume (can exceed if not monodisperse)
- Emulsifier increases droplet lifetime in immiscible liquids and acts as a droplet stabilizer
- Emulsifiers are amphiphilic (have both hydrophilic and hydrophobic parts)
Multiple Emulsions
- Oil in water in oil (o/w/o)
- Water in oil in water (w/o/w)
- Emulsion type can invert (change from o/w to w/o or vice versa) during inversion, they typically form simple emulsions. A w/o/w emulsion normally yields an o/w emulsion.
Emulsion Type
- Phase volume ratio (relative amount of oil and water) determines relative number of droplets and collision probability
- The phase present in greater amount usually becomes the external phase
- Water-soluble emulsifiers generally favor o/w emulsions
- Lipid-soluble emulsifiers generally favor w/o emulsions
Applications
- Oral dosage forms (make medicinal agents more palatable)
- High efficacy (BA or absorption) for unabsorbable macromolecules (e.g., heparin and insulin)
- Topical emulsions (control viscosity, appearance, and greasiness of cosmetics)
- Water washable drug bases and general cosmetic purposes
- Treatment of dry skin and emollient applications
- Intravenous administration of lipid nutrients
- Radiopaque emulsions as diagnostic agents in X-ray examination
- Intramuscular depot injections
- Emulsification of perfluorinated hydrocarbons useful as oxygen carriers in blood replacements
- Dilution with safe, inexpensive diluents (e.g., water) for economic reasons
Formulation Components
- Lipid phase
- Emulsifying agents (surfactants)
- Auxiliary emulsifiers (hydrophilic colloids, finely divided solids)
- Viscosity modifiers
- Antimicrobial preservatives
- Antioxidants
Surface Active Agents (Surfactants)
- Substances with both hydrophilic and hydrophobic regions
- Soluble in both oil and water
- Hydrophilic portion oriented toward polar solvent (water)
- Hydrophobic portion oriented toward non-polar solvent (oil)
- Lower interfacial tension (allows for easier emulsification)
- Classified into anionic, cationic, nonionic, amphoteric based on charge
Anionic Surfactants
- Negatively charged
- Sodium lauryl sulfate is commonly used in o/w emulsions
- Alkali metal soaps (e.g., sodium oleate) often stable w/o emulsions due to low water solubility
- Triethanolamine stearate produces stable o/w emulsions
Cationic Surfactants
- Positively charged
- Expensive
- Used as preservatives due to bactericidal action (sterilizing contaminated surfaces and emulsions)
Nonionic Surfactants
- Neutral (no charge)
- Water-soluble (e.g., polysorbates) often used in o/w emulsions
- Water-insoluble (e.g., sorbitan esters) often used in w/o emulsions
Ampholytic Surfactants
- Possess both cationic and anionic groups
- Stability dependent on pH of the medium
- Lecithin is often used.
Hydrophilic-Lipophilic Balance (HLB) Concept
- System for determining the suitability of a surfactant for a specific emulsion
- Based on the balance between hydrophilic and lipophilic properties of the surfactant
- HLB value can be determined experimentally or calculated from chemical structure
- Values depend on mixing ratios of polyhydric alcohols and fatty acids
Methods for determining Emulsion Type
- Dilution test (useful for liquid emulsions)
- Dye test (fails for ionic emulsifiers)
- CoCl₂/filter paper (fails if emulsion unstable or breaks)
- Fluorescence (not always applicable)
- Conductivity (fails for non-ionic emulsions)
Micro Emulsions
- Dispersions of insoluble liquids appearing clear and homogeneous
- Often called solubilized systems
- Micellar state (oil dissolves in interior of micelle because of hydrophobic nature)
Emulsion Formation Methods
- Dispersion method (separation into droplets by heat, mechanical agitation, ultrasonic vibration, or electricity)
- Condensation method (vapor of a liquid into external phase, limited to dilute emulsions of materials with low vapor pressure)
- Phase inversion method (transition through different temperatures, depends on emulsifier concentration, often stable)
- Low energy emulsification (only portion of internal or external phase is heated, often quite stable with small droplets)
Mechanical Equipment for Emulsification
- Mechanical stirrers (simple for low viscosity systems, more vigorous types for higher viscosity)
- Homogenizers (high pressure forces mixture through small orifice to create a uniform emulsion)
- Ultrasonifiers (ultrasonic vibration to produce dispersion, expensive and limited capacity)
- Colloid mills (high shear between rotor and stator for very fine dispersions)
Production Aspects
- Foaming during agitation (often reduces surface tension for water-soluble emulsifiers)
- Chemical stability (prevention of hydrolytic or oxidative reactions within components)
- Safety (consideration regarding toxicologic clearances for emulsion ingredients)
Stability of Emulsions
- Thermodynamically unstable due reduction of interfacial area by coalescence.
- Stability defined in terms of maintaining the same number of particles per unit volume, invariant with time.
- Product shelf life depends on kinetic stability.
- Stability is generally temperature-dependent
Symptoms of Emulsion Instability
- Creaming (upward movement of denser phase)
- Flocculation (reversible aggregation of droplets)
- Coalescence (irreversible aggregation into larger droplets or complete phase separation)
- Phase inversion (transition from o/w to w/o, or vice versa)
Flocculation
- Revesible aggregation of droplets as 3-dimensional clusters.
- Influenced by surface charges on droplets.
- Requires sufficient emulsifier to prevent coalescence
- Reversibility depends on interaction strength between particles (e.g., electrolytes, ionic emulsifiers).
- often used in practical o/w and w/o emulsions
Creaming
- Particle movement due to differences in density.
- Rate of creaming is described using Stoke's equation (V = d²*(pi-pe)g / 18η)
- . Rate depends on particle size, density difference, viscosity of medium
- Can be reversible (reconstituted by shaking).
Coalescence
- Irreversible aggregation leading to larger droplets and eventually complete phase separation
- Reduced by mechanical barriers
Phase Inversion
- Change in emulsion type (e.g., o/w to w/o)
- Can be induced by adding electrolytes or by changing phase ratios of the emulsion components
- Stability can be affected.
- High-precision critical point needed to produce stable emulsions.
Assessment of Emulsion Shelf Life
- Techniques for speed-up stability program include stressing the emulsion under various conditions
- No quick tests exist to determine the stability of emulsions precisely in the beginning.
- Common stress conditions: Aging and temperature (cycling between different temps), Centrifugation, Agitation
Stress Conditions
- Aging and Temperature Cycling
- Centrifugation
- Agitation
Parameters for Evaluation of Emulsion Stability
- Chemical parameters (e.g., presence of decomposition products)
- Physical parameters (e.g., phase separation, viscosity, and electrical conductivity)
- Particle size analysis
Practical Recommendations for Shelf life predictions
- Develop a realistic program for stability assessment.
- Carefully observe emulsion behavior under various conditions and reasonable time spans.
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