1. UNIT-IV 10 Hrs.
Micromeritics:
Particle size and distribution, mean particle size, number and weight distribution,
particle number, methods for determining particle size by different methods,
counting and separation method, particle shape, specific surface, methods for
determining surface area, permeability, adsorption, derived properties of
powders, porosity, packing arrangement, densities, bulkiness & flow properties.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
3. By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
https://youtu.be/ExvzdUA6Ed0
Micromeritics introduction
4. Definition: It is the science and technology of small particles.
* Given by Dalla Valle
๏ง The unit of particle size used, micrometer (ยตm), micron (ยต).
Micron (ยตm) [ 1 ยตm = 10โ6m = 10โ4 cm = 10โ3 mm]
* As particle size decreases ๏ฏ, area increases ๏ญ
Micromeritics
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
5. 1. Release and dissolution
๏ง Particle size and surface area influence the release of a drug from a
dosage form.
๏ง Higher surface area allows intimate contact of the drug with the
dissolution fluids in-vivo and increases the drug solubility and
dissolution.
2. Absorption and drug action
๏ง Particle size and surface area influence the drug absorption and
subsequently the therapeutic action.
๏ง Higher the dissolution, faster the absorption and hence quicker and
greater the drug action.
Importance of micromeritics in pharmacy
Size and size range of particles are very important in pharmacy.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
6. 3. Physical stability
๏ง The particle size in a formulation influences the physical
stability of the suspensions and emulsions.
๏ง Smaller the size of the particle, better the physical stability of
the dosage form.
4. Dose uniformity
๏ง Good flow properties of granules and powders are important in
the manufacturing of tablets and capsules.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
7. 5. Determination of pore size of filters
๏ง For accurate determination of pore size of filters the size of
particles are required.
4. Coating of Antigens
๏ง Antigens are coated on adsorbent particles where the particle
size is important for uniform dose calculation.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
8. A powder sample is characterized by three things:
a) Shape of the particles,
b) Size of the particles and
c) Particle Size distribution
Fundamental properties
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
9. By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
https://youtu.be/307cVsjR2_k
Shape of the Particles
10. By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
https://youtu.be/_BHE1ma7e78
Particle size and
Equivalent diameter
11. By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
https://youtu.be/8lmWaz7CygU
Average Particle Size
and Particle Size
Distribution
12. Particle Size
The size of a sphere is readily expressed in terms of its diameter.
๏ง Diameter of sphere = 2r
๏ง The surface area of sphere is = 4ฯr2
๏ง Volume of sphere = 4/3ฯr3
๏ง Generally pharmaceutical powders are non-spherical in nature and
their surface area and volume change with dimension. Therefore it is
difficult to express the size as a meaningful diameter.
๏ง In such case it is expressed in terms of equivalent spherical diameters.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
13. ๏ง The Surface diameter, ds, is the diameter of a sphere with the same
surface area as the particle.
๏ง The Volume diameter, dv, is the diameter of a sphere with the same
volume as the particle.
๏ง The Projected diameter, dp, is the diameter of a sphere with the same
projected area observed under microscope.
๏ง The Stokes diameter, dst, is the diameter which describes an equivalent
sphere undergoing sedimentation at the same rate as the particle.
๏ง The Sieve diameter, dsieve, is the diameter of a sphere that will just pass
through the same aperture size as the particle.
๏ง The volume surface diameter, dvs, is the diameter of a sphere that has
the same ratio of surface area to volume as for the particle.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
14. Average Particle Size
๏ง We can conduct a microscopic examination of a sample of a powder
and record the number of particles.
๏ง Particles lying within various size ranges can be obtained and data
can be plotted shown in Table.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
15. ๏ง Edmundson derived a general equation for the average particle size,
whether it be an arithmetic, geometric or a harmonic mean diameter
๐ ๐๐๐๐ =
๐ฎ๐๐ ๐+๐
๐ฎ๐๐ ๐
๐/๐
n = the number of particles.
d = equivalent diameters.
p = an index related to the size of an individual particle ( because d
raised to the power p =1, p =2, or p =3 is an expression of the particle
length, surface, or volume, respectively.)
