Greco lecture 3: particle size distribution

Question 1

Why is particle size so important?

Answer 1

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)

Question 2

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

Answer 2

1. Monosized distribution
2. Normal distribution
3. Positively skewed distribution
4. Bimodal distribution

Question 3

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

Answer 3

1. Percentage of particles that have a certain diameter or less
2. Add up the last percentages

Question 4

How do you separate different particle sizes?

Answer 4

1. Sieving, same method as size analysis

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

Question 5

What’s the aim of size seperation?

Answer 5

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

Question 6

What is the sedimentation method used for?

Answer 6

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

Question 7

What is the elutriation method?

Answer 7

Fluid direction is opposite to the sedimentation direction

Question 8

What is cyclone separation?

Answer 8

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

Question 9

What is the V bulk?

Answer 9

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

Question 10

What is the V true?

Answer 10

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

Question 11

What is the apparent volume before settling?

Answer 11

1. The volume that’s measured before settling (bulk volume)
2. 100g of substance measured in dry cylinder without compacting

3.

Question 12

What is the apparently volume after settling?

Answer 12

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

Question 13

How do you work out the bulk density?

Answer 13

Mass/ V bulk

Question 14

How do you work out the true density?

Answer 14

Mass / V true

Question 15

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

Answer 15

Bulk density/ True density

Question 16

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

Answer 16

Bulk density of powder is always less than true density

Question 17

What is porosity?

Answer 17

The percentage of voidage of the powder bed

Question 18

How do you work out porosity?

Answer 18

1. Volume pores/ Volume Bulk
2. Volume bulk - Volume Particles/ Volume Bulk
3. 1 - Volume particle/ Volume bulk

Question 19

What are the forces that favour powder flow?

Answer 19

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

Question 20

What are the forces that are against powder flow?

Answer 20

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

Question 21

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

Answer 21

1. High surface to mass ratio

2. Higher cohesion

Question 22

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

Answer 22

1. Angle of repose
2. High= powder is cohesive
3. Low= powder is non cohesive

Question 23

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

Answer 23

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