MICROMERITICS AND POWDER RHEOLOGY

MICROMERITICS AND POWDER RHEOLOGY

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 (d_mean):

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 d_mean is arithmetic mean

If p = 0 then d_mean is geometric mean

If p <>mean 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.