Dispersion Systems One Shot Notes and MCQs

Dispersion Systems One Shot Notes

1. Colloidal Dispersions

Definition of Colloids:

A colloidal dispersion is a two-phase system where fine particles (1-1000 nm in size) are dispersed uniformly in a continuous medium.
These dispersed particles do not settle under gravity and are not visible under an ordinary microscope.

Types of Colloids:

Based on the Physical State:
- Sol: Solid in liquid (e.g., Gold sol)
- Gel: Liquid in solid (e.g., Jelly)
- Aerosol: Liquid or solid in gas (e.g., Mist, smoke)
- Foam: Gas in liquid (e.g., Shaving cream)
- Emulsion: Liquid in liquid (e.g., Milk)
Based on Interaction with Dispersion Medium:
- Lyophilic colloids: Particles have an affinity for the dispersion medium (e.g., gum, gelatin).
- Lyophobic colloids: Particles have little or no affinity (e.g., metal sols).

Properties of Colloids:

Optical Properties:
- Tyndall Effect: Scattering of light by colloidal particles makes the path visible (used to distinguish between true solutions and colloids).
Kinetic Properties:
- Brownian Motion: Random movement of particles due to collision with molecules of the dispersion medium.
- Diffusion: Colloidal particles move from higher concentration to lower concentration.
Electrical Properties:
- Electrophoresis: Movement of charged colloidal particles in an electric field.
- Electro-osmosis: Movement of the dispersion medium under the influence of an electric field while particles remain stationary.

Protective Colloids:

Protective colloids prevent the coagulation of a lyophobic sol by forming a protective layer around the particles.
Example: Gelatin stabilizing a silver sol.

Applications of Colloids in Pharmacy:

Drug delivery systems: Liposomes, nanoemulsions, and nanoparticles for targeted delivery.
Suspensions and emulsions: Improved bioavailability of poorly soluble drugs.
Plasma expanders: Colloidal solutions like dextran used in intravenous therapy.

2. Suspensions and Emulsions

Interfacial Properties of Suspended Particles:

Surface Tension: The cohesive force between liquid molecules at the interface affects particle stability.
Wettability: How well a liquid spreads on the surface of particles. A poorly wetted particle may not disperse evenly.

Theory of Sedimentation:

Stokes’ Law: Describes the rate of sedimentation of particles under gravity.
V=2r2(ρp−ρm)g9ηV = \frac{2r^2 (\rho_p – \rho_m) g}{9 \eta}V=9η2r2(ρp−ρm)g
where:
- VVV = sedimentation velocity
- rrr = particle radius
- ρp\rho_pρp = particle density
- ρm\rho_mρm = medium density
- ggg = gravitational constant
- η\etaη = viscosity of the medium
Assumptions: Stokes’ law applies to spherical particles, low concentrations, and no significant interaction between particles.

Effect of Brownian Motion:

Brownian motion counteracts sedimentation by randomizing the movement of particles, keeping them dispersed.

Sedimentation of Flocculated Particles:

Flocculation: Loose aggregation of particles forming flocs that settle faster but can be easily redispersed.
Deflocculated systems: Particles remain separate, leading to slow sedimentation but hard-to-resuspend sediments (cake formation).

Sedimentation Parameters:

Sedimentation volume (F): Ratio of the final volume of sediment to the total volume of the suspension.
F=VsVtF = \frac{V_s}{V_t}F=VtVs
- VsV_sVs = volume of sediment
- VtV_tVt = total volume of suspension

Wetting of Particles:

Wetting agents (surfactants) improve the dispersibility of poorly wettable particles by reducing surface tension.

Controlled Flocculation:

Achieved by adding electrolytes, surfactants, or polymers to create loose aggregates that prevent caking.

Flocculation in Structured Vehicles:

Structured vehicles (viscous solutions) slow down sedimentation and enhance stability. Examples include gums, cellulose derivatives, and carbomers.

Rheological Considerations:

Rheology refers to the flow behavior of suspensions and emulsions:
- Newtonian flow: Viscosity remains constant with shear stress.
- Non-Newtonian flow: Viscosity changes with shear stress (e.g., shear-thinning in polymer solutions).

3. Emulsions

Types of Emulsions:

Oil-in-water (O/W) emulsion: Oil dispersed in water (e.g., milk, lotions).
Water-in-oil (W/O) emulsion: Water dispersed in oil (e.g., creams, butter).
Multiple emulsions: O/W/O or W/O/W systems for specialized applications (e.g., controlled drug release).

Theories of Emulsions:

Surface Tension Theory: Emulsifying agents lower the interfacial tension between two immiscible liquids.
Oriented-Wedge Theory: Emulsifiers orient at the interface, with hydrophilic and hydrophobic parts stabilizing the emulsion.
Interfacial Film Theory: A thin film of emulsifier forms around dispersed droplets, preventing coalescence.

Physical Stability of Emulsions:

Flocculation: Aggregation of droplets without merging.
Coalescence: Merging of droplets, leading to phase separation.
Creaming: Upward movement of dispersed phase due to density differences (can be redispersed by shaking).
Breaking: Complete separation of the phases, making the emulsion irreversible.

Factors Affecting Emulsion Stability:

Emulsifying agents: Surfactants like Tween, Span, or phospholipids.
Temperature: Excessive heat can destabilize emulsions.
Viscosity: Increased viscosity slows down the coalescence of droplets.

Summary of Key Points:

Colloidal dispersions involve particles (1-1000 nm) dispersed in a medium, with important pharmaceutical applications such as drug delivery and plasma expanders.
Suspensions are coarse dispersions where particles settle over time; controlled flocculation and sedimentation parameters influence stability.
Emulsions are mixtures of two immiscible liquids, stabilized by emulsifiers, with physical stability maintained through control of creaming, coalescence, and phase separation.