The value of the index p also decides whether the mean is;
arithmetic (if p is positive),
geometric (if p is zero), or
harmonic (p is negative).
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
16. ๏ง For a collection of particles, the frequency with which a particle in a
certain size range occurs is expressed by nd f.
๏ง When the frequency index, f,
has values of 0, 1, 2, or 3, then
the size frequency distribution
is expressed in terms of the
total number, length, surface,
or volume of the particles,
respectively.
Statistical diameters
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
17. 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.
๏ง Most of the pharmaceutical powders and dispersion are
polydisperse systems.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
18. ๏ง Any collection of particles is usually polydisperse. It is
therefore necessary to know not only the size of a certain
particle, but also how many particles of the same size exist in the
sample.
๏ง Thus, we need an estimate of the size range present and the
number of each particle size.
๏ง This is the particle-size distribution and from it we can calculate
an average particle size for the sample.
Particle Size Distribution
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
19. ๏ง When the number 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.
๏ง The width of histogram represent the size range, and the height
represent the frequency of occurrence in each size range.
๏ง This is important because it is possible to have two samples with
the same average diameter but different distributions.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
20. Number and Weight Distributions
๏ง The data in Table are shown as a number distribution, as they were
collected by a counting technique such as microscopy.
๏ง We can get data based on a weight, rather than a number,
distribution.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
21. ๏ง It is possible to convert the number distribution to a weight distribution and
vice versa.
๏ง As the general shape and density of the particles are independent of the size
range present in the sample, an estimate of the weight distribution of the data
in Table can be obtained by calculating the values shown in columns 9 and
10.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
22. ๏ง The significant differences in the two distributions is apparent, even
though they relate to the same sample.
๏ง For this reason, it is important to distinguish carefully between size
distributions on a weight and a number basis.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
23. Particle Number
๏ง A significant expression in particle technology is the number of
particles per unit weight, N, which is expressed in terms of dvnยท
The number of particles per unit weight is obtained as follows.
Assume that the particles are spheres,
The volume of single particle is = ๐ ๐ ๐
๐
The mass is = ๐ ๐ ๐๐
๐
๏ง The number of particles per gram is then obtained from the equation
๐ต =
๐
๐ ๐ ๐
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
24. Methods for determining particle size
Many methods available for determining particle size such as;
1. Separation methods
A. Sieving.
B. Sedimentation.
2. Counting methods
A. Optical microscopy.
B. Conductivity method.
3. Surface area determination
A. Air permeability
B. Adsorption
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
25. By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
https://youtu.be/MtbfJy3LIsQ
Methods for
determination of Particle
size & Particle size
distribution.
26. A guide to range of particle sizes applicable to each method is;
Particle size Method
< 1 ๏ญm Electron microscope,
Ultracentrifuge,
Adsorption
1 โ 100 ๏ญm Optical microscope,
Sedimentation,
Coulter counter,
Air permeability
๏พ50 ๏ญm Sieving
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
27. By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
https://youtu.be/E4WoS5Vth6o
Sieve Analysis
28. Sieving
(range: 50-1200 ยตm)
๏ง Standard size sieves are available to cover a
wide range of size.
๏ง Powder is placed on the previously arranged
set of sieves in which the sieves are
arranged in decreasing order of their mesh
size.
๏ง The set of sieve is shaken for a fixed period
(1 to 5 min), using mechanical shaker.
๏ง The material which is retained on each sieve
is collected & weigh separately to obtain
frequency distribution curve.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
29. Advantages
๏ง Easy to perform, simple
๏ง Wide size range
๏ง Inexpensive
Disadvantages
๏ง Known problems of reproducibility.
๏ง Wear/damage of sieve in use or cleaning.
๏ง Irregular/agglomerated particles cause misinterpretation.
๏ง Rod-like particles : overestimate of under-size.
