Emulsions and Creams Quiz
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Questions and Answers

What is the significance of Zeta potential in colloidal systems?

  • It determines the size of the particles in suspension.
  • It indicates the repulsive force between particles. (correct)
  • It measures the viscosity of the fluid phase.
  • It assesses the temperature stability of the colloid.

What is generally cited as the threshold for colloidal stability?

  • ±20 mV
  • ±40 mV
  • ±30 mV (correct)
  • ±10 mV

What occurs when the Zeta potential is below ±30 mV?

  • The system achieves maximum stability.
  • Particles exhibit enhanced thermal properties.
  • Particles repel each other strongly.
  • Particles will aggregate due to insufficient repulsion. (correct)

How does the movement of particles affect the outer portion of the diffuse layer?

<p>It is in flux beyond the slipping plane. (A)</p> Signup and view all the answers

What happens to the stability of a colloidal system as the Zeta potential increases?

<p>The repulsion between particles increases. (C)</p> Signup and view all the answers

What does the Bancroft rule state about emulsifiers?

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

What is the phase volume φ in an oil-in-water (o/w) emulsion typically up to?

<p>60% (A)</p> Signup and view all the answers

Which type of surfactants typically stabilize oil-in-water (o/w) emulsions?

<p>Ionic surfactants and those with strong polar heads (HLB = 8-16). (B)</p> Signup and view all the answers

What does the term 'HLB' stand for in emulsion formulation?

<p>Hydrophilic-Lipophilic Balance (D)</p> Signup and view all the answers

In a binary mixture of surfactants for emulsion stability, what effect do surfactants with differing HLB values have?

<p>They create a more stable emulsion than a single surfactant. (D)</p> Signup and view all the answers

What characterizes an emulsion in terms of its phases?

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

What does 'emulsion stability' refer to?

<p>The duration an emulsion remains before separating (C)</p> Signup and view all the answers

Which of the following contributes to thermodynamic instability in emulsions?

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

What role do emulsifiers play in the creation of emulsions?

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

What happens to an emulsion when water and oil separate out?

<p>It is no longer considered an emulsion (D)</p> Signup and view all the answers

What does ΔA represent in the context of emulsions?

<p>The change in surface area between two phases (B)</p> Signup and view all the answers

Which factor increases the energy required to maintain an emulsion?

<p>Increasing interfacial tension (A)</p> Signup and view all the answers

Which statement about colloids is correct?

<p>Colloids maintain their dispersed state without being dissolved. (A)</p> Signup and view all the answers

What does an increase in particle size do to the rate of sedimentation or creaming?

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

What is the consequence of a large difference in density between the internal and external phases?

<p>It will lead to a significant increase in the rate of creaming and sedimentation. (B)</p> Signup and view all the answers

Which of the following statements about creaming is true?

<p>Creaming occurs when the dispersed particles rise to the surface. (B)</p> Signup and view all the answers

What role does viscosity play in the rate of sedimentation and creaming?

<p>The rate of sedimentation and creaming is inversely proportional to viscosity. (C)</p> Signup and view all the answers

What does Stokes Law help to predict regarding sedimentation?

<p>The velocity of dispersed phase particles. (D)</p> Signup and view all the answers

What effect does flocculation have on sedimentation or creaming rates?

<p>It decreases the rate of sedimentation or creaming. (A)</p> Signup and view all the answers

Which process occurs when dispersed particles have a higher density than the continuous phase?

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

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

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

In terms of stability, what is preferable regarding particle size in emulsions?

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

What occurs during coalescence of dispersed phase droplets?

<p>The droplets form larger droplets. (A)</p> Signup and view all the answers

Which phenomenon describes the movement of disperse phase molecules from smaller droplets to larger droplets?

<p>Disproportionation (A)</p> Signup and view all the answers

What role do emulsifiers play in stabilizing emulsions?

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

Which of the following is NOT a method by which emulsifiers stabilize emulsions?

<p>Increasing surface area of droplets. (B)</p> Signup and view all the answers

What is the consequence of smaller droplets having a higher internal pressure than larger droplets?

<p>They contribute to coalescence. (B)</p> Signup and view all the answers

How do emulsifiers create a repulsive force among droplets?

<p>By creating a surface charge on each droplet. (D)</p> Signup and view all the answers

What is a key characteristic of flocculation in the context of dispersed droplets?

<p>Droplets remain separate but move in unison. (D)</p> Signup and view all the answers

What determines the type of emulsion or cream formed?

<p>The relative amount of the two phases and the type of surfactant (B)</p> Signup and view all the answers

What is the Critical Micelle Concentration (cmc)?

