Powders Flashcards
What is a powder
A collection of particles usually 10-100um in diameter
Bulk powder acts as a fluid
Flow isnt completely free
Manufacturing of tablets and capsules will rely on….
Flow properties of the drug and excipients
What are flow properties determined by
Particle size, shape and particle-particle interactions
A drug must ….
dissolve in GIT prior to absorption
Drug dissolution equation
Noyes-Whitney
dm/dt = DA(Cs-c) / h
dm/dt is dissolution rate
D is diffusion coefficient
A is the available surface area
cs is the solubility
c is the concentration
h is the diffusion layer thickness
Wh at is a particle
Is a single solid entity of a material usually in um size rnage
Nature of powders
Crystalline but doesnt mean single crystals
Crystal habit
External shape of Crystal - usually refers to single crystals
Monocrystaline systems
Have well defined habits - cubic, plates, needles
Polycrystalline systems
In practise most systems are poly
Composed of aggregates of micro crystals usually less than 1um
If we looked a a powder particle under a microscope
Non-uniform
Porly defines
Crystalline but composed of many tiny crystals bonded together
Complications
Individual particles can clump together to form aggregates
What does particle size mean
Hard to define
Particles can assume any shape
Each method of size analysis makes assumptions
What does it mean if a particle is monosized or mono disperse
Every particle is same size and hence population may be defined by a singular parameter
In practise we need to….
define both some sort of average size
and also the distribution around that average
Simplest way of displaying size data
Histogram where proportion is plotted against size
Histogram distributions
Symmetrical is normal distribution has one peak int he middle
+skewed and - skewed has a peak and tail on either side
Bimodal has 2 peaks
How to put numbers into distribution charts
Use the mode
Mode, median and interquartile range
Mode = size at which greatest frequency of particles occurs
Median = size at which 50% of particles are larger or smaller
IQ range = rnage between 25% and 75%
What is sieve analysis
- Set of sieves with widest mesh at top, finest at the bottom
- Sieves usually automatically shaken for a predetermined
period of time (usually around 20 minutes) - quick
- accurate
- not used often
Microscopy to measure size
- Light microscope can measure particles down to about 10 um, scanning electron microscope down to about 100 nm
- Old fashioned method
- This is a 2-dimensional technique
- size and shape
Electrical stream sensing one method
- Works on the basis of particles suspended in an electrolyte solution
- Particles pass through an aperture of known diameter. Electrodes either side of aperture will register increase in resistance as particle displaces its own volume of
electrolyte - measure volume diameter (diameter of sphere with the same volume as particle)
- used for range of 0.1-1000um
- quick and easy
- can cause blockages often
Laser light scattering methods
- Use diffraction of light as means of calculating diameter and size range
- Laser applied to suspension of particles in aqueous or non- aqueous medium
Two types: - large-particle analysers (measure 1-1000 um)
- photon correlation spectroscopy (for smaller particles, 10 nm – 1 um)
- rapid and reliable
Why is powder flow important
• Uniform feed into tableting or capsule filling equipment, leading to uniform weight and mechanical properties
• Ease of handling and transfer of powders
