CHAPTER-2
MICROMERITICS AND POWDER RHEOLOGY
Syllabus:
Average particle size, Particle size distribution, Number
and weight distribution.
Methods of determining
particle size: optical microscopy, sieving, sedimentation. Particle volume
measurement, Particle shape, Specific surface area.
Methods of determining
surface area: adsorption method, air permeability method.
Derived properties of
powders: porosity, packing arrangements, densities, bulkiness and flow
properties.
Definition: The
science and technology of small particles is called micromeritics.
Units mostly used in micromeritics: micron (mm) [ 1 mm = 10–6m =
10–4 cm = 10–3 mm]
Importance of micromeritics in pharmacy
Size and size range of particles are very
important in pharmacy.
1. Size
and surface area of particles are related to physical, chemical and
physiological properties of a drug.
2. The
necessary flow properties of solid powders in tablet and capsule manufacture
depends on the particle size, size distribution and size distribution.
3. Particle
size of a drug affects the release from a dosage form administered orally,
parenterally, rectally, and topically.
Dissolution rate is faster from smaller particle size due to its high
specific surface area.
4. Rate
of sedimentation in suspension and rate of creaming in emulsion is faster with
larger particles. Hence, to make a stable suspension or emulsion the particle
size must be controlled.
5. For
accurate determination of pore size of filters the size of particles are
required.
6. Antigens
are coated on adsorbent particles where the particle size is important for
uniform dose calculation.
PARTICLE SIZE AND SIZE DISTRIBUTION
A
powder sample is characterized by three things: (a) shape of the particles, (b)
size of the particles and (c) size distribution of the particles. If the shape
of a particle is perfectly spherical it is easy to express it by its diameter;
but if it is not spherical then it becomes very difficult to express in
diameter. Most of the pharmaceutical particles are not perfectly spherical.
If
all the particles have same diameter then the powder sample is called a monodisperse system, but if all the
particles are not of equal sizes then that powder sample is called polydisperse system. Most of the
pharmaceutical powders and dispersion are polydisperse systems.
Average particle size
Suppose a powder sample is examined under microscope and the
diameter of the particles are measured individually. The average diameter can
be expressed in various ways. Edmundson has derived a general eqution for the average particle size (dmean):
where n = number of
particles in a certain size range
d =
diameter of those particles
p = 1
stands for length, p = 2 stands for surface area and p = 3 stands for volume of
the particle
If p
> 0 (i.e. p is positive) then dmean is arithmetic mean
If p = 0
then dmean is geometric mean
If p
< 0 then dmean is harmonic mean
f = frequency index [f has values
of 0, 1, 2, 3 then the frequency distribution is expressed in number (0),
length (1), surface (2) and volume or weight (3) of the particles,
respectively.]
Let us take the following example:
|
Diameter
(mm) (d)
|
Number (n)
|
nd
|
nd2
|
nd3
|
nd4
|
|
0.75
1.25
1.75
2.25
2.75
3.25
3.75
|
2
10
22
54
17
8
5
|
1.50
12.50
38.50
121.50
46.75
26.00
18.75
|
1.13
15.63
67.38
273.38
128.56
84.50
70.31
|
0.85
19.54
117.92
615.11
353.54
274.63
263.66
|
0.64
24.43
206.36
1384.00
972.24
892.55
988.73
|
|
|
 = 118 |
 = 265.50 |
 = 640.89 |
 = 1645.25 |
 = 4468.95 |
Statistical diameters:
 |
p
|
f
|
Type
of mean
|
Size
parameter
|
Frequency
|
Mean
Diameter
|
Value
of data obtained from the table
|
Comments
|
 |
1
|
0
|
Arithmetic
|
Length
|
Number
|
Length-Number mean,
dln.
|
2.25
|
Rarely
found in pharmaceutical powders
|
 |
2
|
0
|
Arithmetic
|
Surface
|
Number
|
Surface-number
mean, dsn.
|
2.33
|
Refers
to particles having average surface area.
|
 |
3
|
0
|
Arithmetic
|
Volume
|
Number
|
Volume-number mean,
dvn.
|
2.41
|
Refers
to particles having average weight
|
 |
2
|
1
|
Arithmetic
|
Surface
|
Length
|
Surface-length
mean, dsl.2.41
|
2.41
|
No
practical significance
|
 |
3
|
2
|
Arithmetic
|
Volume
|
Surface
|
Volume-Surface
mean, dvs.
|
2.57
|
Inversely
related to specific surface area*
Important
pharmaceutically
|
* Specific surface area =
Particle size distribution
When the number or weight of particles lying within a
certain size range is plotted against the size range or mean particle size, a
graph is obtained, that is called as frequency
distribution curve.
Estimation
of flow properties of a powder:
Two indicators of flow properties are
there (i) Angle of repose (f) and (ii) Flow rate
Angle of repose
This is the maximum angle possible
between the surface of a heap of powder and the horizontal plane.
It is an indicator of the frictional
and cohesive forces in a loose powder.
|
Method-1:
|
Method-2
|
Method-3
|
  
Powder is released slowly through a
funnel on a horizontal round surface. A heap will form. When the particles
will glide over the heap the addition was stopped and the height (h) was
measured. The diameter (D) of the round surface was known previously.
Angle of repose, f =
 |
A hollow cylinder is half-filled
with the powder. One end of the cylinder is sealed with a transparent plate.
The inside surface of the cylinder is lined with sand paper to reduce the
slip of powder. The cylinder is rotated about its horizontal axis, until the
powder surface cascades. The value of f is measured with a
protractor.
|
A rectangular box is lined with
sand paper. The box is filled with powder and tilted slowly until the powder
begins to slide. The angle, f, is measured as the angle of repose.
|
Flow rate measurement:
The flow rate of
granules (less cohesive materials) may be assessed by passing the powder
through a circular orifice fitted in the base of a cylindrical container. The powder is taken in the container and
released through the orifice on the pan of a balance. The weight of powder or
granules falling per unit of time is recorded.
To improve the flow properties of
granules a type of powders are used, they are called glidants. Examples of glidants are talc, corn starch etc.