Greco lecture 3: particle size distribution Flashcards Preview

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Flashcards in Greco lecture 3: particle size distribution Deck (23):

Why is particle size so important?

Can affect:

1. Drug bioavailability

2. Settling rate (suspensions)

3. Possibility of obtaining homogenous mixtures (maintaining it)

4. Flowability

5. Properties that are important for the transformation of powders into a tablet

6. Tolerability of some dosage forms (ocular delivery)


What are the different forms of graphs that are seen for particle size distribution?

1. Monosized distribution

2. Normal distribution

3. Positively skewed distribution

4. Bimodal distribution


What is a cumulative frequency graph in relation to particle size?

1. Percentage of particles that have a certain diameter or less

2. Add up the last percentages


How do you separate different particle sizes?

1. Sieving, same method as size analysis

2. Large column with different levels of mesh size of sieves


What's the aim of size seperation?

1. Separate different sized molecules to pick and process it further


What is the sedimentation method used for?

1. Size analysis and also size separation

2. Bigger particles travel a longer distance than a smaller particle in a given time

3. Bigger will arrive at the bottom faster which means you can collect them


What is the elutriation method?

Fluid direction is opposite to the sedimentation direction


What is cyclone separation?

1. Injection at high speed

2. Particles are in suspension

3. Centrifugal because of the circular spin

4. Sedimentation because the fluid is going down

5. Particles are deposited at the end


What is the V bulk?

The volume occupied by a powder including the pores (empty gap between particles)


What is the V true?

The volume occupied by a powder excluding the volume occupied by the pores


What is the apparent volume before settling?

1. The volume that's measured before settling (bulk volume)

2. 100g of substance measured in dry cylinder without compacting



What is the apparently volume after settling?

1. The volume that's measured after settling (settled volume)


How do you work out the bulk density?

Mass/ V bulk


How do you work out the true density?

Mass / V true


How do you work out the packing fraction (k)?

Bulk density/ True density


How does the bulk density of the powder compare to the true density?

Bulk density of powder is always less than true density


What is porosity?

The percentage of voidage of the powder bed


How do you work out porosity?

1. Volume pores/ Volume Bulk

2. Volume bulk - Volume Particles/ Volume Bulk

3. 1 - Volume particle/ Volume bulk


What are the forces that favour powder flow?

- Gravity
- True density of particles
- Angle of the surface over which particles are flowing


What are the forces that are against powder flow?

- Adhesion forces (particles and container)- the attraction your powders compared to something else

- Cohesion forces (particles to particles)


What do fine powders have that normal powders don't?

1. High surface to mass ratio

2. Higher cohesion


How do you calculate the flowability of a powder and what do the results show?

1. Angle of repose

2. High= powder is cohesive

3. Low= powder is non cohesive


How do the flow properties correspond to higher angle and a smaller angle in terms of angle of repose practical?

1. Higher the angle, the more poor the flow is: 66 degrees

2. Lower the angle, the more excellent it is: 25 to 30 degrees