๏ง Duration of shaking may alter the result.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
30. Sedimentation
(range: 0.08-300 ยตm)
๏ง Methods depend on the fact that the terminal velocity of a
particle in a fluid increases with size.
๏ง Stokesโ equation or Stokesโs law
๐ =
๐
๐
=
๐ ๐๐
๐
๐๐ โ ๐๐ ๐
๐๐๐ผ
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
31. ๐ ๐๐ =
๐๐๐ผ
๐๐ โ ๐๐ ๐
๐
๐
๐ ๐๐ = Stokesโs diameter
๐ = distance of fall in time t
๐๐ = density of particles
๐๐ = density of dispersion medium
๐ผ = viscosity of medium
๐ = gravitational acceleration.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
32. Andreasen Pipette
๏ง A 1% or 2% suspension of the particles in a medium
containing a suitable deflocculating agent is
introduced into the vessel and brought to the 550-ml
mark.
๏ง The stoppered vessel is shaken to distribute the
particles uniformly throughout the suspension, and
the apparatus, with pipette in place, is clamped
securely in a constant-temperature bath.
๏ง At various time intervals, 10 ml samples are
withdrawn and discharged by means of the two-way
stopcock.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
33. ๏ง The samples are evaporated and weighed or analyzed by other
appropriate means.
๏ง The particle diameter corresponding to the various time periods is
calculated from Stokes's law.
๏ง h in equation being the height of the liquid above the lower end of the
pipette at the time each sample is removed.
๏ง The residue or dried sample obtained at a particular time is the weight
fraction.
๏ง The cumulative percentage by weight undersize can then be plotted
on a probability scale against the particle diameter
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
34. Disadvantages
๏ง Sedimentation analyses must be carried out at concentrations
which are sufficiently low for interactive effects between particles
to be negligible.
๏ง Large particles create turbulence.
๏ง Careful temperature control is necessary.
๏ง Particle re-aggregation during extended measurements.
๏ง Particles have to be completely insoluble in the suspending liquid.
Advantages
๏ง Equipment required can be relatively simple and inexpensive.
๏ง Can measure a wide range of sizes with considerable accuracy
and reproducibility.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
35. Conductivity method / Coulter counter
๏ง Particle size ranging from 0.5 to 500 ฮผm is measured by this
method.
๏ง In this method particle volume is measured and converted in to
particle diameter.
๏ง Size is expressed as volume diameter, dv.
๏ง This is quick & accurate method.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
36. ๏ง Particles are suspended in a conducting electrolyte and placed in beaker.
๏ง The electrolyte solution is filled In sample cell, that has an orifice and
maintain contact with the external medium.
๏ง Electrodes are placed in
the solution (in side the
cell) and suspension
outside.
๏ง A known volume of a
dilute suspension is
pumped through the
orifice.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
37. ๏ง A constant voltage is applied across the electrodes to produce a
current.
๏ง As the particle travels through the orifice, it displaces its own volume
of electrolyte, and this results in an increased resistance between the
two electrodes.
๏ง The change in resistance, which is related to the particle volume,
causes a voltage pulse that is amplified and fed to a pulse-height
analyzer calibrated in terms of particle size.
๏ง The instrument records electronically all those particles producing
pulses that are within two threshold values of the analyzer.
๏ง By systematically varying the threshold settings and counting the
number of particles in a constant sample size, it is possible to obtain a
particle-size distribution.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
38. By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
39. Advantages
๏ง 4000 particles per second can be counted. Therefore size
distribution analysis can be completed in relatively short time.
๏ง It gives accurate results.
Disadvantages
๏ง Unsuitable for polar and highly water soluble materials due to
solvation.
๏ง Instrument is expensive.
Applications
In study of;
๏ง Particle growth in suspension and solution.
๏ง Dissolution of drugs in desired medium
๏ง Effect of antibacterial agent on the growth of micro-organism.
๏ง Size distributions of the mineral part of human kidney stones.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
40. By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
https://youtu.be/bzcc_DqHvqM
Optical Microscopy
41. ๏ง The microscope eyepiece is fitted with a micrometer by which
the size of the particles may be estimated.