MCQs: Dispersion Systems

Colloidal Dispersions

What is the size range of colloidal particles?
a) 1-10 nm
b) 1-100 nm
c) 1-1000 nm
d) 10-1000 µm
Answer: c) 1-1000 nm
Which of the following is an example of a lyophilic colloid?
a) Gold sol
b) Gelatin
c) Ferric hydroxide sol
d) Arsenic sulfide sol
Answer: b) Gelatin
The random movement of colloidal particles due to collisions with solvent molecules is called:
a) Diffusion
b) Brownian motion
c) Electrophoresis
d) Tyndall effect
Answer: b) Brownian motion
Which optical property distinguishes a colloidal solution from a true solution?
a) Brownian motion
b) Electro-osmosis
c) Tyndall effect
d) Osmotic pressure
Answer: c) Tyndall effect
What is the function of protective colloids?
a) Increase viscosity
b) Enhance particle sedimentation
c) Prevent coagulation of lyophobic sols
d) Decrease solubility of sol particles
Answer: c) Prevent coagulation of lyophobic sols
Electrophoresis refers to the movement of colloidal particles under the influence of:
a) Gravity
b) Light
c) Electric field
d) Thermal energy
Answer: c) Electric field
What type of colloid is milk?
a) Sol
b) Gel
c) Foam
d) Emulsion
Answer: d) Emulsion
Which of the following is used as a plasma expander in intravenous therapy?
a) Dextran
b) Sodium chloride
c) Glucose
d) Ferric chloride
Answer: a) Dextran
In which pharmaceutical application are liposomes commonly used?
a) Antacid formulations
b) Controlled drug delivery
c) Tablet compression
d) Plasma separation
Answer: b) Controlled drug delivery
Which property of colloids is responsible for their stability?
a) Osmosis
b) Viscosity
c) Charge on particles
d) Surface area
Answer: c) Charge on particles

Suspensions

What does Stokes’ law describe?
a) Diffusion rate of particles
b) Sedimentation rate of particles
c) Interfacial tension of particles
d) Brownian motion of particles
Answer: b) Sedimentation rate of particles
In suspensions, poorly wettable particles are dispersed using:
a) Emulsifiers
b) Buffers
c) Wetting agents
d) Electrolytes
Answer: c) Wetting agents
Which factor can increase the sedimentation rate in suspensions?
a) Smaller particle size
b) Decreased viscosity
c) Increased Brownian motion
d) Lower particle density
Answer: b) Decreased viscosity
What does the sedimentation volume (F) represent?
a) Amount of dispersed drug
b) Volume of particles
c) Ratio of sediment volume to total volume
d) Total volume of the suspension
Answer: c) Ratio of sediment volume to total volume
What is the main purpose of controlled flocculation in suspensions?
a) To increase viscosity
b) To prevent caking
c) To enhance solubility
d) To decrease sedimentation
Answer: b) To prevent caking
What happens to sedimentation when particles exhibit Brownian motion?
a) Sedimentation accelerates
b) Particles form aggregates
c) Sedimentation slows down
d) No effect on sedimentation
Answer: c) Sedimentation slows down
Which parameter influences flocculation in a suspension?
a) Electrolyte concentration
b) pH of the medium
c) Temperature of storage
d) All of the above
Answer: d) All of the above
Flocculation is characterized by:
a) Formation of rigid aggregates
b) Formation of easily redispersible aggregates
c) Formation of cake
d) Increase in solubility
Answer: b) Formation of easily redispersible aggregates
Structured vehicles used in suspensions are designed to:
a) Increase solubility
b) Prevent flocculation
c) Increase viscosity
d) Reduce interfacial tension
Answer: c) Increase viscosity
Which rheological behavior is typical for polymeric suspensions?
a) Newtonian flow
b) Non-Newtonian flow
c) Plastic flow
d) Pseudoplastic flow
Answer: d) Pseudoplastic flow

Emulsions

What is the dispersed phase in an oil-in-water (O/W) emulsion?
a) Oil
b) Water
c) Emulsifier
d) Both oil and water
Answer: a) Oil
What is the role of emulsifying agents?
a) Increase sedimentation
b) Lower interfacial tension
c) Enhance solubility
d) Increase viscosity
Answer: b) Lower interfacial tension
Which of the following is an example of a W/O emulsion?
a) Milk
b) Cream
c) Aerosol
d) Shaving cream
Answer: b) Cream
Which theory of emulsions involves the formation of a thin film at the interface?
a) Surface tension theory
b) Oriented-wedge theory
c) Interfacial film theory
d) Viscosity theory
Answer: c) Interfacial film theory
What phenomenon describes the upward movement of dispersed droplets in an emulsion?
a) Coalescence
b) Creaming
c) Sedimentation
d) Breaking
Answer: b) Creaming
Which factor contributes to the physical instability of emulsions?
a) Increase in viscosity
b) Low temperature storage
c) Coalescence of droplets
d) Addition of stabilizers
Answer: c) Coalescence of droplets
How can the stability of emulsions be enhanced?
a) Adding surfactants
b) Increasing temperature
c) Reducing viscosity
d) Decreasing particle size
Answer: a) Adding surfactants
What is multiple emulsions?
a) Gas dispersed in solid
b) O/W/O or W/O/W systems
c) Solid dispersed in liquid
d) Mixture of two or more gels
Answer: b) O/W/O or W/O/W systems
Which surfactant is commonly used in oil-in-water emulsions?
a) Span 80
b) Tween 80
c) Lecithin
d) Sodium lauryl sulfate
Answer: b) Tween 80
Breaking of an emulsion refers to:
a) Formation of flocs
b) Separation of phases
c) Reduction in viscosity
d) Increase in interfacial area
Answer: b) Separation of phases