emulsions and creams
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Questions and Answers

What does a higher Zeta potential indicate about the particles in a colloidal system?

  • Increased conductivity of the solution
  • More repulsion between the particles (correct)
  • Greater attraction between the particles
  • Enhanced solubility of the particles

    • What is often considered the threshold of colloidal stability in terms of Zeta potential?

  • ±70 mv
  • ±10 mv
  • ±50 mv
  • ±30 mv (correct)

    • What happens to particles when the Zeta potential is below ±30 mv?

  • They exhibit enhanced electrostatic repulsion.
  • They become less reactive in solution.
  • They form stable emulsions.
  • They are likely to coalesce or flocculate. (correct)

    • What defines the Zeta potential in a colloidal system?

    <p>The potential difference across the electric double layer (C)</p> Signup and view all the answers

    Which of the following statements about Zeta potential and particle interaction is false?

    <p>Zeta potential measures only geometric stability. (B)</p> Signup and view all the answers

    How is the phase volume ($ heta$) of an emulsion calculated?

    <p>($ ext{Volume of disperse phase} / ext{Total volume of emulsion}) imes 100$ (D)</p> Signup and view all the answers

    According to the Bancroft rule, what determines the continuous phase of an emulsion?

    <p>The phase that is more soluble in the emulsifier becomes the continuous phase. (A)</p> Signup and view all the answers

    Which HLB (Hydrophilic-Lipophilic Balance) range is typically associated with stabilizing w/o emulsions?

    <p>HLB 3-6 (B)</p> Signup and view all the answers

    What effect does using a binary mixture of surfactants with different HLB values have on emulsion stability?

    <p>It improves stability by forming a complex interfacial film. (A)</p> Signup and view all the answers

    If a mixture of surfactants contains 25% Span 20 (HLB = 8.6) and 75% Tween 20 (HLB = 16.7), what is the overall HLB value of the mixture?

    <p>14.3 (C)</p> Signup and view all the answers

    What does an emulsion consist of?

    <p>A liquid dispersed in another liquid (B)</p> Signup and view all the answers

    What happens to the rate of creaming or sedimentation as viscosity increases?

    <p>It decreases. (B)</p> Signup and view all the answers

    What is the primary factor affecting the stability of emulsions?

    <p>Surface area between the two phases (A)</p> Signup and view all the answers

    What is coalescence in the context of dispersed phase droplets?

    <p>Droplets collide and form a larger droplet. (C)</p> Signup and view all the answers

    What drives the process of disproportionation, often known as Ostwald ripening?

    <p>Diffusion of disperse phase molecules from smaller to larger droplets. (B)</p> Signup and view all the answers

    Thermodynamic instability in emulsions is associated with which of the following?

    <p>Positive change in surface area (D)</p> Signup and view all the answers

    How do emulsifiers primarily stabilize emulsions?

    <p>By lowering interfacial tension and creating repulsive forces. (A)</p> Signup and view all the answers

    What defines the internal phase of an emulsion?

    <p>The liquid that is dispersed (B)</p> Signup and view all the answers

    What physical effect do emulsifiers create to hinder coalescence and flocculation?

    <p>Repulsion due to surface charge. (D)</p> Signup and view all the answers

    In the context of emulsions, what does ΔA represent?

    <p>Change in surface area between phases (D)</p> Signup and view all the answers

    In the Laplace equation, what does 'r' represent?

    <p>The droplet radius. (B)</p> Signup and view all the answers

    Which of the following is an example of an external application of emulsions?

    <p>Skin creams and lotions (A)</p> Signup and view all the answers

    Kinetic instability of emulsions primarily refers to what?

    <p>Separation of phases over time (D)</p> Signup and view all the answers

    What is one effect of emulsifiers on the interfacial tension between two phases?

    <p>They lower interfacial tension. (C)</p> Signup and view all the answers

    What physical mechanism do emulsifiers use to create a barrier against coalescence?

    <p>Entanglement of molecules in the continuous phase. (A)</p> Signup and view all the answers

    What role does an emulsifier play in the formation of an emulsion?

    <p>Reduces interfacial tension between phases (C)</p> Signup and view all the answers

    What is the primary cause of creaming in emulsions?