Optical microscopy
(range: 0.2-100 ยตm)
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
42. ๏ง According to the optical microscopic method, powder is directly
sprinkled on glass slide (or an emulsion or suspension drop is
mounted on ruled slide on a mechanical stage.)
๏ง The microscope eyepiece is fitted with a micrometer (ocular
micrometer or eye piece scale) which is calibrated by using stage
micrometer.
๏ง By matching the particle against the eye piece scale, the size of the
particles can be estimated.
๏ง The ordinary microscope used for measurement the particle-size in the
range of 0.2 to about 100 ยตm.
๏ง For reproducible result atleast 300-500 particles must be observed.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
43. ๏ง The particles are measured along an arbitrarily chosen fixed line,
generally made horizontally across the center of the particle.
๏ง Popular measurements are the
1. Feret diameter
2. Martin diameter
3. projected area diameter.
Martin's diameter is the length of a
line that bisects the particle image.
Feret's diameter is the distance between two tangents on opposite sides of
the particle parallel to some fixed direction, the y direction.
Projected area diameter is the diameter of a circle with the same area as
that of the particle observed perpendicular to the surface on which the
particle rests.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
44. Disadvantages
1. The diameter is obtained from only two dimensions of the particle.
2. The number of particles that must be counted (300-500) to obtain a
good estimation of the distribution makes the method somewhat slow
and tedious.
3. Not suitable for extremely small particles.
4. Unknowingly same particle may be observed repeatedly.
Advantages
1. Simple method.
2. Particles are easily observed.
3. Agglomeration and contamination of particles can be detected.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
45. ๏ง The geometric shape and surface condition of particles refers to shape
of particles.
๏ง Particles may be spherical, rounded, elongated, acicular, angular etc.
๏ง 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. Hence, equivalent diameters are taken in to consideration.
๏ง Most of the pharmaceutical particles are not perfectly spherical.
๏ง Particle shape affects the physicochemical, biopharmaceutical
properties of formulation.
Particle Shape
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
46. Particle shape and surface area
๏ง The shape affects the flow and packing properties of a powder, as
well as having some influence on the surface area.
๏ง The surface area per unit weight or volume is an important
characteristic of a powder in adsorption and dissolution rate
studies.
๏ง A sphere has minimum surface area per unit volume.
๏ง The more asymmetric a particle, the greater is the surface area per
unit volume.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
47. ๏ง The surface area or volume of a sphere can be calculated simply by
๐๐ฎ๐ซ๐๐๐๐ ๐๐ซ๐๐ = ๐ ๐ ๐
๐๐จ๐ฅ๐ฎ๐ฆ๐ =
๐ ๐ ๐
๐
๏ง The surface area and volume of a spherical particle are therefore
proportional to the square and cube, respectively, of the diameter.
๏ง To obtain the surface or volume of a particles whose shape is not
spherical one must choose an equivalent diameter through correction
factor (e.g. for microscopic method, projected diameter, dp)
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
48. ๏ง By means of proportionality constants, we can then write
๐๐ฎ๐ซ๐๐๐๐ ๐๐ซ๐๐ = ๐ถ๐๐ ๐
๐ = ๐ ๐ ๐
๐
where ๐ถ๐ is the surface area factor and ds is the equivalent surface
diameter.
๏ง For volume we write
๐๐จ๐ฅ๐ฎ๐ฆ๐ = ๐ถ๐๐ ๐
๐ =
๐ ๐ ๐
๐
๐
Where ๐ถ๐, is the volume factor and dv, is the equivalent volume
diameter.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
49. ๏ง The surface area and volume factor โshape factorsโ are, in reality,
the ratio of one diameter to another.
๏ง Thus, for a sphere,
๐ถ๐ =
๐ ๐ ๐
๐
๐ ๐
๐
= ๐. ๐๐
and
๐ถ๐ =
๐ ๐ ๐
๐
๐๐ ๐
๐
= ๐. ๐๐๐
๏ง There are as many of these volume and shape factors as there are
pairs of equivalent diameters.
