Lecture 9 - Particle Science I

Question 1

What can particle size effect?

Answer 1

drug bioavailability

settling rate (suspensions)

possibility of obtaining homogenous mixtures (and maintaining it)

flowability

other properties important for transformation into tablets and capsules

tolerability of some dosage forms

Question 2

What is a way to reduce particle size?

Answer 2

Milling

Question 3

How can we select powders of desired size?

Answer 3

Sieving

Question 4

What are cutting methods?

Answer 4

the material is cut by one or more blades

Question 5

Give an example of a cutting method?

Answer 5

cutter mill

• the starting material is fed into this and there is a rotating propellor inside with blades, which cut the material

it also has stationary blades and a screen so the cut product falls through the bottom and is collected

Question 6

What are compression methods?

Answer 6

A pressure is applied to reduce particle size

Question 7

Give an example of a compression method?

Answer 7

Compression mills eg roller mills

one roller is driven and the other is still, we feed our material into the top and this process begins to break down the particles

the second ball moves due to friction

Question 8

What is an example of a traditional compression method?

Answer 8

mortar and pestle

Question 9

What are impact methods?

Answer 9

Particles are hit by a moving surface or moving particles hit a surface

Question 10

Give an example of an impact method?

Answer 10

Hammer mill

starting material is fed into the machine and inside there is a rotating disc with hammers attached

at the bottom there is a milling screen so particles that are small enough can fall through and are collected

Question 11

What are attrition methods?

Answer 11

applying pressure AND friction to break down particles

Question 12

Give an example of attrition methods?

Answer 12

roller mills

3 rollers aligned horizontally with an adjustable gap, the material is fed through the gap and the balls rotate at different speeds

the material is sheered as it is passed through the gap

material is transferred from the slower roller to the faster roller

a scraper is used to collect material

Question 13

What type of force is a ball mill?

Answer 13

combined impact and attrition methods

Question 14

How does a ball mill work?

Answer 14

we put our material in a drum and add balls (made of steel, ceramic etc)

the balls move around within the space to break down the material

the speed at which the drum is turned is critical

Question 15

What happens if a ball mill is moved too slowly?

Answer 15

the balls will sit at the bottom and not come into contact with the material

Question 16

What happens if a ball mill moves too quickly?

Answer 16

the balls will be pushed to the walls of the drum and are not breaking down the material

Question 17

What is the fluid in a fluid energy mill?

Answer 17

Air

Question 18

How does a fluid energy mill work?

Answer 18

there is an inlet of solid material and there is air flowing around the circuit

the inlet creates turbulence so the solid collides with the walls of the system and between themselves which reduces particle size

the material is removed and collected

Question 19

How is the appropriate mill chosen?

Answer 19

depends upon what particle size we need to obtain

characteristics of the material

other factors e.g. cost, time, stability of the ingredients

Question 20

What material are cutter mills good for?

Answer 20

elastic, fibrous materials like roots and wood

Question 21

What materials are attrition methods good for?

Answer 21

Ointments, solids in suspension and pastes

Question 22

What materials are impact methods good for?

Answer 22

brittle materials

Question 23

What is the approximation made when analysing particle size?

Answer 23

particles are spheres (equivalent diameter)

Question 24

What is the projected area diameter?

Answer 24

(da) we squeeze the area of the particle in a circle, this is the diameter of the corresponding circle

Question 25

What is the projected perimeter diameter?

Answer 25

(dp) draw a perfect circle around our particle and measure the diameter

Question 26

What is the size of the particle dependent on?

Answer 26

the orientation

Question 27

What is Feret's diameter?

Answer 27

(df) the mean distance between 2 parallel tangents of the projected outline of our particle

Question 28

What is Martin's diameter?

Answer 28

(dm) the mean short length which are the bounds of 2 separating equal particle diameters

Question 29

Does martin's diameter change with orientation?

Answer 29

No it is the same for the different orientations

Question 30

What are direct ways to analyse particle size?

Answer 30

sieving the particle microscopy - looking down the microscope to determine the size

Question 31

Indirect methods to analyse particle size?

Answer 31

they determine a parameter correlated with size e.g. sedimentation rate and permeability

Question 32

Ideally what particle size distribution do we want?

Answer 32

monosized - all particles are essentially identical

Question 33

what distribution of particle size do we often see?

Answer 33

Normal distribution - most of the particles are of one size

Question 34

What is the size range of light microscopy?

Answer 34

1-1000um

Question 35

What is light microscopy?

Answer 35

A drop of very diluted suspension is placed on a microscope slide and particles are measured in relation to the equivalent diameter chosen (da, dp, df, dm)

Question 36

What is the size range of electron microscopy?

Answer 36

as low as 0.001um

Question 37

What are the types of electron microscopy?

Answer 37

scanning electron microscopy (SEM) transmission electron microscopy (TEM)

Question 38

What is scanning electron microscopy?

Answer 38

this uses electrons to look at our sample and improves the resolution gives a 3D image, gives info on shape expensive, requires a high level of operator expertise

Question 39

What is transmission electron microscopy?

Answer 39

we slice through our particles and look particles are suspended in plastic and we slice them, put them on a microscope and look at them if done properly we can see inside the particles

Question 40

Size range of coulter counter?

Answer 40

0.1 to 1000um

Question 41

What does a coulter counter measure?

Answer 41

the volume of particles

Question 42

How does a coulter counter work?

Answer 42

It has a negative and positive electrode solution phase and a small gap particles are drawn through this gap by a vacuum, as they flow through they block the gap this changes the electrical resistance and by computing this we can tell how big the particles are

Question 43

What are laser light scattering methods?

Answer 43

uses light to measure particle size - the laser light interacts with particles

Question 44

How does laser light scattering work?

Answer 44

light is diffracted by particles by an angle that is inversly proportional to the volume of the particles a detector analyses the radiation diffracted by the particles

Question 45

What are the most commonly used lasers in laser light scattering?

Answer 45

Helium and neon lasers

Question 46

What are sieves classified based on?

Answer 46

the sieve aperture diameter (expressed in um)

Question 47

What does a sieve 1000 mean?

Answer 47

the sieve aperture is 1000um (i.e. 1mm)

Question 48

What is the sieve aperture diameter?

Answer 48

the distance between two consecutive wires

Question 49

How do sieve methods work?

Answer 49

Sieves pull out certain fractions of the material, we can stack sieves with different mesh sizes and all the material goes through, it will separate depending on size

Question 50

What is sedimentation rate based on?

Answer 50

stoke's equation

Question 51

How does separation by sedimentation work?

Answer 51

we give the particles time to settle, the larger particles will travel a longer distance compared to small ones through this process we can measure out particles sizes

Question 52

What is the best way to do separation by sedimentation?

Answer 52

Using an andreasen pipette can take small samples at different times

Question 53

How does cyclone separation work?

Answer 53

particles are suspended in a fluid air flow creates a vortex the large particles will drop out at the bottom because the gravitational pull is stronger than the process as the vortex changes direction at the bottom, it takes small fine particles up with it and larger ones that are too heavy will fall out

Question 54

What is the fluid in cyclone separation?

Answer 54

Air

Question 55

What is elutriation methods?

Answer 55

elutriation force wants to pull the particles up, but for larger particles gravitational pull is stronger sedimentation will win for larger particles but for smaller particles, the driving vector is the elutriation so these will be taken up

Question 56

What is fluid direction in relation to sedimentation direction?

Answer 56

OPPOSITE