    <p>Lower density of dispersed particles compared to the continuous phase (A)</p> Signup and view all the answers

    According to Stokes Law, which factor affects the rate of sedimentation or creaming the most?

    <p>Radius of the dispersed phase particle (C)</p> Signup and view all the answers

    What is the relationship between particle size and stability of an emulsion?

    <p>Smaller particle size increases stability (C)</p> Signup and view all the answers

    What occurs when the dispersed phase has a higher density than the continuous phase?

    <p>Sedimentation occurs (B)</p> Signup and view all the answers

    How does viscosity impact the rate of creaming and sedimentation?

    <p>It is inversely proportional to the rate (D)</p> Signup and view all the answers

    What happens as the difference in density between the internal and external phases increases?

    <p>The rate of creaming and sedimentation increases (B)</p> Signup and view all the answers

    Which process involves dispersed particles rising to the surface of an emulsion?

    <p>Creaming (C)</p> Signup and view all the answers

    What is the effect of flocculation on sedimentation and creaming rates?

    <p>It increases the rate of sedimentation and creaming (D)</p> Signup and view all the answers

    What factor primarily determines the type of emulsion or cream that is formed?

    <p>Relative amount of the two phases (A)</p> Signup and view all the answers

    What is the significance of the Critical Micelle Concentration (cmc)?

    <p>It is the concentration at which surfactant molecules begin to form micelles. (A)</p> Signup and view all the answers

    Which type of emulsifying agent is typically used in water-in-oil (w/o) creams?

    <p>Lipophilic emulsifying agents (B)</p> Signup and view all the answers

    What role do antioxidants play in emulsion formulation?

    <p>They prevent oxidation of the oily phase. (A)</p> Signup and view all the answers

    What is the primary action of 'vanishing creams' in emulsions?

    <p>They are washable and provide a cooling effect. (B)</p> Signup and view all the answers

    Which method involves dissolving surfactants in their respective phases before mixing?

    <p>Solution method (C)</p> Signup and view all the answers

    What is a common feature of creams that indicates their high viscosity?

    <p>They are structured systems containing excess emulsifier. (A)</p> Signup and view all the answers

    Which of the following is NOT a characteristic of creams described in the content?

    <p>They can only be formed with a neutral pH. (B)</p> Signup and view all the answers

    Flashcards

    What is an emulsion?

    A dispersion of two immiscible liquids, one dispersed as droplets within the other.

    Internal Phase

    The substance being dispersed as droplets in an emulsion.

    Continuous Phase

    The substance in which the droplets of the internal phase are dispersed in.

    Emulsion Instability

    The tendency of an emulsion to separate over time, with the internal phase separating from the continuous phase.

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    Change in Surface Area (ΔA)

    The increase in surface area between two immiscible liquids when they are emulsified.

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    Interfacial Tension (ɣAB)

    The force that resists the mixing of two immiscible liquids.

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    Emulsion Stability

    A measure of the stability of an emulsion over a period of time.

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    Thermodynamic Instability

    The tendency of an emulsion to separate due to the thermodynamic properties of the system. This is related to the increase in surface area and interfacial tension during emulsification.

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    Emulsion Formation Energy (ΔA + ɣAB)

    The energy required to create an emulsion with a specific droplet size. It is measured in Joules per week (J/WEEK).

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    Kinetic Instability

    A type of instability in emulsions that occurs when dispersed particles move within the continuous phase due to differences in density.

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    Creaming

    A process of kinetic instability where dispersed particles, less dense than the continuous phase, rise to the surface.

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    Sedimentation

    A process of kinetic instability where dispersed particles, denser than the continuous phase, settle to the bottom.

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    Stokes' Law

    A law that describes the rate of sedimentation or creaming based on particle size, density difference, and viscosity.

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    Particle Size and Sedimentation/Creaming Rate

    Larger particles settle or rise faster due to gravity.

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    Density Difference and Sedimentation/Creaming Rate

    A smaller density difference between the dispersed and continuous phases leads to slower sedimentation or creaming.

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    Viscosity and Sedimentation/Creaming Rate

    A higher viscosity of the continuous phase slows down sedimentation or creaming.