๏ง The ratio ๐ถ๐/๐ถ๐, is also used to characterize particle shape.
๏ง When the particle is spherical, ๐ถ๐/๐ถ๐= 6.0
๏ง The more asymmetric the particle, the more this ratio exceeds the
minimum value of 6.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
50. Specific surface
Specific surface area is defined as the surface per unit weight (Sw) or unit
volume (Sv) of the material.
๏ง It can be derived from equations given above.
๏ง Taking the general case, for asymmetric particles where the
characteristic dimension is not yet defined,
Sv =
Surface area of particles
volume of particles
Sv =
๐ถ๐๐ ๐
๐ถ๐๐ ๐ =
๐ถ๐
๐ถ๐๐
=
๐
๐
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
51. ๐บ๐ =
๐
๐ ๐๐ ๐
๐บ๐ =
๐๐
๐
๏ง The surface area per unit weight therefore,
Or
Sw =
๐ถ๐
๐ถ๐๐ ๐๐๐
Where the dimension is now defined as ๐ ๐๐ the volume-surface
diameter.
๏ง For spherical particles
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
52. ๏ง The above method of estimation is based on particle diameter
data. It can be used to estimate the specific surface area for non
porous solids.
๏ง For porous materials following methods are used;
1. Adsorption method
2. Air permeability method
Powder surface area-Methods
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
53. Adsorption method
๏ง Particles having large specific surface area are good adsorbent
of gases and solute form solution.
๏ง Amount of gas or solute that is adsorbed to form
monomolecular layer on the adsorbent is a function of surface
area of the powder.
Principle
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
54. ๏ง In this method solute from solution is adsorbed on the surface of test
material whose surface area is to be determined.
๏ง A known quantity of material (adsorbent) is added to solution and stirred
for sufficient period to attain adsorption equilibrium.
๏ง The mixture is the filtered and filtrate is analyzed to find unadsorbed
amount of solute. The difference in amount added and that of amount
unadsorbed gives amount of solute adsorbed.
๏ง From the knowledge of cross-sectional area of a molecule and total number
of solute molecules adsorbed, it is possible to calculate the surface area of
test material.
๏ง A practical example of this type is determination of surface area of
activated charcoal by adsorption method using aqueous acetic acid solution.
Monolayer solution adsorption
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
55. ๏ง Determination of surface area is carried out by gas adsorption method using
an instrument called Quantsorb.
๏ง A powder material whose surface area is to be determined is placed into a
cell of the instrument. The nitrogen gas is used as adsorbate.
๏ง Helium, an inert gas, which is not adsorbed, is also passed through the
powder in a cell.
๏ง A thermal conductivity detector measures the amount of nitrogen adsorbed
at every equilibrium pressure.
๏ง A bell-shaped curve is obtained on a strip-chart recording system.
๏ง The height of signal gives the rate of adsorption of nitrogen and the area
under the curve is calculated to obtain the amount of gas adsorbed on
powder sample.
Adsorption of gas on powders
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
56. Air permeability method
๏ง Powder is packed in the sample holder as a compact plug. In this
packing, surface-surface contact between particles appear as a series of
capillaries. The surface of this capillaries is a function of the surface
area of powder.
๏ง When air is allowed to pass through the powder bed at a constant
pressure, the bed resist the flow of air. This results in the pressure drop.
๏ง The greater the surface area per gram of powder, Sw, the greater the
resistance to flow.
๏ง The permeability of air for a given pressure drop is inversely
proportional to specific surface.
Principle
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
57. It consist of sample tube containing the packed powder sample with one end
connected to an air pump through a constant pressure regulator. The other end
is attached to a calibrated manometer.
Fisher subsieve sizer
Pressure
Varies
with
particle
size
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
58. Working
๏ง The air pump buildup air pressure and is connected to a
constant pressure regulator.
๏ง Air is passed through the air dryer to remove moisture.
๏ง Air is then allowed to flow through the packed powder in the
sample tube.