<p>The concentration at which the adsorbed monolayer becomes saturated (C)</p> Signup and view all the answers

Which type of cream is characterized by a cooling action and is washable?

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

Which surfactant type is typically used for w/o emulsions?

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

What is a characteristic feature of the 'Solution method' of emulsion preparation?

<p>Each surfactant is dissolved in the respective phase before mixing (C)</p> Signup and view all the answers

What is the role of antioxidants in emulsion formulations?

<p>To prevent oxidation of the oily phase (D)</p> Signup and view all the answers

Which emulsion has more cleansing action and is less greasy than ointments?

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

Why are emulsions susceptible to microbial growth?

<p>They have significant water content in the aqueous phase (D)</p> Signup and view all the answers

Flashcards

What is an emulsion?

A type of colloid where one liquid is dispersed in another liquid.

Dispersed or Internal Phase

The substance that is dispersed or suspended in another substance.

Continuous or External Phase

The material in which the dispersed phase is suspended.

What is Emulsion Stability?

The length of time a mixture remains an emulsion before it separates out.

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Thermodynamic Instability

The tendency of an emulsion to separate into its original phases due to thermodynamic principles.

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What is ΔA?

The surface area between the dispersed and continuous phases of an emulsion.

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What is ɣAB?

The interfacial tension between the two phases of an emulsion.

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What is Emulsification?

The process of mixing a mixture to create an emulsion.

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ΔA + ɣAB

The energy needed to create an emulsion with a specific droplet size, measured in Joules per week.

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Kinetic Instability

A type of instability where a mixture separates over time due to the movement of particles.

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Creaming

The upward movement of dispersed particles to the surface of an emulsion, caused by the dispersed particles being less dense than the continuous phase.

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Sedimentation

The downward movement of dispersed particles to the bottom of an emulsion, caused by the dispersed particles being denser than the continuous phase.

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Stokes' Law

A mathematical equation that predicts the rate of sedimentation or creaming, based on the size and density of the particles, the viscosity of the continuous phase, and gravity.

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Particle size and rate of sedimentation/creaming

The rate of sedimentation or creaming increases as the size of the particles increases.

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Density difference and rate of sedimentation/creaming

The difference in density between the dispersed and continuous phases affects the rate of sedimentation or creaming.

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Viscosity and rate of sedimentation/creaming

Increased viscosity of the continuous phase slows down the rate of sedimentation or creaming.

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What is Zeta Potential?

The potential difference between the surface of a particle and the surrounding liquid, arising from the electrical double layer. It's crucial for understanding how particles interact and whether a colloid will remain stable.

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What is an Electrical Double Layer?

A region near the surface of a charged particle where electrical charges accumulate, creating an electric field that affects the interaction between particles.

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What is Colloidal Stability?

When particles in a colloid are repelled by each other, preventing them from aggregating and causing the colloid to remain stable.

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What is Coagulation?

When particles in a colloid are attracted to each other, causing them to clump together and separate from the continuous phase.

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What is the ±30 mV Threshold?

A general guideline where a Zeta Potential value above ±30 mV suggests a stable colloid, while below this value, particles may aggregate due to insufficient repulsion.

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Sedimentation/Creaming Rate

The rate at which particles settle or rise in a liquid, typically due to density differences.

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Viscosity

The resistance of a fluid to flow, determined by its internal friction.

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Coalescence

Droplets of the dispersed phase in a colloid merge to form larger droplets, decreasing the total surface area.

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Flocculation

Droplets of the dispersed phase come together but don't merge, forming clumps that move together.

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Disproportionation (Ostwald Ripening)

A process where smaller droplets in a colloid shrink and larger droplets grow due to diffusion of molecules through the continuous phase.

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Emulsifiers

Surface-active agents that lower interfacial tension and create repulsive forces between dispersed phase particles, thus stabilizing the system.

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Emulsifier Extension

The ability of an emulsifier to extend into the continuous phase, creating a physical barrier to droplet collisions.

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Surface Charge Repulsion

Emulsifiers create a repulsive force by generating a charge on the surface of each droplet, preventing them from coming together.

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Viscosity of Emulsions

The viscosity of the emulsion is directly proportional to the viscosity of the continuous phase and the volume fraction of the dispersed phase. Increased volume of dispersed phase leads to a higher viscosity.

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Phase Volume (φ)

The volume percentage of the dispersed phase (e.g., oil droplets) in the total volume of the emulsion. This value influences the emulsion type (o/w or w/o).