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    Coalescence

    A phenomenon where droplets in a colloid collide and join together, forming larger droplets, leading to a decrease in overall surface area.

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    Flocculation

    A process where droplets in a colloid collide and associate with each other, moving together as a group, but their individual surface areas remain unchanged.

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    Disproportionation (Ostwald Ripening)

    The process by which smaller droplets in a dispersion transfer their molecules to larger droplets through the continuous phase, driven by the difference in internal pressure between droplets of different sizes.

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    Emulsifier

    A surface-active ingredient (surfactant) that migrates to the interface between two phases, reducing interfacial tension and creating repulsive forces between the dispersed particles.

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    Physical Barrier (Emulsifiers)

    The ability of an emulsifier to surround dispersed particles with a physical barrier that prevents them from colliding and coalescing. This usually involves extending into the continuous phase, creating a 'shield' around each particle.

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    Surface Charge (Emulsifiers)

    The ability of an emulsifier to create an electrical charge on the surface of each dispersed particle, creating repulsive forces that hinder coalescence and flocculation.

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    Interfacial Tension

    The tendency of a system to resist changes in its surface area. It is influenced by the type of molecules involved and the temperature.

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    Viscosity and Creaming/Sedimentation

    As viscosity increases, the rate of creaming or sedimentation decreases, because the viscous fluid hinders the movement of dispersed particles.

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    Zeta Potential

    The electrical potential that exists between the surface of a particle and the surrounding liquid, influencing the stability of a colloidal system.

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    Slipping Plane

    The diffuse layer surrounding a charged particle in a liquid is constantly moving, creating a slipping plane which separates the bound layer from the diffuse layer. The Zeta Potential is a measure of the electric potential at this slipping plane.

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    Colloidal Stability

    A system is considered colloidally stable when particles repel each other enough to remain dispersed in the liquid. This typically happens when the Zeta Potential is above ±30 mV.

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    Colloidal Instability

    When the Zeta Potential is below ±30 mV, there is not enough repulsion between particles, leading to them clumping together and causing instability in the system.

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    Zeta Potential Limitations

    Zeta potential only measures stability related to particle aggregation, such as flocculation and coalescence. It doesn't consider other factors like sedimentation.

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    Viscosity of Emulsions

    The viscosity of an emulsion is directly proportional to the volume fraction of the dispersed phase. Larger volume fractions result in greater viscosity.

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    Phase Volume (φ) in Emulsions

    The phase volume (φ) is the percentage volume of the dispersed phase in an emulsion. This value is key in understanding whether the emulsion is water-in-oil or oil-in-water.

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    Bancroft Rule

    The Bancroft Rule predicts the type of emulsion based on the emulsifier's solubility. The emulsifier is more soluble in the continuous phase.

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    Surfactant Mixtures in Emulsions

    A surfactant mixture with both high and low HLB values can create a more stable emulsion compared to a single surfactant.

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    HLB of a Surfactant Mixture

    The HLB of a surfactant mixture can be calculated using the weighted average of the HLB values of the individual surfactants.

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    What is the internal phase?

    The substance being dispersed as droplets in an emulsion.

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    What is the continuous phase?

    The substance in which the droplets of the internal phase are dispersed.

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    What is the Critical Micelle Concentration (cmc)?

    The concentration at which the adsorbed monolayer becomes saturated with surfactant molecules. Any additional surfactant molecules will form micelles.

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    What are Micelles?

    Spherical aggregates of surfactant molecules in an aqueous solution.

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    What is an emulsifier?

    Emulsifying agent, which stabilizes the mixture by reducing interfacial tension.

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    What is Bancroft's rule?

    Determines whether the emulsion is oil-in-water (o/w) or water-in-oil (w/o).

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    How does droplet size affect emulsion stability?

    A general principle of emulsion stability where smaller droplets are more stable, as they have a larger surface area-to-volume ratio.