๏ง The flow of air is measured by manometer.
๏ง The level of the fluid in manometer indicate the average
diameter of the particles.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
59. The Koezeny-carman equation is used to estimate the surface area by
this method
๐ฝ =
๐จ
๐ผ ๐๐
๐
โ๐ท๐
๐ฒ๐
๐บ
(๐ โ ๐บ)๐
Where,
A = cross sectional area of bed (pack), cm2
โ๐ท = pressure difference of plug
t = time of flow, seconds
l = length of the sample holder
๐บ = porosity of powder
Sw = surface area per gram of powder, cm2
/g
๐ผ = viscosity of air
K = a constant of irregular capillaries
V = volume of air flowing through the bed, cm3
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
60. Derived properties of powders
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
61. 1. True density: The true density of a sample excludes the volume
of the pores and voids within the powder sample.
๐ป๐๐๐ ๐ ๐๐๐๐๐๐ =
๐พ๐๐๐๐๐ ๐๐ ๐๐๐๐ ๐๐
๐๐๐๐ ๐๐๐๐๐๐ ๐๐ ๐๐๐๐ ๐๐
2. Bulk density: The bulk density value includes the volume of all
of the pores within the powder sample.
๐ฉ๐๐๐ ๐ ๐๐๐๐๐๐ (๐) =
๐ด๐๐๐ ๐๐ ๐๐๐๐ ๐๐ (๐)
๐ฉ๐๐๐ ๐๐๐๐๐๐ (๐ฝ๐)
Density of powders
True volume (VP) = volume of powder itself.
Bulk volume (Vb) = true volume + volume of spaces between particles
Density is defined as weight per unit volume (W/V).
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
62. ๏ง Bulk density is used to check the uniformity of bulk chemical
(Q.C. measures).
๏ง The size of capsule is mainly determined by bulk volume for a
given dose of material. The higher the bulk volume, lower will be
bulk density & bigger the size of capsule.
๏ง It helps in selecting the proper size of container, packing material,
mixing apparatus in the production of tablet and capsule.
Applications of Bulk density
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
63. Porosity
๏ง Porosity or void fraction is a measure of the void (i.e. "empty")
spaces in a material, and is a fraction of the volume of voids over
the total volume,
๏ง Values between 0 and 1, or as a percentage between 0% and 100%.
๏ง Suppose a powder, such as zinc oxide, is placed in a graduated
cylinder and the total volume is noted. The volume occupied is
known as the bulk volume, Vb.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
64. Void volume = Bulk volume โ True volume
๐ฝ = ๐ฝ๐ โ ๐ฝ๐
๏ง If the powder is nonporous, that is, has no internal pores or
capillary spaces, the bulk volume of the powder consists of the
true volume of the solid particles plus the volume of the spaces
between the particles.
๏ง The volume of the spaces, known as the void volume, V, is given
by the equation
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
65. The porosity, or voids, ฮต, of powder is defined as; the ratio of the
void volume to the bulk volume of the packing.
๐ท๐๐๐๐๐๐๐(๐บ) =
๐๐๐๐ ๐๐๐๐๐๐
๐๐๐๐ ๐๐๐๐๐๐
๐ท๐๐๐๐๐๐๐(๐บ) =
๐๐๐๐ ๐๐๐๐๐๐ โ ๐๐๐๐ ๐๐๐๐๐๐
๐๐๐๐ ๐๐๐๐๐๐
๐บ =
๐ฝ๐ โ ๐ฝ๐
๐ฝ๐
Porosity is frequently expressed in percent
% ๐บ = ๐ โ
๐ฝ๐
๐ฝ๐
ร ๐๐๐
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
66. Porosity measurement
Porosity of powders can be measured by;
1. Direct method/Taping method
2. Water saturation method
3. Water evaporation method
4. Computed tomography method /industrial CT scanning
5. Mercury intrusion porosimetry
6. Gas expansion method
Applications of Porosity
๏ง Porosity influences the rate of disintegration and dissolution. The
higher the porosity, the faster the rate of dissolution.
