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?

The potential difference across the electric double layer Signup and view all the answers

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

Zeta potential measures only geometric stability. Signup and view all the answers

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

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

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

The phase that is more soluble in the emulsifier becomes the continuous phase. Signup and view all the answers

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

HLB 3-6 Signup and view all the answers

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

It improves stability by forming a complex interfacial film. 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?

14.3 Signup and view all the answers

What does an emulsion consist of?

A liquid dispersed in another liquid Signup and view all the answers

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

It decreases. Signup and view all the answers

What is the primary factor affecting the stability of emulsions?

Surface area between the two phases Signup and view all the answers

What is coalescence in the context of dispersed phase droplets?

Droplets collide and form a larger droplet. Signup and view all the answers

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

Diffusion of disperse phase molecules from smaller to larger droplets. Signup and view all the answers

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

Positive change in surface area Signup and view all the answers

How do emulsifiers primarily stabilize emulsions?

By lowering interfacial tension and creating repulsive forces. Signup and view all the answers

What defines the internal phase of an emulsion?

The liquid that is dispersed Signup and view all the answers

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

Repulsion due to surface charge. Signup and view all the answers

In the context of emulsions, what does ΔA represent?

Change in surface area between phases Signup and view all the answers

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

The droplet radius. Signup and view all the answers

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

Skin creams and lotions Signup and view all the answers

Kinetic instability of emulsions primarily refers to what?

Separation of phases over time Signup and view all the answers

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

They lower interfacial tension. Signup and view all the answers

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

Entanglement of molecules in the continuous phase. Signup and view all the answers

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

Reduces interfacial tension between phases Signup and view all the answers

What is the primary cause of creaming in emulsions?

Lower density of dispersed particles compared to the continuous phase Signup and view all the answers

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

Radius of the dispersed phase particle Signup and view all the answers

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

Smaller particle size increases stability Signup and view all the answers

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

Sedimentation occurs Signup and view all the answers

How does viscosity impact the rate of creaming and sedimentation?

It is inversely proportional to the rate Signup and view all the answers

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

The rate of creaming and sedimentation increases Signup and view all the answers

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

Creaming Signup and view all the answers

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

It increases the rate of sedimentation and creaming Signup and view all the answers

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

Relative amount of the two phases Signup and view all the answers

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

It is the concentration at which surfactant molecules begin to form micelles. Signup and view all the answers

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

Lipophilic emulsifying agents Signup and view all the answers

What role do antioxidants play in emulsion formulation?

They prevent oxidation of the oily phase. Signup and view all the answers

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

They are washable and provide a cooling effect. Signup and view all the answers

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

Solution method Signup and view all the answers

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

They are structured systems containing excess emulsifier. Signup and view all the answers

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

They can only be formed with a neutral pH. Signup and view all the answers

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 γ_AB (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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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.