Powders&Sachets Flashcards
What is a powder ?
A pool of solid particles or same/ different chemical composition.
complex + heterogeneous
In what ways do powders act similar to: 1) liquids, 2) solids and 3) gases?
- FLOW like LIQUIDS
2) RESIST DEFORMATION (to a point) like SOLIDS
3) can be COMPRESSED like GASES
How can particle size be
a) estimated
b) determined?
a) equivalent sphere concept
1. Simplify size analysis for powders
2. Allow comparisons based on a single value
b) sieving- air jet sieve, sieving diameter (ds) corresponds to minimum apertrue size
what do Microscopy-based methods allow the determination of?
- Projected diameters =
independant of particle orientation (dp, da)
OR
dependant on particle orientation (dM. dF)
see video images
What is particle size distribution related to & whats it based on?
Relates to heterogeneity of a powder = impact on behaviour (dissolution rate, mixing, demixing, flow…)
- number distribution
- weight distribution
(* length, surface, volume distributions)
Particle size distribution: frequency curves-
Name for when particles all in same size class?
Narrow size distribution
Relation between particle sizes and size distribution?
higher variety of particle sizes detected in the powder = likely wider size distribution
obtained by generating a frequency histogram or curve.
how may size distribution be normalised for some powders?
by plotting the diameter values on a log scale
what do frequency curves + histograms allow the determination of?
whats the exception?
allow the mode, rather than the mean, to be determined, unless (of course) sizes are normally distributed
How to caluclate % frequency for particle size distribution table?
n (num of particles/freq) / total freq (add up whole column) x 100
normal distribution unlikely for powders but whats special about it?
mean, mode, median have same value
what is a skewed size distibution and what can be determined?
+ positively skewed: left peak = smaller mode, mean HIGHER than mode.
larger particles
- negatively skewed: right peak = higher mode value, mean LOWER than mode.
smaller particles
can only determine MODE directly
what does multimodal distribution imapct on?
2 modes- 2 peaks, use highest mode
impl on:
behaviour, properties of powder and final dosage form
what does cumulative frequence allow?
what does curve shape depend on
sigmoidal shape, MEDIAN rather than mean, to be determined.
shape changes depending on narrow/wideness of size distribution
what does sieving analysis allow?
a frequency can be determined for the undersize or oversize fractions
view table in noted about calculating frequency and %s.
how to obtain
a) number size distibution
b) weight distribution
a) microscopy based methods
b) sieving methods
T/F: BP clasisfication of powder fineness is based on mean particle size?
FALSE
A 250 micrometer sieve is placed on top of a receiver pan. The fraction retained on the sieve corresponds to the XXX fraction; the fraction of powder recovered in the receiver pan corresponds to the YYY fraction.
Retained XXX = oversize
Recovered YYY = undersize
For powders, flowability will depend on the balance between what two types of forces?
Flow promoting: drive flow
- gravity
- particle mass (related to density)
- angle of inclination
- applied mechanical force
and drag forces: resist flow x adhesion (diff) x cohesion (like) x electrostatic interactions x surface forces
Name the 2 drag forces that vary with particle size and moisture content
adhesion and cohesion
in what condition will a powder flow on?
when (flow promoting forces) > (drag forces)
what powder properties affect powder flow and whats the IDEAL for each?
particle shape: spherical
particle size: big (free flowing, more gravity, less adhesion+cohesion)
packing and porosity:
density: higher density
(moisture content: Low but not too dry)
ONLY IF ALL ELSE EQUAL
what does packing influence and what are the loosest and tightest forms?
how solid partcicles arranged in powder bed, will influence volume of powder bed.
Cubic: loosest, most porous
Rhombohedral: tightets, least porous
whats porosity and what is it for aggregates, real powder and polydisperse:
Porosity (e)= measure of how loosely/ tightly particles are packed.
For particles arranged in a cubic geometry: 48% of the powder volume is void.
Rhombohedral: 26%.
highest porosity: aggregates
then real powder
then polydisperse