๏ง Porosity is applied in the studies of adsorption & diffusion of drug
materials.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
67. Packing Arrangements
๏ง Powder beds of uniform-sized spheres can assume either two ideal
packing arrangements:
(a) closest or rhombohedral and
(b) open, loosest, or cubic packing.
(a) (b)
๏ง The theoretic porosity of a powder consisting of uniform spheres
in close packing is 26% and for loosest packing is 48%.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
68. ๏ง The particles in real powders, it is to be expected that the particle of
ordinary powders may have any arrangement intermediate between the
two ideal packings of Figure, an most powders in practice have
porosities between 30% and 50%.
๏ง If the particles are of greatly different sizes, the smaller ones may sift
between the larger ones to give porosities below the theoretical
minimum of 26%.
๏ง If powders containing flocculates or aggregates, which lead to the
formation of bridges and arches in the packing, the porosity may be
above the theoretical maximum of 48%.
๏ง In real powder systems, almost any degree of porosity is possible.
๏ง Crystalline materials compressed under a force of 100,000 lb/in.2 can
have porosities of less than 1%.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
69. Bulkiness
Specific bulk volume, the reciprocal of bulk density, is often called
bulkiness or bulk.
๏ง It is an important consideration in the packaging of powders.
๏ง The bulk density of calcium carbonate can vary from 0.1 to 1.3, and
the lightest or bulkiest type would require a container about 13
times larger than that needed for the heaviest variety.
๏ง Bulkiness increases with a decrease in particle size.
๏ง In a mixture of materials of different sizes, however, the smaller
particles sift between the larger ones and tend to reduce the
bulkiness.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
70. Flow properties
๏ง It depends on particle size, shape, porosity , moisture content and
density.
๏ง Powders may be free-flowing or cohesive (โstickyโ).
๏ง Powder transfer through large equipment such as hopper.
Uneven powder flow may results inโฆ
๏ excess entrapped air within powders โ capping or lamination.
๏ increase particleโs friction with die wall causing lubrication
problems, and increase dust contamination risks during powder
transfer.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
71. Tests to evaluate the flowability of a powder:
1. Carrโs compressibility index
๏ง A volume of powder is filled into a graduated glass cylinder and
repeatedly tapped for a known duration. The volume of powder
after tapping is measured.
๐ช๐๐๐โ๐ ๐๐๐ ๐๐ % =
๐ป๐๐๐๐๐ ๐ ๐๐๐๐๐๐ โ ๐ท๐๐๐๐๐ ๐๐ ๐๐๐๐ ๐ ๐๐๐๐๐๐
๐ป๐๐๐๐๐ ๐ ๐๐๐๐๐๐
ร ๐๐๐
Flow description % Compressibility
Excellent flow 5-15
Good 16-18
Fair 19-21
Poor 22-35
Very poor 36-40
Extremely poor >40
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
72. 2. Hausner's ratio
Hausner's ratio=Tapped density /Poured or bulk density
๏ง Hausner's ratio is related to interparticle friction
๏ Value less than 1.25 indicates good flow.
๏ Value greater than 1.5 indicates poor flow.
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
73. 3. The angle of repose ๐ฝ
๏ง The frictional forces in a loose powder can be measured by the
angle of repose ๐ฝ.
๐ญ๐๐๐ฝ =
๐
๐
๐ฝ = the maximum angle possible between the surface of a pile of
powder and horizontal plane
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa
74. ๏ง The sample is poured onto a horizontal surface and the angle of the
resulting pyramid is measured.
๏ง The user normally selects the funnel orifice through which the
powder flows slowly and reasonably constantly.
Significance
๏ Angle of repose less than 20 (excellent flow)
๏ Angle of repose between 20-30 (good flow)
๏ Angle of repose between 30-34 (Pass flow)
๏ Angle of repose greater than 40 (poor flow)
By; Khalifa M. Asif Y. Asst. Professor Ali-Allana College of Pharmacy, Akkalkuwa