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Bancroft Rule

The emulsifier that is more soluble in one of the phases tends to be the continuous phase. For example, a hydrophilic emulsifier creates an o/w emulsion, while a hydrophobic emulsifier favors a w/o emulsion.

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Mixed Surfactants for Stability

A mixture of surfactants with different HLB values can create a more stable emulsion than a single surfactant. This is because the mixed surfactants form a more complex and stable interfacial film.

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HLB Mixture Calculation

A method to calculate the overall HLB value of a mixture of surfactants. It involves multiplying the HLB value of each surfactant by its weight fraction and summing the products.

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What are the two phases in an emulsion?

The smaller liquid phase is dispersed as tiny droplets within the larger, continuous liquid phase.

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What is an emulsifying agent?

A substance that helps to stabilize an emulsion by reducing the interfacial tension between the two phases. Enables the formation of fine droplets and prevents them from separating.

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What is an Oil-in-Water (O/W) emulsion?

A type of emulsion where oil droplets are dispersed in a continuous water phase. Think 'vanishing creams'.

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What is a Water-in-Oil (W/O) emulsion?

A type of emulsion where water droplets are dispersed in a continuous oil phase. Think 'creamy lotions'.

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What is HLB (Hydrophilic-Lipophilic Balance)?

A measure of the balance between hydrophilic and lipophilic properties of a surfactant. Determines whether an emulsion will be O/W or W/O. A higher HLB number usually corresponds to an O/W emulsion, while a lower number indicates a W/O emulsion.

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What is the Critical Micelle Concentration (CMC)?

The concentration at which surfactant molecules in a solution begin to form micelles, spherical structures with hydrophobic tails facing inward and hydrophilic heads facing outward. This point marks a significant shift in the behavior of surfactant molecules.

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Explain what a micelle is?

Spherical aggregates of surfactant molecules in an aqueous solution. Their hydrophobic tails point inwards, enclosing a core that can solubilize oils, while their hydrophilic heads point outwards, interacting with the surrounding water.

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Study Notes

Emulsions & Creams

  • Emulsions are low-viscosity disperse systems, often O/W or W/O.
  • Used in external applications.
  • Composed of two immiscible phases plus an emulsifier (less than cmc).
  • Emulsions are macro-heterogeneous colloidal systems.
  • Colloids comprise one substance dispersed in another.
  • Dispersed particles do not dissolve but are dispersed/suspended.
  • Dispersed particles are in the range of 1–1000 nm.
  • The dispersed phase is the internal phase, while the continuous phase is external.

Instability of Emulsions

  • Emulsion stability is the duration a mixture remains an emulsion before separating.
  • Emulsions are unstable due to thermodynamic and kinetic factors.
  • Thermodynamic instability (ΔA) is due to the increased surface area between oil and water. - Creating an emulsion requires a large increase in surface area (ΔA). - ΔA is always positive and large for emulsification or negative and large for separation. - Energy (W) is required to overcome interfacial tension between phases and create additional surface area.
  • Kinetic instability includes: - Creaming (particles rise due to lower density). - Sedimentation (particles settle at the bottom due to higher density). - Coalescence (droplets merge into larger entities). - Flocculation (droplets associate). - Ostwald ripening (smaller droplets dissolve into larger ones).

Stabilising Methods - Emulsifiers

  • Emulsifiers (surfactants) migrate to the interface between phases.
  • They lower interfacial tension between phases.
  • They create repulsive forces between internal phase particles.
    • These stop coalescence and flocculation.
  • Emulsifiers create a repulsive force by creating a surface charge on droplets and thus an electric double layer.
  • Zeta Potential of > ±30 mV is critical for colloidal stability.

Micelles and Critical Micelle Concentration (cmc)

  • Surfactants accumulate at the interface of immiscible phases.
  • CMC is where the adsorbed monolayer saturates with surfactant molecules.
  • Additional surfactants form micelles.

Formulation Principles

  • The type of emulsion depends on the relative amounts of dispersed and continuous phases and the type of surfactant.
  • Emulsions are susceptible to microbial growth.

Preparation Methods

  • Solution method: Each surfactant is dissolved in the respective phase, then mixed.
  • Dispersion method: Surfactant is dispersed into the less soluble phase, then mixed slowly with the other.

Other information

  • Creams are semi-solid emulsions used for external applications.
  • There are o/w creams (washable cooling action) and w/o creams (emollient cleansing action).
  • Creams often contain excess emulsifier resulting in gel crystalline phase or liquid crystal structures such as waxes.
    • This contributes to their semi-solid consistency.

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