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    Study Notes

    Emulsions & Creams

    • Emulsions are low viscosity disperse systems, often O/W or W/O, used for external application. They consist of two immiscible phases and a suitable amount of emulsifier (≤ cmc).
    • An emulsion is a type of colloid. Colloids are macro-heterogeneous systems that comprise of one substance dispersed in another. Dispersed particles are not dissolved, instead suspended. Dispersed particles have a size of 1-1000nm (10⁻⁹m to 10⁻⁶m) that is too small to see without a microscope.
    • The dispersed substance is the internal phase, and the material it's dispersed in is the continuous, or external, phase. The physical state of the internal and external phases determines the type of colloid.
    • Emulsion stability refers to the length of time a mixture remains an emulsion before separation. Instability arises from thermodynamic and kinetic factors.
    • Thermodynamic instability (ΔA): the surface area between two surfaces (e.g., oil and water) increases significantly. An emulsion requires a large increase in surface area between two phases during emulsification. ΔA is always positive for emulsification or negative for separation, and is always large.
    • Interfacial tension between phases necessitates overcoming energy, requiring more energy to make more surface area between phases. ΔA (increase surface area) and γAB (interfacial tension) are the forces that need to be overcome for emulsification. These together represent the work (W) required to create an emulsion with a certain droplet size.
    • Kinetic instability includes creaming, coalescence, flocculation, sedimentation, and Ostwald ripening.
    • Creaming and sedimentation are vertical processes occurring because of density differences. Particles less dense than the continuous phase will tend to rise (creaming) or oppositely, heavier ones settle (sedimentation). The rate is influenced by particle size, viscosity differences, and gravity.
    • Coalescence is droplet merging, reducing surface area and leading to phase separation.
    • Flocculation involves dispersed particles clumping, increasing particle size and resulting in instability.
    • Ostwald ripening describes the diffusion of disperse phase molecules from smaller to larger droplets, driven by differing internal pressures.
    • Emulsifiers are surface active ingredients (surfactants) and migrate to the interface between the two phases. They reduce interfacial tension and create repulsive forces between internal-phase particles to increase stability, preventing coalescence and flocculation, by acting as a physical barrier.
    • Zeta potential is crucial in stabilizing emulsions. It's the electric potential at the slipping plane of the diffuse (outer) layer of the Electric Double Layer around the particles and is influenced by the particle and the viscosity of the continuous phase. Emulsions with a higher zeta potential have more repulsive forces between particles and are more stable. A ±30mV threshold is often cited as a stability threshold for a colloid.
    • Emulsion rheology relates viscosity of emulsion to viscosity of continuous phase and the phase volume: η = η0(1 + 2.5φ), where η is viscosity of emulsion, η0 is viscosity of continuous phase, and φ is phase volume.
    • Factors affecting emulsion type include the HLB (hydrophilic-lipophilic balance) value of the emulsifying agent, the relative amounts of the two immiscible phases, and the Bancroft rule (the liquid the emulsifier is more soluble in becomes the continuous phase).
    • Stable emulsions can result from mixtures of surfactants with different HLB values, forming a more complex interfacial film.
    • Creams are semi-solid emulsions for external application. Classified as o/w (easily washed off) or w/o (oily). Important for creams is the presence of excess emulsifiers that leads to a structured system with gel crystalline phases and waxes.
    • Micelles are spherical aggregates of surfactant molecules that form when the concentration exceeds the Critical Micelle Concentration (cmc). At the cmc, the adsorbed monolayer of surfactants becomes saturated.
    • Formulation principles include proper emulsifier selection, and measures for microbial growth, using antimicrobial preservatives which can include, phenoxyethanol, parabens, chlorocresol, benzoic acid, or cetrimide.
    • Antioxidant use protects against oily phase oxidation.
    • Preparation methods for emulsions and creams include a "solution method" or a "dispersion method." The solution method dissolves the surfactants in the respective phases. In the Dispersion method the surfactants disperse in the less soluble phase before mixing.

    Clotrimazole Cream B.P.

    • Clotrimazole is dispersed in an O/W cream base.
    • Ingredients of the cream base: Sorbitan monostearate, polysorbate 60, cetyl esters wax, cetostearyl alcohol, 2-octyldodecanol, purified water, and benzyl alcohol.

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    Week 13 Emulsions & Creams 2024 PDF

    Description

    Test your knowledge on Zeta potential and its significance in colloidal systems. This quiz covers definitions, stability thresholds, and the effects on particle interactions. Perfect for students of colloidal chemistry.

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